Quoted message said:Quoted message said:On Wed, 07 Mar 2007 15:46:32 -0600, A Muzi <[email hidden]>
wrote:
[...]
Quoted message said:Quoted message said:Quoted message said:With a 20" wheel you can skid a drum, can't with a 'full size' wheel
Dear Andrew,
Any idea why?
Can you skid a 20" rim caliper? (Fogel Labs does not stock the wheel
or courage needed to test what I presume are front wheel skids.)
I'm not arguing, just curious whether all brakes are more effective on
smaller wheels or if reducing the wheel size favors drums over rim
calipers or what.
Cheers,
Carl Fogel
Looking at what Andrew wrote one more time, I wonder if the idea is
that a drum can skid a 20" wheel, but a drum can't skid a 27" or 700c
wheel?
So before I find out why, I need to figure out what people are saying.
My guess is that almost any brake can skid any rear tire.
It should be able to, especially if you jam it on quite sharply, to be
sure to lock the wheel before the braking surfaces get a chance to heat
up too much.
Whether you can skid the front wheel depends on whether you endo first,
which you might expect to be the case on a dry road with a good tyre. It
also depends on how far back/forward your weight is.
But there is another question about wheel size which I have been
puzzling over and still don't have an answer to.
If the front wheel is locked, the bicycle and its front wheel can be
regarded as a single rigid body. There is a force rearwards
acting at the contact patch, and from that you can work out the torque
on the bike, and at what rate of deceleration it will endo.
But what if the front wheel isn't locked? In this case the system is not
a single rigid body, but is more complex. I initially started modeling
this as a rearwards force on the bicycle acting at the front axle
instead of at the ground, but I won't try to explain this further until
I feel more like I've figured out whether it's right or not. And if you
think about it, there are two ways to endo-- you can pivot around the
front tyre contact patch, or you can pivot around the front axle, with
the front wheel still turning.
If you are pivoting around the front axle, a smaller front wheel will
tend to make it easier to endo (and hence harder to skid the front
wheel), since the perpendicular distance from where the force is acting
to the centre of mass is greater.
Here is a herring to think about: imagine a bike with enormous wheels
and a low-slung seating position, such that you are 1m above the ground,
but half a metre _below_ the centres of the huge wheels. Now imagine a
similar bike, with the same wheelbase, but with small wheels. You are
still 1m above the ground, but now you are also half a metre above the
centres of the wheels. Will the second bike be easier to endo? Either
bike can theoretically endo around the contact patch, the first one
you'd think can't endo around the axle.
Quoted message said:700c front rim caliper
Can or can't skid?
(Can't is my understanding.)
My estimate is can if the road is a bit slippery.
Quoted message said:700c front drum
Can or can't skid?
(I'm guessing can't.)
Can if the road is slippery and the drum effective.
Quoted message said:20" front rim caliper
Can or can't skid?
(I'd guess can't, but maybe people are saying can?)
20" front drum
Can or can't skid?
(I'm surprised, but people seem to be saying it can?)
If the drum brake is only able to exert a certain maximum force, then
that will result in a smaller force at the contact patch for a bigger
wheel as in a bigger wheel the drum is at a greater mechanical
disadvantage. This may be a reason why it might be easier in practice to
skid the smaller wheel with a drum brake.
But if you believe my herring about where braking forces act, then you
would expect the smaller wheeled bike to endo easier, and that there
will therefore be some road conditions in which its ability to skid is
limited by its tendency to endo.
