Article on tire pressure and power generation
Robert Virzi
(1193, Vol 36, No 6, pp 661 - 666) In it the authors evaluate
the effect tire pressure has oxygen uptake and energy expenditure.
To cut to the chase, they found that
"We conclude that differences in rolling resistance caused by
varying tire pressure between 552 and 965 kPa, are too small to
be detected physiologically."
That surprised me. If my memory is correct, the range they were
testing in translates to 80 - 130 psi, which pretty much covers
most of the roadies in this group. Their measures were fairly
sensitive, and unlike previous studies, they factored out most
of the variation that may have influenced earlier reports (e.g.,
that by Schwinn Bicycle Company, 1987).
So pumping up to the point of bursting may give you a psychological
edge, but these authors claim it isn't measurably better in terms
of energy consumption.
By the way, the authors are Ryschon & Stray-Gundersen.
-Bob Virzi
--
rvi...@gte.com Think Globally. ===
+1(617)466-2881 === Act Locally!
Jobst Brandt
X-Newsreader: TIN [version 1.2 PL1.1]
> I came across an interesting article in the journal Ergonomics.
> (1193, Vol 36, No 6, pp 661 - 666) In it the authors (Ryschon &
> Stray-Gundersen) evaluate the effect tire pressure has on oxygen
> uptake and energy expenditure. To cut to the chase, they found that
> ...
> "We conclude that differences in rolling resistance caused by
> varying tire pressure between 552 and 965 kPa, are too small to be
> detected physiologically."
> ...
> So pumping up to the point of bursting may give you a psychological
> edge, but these authors claim it isn't measurably better in terms of
> energy consumption.
I suppose it would be more apparent on what they base their findings
if the data were available. I am certain that they are wrong in the
quote presented above. I have data and measurements that show beyond
a doubt that there are substantial differences. If the authors are
talking about going shopping at 10 mph or less they may have a point
but then I can't imagine why one would measure energy expenditure and
oxygen uptake of a non athlete in a non athletic endeavor.
For those who wish to verify the results, I will post the data that
can be graphed in a power curve-fit to easily see the characteristics
of various weights of tires.
Jobst Brandt
that contain drag values at incresing inflation pressures. The
variations are not insignificant and the drag forces aren't either.
As is apparent from the date, this is not new and does not represent
current tires from either Specialized or Avocet but the values are
representative of typical tires of this quality.
jobst_...@hplabs.hp.com
----------------------------------------------------------------------
Tires, AVOCET and SPECIALIZED, 18 Apr 86
Rolling resistance (g) vs. inflation pressure (kg/cm2) @ 50 kg load
Tire ID Size Sample Nominal wt Measured wt Width +
------- ---- ------ ---------- ----------- -------
Col 1 - Air Pressure (kg/cm2)
Col 2 - S Turbo/LR 700x25C (A) 205 234 21.
Col 3 - S Turbo/LR 700x25C (B)
Col 4 - S Turbo/LS 700x25C (A) 205 243 21.
Col 5 - S Turbo/LS 700x25C (B)
Col 6 - A Criterium/20 700x25C (A) 225 236 23.
Col 7 - A Criterium/20 700x25C (B)
Col 8 - A Timetrial/20 700x20C (A) 215 214 21.
Col 9 - A Timetrial/20 700x20C (B)
Col 10 - S Turbo/LR 700x28C (A) 225 291 24.
Col 11 - S Turbo/LR 700x28C (B)
Col 12 - S Turbo/LS 700X28C (A) 225 299 24.
Col 13 - S Turbo/LS 700X28C (B)
Col 14 - A Road/20 700x28C (A) 265 272 25.
Col 15 - A Road/20 700x28C (B)
Col 16 - S Turbo/R 700x25C (A) 180 188 21.
Col 17 - S Turbo/R 700x25C (B)
Col 18 - S Turbo/S 700x25C (A) 180 193 21.
Col 19 - S Turbo/S 700x25C (B)
Col 20 - A Criterium/30 700x25C (A) 190 182 22.
Col 21 - A Criterium/30 700x25C (B)
Col 22 - A Timetrial/30 700x20C (A) 165 168 20.
Col 23 - A Timetrial/30 700x20C (B)
Col 24 - S Turbo/R 700x28C (A) 220 248 24.
Col 25 - S Turbo/R 700x28C (B)
Col 26 - S Turbo/S 700x28C (A) 220 253 24.
Col 27 - S Turbo/S 700x28C (B)
Col 28 - A Road/30 700x28C (A) 230 241 25.
Col 29 - A Road/30 700x28C (B)
c1 c2 c3 c4 c5 c6 c7 c8 c9 c10 c11 c12 c13 c14 c15 c16 c17 c18
3.5 356 365 395 369 367 392 348 347 430 443 423 432 423 406 355 364 382
4.0 322 329 360 343 337 361 319 316 393 401 391 393 386 376 325 333 352
4.5 298 302 330 319 316 336 296 290 366 373 362 368 354 347 297 309 326
5.0 279 282 311 298 294 314 280 271 341 348 335 345 335 320 275 289 311
5.5 265 268 293 287 279 297 265 255 324 329 319 327 319 305 262 276 295
6.0 255 253 282 274 267 282 249 242 310 312 306 310 303 293 248 260 283
6.5 244 242 274 264 254 269 239 230 297 294 292 297 290 283 237 253 275
7.0 238 232 263 255 247 256 229 221 287 284 282 288 280 273 231 245 267
7.5 231 226 255 250 238 247 222 213 272 277 272 279 272 264 222 236 260
8.0 222 219 248 244 233 239 215 205 265 267 266 272 264 258 216 230 253
8.5 216 212 244 237 226 231 209 201 259 259 259 266 257 252 208 223 245
9.0 213 211 241 236 223 224 204 195 255 256 259 259 254 245 204 219 245
Bruce Prickett
I don't think that any of the tires you've listed are (were) belted.
Your argument against belted tires and Mr. Tuffy strips was based on rolling
resistance, but if there was any data presented I missed it. I believe your
assertion that there is a noticeable increase in rolling resistance when using
thicker tires or Tuffy strips, but I'm curious if it's been quantified. Since I
use BOTH Mr. Tuffys and and kevlar belted tires, I guess I'm not performance
oriented :)
It's also interesting that 2 samples of the same model tire vary as much as
10% in rolling resistance. It's always fun to see real data to supplement
the opinions that are posted.
Bruce Prickett
Jobst Brandt
my directory I notice that it is only 72 bytes wide whereas it should
be 102 bytes. I must retrieve the data from a tape. When I get it
back in shape, I'll post the data. Meanwhile, you can assure yourself
from the 18 columns (17 tires) that the inflation pressure makes a
substantial difference.
Kevlar belted tires were not in this series but they were tested and
found to be inferior to the same model without Kevlar belts (not to
be confused with beads that have no effect on RR). That the two
examples of some models listed are different arises that this was
for some models a before and after modification test. The difference
was in most cases a change in thickness or compound in the tread.
The tests were all done on the standard Japanese tire research drum
testers and loaded as described. Any changes that would occur from
changes in surface configuration would not change the relative
positions of any of the tires. As is apparent, the better the tire,
the less effect inflation has because the tire has less losses and
a percentage of less is less. New data will be gathered soon as the
local testing lab is completed. We can measure values for which the
Japanese testing institute is not instrumented. It will be revealing
no doubt as wear rate and cornering ability is quantified.
Keith Erskine
> Meanwhile, you can assure yourself
> from the 18 columns (17 tires) that the inflation pressure makes a
> substantial difference.
Thanks for the data. Maybe someone who has the bike power program can
show us the % difference in effort required to reach 25mph on flat
ground with no wind at 80psi vs. 110psi. Anyone? Anyone?
> New data will be gathered soon as the
> local testing lab is completed.
If you could post this as well I'm sure myself and many others will
be very grateful.
Keith Erskine
Robert Virzi
paper I didn't write, but I should point out that you are totally
missing the point of their work.
Here is the quote from the article, with the reference if you want to
go look it up (most research libraries will carry this journal):
> ...
> "We conclude that differences in rolling resistance caused by
> varying tire pressure between 552 and 965 kPa, are too small to be
> detected physiologically."
> ... Ryschon & Stray-Gundersen in Ergonomics, 1993, Vol 36, No 6, pp 661 - 666.
They are specifically talking about measuring the PHYSIOLOGICAL effect,
that is, the effect on the human rider.
Here is one of your replies to an earlier posting of this:
JB> I suppose it would be more apparent on what they base their findings
JB> if the data were available. I am certain that they are wrong in the
JB> quote presented above. I have data and measurements that show beyond
JB> a doubt that there are substantial differences. If the authors are
JB> talking about going shopping at 10 mph or less they may have a point
JB> but then I can't imagine why one would measure energy expenditure and
JB> oxygen uptake of a non athlete in a non athletic endeavor.
Followed by this a couple of days later (I presume)
JB> As I look at the data that I posted and the file as it resides in
JB> my directory I notice that it is only 72 bytes wide whereas it should
JB> be 102 bytes. I must retrieve the data from a tape. When I get it
JB> back in shape, I'll post the data. Meanwhile, you can assure yourself
JB> from the 18 columns (17 tires) that the inflation pressure makes a
JB> substantial difference.
You raise a couple of good points that the authors addressed, but that I
did not take the time to put into my summary. First off though, the data
is available, that is what a reference is. While your data quite clearly
show that there is a measureable and significant difference in rolling
resistance, as measured on a rotating drum type arrangement, I do not see
at all how it applies to what the authors are claiming, which is that
just because you can measure it to N digits doesn't mean it affects human
performance. They specifically looked at oxygen uptake, a reasonably
sensitive measure, and found NO DIFFERENCE over the ranges shown. They
are not saying the rolling resistance doesn't change, they are saying it
doesn't change enough to affect the rider. It is negligible.
Some more info from the article, for those of you who can't get a hold
of it. They had 7 "racing cyclists" (USCF cat 2 - 4) ride their own
bikes outfitted with the experimenters rims and tires inflated to various
pressures in the range shown (~80 - 140psi). The riders rode on a
motor driven treadmill while oxygen uptake was measured. They rode for
10 minutes at 19kph up a 4% grade at ~ 75 rpm during the pretest part
of the trial. They were then tested 4 times for 5 minutes each time
at each pressure (total 16 measurement periods).
The authors refer to past work, as is the custom in scientific and
engineering literature. They point out that tire pressure and tire
type (balloon vs narrow) can have an effect on oxygen uptake, but
not in the range tested (i.e., at very low pressures or changing
tire width dramatically). Between ~80-140psi, in a well controlled
study, they found no physiological effects. They also raise some
question regarding how drum-type measurements relate to real riding.
To put this in perspective, I was quite skeptical while reading the
study. I have not been able to find fault with their methodology or
statistics. It is an intriguing study, and I would urge people in
this group to track it down, if only to find something that I am
missing. Tire pressure sure SEEMS to make a difference in the way
a ride feels. But then again, isn't there a religious war regarding
how aluminum frames sure SEEM to be harsher than cromoly? ;-}
Jobst Brandt
Rolling resistance (g) vs. inflation pressure (kg/cm2) @ 50 kg load
------- ---- ------ ---------- ----------- ----------- ---
Col 1 - Air Pressure (kg/cm2)
Col 3 - S Turbo/LR 700x25C (B)
Col 5 - S Turbo/LS 700x25C (B)
Col 7 - A Criterium/20 700x25C (B)
Col 9 - A Timetrial/20 700x20C (B)
Col 11 - S Turbo/LR 700x28C (B)
Col 13 - S Turbo/LS 700X28C (B)
Col 15 - A Road/20 700x28C (B)
Col 17 - S Turbo/R 700x25C (B)
Col 19 - S Turbo/S 700x25C (B)
Col 21 - A Criterium/30 700x25C (B)
Col 23 - A Timetrial/30 700x20C (B)
Col 25 - S Turbo/R 700x28C (B)
Col 27 - S Turbo/S 700x28C (B)
Col 29 - A Road/30 700x28C (B)
c1 c2 c3 c4 c5 c6 c7 c8 c9 c10 c11 c12 c13 c14 c15 c16 c17 c18 c19 c20 c21 c22 c23 c24 c25 c26 c27 c28 c29
3.5 356 365 395 369 367 392 348 347 430 443 423 432 423 406 355 364 382 386 378 387 361 369 419 432 427 442 394 403
4.0 322 329 360 343 337 361 319 316 393 401 391 393 386 376 325 333 352 356 350 357 325 331 390 390 395 401 360 363
4.5 298 302 330 319 316 336 296 290 366 373 362 368 354 347 297 309 326 332 322 329 301 302 364 362 369 375 334 331
5.0 279 282 311 298 294 314 280 271 341 348 335 345 335 320 275 289 311 311 302 307 282 284 340 339 342 351 311 310
5.5 265 268 293 287 279 297 265 255 324 329 319 327 319 305 262 276 295 296 284 295 268 267 320 322 325 333 290 289
6.0 255 253 282 274 267 282 249 242 310 312 306 310 303 293 248 260 283 285 269 281 253 252 305 304 311 318 275 273
6.5 244 242 274 264 254 269 239 230 297 294 292 297 290 283 237 253 275 277 257 270 244 242 292 291 298 309 266 262
7.0 238 232 263 255 247 256 229 221 287 284 282 288 280 273 231 245 267 270 247 261 235 232 281 283 286 299 254 249
7.5 231 226 255 250 238 247 222 213 272 277 272 279 272 264 222 236 260 260 238 249 228 224 272 272 274 289 248 242
8.0 222 219 248 244 233 239 215 205 265 267 266 272 264 258 216 230 253 253 231 241 223 217 263 264 273 281 243 235
8.5 216 212 244 237 226 231 209 201 259 259 259 266 257 252 208 223 245 246 223 233 216 209 256 257 0 274 235 229
9.0 213 211 241 236 223 224 204 195 255 256 259 259 254 245 204 219 245 245 222 231 212 208 252 256 0 273 233 226
Jobst Brandt
> To put this in perspective, I was quite skeptical while reading the
> study. I have not been able to find fault with their methodology or
> statistics. It is an intriguing study, and I would urge people in
> this group to track it down, if only to find something that I am
> missing. Tire pressure sure SEEMS to make a difference in the way
> a ride feels. But then again, isn't there a religious war regarding
> how aluminum frames sure SEEM to be harsher than cromoly?
The whole article, that I didn't see, seems to be based on whether the
authors could measure rolling resistance in the most obscure way
possible. Instead of measuring it directly they put the wheels on a
bicycle and then instrumented a rider who rode at a speed
substantially below his aerobic limit and then looked for a detectable
signal amidst all that interference that included the psyche of the
rider and a slew of other effects.
I think researchers get carried away on an idea sometimes and lose
sight of the objective. Let me propose that an exercise bike be
instrumented to chart a rider's response to varying loads as the
effort is increased at constant speed and when the speed is increased
at constant load. With a statistical spread for a significant sample
of riders it should be possible to characterize typical response to
varying bicycling demands. Because all the external effects to which
a bicyclist is subjected can be accurately measured, these can be
classified in importance by their relative power demands rather than
measuring the effect of bartape on the rider, for instance, this could
be separately evaluated by this method.
To measure as they did is highly suspect to me. I don't know what the
goal of the effort was but the way it came across on the original
posting I'll stick by my original statement. That is, that there is
no sudden cutoff point where inflation makes no difference and that
the changes in RR are measurable.
Paul Barton-Davis
>The whole article, that I didn't see, seems to be based on whether the
>authors could measure rolling resistance in the most obscure way
>possible.
It would be nice to be able to expect more from Mr. Brandt than a
bunch of highly jaundiced comments on a paper "he didn't see". I look
forward to the day that someone criticizes one of Mr. Brandt's works
based on a third-party summary of it, in particular to the vehemence I
expect as Jobst tells us all that we should read it before we
criticize it.
But down to the details: "the most obscure way possible" ? Is that so,
Jobst ? Why would one care about rolling resistance if it made no
physiological difference to a rider ? The primary reason for caring about
such details on a bike is normally that they make it harder to ride.
Give this, it is surely the most *obvious* thing to do to see if some
specific component actually has any effect in this regard.
Instead of measuring it directly they put the wheels on a
>bicycle and then instrumented a rider who rode at a speed
>substantially below his aerobic limit and then looked for a detectable
>signal amidst all that interference that included the psyche of the
>rider and a slew of other effects.
When *I* ride my bike, I have to deal with that "slew of other
effects". Why should I care if Jobst Brandt has shown that rolling
resistance varies from tire to tire and from tire pressure to tire
pressure, if the difference it makes when riding is lost in the "slew
of other effects" ? I don't. Aerodynamic water bottles no doubt have a
measurable effect on riding, but are utterly irrelevant as far as
physiological effort is concerned, since the difference they make is
lost in the noise.
Your only argument with these people must surely be whether or not the
difference *you've* measured are effectively noise in the overall
physiological effort story, and if you claim that they are not, then
it seems incumbent upon *you* to explain how, in the experiments they
performed, they were unable to see an effect that you apparently
believe it significant enough to appear in the presence of general
riding noise ("a slew of other effects").
>I think researchers get carried away on an idea sometimes and lose
>sight of the objective. Let me propose that an exercise bike be
>instrumented to chart a rider's response to varying loads as the
>effort is increased at constant speed and when the speed is increased
>at constant load. With a statistical spread for a significant sample
>of riders it should be possible to characterize typical response to
>varying bicycling demands. Because all the external effects to which
>a bicyclist is subjected can be accurately measured, these can be
>classified in importance by their relative power demands rather than
>measuring the effect of bartape on the rider, for instance, this could
>be separately evaluated by this method.
This sounds like a good idea. However, these experimenters took a
perfectly reasonably line: they varied certain things given an
assumption that other factors remained constant. Either their
assumptions are wrong, or there is a lot more complexity to the story
than either your or their method will elucidate.
>To measure as they did is highly suspect to me. I don't know what the
>goal of the effort was but the way it came across on the original
>posting I'll stick by my original statement. That is, that there is
>no sudden cutoff point where inflation makes no difference and that
>the changes in RR are measurable.
They don't deny the last point. They claim merely that the measurable
differences in RR have no impact on physiological effort, and in my
book, thats absolutely the most important thing.
-- paul
--
"It's the dice that play God with the Universe" - Debil
Tim McBrayer
>
>Some more info from the article, for those of you who can't get a hold
>of it. They had 7 "racing cyclists" (USCF cat 2 - 4) ride their own
>bikes outfitted with the experimenters rims and tires inflated to various
>pressures in the range shown (~80 - 140psi). The riders rode on a
>motor driven treadmill while oxygen uptake was measured. They rode for
>10 minutes at 19kph up a 4% grade at ~ 75 rpm during the pretest part
>of the trial. They were then tested 4 times for 5 minutes each time
>at each pressure (total 16 measurement periods).
I realize I shouldn't butt into the middle of a thread I haven't followed
completely, but shouldn't the underlined phrase make the study fairly
meaningless? It seems to me that there is little difference between powering the
road beneath the wheels and powering the wheels themselves. This would mean
that the cyclist was not the only (or even the primary?) source of power.
Depending on the motor's impact, this would decrease the significance of the
rider's power input (and hence O2 intake), possibly to nothing.
My intuition tells me that tire pressure does have a noticable impact, but I'm
certainly willing to be convinced. However, when the rider is not the only
power source propelling the bike, I have to wonder about the significance of the
study. Any comments?
--
Tim McBrayer tmcb...@thor.ece.uc.edu
Computer Architecture Design Laboratory (513) 556-0904
University of Cincinnati
Harry Phinney
<much argument deleted>
: They claim merely that the measurable
: differences in RR have no impact on physiological effort, and in my
: book, thats absolutely the most important thing.
I disagree that this is necessarily the most important thing. The
rolling resistance exists, and is a force which must be overcome by the
rider. Even if one cannot measure the difference in VO2 from a change
to this force, it does not in any way eliminate the force or the power
that the rider must produce to overcome the force. In fact, if one
knows the force then it is trivial to compute the power requirement
necessary for the rider to counter the force at a given speed, and thus
it should be possible to predict whether one could measure a change in
VO2. In other words, you could just as well take Jobst's data, figure
out the power change due to different inflation pressures at a given
speed, and see whether this level of power change can be measured via
VO2.
This study seems a little like measuring the VO2 change due to changing
or eliminating the aerodynamic drag of each frame tube and spoke
individually, and each body part individually, and deciding that
aerodynamic drag doesn't matter because none of the individual changes
could be measured via VO2. I'm sure that a similar study would find no
VO2 change due to different bearing qualities in the front hub, or the
rear hub, or the BB, or the chain. Put all of these "rolling
resistances" together, and _maybe_ you could measure a change. If so,
then how do you reduce the resistance? By reducing the drag from each
component, regardless of the fact that you can't measure a VO2 change
from any single component.
Harry Phinney ha...@cv.hp.com
Your Name Here
[Shouldn't the use of a motor driven treadmill invalidate human power
measurements?]
No, for example walking up a 4% hill is JUST THE SAME as walking up
a 4% grade on a CONSTANT SPEED motorized treadmill. That is, as far as
anything
mechanical or bio-mechanical is concerned. Shut your eyes when you
walk up a down escalator and see if it feels different from walking
up a stopped escalator. It should feel pretty much the same. Likewise
your O2 consumption will be the same and so on.
But what about energy conservation? Your body cannot feel the difference
between pushing a motor (soing work on it) and raising its gravitational
potential energy.
But you are not alone in being confused by energy balance issues when
comparing systems that move at constant velocity with respect to each
other (like the stationary and moving escalators).
-Andy Ruina, ru...@cornell.edu, 607-255-7108,
TAM Kimball Hall, Cornell, Ithaca, NY 14850
Daniel Connelly
>Thanks for the data. Maybe someone who has the bike power program can
>show us the % difference in effort required to reach 25mph on flat
>ground with no wind at 80psi vs. 110psi. Anyone? Anyone?
I don't think bike_power is needed for a back-of-the-envelope to
determine significance...
The differences in RR are of order "50g" or more correctly 50g * 10
m/s^2 = 0.5 N. Velocities are typically 36kph = 36000m/h = 10 m/s.
Power=force*vel => power ~= 5 watts. This is approx 2% of a total
power output of 250W. Since power goes roughly with v^n for n close
to but somewhat less than 3, this translates into a 0.7% to 1.0%
difference in velocity. This will scale linearly with the difference
in RR relative to the chosen value of 0.5N.
Whether 250 watts corresponds to 36k/h is another issue.
Dan
--
Paul Barton-Davis
>Paul Barton-Davis (pa...@cs.washington.edu) wrote:
><much argument deleted>
>: They claim merely that the measurable
>: differences in RR have no impact on physiological effort, and in my
>: book, thats absolutely the most important thing.
>
>I disagree that this is necessarily the most important thing. The
>rolling resistance exists, and is a force which must be overcome by the
>rider. Even if one cannot measure the difference in VO2 from a change
>to this force, it does not in any way eliminate the force or the power
>that the rider must produce to overcome the force.
Sorry. I wasn't clear enough in my use of the term "physiological
effort". I think that a legitimate criticism of this work is that they
only used VO2 measurements as an indicator of physiological effort.
However, Jobst's criticism seemed to be of the whole idea that one
would want to measure physiological effort directly, and this seems
misplaced to me.
>This study seems a little like measuring the VO2 change due to changing
>or eliminating the aerodynamic drag of each frame tube and spoke
>individually, and each body part individually, and deciding that
>aerodynamic drag doesn't matter because none of the individual changes
>could be measured via VO2. I'm sure that a similar study would find no
>VO2 change due to different bearing qualities in the front hub, or the
>rear hub, or the BB, or the chain. Put all of these "rolling
>resistances" together, and _maybe_ you could measure a change. If so,
>then how do you reduce the resistance? By reducing the drag from each
>component, regardless of the fact that you can't measure a VO2 change
>from any single component.
Absolutely. I couldn't put it better.
But the other point one would take home from such a study is that
changing any of the components alone won't do very much for you (and
perhaps, that even changing the entire bike won't make as big a
difference either). This would be of some consequence for a large
class of well-financed recreational riders :-)
Charles R. Sullivan
omitted from most of the discussion so far. That is, the influence
of the road (or test equipment) surface characteristics on rolling
resistance. Specifically, rough or compliant surfaces can be expected
to increase rolling resistance over that measured in a steel drum.
Furthermore, this effect becomes greater at higher tire pressures.
Thus, increasing tire pressure from 80 to 140 psi could easily do less
good in the real world than it does in a steel drum. Furthermore,
on some surfaces, it could actually make the rolling resistance worse.
This might sound like I am criticizing steel drum experiments.
But actually, the VO2 study sounds like it was worse in this regard.
I'm not sure what the surface of the treadmill they used was, but
if it was rubber, for example, the hysteresis in that rubber could
have dominated the losses in the tires, particularly at high pressure.
(The issues regarding what power comes from the treadmill motor, and
what comes from the bicycle rider, are somewhat subtle, but careful
thought should show that this loss is supplied by the rider.)
Likewise, if it was a coarsely textured surface, that could
have substantial effects also.
Steel drum experiments are, IMHO, more useful than experiments with
an unknown treadmill surface. But they do have their limitations.
As Jobst said:
>Any changes that would occur from
>changes in surface configuration would not change the relative
>positions of any of the tires.
This means they can be very useful in designing and selecting tires.
However, they are less useful in selecting a tire pressure.
Jobst's statement is not necessarily true of the relative rankings of
a given tire, used on different surfaces. The steel drum experiment
will always show lower rolling resistance at higher pressure, but
that need not be true on real roads which are neither as hard
as steel or as smooth as these drums.
In conclusion, the steel drum experiments, while useful and repeatable,
are limited in their applicability to selecting tire _pressure_
for the real world. However, they are good for selecting _tires_.
Tests on some random surface that is part of the treadmill
could possibly by chance correspond to real road surfaces, but
they hardly prove anything.
Charlie Sullivan char...@power.berkeley.edu
Jobst Brandt
> This means they can be very useful in designing and selecting tires.
> However, they are less useful in selecting a tire pressure. Jobst's
> statement is not necessarily true of the relative rankings of a
> given tire, used on different surfaces. The steel drum experiment
> will always show lower rolling resistance at higher pressure, but
> that need not be true on real roads which are neither as hard as
> steel or as smooth as these drums.
This has been thrashed around before. It would be difficult to create
a surface where rolling resistance did not increase with random
roughness and decrease with inflation pressure. Exceptions I can
imagine would be such that the tire begins bouncing on a regular
pattern or that the roughness exceeds the absorption of the tire and
strikes the rim. It is important to keep in mind that RR arises from
internal losses in the tread, casing, and tube and not from mere
contact between tread and road.
The last time we got on this subject, it was suggested that at the
higher inflation pressures the road would deform plastically and
cause more RR. This is patently absurd since the pressure of a high
heeled shoe exceeds any tire pressure tenfold and they only sink into
fresh tar. THe other contention was that if the surface has large
stones on it... This is equally far afield because the test becomes
discontinuous under such conditions and besides, with such rough
surfaces RR is no longer an issue but rather the control of the
bicycle.
> In conclusion, the steel drum experiments, while useful and
> repeatable, are limited in their applicability to selecting tire
> _pressure_ for the real world. However, they are good for selecting
> _tires_. Tests on some random surface that is part of the treadmill
> could possibly by chance correspond to real road surfaces, but they
> hardly prove anything.
If you are selecting tire pressure to minimize RR then these tests are
entirely valid. They are not valid for making other decisions such as
cornering ability and comfort.
Dick King
>
Putting things into perspective, the numbers were all in the 200-300 range,
except at pressures low enough that you will snakebite for sure. In fact, few
numbers were outside of 220-280.
100gm for a 50kg load is equivalent to a 0.2% slope, or a ten foot rise per
mile. It wouldn't surprise me if noone noticed it.
-dk
Dick King
>Tim McBrayer wrote (approximately):
>
> [Shouldn't the use of a motor driven treadmill invalidate human power
> measurements?]
>
>No, for example walking up a 4% hill is JUST THE SAME as walking up
>a 4% grade on a CONSTANT SPEED motorized treadmill. That is, as far as
>anything
>mechanical or bio-mechanical is concerned.
Except for the wind which you don't feel on the treadmill. I suppose that's
why you need a grade.
-dk
Jobst Brandt
Dick King writes:
> Putting things into perspective, the numbers were all in the 200-300
> range, except at pressures low enough that you will snakebite for
> sure. In fact, few numbers were outside of 220-280.
That depends on which tires you chart. Whether you snake bite or not
depends on the weight of the rider and the road. In any case the
curves are not accurately described by the above "perspective". RR
decreases with increasing pressure and continues to do so far beyond
the pressures these tires can withstand. That it is an asymptotic
function that goes to zero is also apparent.
> 100gm for a 50kg load is equivalent to a 0.2% slope, or a ten foot
> rise per mile. It wouldn't surprise me if noone noticed it.
That an 0.2% gradient is insignificant is not accurate. This is
equivalent to a 0.36 lb force pushing on a 180 lb rider/bike. I am
sure that most riders would not find this insignificant if they were
to receive such an assist in a TT.
On the other hand, since it's about "noone", I think I'll go get some
lunch now. What language are these people speaking.
Bradley D. Thayer
(Charles R. Sullivan) wrote:
> I think that there is an important aspect of this that has been
> omitted from most of the discussion so far. That is, the influence
> of the road (or test equipment) surface characteristics on rolling
> resistance.
.
.
.
> The steel drum experiment
> will always show lower rolling resistance at higher pressure, but
> that need not be true on real roads which are neither as hard
> as steel or as smooth as these drums.
This posting piqued my interest and reminded me of something that I read
(in Bicycling, I think) about one of the advantages of suspension on MTBs
(hang with me here, I think this is relevant to roadies). The advantage of
a suspended bike over a non-suspended bike was that it allowed the bike to
go over obstacles rather than try to go through them (with the result that
the entire bike must go over the obstacle). Since the point for most of us
is to turn our pedalling energy into forward motion rather than vertical
(perpendicular to the road) motion, it seems to me that allowing a part of
your bike to "flex" over the bumps is a good idea. Now the relevancy. Is
their an anlaogy between suspended bikes and tires? Might not there be a
point where higher tire inflation pressure will increase resistance on real
road surfaces because your entire bike must rise over an obstacle rather
than just flexing the sidewalls? Jobst maintains that rolling resistance
asymptotically goes to zero with higher pressure. On a real road surface
(like few-years-old asphalt), I am not sure this is true.
The key here is the tradeoff in energy lost in flexing the tire sidewalls
vs. climbing millions of little hills during a ride. My intuition says
that in the extreme, riding a solid steel wheel over a rough road, you are
better off with a lower pressure tire.
The only leak that I can see in my asbestos underwear is that this may not
be considered "rolling resistance," but rather some other resistance.
Flame away.
--
"Rebel without a cause"
IDA--your tax dollars at work!
bui tho xuan
in rolling resistance with increasing pressure (as a func. of road
csurface) is a partial product of semantics.
If you measure the rolling resistance as a scientific parameter,
you'll find that it'll increase with pressure of the tire as Jobst
reported.
Now, if the tire slips in actual operating condition, you're
wasting energy, but you're not changing the rolling resistance.
Actually, it's dropping, cause the tire is spinning faster.
You, on the other hand, is going slower.
So next time, when your Vittoria 700x18c's that are pumped to
6 gazillion psi slips on wet pavement or sand, tell yourself
that you're reducing rolling resistance :-)
tho
Thomas H. Kunich
>(Letting) your bike to "flex" over the bumps is a good idea. Now the relevancy. Is
>their an anlaogy between suspended bikes and tires? Might not there be a
>point where higher tire inflation pressure will increase resistance on real
>road surfaces because your entire bike must rise over an obstacle rather
>than just flexing the sidewalls? Jobst maintains that rolling resistance
>asymptotically goes to zero with higher pressure. On a real road surface
>(like few-years-old asphalt), I am not sure this is true.
Well, nice try, and if the tire became completely ridgid you'd probably have
something there. But those small bumps in the road will flex the sidewalls
no matter how hard you inflate the tire because the spot loadings are very
high and the deflection almost too small to measure.
BTW, there is much more power lost to those suspension forks than is
made up for by the suspension. Not to denigrate suspensions, mind you,
but power loss is the biggest strike they have against them.
Handling is improved, ride is improved, tire problems disappear, but
they do absorb power and it's plain to see because most of these bikes
will _stop_ if they hit the right kind of small bump. The fork will
absorb all of the forward momentum.
Jobst Brandt
> The advantage of a suspended bike over a non-suspended bike was that
> it allowed the bike to go over obstacles rather than try to go
> through them (with the result that the entire bike must go over the
> obstacle)....
> Is their an analogy between suspended bikes and tires? Might not
> there be a point where higher tire inflation pressure will increase
> resistance on real road surfaces because your entire bike must rise
> over an obstacle rather than just flexing the sidewalls? Jobst
> maintains that rolling resistance asymptotically goes to zero with
> higher pressure. On a real road surface (like few-years-old
> asphalt), I am not sure this is true. The key here is the tradeoff
> in energy lost in flexing the tire sidewalls vs. climbing millions
> of little hills during a ride. My intuition says that in the
> extreme, riding a solid steel wheel over a rough road, you are
> better off with a lower pressure tire.
For every bump the wheel climbs there must be one that it descends.
You may visualize a saw toothed road where the tire constantly lands
on inclined ramps but this is not reality. Besides, the suspension
bike may be more comfortable and manageable but it absorbs more energy
than the unsprung bicycle. The heat generated in the shock absorbers
is lost energy that substantially exceeds that lost to the tires.
In the previous round of arguments on this subjects, it was proposed
that the tire, if hard enough, would deform the pavement and that
these losses would exceed the losses of the tire by far. This, of
course, is hypothetical and unreal because the roads we choose to
ride on are not to any substantial amount ever made of such pavement.
Since asphalt pavement is plastic rather than elastic, the tire would
leave a rut in the surface as one would expect to see when riding over
freshly laid hot-mix asphalt.
However, trying to get this thread back from outer space to the
surface of the road, let me reiterate that for pavement on which
bicycles are commonly ridden, rolling resistance decreases with
increasing inflation pressure until the tire bursts.
Norman Yarvin
>In the previous round of arguments on this subjects, it was proposed
>that the tire, if hard enough, would deform the pavement and that
>these losses would exceed the losses of the tire by far.
Actually it would! Consider a perfectly elastic bike (perhaps made of
steel throughout). Upon riding along a bumpy road, a great amount of
energy would be deposited in the frame by the bumps on the road. This
energy could only be manifested as vibrations of the bicycle. Thus,
the steel 'tire' would bounce so much it would hardly ever be in
contact with the road, and when it was it would deliver enormous forces
in the manner of a pneumatic hammer. These would surely deform the
pavement.
Of course to achieve that effect the a bike would have to have a
perfectly elastic rider...a man of steel! Otherwise the man would
supply the necessary damping of the frame---which would be about as
pleasant as riding a pneumatic hammer.
Actually with a normal man riding a perfectly elastic bike on a normal
road, the question would be which gave first, the man or the road. One
of them would need to absorb the shocks. My guess is that it would be
the man.
--
Norman Yarvin yar...@cs.yale.edu
Greg Holley
> Some more info from the article, for those of you who can't get a hold
> of it. They had 7 "racing cyclists" (USCF cat 2 - 4) ride their own
> bikes outfitted with the experimenters rims and tires inflated to various
> pressures in the range shown (~80 - 140psi). The riders rode on a
> motor driven treadmill while oxygen uptake was measured. They rode for
> 10 minutes at 19kph up a 4% grade at ~ 75 rpm during the pretest part
> of the trial. They were then tested 4 times for 5 minutes each time
> at each pressure (total 16 measurement periods).
>
> The authors refer to past work, as is the custom in scientific and
> engineering literature. They point out that tire pressure and tire
> type (balloon vs narrow) can have an effect on oxygen uptake, but
> not in the range tested (i.e., at very low pressures or changing
> tire width dramatically). Between ~80-140psi, in a well controlled
> study, they found no physiological effects. They also raise some
> question regarding how drum-type measurements relate to real riding.
I've only read Robert's article (and not the original), but I have some
questions about the method of this test. First, 12mph @ 4% grade isn't
particularly fast even for me (equivalent, as far as power required to
overcome gravity, to 8mph @ 6% or 6mph @ 8%). For "racing cyclists," this
should be well short of maximum ability. Was that intentional on the
part of the researchers? (That is, does this sort of test yield more
accurate results when the subjects are comfortable or being stressed.)
Second, I'm more worried about rolling resistance when riding level or
downhill, where friction losses in the tires make up a proportionately
larger part of the expenditure. I'd be interested in figures for
21mph level in dead air, 24mph level with a tailwind/drafting someone,
40mph downhill.
--
Greg Holley hol...@acuson.com
Charles R. Sullivan
>In the previous round of arguments on this subjects, it was proposed
>that the tire, if hard enough, would deform the pavement and that
>these losses would exceed the losses of the tire by far. This, of
>course, is hypothetical and unreal because the roads we choose to
>ride on are not to any substantial amount ever made of such pavement.
>Since asphalt pavement is plastic rather than elastic, the tire would
>leave a rut in the surface as one would expect to see when riding over
>freshly laid hot-mix asphalt.
I am the person who proposed that "absurdity." Note that the visble
rut is not essential, as an extremely small deformation of the
pavement could result in large losses, because the pavement is
not elastic like the tire.
I followed up my original post with several refernces I looked up.
One showed a finite-element analysis of pavement deformation
under car or truck tires. I had to extrapolate to guess what
a bicycle tire would do, but that is better than asserting:
>[...] let me reiterate that for pavement on which
>bicycles are commonly ridden, rolling resistance decreases with
>increasing inflation pressure until the tire bursts.
based on evidence only from a surface that is neither pavement,
nor one on which bicycles are commonly ridden.
If anyone wants my post from months ago with data, calculations,
and references, send me email, but I will be away for a few
weeks coming up, so be patient.
BTW, I should say that despite my disagreement with Jobst about this,
he is a really smart and helpful guy, and is extremely
knowledgeable about rolling resistance. He may even turn
out to be right about this, but so far he has offered no convincing
evidence.
Charlie Sullivan char...@power.berkeley.edu
Andy Ruina
If you take a given set of
a) real commercially available tires and test them
b) at pressures that they can stand on both
c) a smooth steel surface and
d) on any realistic riding surface
that the serial ranking of the tires rolling resistances will be the
same on both surfaces.
I do not know whether or not this is true. Lacking actual tests or sound
reasoning we don't know. My guess is that the serial rankings of tires
from a steel drum test and would be highly correlated with those from a
real-road test. Why should they be perfectly correlated?
Here are some thoughts.
I. If you took a solid steel wheel (a train wheel) it would have lower
rolling resistance than any air-filled rubber tire when tested on a
smooth steel surface. That same wheel would have higher rolling
resistance than any of the air-filled tires when tested on pavement.
This is not a strict counter-example of course because no-one is selling
or testing solid steel tires. But why do the combination of reasons that
would shift the serial ranking of a steel wheel from test to test not
also apply to real wheels?
II. If tire cording had infinite tensile strength one could imagine
inflating tires to arbitrarily high pressures. What would happen?
The outer layer of cord would become close to rigid. The only deformation
would be in the rubber outside of the cording, the tread etc. This
rubber does contribute to rolling resistance, however. This would
be like a steel wheel with a rubber layer glued on. A pre-pneumatic
tire. Such would undoubtedly score very high on a smoothe rigid
test bed. But
1) it would have some rolling resistance no matter
how rigid the cording layer (no matter how high
the internal pressure). That is, rolling resistance
does NOT go to zero with tire pressure unless the
tread is non-existant.
2) It would have pretty high rolling resistance on
roads of the roughness that some people actually
do ride on.
III. Rolling resistance does not just depend on the lossiness
of the tire material, but on the deformation of the material.
Different tires are not only constructed of different material,
but are constructed differently. That is, two tires with different
winding patterns have different strains in the tire patch region.
The deformation pattern associated with rolling on smoothe surfaces is
not the same as the pattern on rough surfaces. That is the deformation
near a road asperity (a gravel corner protruding from tar) is different
than the overall deformation from supporting a rider. Why should
different tires (that are not just constructed of different materials but
are also constructed differently) have the same relative dissipation for
two different types of loading (weight of rider on smooth surface and
deformation near an asperity)?
Is the effect of cord layout on rolling resistance will understood?
(Aside: are there radial wound bike tires for which the tire cross section
is non-circular and the patch deformation might be largely different?)
*******
Since pneumatic tires are such an important part of most bikes suspension
systems it seems a shame to be stuck thinking of suspension and rolling
resistance as direct trade-offs. If one relied on smooth-steel
test beds one would un-invent the pneumatic tire!
- Andy Ruina, ru...@cornell.edu, 607-255-7108
TAM, Kimball Hall, Cornell, Ithaca, NY 14850
bui tho xuan
>Here are some thoughts.
>
>I. If you took a solid steel wheel (a train wheel) it would have lower
>rolling resistance than any air-filled rubber tire when tested on a
>smooth steel surface. That same wheel would have higher rolling
>resistance than any of the air-filled tires when tested on pavement.
>This is not a strict counter-example of course because no-one is selling
>or testing solid steel tires. But why do the combination of reasons that
>would shift the serial ranking of a steel wheel from test to test not
>also apply to real wheels?
I think this illustrates the problem I tried to state before (correct me
if I'm wrong).
A steel wheel loaded to the same force on pavement as a air-filled tire
will still have a lower rolling resistance (assuming not too much
pavement deformation). The reason that no one is selling them is that
the rolling friction is _So low_ that you have no traction.
So, you'll have to work harder, but your rolling resistance is less.
the tests measure rolling resistance, not how hard a bicyclist is
working to move a bike. The wheel spins faster, but the biker moves
slower. Not unlike trying to drive when you're stuck in the snow. The
rolling resistance is very little, the tires spins at 6 gazillion rpm,
but you ain't goin' nowhere.
How else can I say it to make it more confusing? :-)
tho
Andy Ruina
> (correct me if I'm wrong).
>
>A steel wheel loaded to the same force on pavement as a air-filled tire
>will still have a lower rolling resistance (assuming not too much
>pavement deformation). The reason that no one is selling them is that
>the rolling friction is _So low_ that you have no traction.
>
Rolling resistance and traction are, for the most part, different
concepts. When riding energy is lost both at the front wheel and
the driven rear wheel. I think that most, if not all, rolling
resistance tests that people perform are undriven. That is they
correspond to the dissipation at the front wheel or a coasting
rear wheel. The extra loss from driving a wheel (due basically
to slip something like you describe) is probably best not
attributed to rolling resistance but rather dissipation in power
transmission.
A rigid steel wheel would have pretty high rolling resistance on a
rough rigid road even if it had perfect traction and was not being
driven. This loss is due to collisions, the exact lossiness
depends not only on the roughness of the terain and the radius of
the wheel but also on the rest of the bike and rider system. But
an estimate of the rolling resistance of a rigid wheel on a rough
terain is the ratio of the spacing between contact points on the
road and the wheel radius. Rolling coefficient of .004 (150 grams
drag for 50kg load) corresponds to a little less than a sixteenth
of an inch between contact asperities, probably noticably less
than the roughness of many roads would give.
That is, I think (but am not absolutely sure) that bikes coast
faster with tires on the rims.
In short, I think you are basically wrong. You do have a point,
though, that losses at the tire due to power transmission
might not be perfectly correlated with losses do to simple
rolling.
-Andy Ruina, TAM, Cornell, Ithaca, NY 14850, 607-255-7108
Jobst Brandt
> A steel wheel loaded to the same force on pavement as a air-filled
> tire will still have a lower rolling resistance [...]
> So, you'll have to work harder, but your rolling resistance is less.
> the tests measure rolling resistance, not how hard a bicyclist is
> working to move a bike. The wheel spins faster, but the biker moves
> slower. Not unlike trying to drive when you're stuck in the snow.
> The rolling resistance is very little, the tires spins at 6
> gazillion rpm, but you ain't goin' nowhere.
This is not what RR is about. RR is measured by rolling a freely
rotating wheel against a roadway or drum and measuring the retarding
force either directly or by how quickly the rolling wheel slows down.
One method is to roll a runners three-wheeled-stroller with a known
load down a smooth and level hallway while measuring its time of
passage at two intervals. From this, it's acceleration can be
measured and from F=ma the drag force can be determined. The same
method is employed in drum testers where the speed of the drum is
measured at intervals under free rolling. In both methods the rate of
change is measured with and without the test wheel in order to
subtract the equipment overhead. RR can also be directly measured by
strain gauge dynamometer but this also requires smoothing to get rid
of measurement noise and tire irregularities.
Now then, the steel wheel analogy is specious because there is no
parallel between a highly inflated pneumatic tire and steel. Most of
us have experienced riding on a flat at one time or another. Riding
on a flat tire down a road is substantially different from riding a
highly inflated tire. I think you'll agree that there is no
comparison and even at that, the flat tire did not make any sort of
groove in the road. An inflated tire does even less.
I think I am beginning to hear voices, voices that say "But what if?".
Maybe these voices should begin to say "So what?". The whole line of
argumentation sounds to me like excuses for not pumping tires. Just
think of all those bike racers out there pumping up their tires before
a TT to pressures that only increase RR. I'm sure they want to know
about this effect, but watch it, you might get some rude replies.
Jobst Brandt
> I am the person who proposed that "absurdity." Note that the visible
> rut is not essential, as an extremely small deformation of the
> pavement could result in large losses, because the pavement is not
> elastic like the tire.
So let's not get lost in minutia and move on to board and concrete
bicycle tracks. What we are trying to determine is not whether a
pavement can be found that would reverse the tire pressure / RR
equation, but rather whether increased inflation pressure reduces
RR for practical tires. Since tires have a relatively limited range
of pressures in which they can operate (about 2:1) for reasons of
bottoming or exploding, most of the hypothetical proposals are less
than practical.
> I followed up my original post with several references I looked up.
> One showed a finite-element analysis of pavement deformation under
> car or truck tires. I had to extrapolate to guess what a bicycle
> tire would do, but that is better than asserting:
>
>> [...] let me reiterate that for pavement on which bicycles are
>> commonly ridden, rolling resistance decreases with increasing
>> inflation pressure until the tire bursts.
>
> based on evidence only from a surface that is neither pavement, nor
> one on which bicycles are commonly ridden.
I am fairly sure the finite element analysis was primarily involved in
the beam strength of pavement rather than its plastic deformation at
the contact point. Pavement is assumed to be dimensionally stable in
its cured state. It fails from flexing under bending stresses to
which it is subjected as it distributes loads from the contact patch
to the subgrade. Inelastic losses in the pavement and subgrade are no
doubt incurred by these deformations. Maybe you could cite something
from the report that referred to plastic flow at the surface from
contact pressure if there was any.
Brad Thayer
Brandt) wrote:
> Now then, the steel wheel analogy is specious because there is no
> parallel between a highly inflated pneumatic tire and steel. Most of
> us have experienced riding on a flat at one time or another. Riding
> on a flat tire down a road is substantially different from riding a
> highly inflated tire. I think you'll agree that there is no
> comparison and even at that, the flat tire did not make any sort of
> groove in the road. An inflated tire does even less.
>
We are probably drawing to the end of this thread, but I would like to be
sure that the combatants are clear on what the fight was about. My earlier
point on steel wheels was this: if you could continue inflating a tire to
very high pressures, it would effectively become a rigid steel tire with a
rubber covering (thanks for the clarification on this Andy). Now, I am not
convinced that said tire would have a lower rolling resistance than a lower
pressure tire on _real road surfaces_. Although it may occur at an
unattainably high and uninteresting pressure, I do feel that there is a
crossover that is a function of the roughness of the surface. If this is
not true, why don't TTers and hour record holders and others wanting very
high speed use rubber coated steel tires? Is it because the other benefits
of a real tire outweigh the small differences between very high attainable
pressures and steel tires? Or is it because people seeking very high speed
do it on smooth surfaces, pushing this transition pressure beyond current
attainable limits?
Does anybody want to sacrifice a set of rims to check the speed of a bare
rimmed bike (I assume no rubber covered steel wheels are available, and a
flatted tire is not the same) on a real road and return breathless with the
next breakthrough? It could settle this argument. I am sure that the
steel wheeled bike will be slower than a high pressure tired bike. You can
even go back and look for grooves!
Garrick F. Mitchell
rider of a certain reputably stiff aluminum frame, the idea of rubber-
coated steel tires makes me shudder.
In article <bthayer-28...@bthayer-mac.ida.org>, Brad Thayer writes:
|>
|> ...If you could continue inflating a tire to
|> very high pressures, it would effectively become a rigid steel tire with a
|> rubber covering.
Remember that a pneumatic tire is filled with air, a gas that behaves
somewhat like PV = nRT. In order to get the air-filled tire anywhere
near the "rigid" behavior you describe you'd basically have to pressurize
the air until it became SOLID. I won't even venture to guess a pressure.
|> ...why don't TTers and hour record holders and others wanting very
|> high speed use rubber coated steel tires?
Because a steel tire would be so unspeakably uncomfortable as to render
even the best athlete a quivering blob of jelly in a very short time.
Personally, I'll take some infinitesimal increase in rolling resistance
if the resulting extra comfort allows me to keep my aero position (in a
TT) or simply to think more clearly (in a RR or crit)!
It seems to me if you want to minimize your rolling resistance, you should
take an airplane.
--
----------------------------------) --------------)
Garrick Mitchell / / __ __ __ /
Rice U. Cycling Team / / /_/ / / /
gar...@owlnet.rice.edu / / / \ (__/ /
------------------------------/ / ___ ____ /
/ / / /
/ (___ / /
(_____________/
Charles R. Sullivan
>So let's not get lost in minutia
Yes, good point. I'm afraid I'm guilty of that.
So, stepping back, this is my view of the forest:
*By and large* inflation pressure reduces rolling R.
The furtherest I get from agreeing with higher presssure
always giving lower rolling r is that I think that even though the
tire's intrinsic losses are still decreasing with higher pressure at
the upper limit of safe inflation, it is likely (though not proven)
that there is a *slight* increase in rolling R on *some*
real world surfaces, as the pressure
goes from very high to extremely high. This "*some* real
world surfaces," I think includes enough of what typical riders
ride on that a lot of them might get a *very slight* decrease in average
rolling resistance by inflating there tires somewhat below the
upper safe limit (e.g. 110 instead of 150). But I would not
be confident enough to give anyone a definite recommendation
about what pressure to run. I don't know if this really happens,
and I don't know whether, if it does happen, it happens at
80 psi, or 150 psi, or 500 psi.
All I know for sure is that a steel drum test is not adequate to
answer this (probably unimportant) question.
I don't want to mislead people such that they
try to run at 60 psi (with their kevlar and Mr. Tuffy's)
and wonder why they can't keep up with their friends, and think
that there is something wrong with their Kevlar because they
get (snakebite) flats.
The bottom line is that high pressure is generally good for low
rolling resistance.
Now for the minutia below the bottom line:
>I am fairly sure the finite element analysis was primarily involved in
>the beam strength of pavement rather than its plastic deformation at
>the contact point. Pavement is assumed to be dimensionally stable in
>its cured state. It fails from flexing under bending stresses to
>which it is subjected as it distributes loads from the contact patch
>to the subgrade. Inelastic losses in the pavement and subgrade are no
>doubt incurred by these deformations. Maybe you could cite something
>from the report that referred to plastic flow at the surface from
>contact pressure if there was any.
The analysis included three or four different layers of
different materials involved, and all the properties of all
layers were in the analysis. But the results were just given
as the profile of the surface under load. So the plastic flow
at the surface is in there, and I agree that that is probably the
only part that gives losses with a bike. Unfortunately separting
it out was just guesswork on my part. Better than nothing
is the only support I have for my guess work. If anyone
wants to anylyze it more rigorously, please do so.
charlie sullivan char...@power.berkeley.edu
Hans Marggraff
III. Rolling resistance does not just depend on the lossiness
of the tire material, but on the deformation of the material.
Different tires are not only constructed of different material,
but are constructed differently.
(Aside: are there radial wound bike tires for which the tire cross section
is non-circular and the patch deformation might be largely different?)
somewhere, but experience and my racing colleagues tell me that there is
a considerable difference between tyres performance on drum and road.
Tyres with an integral inner tube with a thick rubber layer and even steel
for protection perform quite well on a road, but are no good on a drum
where the good old silk/cotton woven tyre performs best. This is usually
attributed to the fact that tyres deform more on a drum with a small diameter.
The effect is alarge enough to be tested by yourself.
Hans
--
Hans Marggraff speaking for himself but working for:
Straessle Informationssysteme GmbH & Co. __o
Alte Bahnhofstrasse 10;77933 Lahr; Germany -\<,
Tel: +49 7821 284 340 Fax: +49 7821 284 363 .....O / O
email h...@straessle-lahr.de
Jobst Brandt
X-Newsreader: TIN [version 1.2 PL1.1]
> I do not know if effect of cord layout on rolling resistance is
> quantified somewhere, but experience and my racing colleagues tell
> me that there is a considerable difference between tyres performance
> on drum and road. Tyres with an integral inner tube with a thick
> rubber layer and even steel for protection perform quite well on a
> road, but are no good on a drum where the good old silk/cotton woven
> tyre performs best. This is usually attributed to the fact that
> tyres deform more on a drum with a small diameter. The effect is
> large enough to be tested by yourself.
It sounds to me that you believe that there is no correlation between
RR tests on a drum and RR on the road. That is to say, the tire that
shows higher RR on the drum does not necessarily have the higher RR
on the road. Is that correct?
As I stated before, RR arises from flexing in the casing, tread, and
tube of a tire and in addition from tread squirm in which a non slick
tread has voids into which rubber displaces when loaded. Tread squirm
causes substantial losses in bicycle tires where the ratio between
lands and grooves is closer to 1:1 than with good auto tires. The
amount of flex as the tire passes the contact point is proportional to
inflation and the characteristics of the specific tire. This is true
both on the road and the drum tester.
I am interested in the source of conflicting data to that which I
posted. The information I posted was typical of tests done by the
tire industry. I believe it is the state of the art and I see no
breakthroughs coming along soon. Recent advances in this field were
the addition of tests for maximum lean angle and in situ wear-out
tests that test tread compounds while on the tire. These advances
that were made by Avocet and were demonstrated at InterBike the past
two years.