Durability of shaped tubing (eg Columbus Megatube)
Dietrich Wiegmann
round downtube of the same weight, thickness and material?
I thought downtubes should be round to resist torsion as well as
possible, but two failures that started on the underside of the
downtube near the head tube have me wondering. Are vertically flared
or shaped downtubes fashion or function?
Thanks,
Dietrich
Jim Sharrock
the handlebars swing around and the metal adjuster on the veloce stirrups
hits the side wall. That such a small force could cause such a large dent
amazed me. Stupid design on the stirrups I would say, why it couldn't be
rubber, who knows.
I like the look of the downtube but I can't imagine it'll stand up well to a
forceful side impact.
Jim.
"Dietrich Wiegmann" <dwie...@anteon.com> wrote in message
news:1ee1aae4.02012...@posting.google.com...
Sheldon Brown
That's an easier question to ask than to answer.
In practice, you don't see a head-to-head comparison, because ovalized down
tubes are generally larger in diameter than round tubes, and thus have to
have thinner walls for the same weight.
There are different kinds of "durability." A frame with an oversided,
vertically ovalized down tube at the front end will generally be more
_crashworthy_ and less likely to get the classic buckle in the event of a
front-end collision.
The failures you describe starting on the underside of the downtube, however
are more likely related to fatigue and/or the difficulty of welding a good
joint with such thin wall tubing.
Ovalization is generally represented as a way of making the frame stronger,
but you shouldn't lose track of what may be the more important reason: joint
fitting.
Cannondale used to make frames where the (round) down tube was larger in
diameter than the head tube, and the result was a seriously ugly overhang at
this highly visible joint. Vertical ovalization solves this problem.
Meanwhile, down at the bottom bracket, The seat tube and down tube are
fighting it out for material to attach to. Ovalizing these in a horizontal
orientation makes everybody happy.
It is my understanding that the torsional rigidity is roughly a function of
the smaller diameter of an oval tube. Ovalized, oversized downtubes don't
usually get that flat that torsional rigidity becomes a problem.
Sheldon "Ovality" Brown
+-------------------------------------------------+
| Some of my mother's paintings may be seen at: |
| http://sheldonbrown.com/joyce |
+-------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772, 617-244-1040 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com
Jon Isaacs
>but you shouldn't lose track of what may be the more important reason: joint
>fitting.
This is an interesting issue. Since out of plane stiffness seems to be the
important factor, it seems to me that the common orientation of flattened
tubing is wrong for this a bending viewpoint and flattened tubing in general
reduces the torsional stiffness. It may be more aerodynamic though.
jon isaacs
jobst....@stanfordalumni.org
100% fashion! However, bike riders eat it up as though it were nirvana.
Jobst Brandt <jobst....@stanfordalumni.org> Palo Alto CA
Mark Hickey
Ever notice that most frames with "multi-squished down tubes" have it
aligned "wrong" for the forces at work at that part of the frame
(horizontally at the bottom bracket, vertically at the head tube), and
that it just happens to make 'em easier to weld that way?
Gotta be a coincidence there somewhere... ;-)
And yes, if you want to design a structure to resist torque with the
greatest efficiency, you make it round. Progress!
Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame
Jon Isaacs
>greatest efficiency, you make it round. Progress!
So are the new Habanero's going to feature special "round" tubing? I have
several older bikes that were built with round tubing.
What goes around come around huh??
jon isaacs
Tom H
> joni...@aol.com (Jon Isaacs) wrote:
>
>
> Ever notice that most frames with "multi-squished down tubes" have it
> aligned "wrong" for the forces at work at that part of the frame
> (horizontally at the bottom bracket, vertically at the head tube), and
> that it just happens to make 'em easier to weld that way?
>
> Gotta be a coincidence there somewhere... ;-)
>
> And yes, if you want to design a structure to resist torque with the
> greatest efficiency, you make it round. Progress!
>
> Mark Hickey
> Habanero Cycles
> http://www.habcycles.com
> Home of the $695 ti frame
Doesn't the downtube resist bending, and isn't this principally
laterally at the bb (I fail to see how it could bend "vertically" with
all the other tubes supporting it)? I have also noticed frames with
reinforcement at the head-tube. This tends to be place to support the
down-tube under "vertical" stresses. Are you saying these gussets
should be placed at the side?
Tom
Mark Hickey
>ma...@habcycles.com (Mark Hickey) wrote in message news:<3c56526d....@netnews.att.net>...
>> Ever notice that most frames with "multi-squished down tubes" have it
>> aligned "wrong" for the forces at work at that part of the frame
>> (horizontally at the bottom bracket, vertically at the head tube), and
>> that it just happens to make 'em easier to weld that way?
>>
>> Gotta be a coincidence there somewhere... ;-)
>>
>> And yes, if you want to design a structure to resist torque with the
>> greatest efficiency, you make it round. Progress!
>Doesn't the downtube resist bending, and isn't this principally
>laterally at the bb (I fail to see how it could bend "vertically" with
>all the other tubes supporting it)?
Think about the forces that are going to try to bend the bottom
bracket shell out of alignment. By FAR the largest are the weight of
the rider lurching from side to side on the cranks. That will attempt
to "twist" the bottom bracket across its width. The forces that will
try to turn it to the left and right (from the rider's perspective)
are much lower.
>I have also noticed frames with
>reinforcement at the head-tube. This tends to be place to support the
>down-tube under "vertical" stresses. Are you saying these gussets
>should be placed at the side?\
The gussets are there because of the natrual stress riser at the
junction. When you think about it, the stresses at that critical
point are better dispersed with a larger diameter tube. This is
accomplished with a round tube vs. the smaller radius of the "skinny
end of a squished tube". The alignment of the squished downtube at
the head tube junction would theoretically be better at resisting
fore/aft deflection, but since that tubes in tension (think of it as a
cable preventing the bottom of the head tube from moving), it makes
little difference. However, the torque from the rider cranking on the
bars in a sprint or climb will attempt to displace the head tube
laterally. If you wanted to prevent this by squishing the down tube,
you'd make the upper junction wide, not tall.
A Muzi
with the
> >greatest efficiency, you make it round. Progress!
>
"Jon Isaacs" <joni...@aol.com> wrote in message
news:20020129074559...@mb-mq.aol.com...
So are the new Habanero's going to feature special "round" tubing? I have
> several older bikes that were built with round tubing.
>
> What goes around come around huh??
Uh, the Habanero in my front window features round tubes except the
chainstays. Sort of like SR-Sakae's "RoundTech" chainrings . . .
--
Andrew Muzi
http://www.yellowjersey.org
Open every day since 1 April 1971
Sheldon Brown
> >> Ever notice that most frames with "multi-squished down tubes" have it
> >> aligned "wrong" for the forces at work at that part of the frame
> >> (horizontally at the bottom bracket, vertically at the head tube), and
> >> that it just happens to make 'em easier to weld that way?
> >>
> >> Gotta be a coincidence there somewhere... ;-)
> >>
> >> And yes, if you want to design a structure to resist torque with the
> >> greatest efficiency, you make it round. Progress!
Thomas Harrigan demurred:
> >Doesn't the downtube resist bending, and isn't this principally
> >laterally at the bb (I fail to see how it could bend "vertically" with
> >all the other tubes supporting it)?
Mark riposted:
> Think about the forces that are going to try to bend the bottom
> bracket shell out of alignment. By FAR the largest are the weight of
> the rider lurching from side to side on the cranks. That will attempt
> to "twist" the bottom bracket across its width. The forces that will
> try to turn it to the left and right (from the rider's perspective)
> are much lower.
Oh, I'm not so sure about that. The magnitude of the force from chain pull
is clearly greater than that of the rider's foot on the pedal, according to
the ratio of crank length to chainring radius.
This is somewhat countered by the considerably greater moment arm of the
pedal vs the chain.
It seems to me that the horizontal ovalization of the seat tube should
pretty much deal with the rider-weight stresses, but that the horizontal
ovalization of the lower down tube is more about chain pull. (To the extent
that it's about stress at all, and not about getting everything to fit in a
confined space.)
> >I have also noticed frames with
> >reinforcement at the head-tube. This tends to be place to support the
> >down-tube under "vertical" stresses. Are you saying these gussets
> >should be placed at the side?\
>
> The gussets are there because of the natrual stress riser at the
> junction. When you think about it, the stresses at that critical
> point are better dispersed with a larger diameter tube. This is
> accomplished with a round tube vs. the smaller radius of the "skinny
> end of a squished tube". The alignment of the squished downtube at
> the head tube junction would theoretically be better at resisting
> fore/aft deflection, but since that tubes in tension (think of it as a
> cable preventing the bottom of the head tube from moving), it makes
> little difference.
I can't agree. Ovalization and/or gussetting at the front is mainly about
making a reasonably crashworthy frame while also keeping the weight down.
Sheldon "Only Rarely Disagrees With Mark" Brown
+---------------------------------------------------+
| In theory, there's no difference between theory |
| and practice; but, in practice, there is. |
+---------------------------------------------------+,
Toby Hamilton
> So are the new Habanero's going to feature special "round" tubing? I have
> several older bikes that were built with round tubing.
The trouble there is that "round" is such a boring sounding word. It needs
to be something like Axial Symmetry and maybe throw in a gratuitous "Pro"
somwhere as well. ;)
--
Toby Hamilton (th-...@rogers.com)
Mark Hickey
>Jon Isaacs wrote:
>
>> So are the new Habanero's going to feature special "round" tubing? I have
>> several older bikes that were built with round tubing.
>
>The trouble there is that "round" is such a boring sounding word. It needs
>to be something like Axial Symmetry and maybe throw in a gratuitous "Pro"
>somwhere as well. ;)
Toby, you have a great future in marketing!
Also, I was thinking "roundtech" was a nice term....
Sheldon Brown
> >The trouble there is that "round" is such a boring sounding word. It needs
> >to be something like Axial Symmetry and maybe throw in a gratuitous "Pro"
> >somwhere as well. ;)
>
> Toby, you have a great future in marketing!
Mark Hickey wrote:
> Also, I was thinking "roundtech" was a nice term....
Too late for that one, that's a trademark of SR/SunTour, dating back to when
they were fighting Shimano's Biopace design with ridicule instead of science.
How 'bout "EquiRadial" or "C=2*R" or "Guaranteed Built With 3.1416 Tubing"
Another great feature is "full-butted" tubing: Many bikes are made with
double-butted tubing, that's stronger on each end. Wouldn't you rather have
a bike with full-butted tubing, stronger all the way from one end of the
tube to the other?
Sheldon "Marketeer" Brown
+----------------------------------------+
| Lottery: the closest thing we have |
| to a tax on stupidity. |
+----------------------------------------+
Mark Hickey
>Mark Hickey asserted:
>
>> >> Ever notice that most frames with "multi-squished down tubes" have it
>> >> aligned "wrong" for the forces at work at that part of the frame
>> >> (horizontally at the bottom bracket, vertically at the head tube), and
>> >> that it just happens to make 'em easier to weld that way?
>> >>
>> >> Gotta be a coincidence there somewhere... ;-)
>> >>
>> >> And yes, if you want to design a structure to resist torque with the
>> >> greatest efficiency, you make it round. Progress!
>
>Thomas Harrigan demurred:
>
>> >Doesn't the downtube resist bending, and isn't this principally
>> >laterally at the bb (I fail to see how it could bend "vertically" with
>> >all the other tubes supporting it)?
>
>Mark riposted:
>
>> Think about the forces that are going to try to bend the bottom
>> bracket shell out of alignment. By FAR the largest are the weight of
>> the rider lurching from side to side on the cranks. That will attempt
>> to "twist" the bottom bracket across its width. The forces that will
>> try to turn it to the left and right (from the rider's perspective)
>> are much lower.
>
>Oh, I'm not so sure about that. The magnitude of the force from chain pull
>is clearly greater than that of the rider's foot on the pedal, according to
>the ratio of crank length to chainring radius.
I thought through that, and concluded that the effect of chain pull is
concentrated mainly in the rear triangle - that is, think of it as the
bottom bracket and hub having a fight. I can't believe this causes
much of a displacement of the bottom bracket. Another reason I don't
believe there's much displacement is that on bikes I've built with
VERY little right crank to chainstay clearance (as little as 1mm),
there's never been enough deflection in the bottom bracket to cause
contact - even when "multiplied" by the full length of the crank.
You'd expect to see contact if there was deflection, as the potential
contact point coincides with the peak power input at the left crank.
On the other hand, it's not hard to see lateral movement of the bottom
bracket on most frames (and I can even discern a slight amount on my
stiffer-than-average frames, though I'm clearly not pushing as hard as
I should since I only weigh 150 pounds). This causes chain to front
deraille...errr, derailer contact, which is a much more common
complaint than any other bottom bracket flex problem (IMHO of course).
>This is somewhat countered by the considerably greater moment arm of the
>pedal vs the chain.
>
>It seems to me that the horizontal ovalization of the seat tube should
>pretty much deal with the rider-weight stresses, but that the horizontal
>ovalization of the lower down tube is more about chain pull. (To the extent
>that it's about stress at all, and not about getting everything to fit in a
>confined space.)
>
>> >I have also noticed frames with
>> >reinforcement at the head-tube. This tends to be place to support the
>> >down-tube under "vertical" stresses. Are you saying these gussets
>> >should be placed at the side?\
>>
>> The gussets are there because of the natrual stress riser at the
>> junction. When you think about it, the stresses at that critical
>> point are better dispersed with a larger diameter tube. This is
>> accomplished with a round tube vs. the smaller radius of the "skinny
>> end of a squished tube". The alignment of the squished downtube at
>> the head tube junction would theoretically be better at resisting
>> fore/aft deflection, but since that tubes in tension (think of it as a
>> cable preventing the bottom of the head tube from moving), it makes
>> little difference.
>
>I can't agree. Ovalization and/or gussetting at the front is mainly about
>making a reasonably crashworthy frame while also keeping the weight down.
I agree with you on the gussetting, but am not convinced that a "tall"
down tube to head tube junction makes a more crash-resistant frame,
since they always break at the bottom of the head tube/ down tube
junction. The "sharper" that junction is, the more stress will be
concentrated at that critical point.
I don't know how significant the difference is (and I'm not about to
build a few dozen frames for destructive testing to find out...) but
just don't buy the reasons given for the "improved tubing shapes".
The rounder the down tube, the better it is at resisting torque (which
is what's really important at the dt/ht junction).
>Sheldon "Only Rarely Disagrees With Mark" Brown
Mark "hardly every disagrees with Sheldon" Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame
>+---------------------------------------------------+
Tom Thompson
"Sheldon Brown" <capt...@sheldonbrown.com> wrote in message > Another great
feature is "full-butted" tubing: Many bikes are made with
> double-butted tubing, that's stronger on each end. Wouldn't you rather
have
> a bike with full-butted tubing, stronger all the way from one end of the
> tube to the other?
>
Tom Thompson
Matt O'Toole
> Another great feature is "full-butted" tubing: Many bikes are made with
> double-butted tubing, that's stronger on each end. Wouldn't you rather
have
> a bike with full-butted tubing, stronger all the way from one end of the
> tube to the other?
It worked for Fat City...
Matt O.
A Muzi
>
> > >The trouble there is that "round" is such a boring sounding word. It
needs
> > >to be something like Axial Symmetry and maybe throw in a gratuitous
"Pro"
> > >somwhere as well. ;)
> >
> > Toby, you have a great future in marketing!
>
> Mark Hickey wrote:
>
> > Also, I was thinking "roundtech" was a nice term....
>
news:3C574FAF...@sheldonbrown.com...
> Too late for that one, that's a trademark of SR/SunTour, dating back to
when
> they were fighting Shimano's Biopace design with ridicule instead of
science.
> How 'bout "EquiRadial" or "C=2*R" or "Guaranteed Built With 3.1416 Tubing"
>
> Another great feature is "full-butted" tubing: Many bikes are made with
> double-butted tubing, that's stronger on each end. Wouldn't you rather
have
> a bike with full-butted tubing, stronger all the way from one end of the
> tube to the other?
>
> Sheldon "Marketeer" Brown
But it's so hard to create satire in an industry that seriously gave us
"triple butted", then "quad butted" concurrent with the famous "one tube
CrMo" frames (the minimum cost to get a sticker). Our reality is both sad
and funny.
--
Andrew "cynique? moi?" Muzi
Neacalban1
the megapro down-tube is definitely stiffer than my 85 Colnago Profil,with no
discernible movement(I'm sure its there, but I don't feel it) from side to side
at the BB. The Profil has top and down tubes(probably Columbus SL) that are
pinched in from the side,supposedly to make em stiffer? And the chain-stays are
flattened,but there's no bridge behind the BB.
Anyone think there's any stiffness differential between a 91 Chorus crank,
and a 2000 Record? whatever, I cannot get any chain-rub on the front derailleur
when hammering(Yeah, I know I'm 11 years older.... but I can still do it on the
Profil.
James Thomson
news:Hhj58.14139$TI3.1...@typhoon.sonic.net...
> 100% fashion! However, bike riders eat it up as though it were nirvana.
Strictly speaking, can nirvana be taken orally?
James Thomson
Marjan Sterk
<sna...@jcet.freeserve.co.uk> wrote:
>Strictly speaking, can nirvana be taken orally?
The former Seattle rock band's music can be taken orally, but
cassettes are preferred to CDs for that unless you have very wide
mouth. On the other hand, Nirvana ceased to exist when its frontman
took a piece of metal orally.
James Thomson
news:3c57f121....@news.ijs.si...
> On Wed, 30 Jan 2002 04:06:48 +0100, "James Thomson"
> <sna...@jcet.freeserve.co.uk> wrote:
>
> >Strictly speaking, can nirvana be taken orally?
>
I was hoping someone might supply the punchline 'ask Courtney Love' ...
James Thomson
Tom H
> Mark Hickey asserted:
>
> > >> Ever notice that most frames with "multi-squished down tubes" have it
> > >> aligned "wrong" for the forces at work at that part of the frame
> > >> (horizontally at the bottom bracket, vertically at the head tube), and
> > >> that it just happens to make 'em easier to weld that way?
> > >>
> > >> Gotta be a coincidence there somewhere... ;-)
> > >>
> > >> And yes, if you want to design a structure to resist torque with the
> > >> greatest efficiency, you make it round. Progress!
>
> Thomas Harrigan demurred:
>
> > >Doesn't the downtube resist bending, and isn't this principally
> > >laterally at the bb (I fail to see how it could bend "vertically" with
> > >all the other tubes supporting it)?
>
> Mark riposted:
>
> > Think about the forces that are going to try to bend the bottom
> > bracket shell out of alignment. By FAR the largest are the weight of
> > the rider lurching from side to side on the cranks. That will attempt
> > to "twist" the bottom bracket across its width. The forces that will
> > try to turn it to the left and right (from the rider's perspective)
> > are much lower.
>
Shelon opined:
> Oh, I'm not so sure about that. The magnitude of the force from chain pull
> is clearly greater than that of the rider's foot on the pedal, according to
> the ratio of crank length to chainring radius.
>
> This is somewhat countered by the considerably greater moment arm of the
> pedal vs the chain.
>
> It seems to me that the horizontal ovalization of the seat tube should
> pretty much deal with the rider-weight stresses, but that the horizontal
> ovalization of the lower down tube is more about chain pull. (To the extent
> that it's about stress at all, and not about getting everything to fit in a
> confined space.)
>
Pedals, being out of plane with the frame, produce a torque orthogonal
to the bb axis. I always thought it was principally this torque that
caused the bb area to flex, and that tubes were ovalised (made
laterally fatter) to resist it. Frames are plenty strong enough to
resist flexing in their plane, be it rider weight or chain-pull.
Frames can of course be left round, but at the bb, surely the wider
the better?
> Sheldon "Only Rarely Disagrees With Mark" Brown
Tom "saving up for a Habanero" Harrigan
Mark Hickey
I think we all agree the torque's always 90 degrees out of plane
relative to the BB axis - but it's not constant throughout the
rotation of the crank. There's certainly some tendency for the BB
shell to "wag" from front to back under heavy torque, but I believe a
lot higher tendency for it to "wag" from top to bottom. And I also
believe this is clearly visible (the relative movement of one
direction vs. the other) on most bikes. If this is so, then the
typical "wide at the BB down tube" doesn't make sense.
I'd agree that wider is always better, but when you start with a
finite tube, you have to balance the axis you're stiffening with the
one your making more flexible. In the case of the down tube, I feel
that the tube's primary job is to resist torque very close to
perpendicular to its centerline at the BB, and the best shape to do so
would be round.
If wider was alway better, the best thing to do would be to squish the
bottom of the tube flat! ;-)
Jon Isaacs
>resist flexing in their plane, be it rider weight or chain-pull.
>Frames can of course be left round, but at the bb, surely the wider
>the better?
This issue is really quite complicated and IMHO cannot be resolved without some
actual analysis.
However I think one can say that if the BB deflection is primarily a function
of the torsional stiffness of the downtube, then for a given wall thickness and
tube mass, a round tube is a better choice.
jon isaacs
Tom H
> thomas....@bakeratlas.com (Tom H) wrote:
>
>
> I think we all agree the torque's always 90 degrees out of plane
> relative to the BB axis - but it's not constant throughout the
> rotation of the crank.
You've lost me here. The component of the torque produced by pedal
action that moves the bicycle forward is directed _along_ the axis of
the bb (directed, if my memory serves me correctly, from right to left
of the frame). If the pedals were in the plane perpendicular to the bb
at its centre, then this would be the only torque. For obvious reasons
the pedals must move in planes parallel to this and thus produce a
torque orthogonal to bb torque. You could take the seat-tube and down
tube as convenient axes to represent this torque.
> There's certainly some tendency for the BB
> shell to "wag" from front to back under heavy torque, but I believe a
> lot higher tendency for it to "wag" from top to bottom. And I also
> believe this is clearly visible (the relative movement of one
> direction vs. the other) on most bikes. If this is so, then the
> typical "wide at the BB down tube" doesn't make sense.
>
I confess to never having noticed this front-to-back wagging!!
> I'd agree that wider is always better, but when you start with a
> finite tube, you have to balance the axis you're stiffening with the
> one your making more flexible. In the case of the down tube, I feel
> that the tube's primary job is to resist torque very close to
> perpendicular to its centerline at the BB, and the best shape to do so
> would be round.
>
This torque is precisely that produced by the pedals being separated.
> If wider was alway better, the best thing to do would be to squish the
> bottom of the tube flat! ;-)
>
> Mark Hickey
> Habanero Cycles
> http://www.habcycles.com
> Home of the $695 ti frame
The seat and down tubes both have to resist torque and bending, so I'm
sure flat is not the optimal solution.
Tom
Mark Hickey
>ma...@habcycles.com (Mark Hickey) wrote in message news:<3c5945c8...@netnews.att.net>...
>> thomas....@bakeratlas.com (Tom H) wrote:
>>
>> I think we all agree the torque's always 90 degrees out of plane
>> relative to the BB axis - but it's not constant throughout the
>> rotation of the crank.
>
>You've lost me here. The component of the torque produced by pedal
>action that moves the bicycle forward is directed _along_ the axis of
>the bb (directed, if my memory serves me correctly, from right to left
>of the frame). If the pedals were in the plane perpendicular to the bb
>at its centre, then this would be the only torque. For obvious reasons
>the pedals must move in planes parallel to this and thus produce a
>torque orthogonal to bb torque. You could take the seat-tube and down
>tube as convenient axes to represent this torque.
I see where you're coming from now - I thought you were talking about
drive line torque in your original post.
>> There's certainly some tendency for the BB
>> shell to "wag" from front to back under heavy torque, but I believe a
>> lot higher tendency for it to "wag" from top to bottom. And I also
>> believe this is clearly visible (the relative movement of one
>> direction vs. the other) on most bikes. If this is so, then the
>> typical "wide at the BB down tube" doesn't make sense.
>I confess to never having noticed this front-to-back wagging!!
This is primarily what convinces me the "wide at the BB shell
downtube" is unnecessary and counterproductive.
>> I'd agree that wider is always better, but when you start with a
>> finite tube, you have to balance the axis you're stiffening with the
>> one your making more flexible. In the case of the down tube, I feel
>> that the tube's primary job is to resist torque very close to
>> perpendicular to its centerline at the BB, and the best shape to do so
>> would be round.
>This torque is precisely that produced by the pedals being separated.
Exactly.
>> If wider was alway better, the best thing to do would be to squish the
>> bottom of the tube flat! ;-)
>The seat and down tubes both have to resist torque and bending, so I'm
>sure flat is not the optimal solution.
Exactly. It's sometimes easier to visualize the advantages (or
disadvantages) of non-round tubes by taking the non-roundness to its
logical conclusion.
Todd Holland
> I thought through that, and concluded that the effect of chain pull is
> concentrated mainly in the rear triangle - that is, think of it as the
> bottom bracket and hub having a fight. I can't believe this causes
> much of a displacement of the bottom bracket. Another reason I don't
> believe there's much displacement is that on bikes I've built with
> VERY little right crank to chainstay clearance (as little as 1mm),
> there's never been enough deflection in the bottom bracket to cause
> contact - even when "multiplied" by the full length of the crank.
> You'd expect to see contact if there was deflection, as the potential
> contact point coincides with the peak power input at the left crank.
I've experienced right crank-to-chainstay interference on two
different bikes, a Klein Quantum (road) and Otis Guy Ti (MTB). These
bikes had over a millimeter of static clearance that turned into a
click when climbing hard. I respaced one at the BB and filed the edge
of the crank off the other.
Todd H.
Mark Hickey
>Mark Hickey wrote:
>
>> Another reason I don't
>> believe there's much displacement is that on bikes I've built with
>> VERY little right crank to chainstay clearance (as little as 1mm),
>> there's never been enough deflection in the bottom bracket to cause
>> contact - even when "multiplied" by the full length of the crank.
>> You'd expect to see contact if there was deflection, as the potential
>> contact point coincides with the peak power input at the left crank.
>
>I've experienced right crank-to-chainstay interference on two
>different bikes, a Klein Quantum (road) and Otis Guy Ti (MTB). These
>bikes had over a millimeter of static clearance that turned into a
>click when climbing hard. I respaced one at the BB and filed the edge
>of the crank off the other.
Would it be safe to assume that you probably saw a lot more than 1mm
of "wobble" when looking at the clearance between the chain/chain ring
and the front derailleur cage? Of course, 1mm of deflection there
would be equivalent to 2mm at the end of the crank arm, and often
occurs when the cranks aren't in the "peak power" positions (3 and 9
o'clock).
Mark Hickey