Lateral G-Forces
Janny
I am given to understand from watching Motor Trend TV, that a reasonable
measure of a car's cornering capabilities can be found in the measure of
lateral g-forces that a vehicle can sustain before breaking traction on
either the front or rear.
Assuming this is true, (and I'm sure someone will point out that this is a
grotesque generalization) what sort of lateral g-forces will the average
family vehicle withstand? And, what sort of lateral g-forces will the
average street driven sports car withstand?
Is there some down and dirty measure that one can use to make quick
calculations of this?
TTYL
John (aka Janny)
jvriesi...@tct.net
remove -nospam to mail
Chuck Tomlinson
In article <69jhpl$b...@sabik.tct.net>, "Janny" <jvri...@tct.net> wrote:
>I am given to understand from watching Motor Trend TV, that a reasonable
>measure of a car's cornering capabilities can be found in the measure of
>lateral g-forces that a vehicle can sustain before breaking traction on
>either the front or rear.
True. This measure is usually called "roadholding" or "grip". Note: this is
not the same thing as "handling", which refers primarily to the car's *feel*
rather than its sheer traction potential.
>Assuming this is true, (and I'm sure someone will point out that this is a
>grotesque generalization) what sort of lateral g-forces will the average
>family vehicle withstand? And, what sort of lateral g-forces will the
>average street driven sports car withstand?
Typical family sedans can generate about 0.7-0.8 g, with sports cars often in
the 0.9-1.0 g range. The road test summary in magazines like Motor Trend and
Road & Track will usually list lateral accel.
It may be interesting to note that, since lateral accel increases with the
square of the cornering speed, the difference in max cornering speed between a
mundane 0.7 g sedan and a superb 1.0 g sports car is only about 20%.
>Is there some down and dirty measure that one can use to make quick
>calculations of this?
No. Cornering traction depends largely on the friction properties of the
tires, and to a lesser extent on chassis tuning. Neither of these dominant
factors can be quantified easily, and the readily available vehicle properties
(like weight, weight bias, tire size) have relatively little impact on
ultimate grip.
--
Chuck Tomlinson
Grass Wipe
In response to you question pertaining to to what a car's G-force handling
capabilities are, and through my reading of magazines, almost all cars are
between .70 and 1.00 g's. With the 1.00 reserved for only the serious street
racers.
Harris Yong
Although the lateral acceleration ratings does not take into account many
factors, it does measure the maximum, steady (best) state cornering
ability. This is a little subjective too as cars may feel very different at
this maximum limit. Typical not that great values are in the low 0.7s,
generally obtained by tall and heavy vehicles. The mid 0.8s reflect pretty
good handling, while 0.9s are usually only held by the higher performance
sports cars in the market. Some street cars have "broken" the 1.0 barrier.
I put the word in quotations because there is really not a barrier there;
it just means that a driver will feel as much lateral force as the force of
gravity downwards.
If you know the radius of the circle you are travelling in, then you can
obtain lateral acceleration by using the formula speed squared / radius.
The speed is typically the fastest "lap" around the circle, I think.
Keeping everything in metric/SI units, you can use meters / second for
speed and meters for the radius. You will then get a result in meters /
second squared. Divide this by 9.8 and you will normalize your lateral
acceleration and get the "g" force.
Note that the radius is used in this calculation, and this can play a major
role in the number achieved. A small circle will cause all but the most
powerful rear drive cars to understeer (run wide), while a large circle
will cause many cars to oversteer (tendency to spin out). Depending on a
car and track, the results posted by different tests will be different.
Common radius is 100 ft (diameter 200 ft).
Hope this helps :)
--
Harris Yong
sine theta [d, r]
--
Janny <jvri...@tct.net> wrote in article <69jhpl$b...@sabik.tct.net>...
> I am given to understand from watching Motor Trend TV, that a reasonable
> measure of a car's cornering capabilities can be found in the measure of
> lateral g-forces that a vehicle can sustain before breaking traction on
> either the front or rear.
> Assuming this is true, (and I'm sure someone will point out that this is
a
> grotesque generalization) what sort of lateral g-forces will the average
> family vehicle withstand? And, what sort of lateral g-forces will the
> average street driven sports car withstand?
BrianofKY
The max. steady state cornering force that can be generated without down force
(ground effects, wings, or vacuum) or adhesive tires is 1g. Some race cars
(F1, WSC) using wings, ground effects and very sticky tires at very high speeds
can generate 3 to 4 g's.
One of the commercial car mags (R&T, C&D or MT I can't recall) has a skid pad
showdown every couple of years now. Everyting from indy cars, vacuum carts,
sail cars. Lots of discussion of steady state cornering g-forces.
It is important to note that this is a steady state figure and is not the best
real world indication of handling capability. Something like slalom speed is
probably a better indication of how stable a vehicles handling is. This method
includes more driver factor however.
Brian
David Hillman
In article <19980116001...@ladder02.news.aol.com>,
BrianofKY <bria...@aol.com> wrote:
>One of the commercial car mags (R&T, C&D or MT I can't recall) has a skid pad
>showdown every couple of years now. Everyting from indy cars, vacuum carts,
>sail cars. Lots of discussion of steady state cornering g-forces.
Road and Track. Co-sponsored by the Tirerack, I believe. Won the past
years by George Bowland in an A-modified Special at just shy of 2 G.
See <URL:http://www.tirerack.com/features/solo2/skidpad.htm>.
--
David Hillman, alfa romeo 164 l, nissan 240 sx se
"To be a serious automotive journalist, it is said,
you must have owned an Alfa Romeo." - John Simister
Chuck Tomlinson
bria...@aol.com (BrianofKY) wrote:
>
>The max. steady state cornering force that can be generated without down force
>(ground effects, wings, or vacuum) or adhesive tires is 1g.
Sorry, that is not correct. Even some street cars can exceed 1.0g
without aerodynamic downforce, simply because the coefficient of
friction between tires and road can be greater than 1.0.
>One of the commercial car mags (R&T, C&D or MT I can't recall) has a skid pad
>showdown every couple of years now. Everyting from indy cars, vacuum carts,
>sail cars. Lots of discussion of steady state cornering g-forces.
It's R&T, and several cars have exceeded 1.0 lateral g with no
aerodynamic downforce whatsoever.
--
Chuck Tomlinson
Ben Lanson
How repeatable are skidpad numbers? If I take my car to ten different
skidpads (all of the same radius) how closely are those numbers likely
to be grouped? Can one compare, say R&T numbers to C&D numbers, or even
numbers in the magazines which are taken from different skidpads?
Ben
Henri Helanto
bria...@aol.com (BrianofKY) writes:
>The max. steady state cornering force that can be generated without down force
>(ground effects, wings, or vacuum) or adhesive tires is 1g.
No, not really. Remember that neither tire nor asphalt is 100%
smooth and small irregularities in both put the real-life
absolute maximum to over 1.5G. (Open karts and non-spoilered
monopostos can attain 1.2-1.4G rather easily, for example)
-Henri
--
# Henri Helanto ; he...@muncca.fi ; hhel...@cc.hut.fi #
# Nissan Skyline GT-R ; '71 Corvette LS-6 ; GMC Typhoon ; etc...#
CAUTION: Before engaging mouth make sure that the brain is in gear.
Pedro Roque
I don't believe that this value means a great deal in the real world. What it
tell's you is how fast a car can run in a circle. I think american magazine
use this value only because the feel the need to put a number in everything.
Thy should do like european magazines: you take a car to twisted road, and see
how fast it goes.
Pedro
Janny wrote:
> I am given to understand from watching Motor Trend TV, that a reasonable
> measure of a car's cornering capabilities can be found in the measure of
> lateral g-forces that a vehicle can sustain before breaking traction on
> either the front or rear.
>
> Assuming this is true, (and I'm sure someone will point out that this is a
> grotesque generalization) what sort of lateral g-forces will the average
> family vehicle withstand? And, what sort of lateral g-forces will the
> average street driven sports car withstand?
>
> Is there some down and dirty measure that one can use to make quick
> calculations of this?
>
John Alexiou
"Janny" <jvri...@tct.net> writes:
>I am given to understand from watching Motor Trend TV, that a reasonable
>measure of a car's cornering capabilities can be found in the measure of
>lateral g-forces that a vehicle can sustain before breaking traction on
>either the front or rear.
Lateral G's a very important. On the road they provide the safety
marigin for you to avoid trouble (when trouble arises). On the
track it cut's seconds off even from the weakest cars.
and remember braking distances and lateral G s are quite closely
correlated also.
>Assuming this is true, (and I'm sure someone will point out that this is a
>grotesque generalization) what sort of lateral g-forces will the average
>family vehicle withstand? And, what sort of lateral g-forces will the
>average street driven sports car withstand?
A typical passenger car will have anything from 0.7 to 0.9 G. Typically
trucks, SUV, and vans have the lowest figures. Sports cars will have
0.85 to 1.0 G. Race ONLY cars (with slick tires) can get up to 1.5 G.
Now the behavior of a pneumatic tire under pressure, and side forces
is extreemly complicated. But figures as high as 1.5 G ARE attanable
under the right conditions, and without aerodynamic downforce.
>Is there some down and dirty measure that one can use to make quick
>calculations of this?
In generaly there is peak G, and sustained G figures. One measuring
device is the G-Tech pro ($140) from TESLA. (www.gtechpro.com)
Another way of measuring is the skidpad, where you bastically run in
a fixed radious circle as fast as the car can, and then use a simple
formula to calculate the G from the speed, or the time around.
typically G = mph*mph/(14.81*ft)
where G is acceleration, mph is the speed around the curve,
and ft the RADIUS (not diameter) of the curve.
Average G from braking can be calculated from
G = mph*mph/(29.62*ft) where
mph is the speed where you started braking (60mph typically)
ft is the distance to a stop.
this gives G = 121.5/ft for a 60mph-0mph stop.
JA
>TTYL
>John (aka Janny)
>jvriesi...@tct.net
>remove -nospam to mail
--
---------------------------------------
| John Alexiou. ja...@prism.gatech.edu. |
---------------------------------------
John Alexiou
bria...@aol.com (BrianofKY) writes:
>>n response to you question pertaining to to what a car's G-force handling
>>capabilities are, and through my reading of magazines, almost all cars are
>>between .70 and 1.00 g's. With the 1.00 reserved for only the serious street
>>racers.
>The max. steady state cornering force that can be generated without down force
>(ground effects, wings, or vacuum) or adhesive tires is 1g. Some race cars
>(F1, WSC) using wings, ground effects and very sticky tires at very high speeds
>can generate 3 to 4 g's.
Here is an example of a person that hasn't done their homework.
1g is not any limit in pneumatic tires. A soft compount can easily
achieve over 1g latteral acceleration. For dry (coulomb) friction
there might be a limit, but when a deformable body like a tire
grabs a rough surface like asphalt or concrete the results are
harly linear and ideal. In fact if you have ever played with a
big rubber eraser on a flat surface you will know what I mean.
C.R. Krieger
On Sun, 18 Jan 1998 03:25:01 +0000, Pedro Roque
<nop3...@mail.telepac.pt> brazenly asserted:
>I don't believe that this value means a great deal in the real world. What it
>tell's you is how fast a car can run in a circle. I think american magazine
>use this value only because the feel the need to put a number in everything.
>Thy should do like european magazines: you take a car to twisted road, and see
>how fast it goes.
>
>Pedro
HAHAHAHAHA! OK, Pedro, you bring your .75 G-capable Corolla (or Lada,
or whatever) and we'll play around on a racetrack or twisty road of
your choice with my 1.0 G Audi and see who's faster.
Has it occurred to you that the higher speed in a CIRCLE might mean a
higher speed in PART of a circle (Let's call it a 'turn', just for
laughs.)? So what are you; about 25 watts?
>
>Janny wrote:
>
>> I am given to understand from watching Motor Trend TV, that a reasonable
>> measure of a car's cornering capabilities can be found in the measure of
>> lateral g-forces that a vehicle can sustain before breaking traction on
>> either the front or rear.
>>
>> Assuming this is true, (and I'm sure someone will point out that this is a
>> grotesque generalization) what sort of lateral g-forces will the average
>> family vehicle withstand? And, what sort of lateral g-forces will the
>> average street driven sports car withstand?
>>
>> Is there some down and dirty measure that one can use to make quick
>> calculations of this?
>>
>> TTYL
>> John (aka Janny)
>> jvriesi...@tct.net
>> remove -nospam to mail
>
>
>
C.R. Krieger
"Ignore 'em, m'dear; they're beneath your dignity." - W.C. Fields
Reply to warp1 at lakefield dot net
Marc
Just_r...@my.sig_line (C.R. Krieger) wrote:
>On Sun, 18 Jan 1998 03:25:01 +0000, Pedro Roque
><nop3...@mail.telepac.pt> brazenly asserted:
>>I don't believe that this value means a great deal in the real world. What it
>>tell's you is how fast a car can run in a circle. I think american magazine
>>use this value only because the feel the need to put a number in everything.
>>Thy should do like european magazines: you take a car to twisted road, and see
>>how fast it goes.
>HAHAHAHAHA! OK, Pedro, you bring your .75 G-capable Corolla (or Lada,
>or whatever) and we'll play around on a racetrack or twisty road of
>your choice with my 1.0 G Audi and see who's faster.
>Has it occurred to you that the higher speed in a CIRCLE might mean a
>higher speed in PART of a circle (Let's call it a 'turn', just for
>laughs.)? So what are you; about 25 watts?
A "turn" generally has an entrance and an exit. The point isn't to
get around that particular turn as quickly as possible (as it is in a
skidpad), but to minimize the time around a course. It is entirely
possible that a car with worse steady state grip can get a faster time
than a car with better steady state grip because of the
characteristics of the cars in braking/accelerating transitions (as
well as turn in and other factors).
Marc
For email, remove second "y" from Gum...@ticnet.com
Johnson, J.
Marc wrote:
>
> Just_r...@my.sig_line (C.R. Krieger) wrote:
>
> >HAHAHAHAHA! OK, Pedro, you bring your .75 G-capable Corolla (or Lada,
> >or whatever) and we'll play around on a racetrack or twisty road of
> >your choice with my 1.0 G Audi and see who's faster.
I bet you could find a course where this "0.75 G-capable Corolla"
wouldnt let your "1.0G Audi" shine as bright as you think it would.
And what if this Pedro guy had more skill than you? The difference
in skill alone could easily even out any preconceived advantages.
Id like to see the look on your face if you went wide and he cut
right under you.
>
> >Has it occurred to you that the higher speed in a CIRCLE might mean a
> >higher speed in PART of a circle (Let's call it a 'turn', just for
> >laughs.)? So what are you; about 25 watts?
Thats a little to simplistic.
>
> A "turn" generally has an entrance and an exit. The point isn't to
Just to add a few things to this very true statement :)
And most important: an apex. Next important is the amount of available
power and your cars balance which will dictate the fastest way thru
the curve for your car. The type of curves that come closest to the
skid pad scenario are long constant radius bends. The radius of which
depends on your cars power since in some bends you are power limited
not grip limited. In such cases, I guess one could talk about "steady
state" (by ignoring other effects). But, if you are early/late apexing,
then you are varying the inputs (= steering, throttle, line etc.) and
anything but in the "max. lateral G steady state". Actually you are
not in any kind of steady state - per definition [well someone could
take a cheap shot, redefine what constitutes the system under
investigation, and then claim a special form of steady state].
Considering just a few other points:
1. Varying available road grip
2. The fact that no suspension is ideal (whats ideal anyway?) for all
possible road conditions and driver style
3. The dynamical aspect of the drivers reactions
and the notion of "max. lateral G steady state" becomes
purely theoretical valid only for a very simplified model of reality.
Available grip is not a constant as measured on the skidpad but a
dynamic. The driver tries to follow the envelope of this dynamic.
In most cases, exceeding the limit is not the fastest way around the
track (shortening up some very tight s-curves would be a
counter example). If longevity, endurance, etc. is an issue then
strategy & consistency become more important than setting fastest lap
times.
> get around that particular turn as quickly as possible (as it is in a
> skidpad), but to minimize the time around a course. It is entirely
> possible that a car with worse steady state grip can get a faster time
> than a car with better steady state grip because of the
> characteristics of the cars in braking/accelerating transitions (as
> well as turn in and other factors).
>
True. Porsches demonstrated this for corvettes some years ago
(back when some owners were so proud of their .96 test results).
A stock euro M3 exceeds 1.0G lateral (according to BMW), but that
doesnt mean you're going to be able to get close in curves all the
time. Even with the not so bad handling. When I travel bumpy
serpentines in the Alps I couldnt care much about the 1.0G. Its
more important to be (know how to) setup for turns than knowing
what your car can pull on the skid pad.
David Hillman
In article <34ca5248...@NEWS.TICNET.COM>, Marc <Gum...@ticnet.com> wrote:
>A "turn" generally has an entrance and an exit. The point isn't to
>skidpad), but to minimize the time around a course. It is entirely
>possible that a car with worse steady state grip can get a faster time
>than a car with better steady state grip because of the
>characteristics of the cars in braking/accelerating transitions (as
>well as turn in and other factors).
A turn also has a period of neutral throttle, which varies greatly in
length. In a 90 degree street course turn, it is virtually non-existant.
But in a 180 sweeper, it can be considerable ( the Carousel at Road
America, for example ).
This is the part of the turn where maximum lateral acceleration is
most meaningful.
Noel
I don't think the skidpad is a good test for a car's handling abilities;
it's just too subjective. A car like the Maclaren F1, which is capable
of more than 1.01g, records only a 0.88 when driven by a journalist who
is intimidated by the car.
I think each magazine should choose one road course at which they test
all the cars they review. A course with a good amount of differing
turns to test cornering ablities. A straight-away to test acceleration,
high speed and braking. And a chicane, that will test turn-in and
control. They should employ the talents of a professional race car
driver to pilot the car around the track as fast as possible for several
laps.
Noel
David Hillman
In article <34C67F...@neversoft.comNOSPAM>,
Noel <No...@neversoft.comNOSPAM> wrote:
>I don't think the skidpad is a good test for a car's handling abilities;
>it's just too subjective. A car like the Maclaren F1, which is capable
>of more than 1.01g, records only a 0.88 when driven by a journalist who
>is intimidated by the car.
I completely agree with your conslusion, although not necessarily
with your logic. A skidpad test is not subjective, although the
results can tend to be inconsistent, as you mention.
>I think each magazine should choose one road course at which they test
>all the cars they review. A course with a good amount of differing
>turns to test cornering ablities. A straight-away to test acceleration,
>high speed and braking. And a chicane, that will test turn-in and
>control. They should employ the talents of a professional race car
>driver to pilot the car around the track as fast as possible for several
>laps.
Again, I agree, and would love to see this. Currently, the only
magazines that regularly do this, AFAIK, are Grassroots Motorsports
and Car magazine.
Then again, I am in the minority of car buyers who actually care that
a car with less horsepower might be faster around a circuit. Zero to
sixty is more than enough information for most buyers.
Chuck Tomlinson
Noel <No...@neversoft.comNOSPAM> wrote:
>I don't think the skidpad is a good test for a car's handling abilities;
>it's just too subjective. A car like the Maclaren F1, which is capable
>of more than 1.01g, records only a 0.88 when driven by a journalist who
>is intimidated by the car.
I find it hard to believe that any journalist who tests supercars on the
street would be intimidated by a McLaren F1 on a wide-open skidpad. Also, I
think skidpad tests are far more objective measures of lateral grip than,
say, purely verbal descriptions of "steering feel" or "stability".
>I think each magazine should choose one road course at which they test
>all the cars they review. A course with a good amount of differing
>turns to test cornering ablities. A straight-away to test acceleration,
>high speed and braking. And a chicane, that will test turn-in and
>control. They should employ the talents of a professional race car
>driver to pilot the car around the track as fast as possible for several
>laps.
I agree that the magazines need some way to numerically measure a car's
performance during combined steering and acceleration at a car's traction
limits. This regime is obviously important during turn entrances and exits,
but isn't captured by any of the "normal" tests (eg, skidpad, 100' slalom).
Maybe the magazines should agree on a standard slalom with varying cone
spacing (e.g., each gap is 10' shorter or longer than the adjacent gap).
Car and Driver seems to do something like this, with their increasing and
decreasing slalom tests.
As for a professional driver's lap times: I think that would border on
purely academic as a measure of a street car's handling. A chassis tuner
can squeeze every ounce of grip out of a car by making it marginally stable
near the lateral limits. A pro driver could make such a car shine on the
track, but the typical customer (on the street) would likely be scared
witless, or fly off the road.
IOW, lap times are technically interesting, but on the *street*, a car that
inspires driver confidence can often outrun a more "nervous" car with better
track times. That's just my 2c...
--
Chuck Tomlinson
Matthew T. Russotto
In article <6a84np$7...@singer.cent.gla.ac.uk>, Ian <ia...@earthling.net> wrote:
}I'm afraid that Chuck's right.
}I'm doing a course in aero engineering at uni and it's a blatant fact if you
}know the relevant facts.
}The force of friction=constant x perpendicular force
}This constant cannot exceed 1
There's the error in the reasoning. The constant CAN exceed 1.
--
Matthew T. Russotto russ...@pond.com
"Extremism in defense of liberty is no vice, and moderation in pursuit
of justice is no virtue."
Pedro Roque
Noel wrote:
> I don't think the skidpad is a good test for a car's handling abilities;
> it's just too subjective. A car like the Maclaren F1, which is capable
> of more than 1.01g, records only a 0.88 when driven by a journalist who
> is intimidated by the car.
>
> I think each magazine should choose one road course at which they test
> all the cars they review. A course with a good amount of differing
> turns to test cornering ablities. A straight-away to test acceleration,
> high speed and braking. And a chicane, that will test turn-in and
> control. They should employ the talents of a professional race car
> driver to pilot the car around the track as fast as possible for several
> laps.
>
> Noel
My feeling exactly
Pedro
Pedro Roque
C.R. Krieger wrote:
> On Sun, 18 Jan 1998 03:25:01 +0000, Pedro Roque
> <nop3...@mail.telepac.pt> brazenly asserted:
>
> >I don't believe that this value means a great deal in the real world. What it
> >tell's you is how fast a car can run in a circle. I think american magazine
> >use this value only because the feel the need to put a number in everything.
> >Thy should do like european magazines: you take a car to twisted road, and see
> >how fast it goes.
> >
> >Pedro
>
> HAHAHAHAHA! OK, Pedro, you bring your .75 G-capable Corolla (or Lada,
> or whatever) and we'll play around on a racetrack or twisty road of
> your choice with my 1.0 G Audi and see who's faster.
>
> Has it occurred to you that the higher speed in a CIRCLE might mean a
> higher speed in PART of a circle (Let's call it a 'turn', just for
> laughs.)? So what are you; about 25 watts?
>
Some clues about my car: It's italian, and as 355 in the name. Care to guess ?
I don't mean that lateral G-force isn't a important measure of a car's cornering
abilities. I just don't agree about the way american car magazines measure it. In a
circle there's virtually no weight transfer, and the suspension doesn't have to
work as hard as in a succession of different corners. You might want to think about
why european magazines, which is the land of really sports cars, they just don't
care about lateral G-force. (not in circle, anyway)
Brandon Sommerville
Pedro Roque had the nerve to say:
>
>Some clues about my car: It's italian, and as 355 in the name. Care to guess ?
>
(sorry, couldn't resist. :) )
Brandon
Laugh and the world laughs with you.
Cry, and someone yells, "Shut up!"
Ben Lanson
The equation F = mN (friction = mu * normal force) does not hold on
irregular surfaces (roads) for deformable objects (tires). Hence, with
grippy tires the 1.0 g limit is not a limit at all, and has in fact been
exceeded.
Ben
Ian wrote:
>
> I'm afraid that Chuck's right.
> I'm doing a course in aero engineering at uni and it's a blatant fact if you
> know the relevant facts.
> The force of friction=constant x perpendicular force
> This constant cannot exceed 1, and so the max force of friction is equal to
> the perpendicular force (the weight of the car in this case). It should be
> obvious from this that the max acc round the corner is 1g.
>
> What isn't obvious is that even a car with no wings or downforce aids will
> infact still generate some downforce. Also note that it was the sustained
> acceleration that cannot exceed 1g.
Henri Helanto
"Ian" <ia...@earthling.net> writes:
>I'm doing a course in aero engineering at uni and it's a blatant fact if you
>know the relevant facts.
>The force of friction=constant x perpendicular force
>This constant cannot exceed 1, and so the max force of friction is equal to
>the perpendicular force (the weight of the car in this case). It should be
>obvious from this that the max acc round the corner is 1g.
Read again what's been written about the smoothness of the surface
of the tire and the asphalt and deformation. Simple physics don't
apply here.
>What isn't obvious is that even a car with no wings or downforce aids will
>infact still generate some downforce. Also note that it was the sustained
>acceleration that cannot exceed 1g.
How do you explain that my GT-R was _measured_ (60ft skidpad and
a lateral acceleration sensor) 1.31G:s on slicks? Do you really
believe that the body of the car could generate over NINE HUNDRED
LBS of downforce at under 50mph?
C97Deard
The racing cars do not only have friction on their side, they also have down
forces so they can corner at hicher velocitys with out over coming the
frictional forces.
dennis...@dwt.csiro.au
In article <6a84np$7...@singer.cent.gla.ac.uk>,
>know the relevant facts.
>The force of friction=constant x perpendicular force
>This constant cannot exceed 1, and so the max force of friction is equal to
>the perpendicular force (the weight of the car in this case). It should be
>obvious from this that the max acc round the corner is 1g.
>
>infact still generate some downforce. Also note that it was the sustained
>acceleration that cannot exceed 1g.
>
Ian
I am afraid you are wrong on this. This is a common misconception, and is
the reason that many people resort to overly complex explanations WRT
tyre traction when lateral acceleration is over 1G.
I know that you are not going to believe me on this, so my suggestion is
to go and pull out your trusty CRC Handbook, and have a look at some of
the coefficients of friction. Although not particularly common, there are
interfaces that generate mu figures of SIGNIFICANTLY more than 1, yet
still obey Amonton's Law (F= mu*N). Natural rubber on glass and bitumen
even generate kinetic coefficients of friction of more than 1, in some
cases even on a wet surface!! The problem is, your understanding of mu is
very common, and had many scrabbling for alternative suggestions WRT drag
racing averaging over 1 G over the 1/4 (needlessly, I might add).
Some static coefficients of friction (in air unless otherwise stated.
Material-Material mu
Al-Al 1.9
Cu-Cu (in H2 or N2) 4.0
(in air or O2) 1.6
Au-Au (H2 or N2) 4.0
(air or O2) 2.5
Ni-Ni (H2 or N2) 5.0
(air or O2) 1.6
Pt-Pt 3.0
Ag-Ag 1.5
No note is made WRT these coefficients of friction (i.e. they are
regarded as coefficients of friction, nothing more, nothing less, by the
CRC Handbook). So, coefficients of friction >1, while unusual, are
certainly not exceptional, and there is no area dependence to these
numbers. A note is made that the metals were spectroscopically pure and
were outgasses prior to other gases being admitted. They do state that
some of the materials that they have tested do not follow Amonton's Law,
but they are not included in the list above.
Now, the CRC handbook gives kinetic coefficients of friction. Two points
to be made here. First, the generally used coefficient of kinetic
friction of a tyre on the road is taken to be 0.8, and significantly less
if there is water on the road.
The second point to be made is that this figure is for natural vulcanised
rubber; for modern tyres with synthetic rubbers, and other additives to
the tyre compound, the coefficient of friction would undoubtably be
higher. Rates are 100m/min (remember, this is kinetic friction)
Material-Material mu (kinetic)
rubber-glass 1.07
""""""""""" (wet) 0.94
rubber-concrete 1.02
"""""""""""" (wet) 0.97
rubber-bitumen 1.07
"""""""""""""" (wet) 0.95 (looks pretty similar to rubber-glass)
rubber-bitumen containing rubber 1.15
(wet) 1.03
So, with natural vulcanised rubber, skidding over wet, rubberised
bitumen, mu>1.
Note that there is no statement in the CRC handbook that rubber/other
material interactions do not obey Amonton's Law.
I hope this helps somewhat.
Dennis
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BrianofKY
I think each magazine should choose one road course at which they test
> all the cars they review. A course with a good amount of differing
> turns to test cornering ablities. A straight-away to test acceleration,
> high speed and braking. And a chicane, that will test turn-in and
> control. They should employ the talents of a professional race car
> driver to pilot the car around the track as fast as possible for several
> laps.
>
> Noel
And though not a 'professional' I would like to volunteer my services.
Brian
BrianofKY
<There's the error in the reasoning. The constant CAN exceed 1.
Yes, it's called adhesion and believe me a good racing slick at temperature
will definitly stick like glue.
Also 'aero' engineer, I would expect you to know something about lift. It's
the aerodynamic force generated by air flowing at a relative velocity over a
surface (ie. a vehicle.) While it is true that this lift in most vehicle is
up, a race car with ground effects and wings can easily generate two to three
times its weight in lift.
Also, your explanation would mean that a car could not accelerate in any
direction at greater that 1g. Explain top fuel dragsters 0-300MPH+ et< 5.0s =
(Vf-Vi)/t = a => 300(miles/hour)*5280(ft/mile)/3600(sec/hr) =88 (ft/sec^2) or
about 8g.
Brian
Ben Lanson
BrianofKY wrote:
> Also, your explanation would mean that a car could not accelerate in any
> direction at greater that 1g. Explain top fuel dragsters 0-300MPH+ et< 5.0s =
>
> (Vf-Vi)/t = a => 300(miles/hour)*5280(ft/mile)/3600(sec/hr) =88 (ft/sec^2) or
> about 8g.
>
>
> Brian
I agree with your point, but not with your numbers. 1g = 9.8 m/s^2 or
roughly 32 f/sec^2, which makes 88 ft/sec^2 about 2.75 g. Note that a
sustained 8g force on a driver would cause many to black out unless they
were wearing military type g-suits.
Ben
BrianofKY
>I agree with your point, but not with your numbers. 1g = 9.8 m/s^2 or
>roughly 32 f/sec^2, which makes 88 ft/sec^2 about 2.75 g. Note that a
>sustained 8g force on a driver would cause many to black out unless they
>were wearing military type g-suits.
>
>Ben
>
far as sustained, 4.7seconds isn't really a long time.
And that 2.75 is average, I doubt that it is linear acceleration so the peak
could be easily twice as high.
Thanks for the correction
Brian
Chuck Tomlinson
bria...@aol.com (BrianofKY) wrote:
>>I agree with your point, but not with your numbers. 1g = 9.8 m/s^2 or
>>roughly 32 f/sec^2, which makes 88 ft/sec^2 about 2.75 g. Note that a
>>sustained 8g force on a driver would cause many to black out unless they
>>were wearing military type g-suits.
>>
>far as sustained, 4.7seconds isn't really a long time.
>
>And that 2.75 is average, I doubt that it is linear acceleration so the peak
>could be easily twice as high.
AFAIK, fuel dragsters leave the line at between 4 g and 5 g. Their
initial *decel* (just after the 'chute opens) might be even higher
in magnitude than the launch accel. That has nothing to do with
tire friction, of course; just a point of interest.
--
Chuck Tomlinson
John Alexiou
Pedro Roque <nop3...@mail.telepac.pt> writes:
>C.R. Krieger wrote:
>> On Sun, 18 Jan 1998 03:25:01 +0000, Pedro Roque
>> <nop3...@mail.telepac.pt> brazenly asserted:
>Some clues about my car: It's italian, and as 355 in the name. Care to guess ?
>I don't mean that lateral G-force isn't a important measure of a car's cornering
>abilities. I just don't agree about the way american car magazines measure it. In a
>circle there's virtually no weight transfer, and the suspension doesn't have to
>work as hard as in a succession of different corners. You might want to think about
Actually it measures how well the suspension keeps the tires perpendicular
to the road, which is one of the MAJOR functions of a suspension system.
It is an IDEAL situation, that does not emulate real life, but that's the
point. It's trying to extract as many unknown variables out of the
picture showing what this car/tire combo can do MAX at the limit IDEALLY.
A good driver might be able to approach this, just as he might be able to
approach the 0-60mph (0-100km/h) times they post.
>why european magazines, which is the land of really sports cars, they just don't
>care about lateral G-force. (not in circle, anyway)
Of course when CAR magazine takes a dozen cars, through like 8 race
tracks, and checks their handling, we all listen, since these guys
really know how to drive, feel, and write about cars. But handling
is subjective, and roadholding should be as objective as posible.
In a recent CAR magazine they declared the Lotus Elise as the ULTIMATE
in handling ("a new datum." I quote) although it did not have the best
roadholding, or the fastest times around the corners. The reason:
handling is an evaluation of the car/driver system not just the car.
my 6 Greek Drachmas (aprox. 2 cents)