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From: Larrybud on 15 Jul 2008 12:37 > So, considering I can brake hard enough with the front to > unweight the rear without going over the bars, please explain > where skidding comes into play or how I'd stop any faster using > the rear brake. With both tires on the pavement, you've increased your coefficient of friction (assuming you're using both brakes).
From: Paul Hovnanian P.E. on 15 Jul 2008 13:44 "DanKMTB(a)gmail.com" wrote: > [snip] > Without question. And if we allow over-the-bar tumbles (which still > won?t equal a face plant, at least with me riding), I guarantee I can > out stop most cars, [snip] Doubtful. Your ability to stop is limited by the amount of braking torque you can develop at the front wheel before you go over the handlebars. This is due to the height of your CG relative to wheelbase. Bicycles have higher CGs than SUVs relative to wheelbase. Cars, although they suffer from the same weight transfer effect, are able to develop the maximum braking torque allowed by the tires without suffering from this phenomena. In fact, cars maintain some weight on their rear wheels even at maximum braking, producing a braking force that does not contribute to the forward weight transfer torque. In fact, I'm somewhat suspicious of your cycling technique. When I ride, I apply as much braking force as possible to the rear wheel without locking it for panic stops, staying off the front brake to prevent unloading the rear. -- Paul Hovnanian paul(a)hovnanian.com ----------------------------------------------------------------------- Have gnu, will travel.
From: Matthew T. Russotto on 15 Jul 2008 14:09 In article <N_SdnQZu1-7jdOrVnZ2dnUVZ_ojinZ2d(a)comcast.com>, Brent P <tetraethylleadREMOVETHIS(a)yahoo.com> wrote: >On 2008-07-11, Matthew T. Russotto <russotto(a)grace.speakeasy.net> wrote: >> In article <FKKdnWUbKYHBEerVnZ2dnUVZWhednZ2d(a)comcast.com>, >> Brent P <tetraethylleadREMOVETHIS(a)yahoo.com> wrote: >>> >>>If you were correct, that means the rear brake and rear wheel would be >>>entirely useless for slowing. > >> They are not useless for slowing. They are useless for a >> maximum-effort stop. > >Not at all if you want to remain stable. The rear brake does not assist in stability in a maximum effort stop >>>Once you've weight shifted everything >>>on to the front wheel you've past the point of maximum braking because >>>now the rear wheel is doing nothing. > >> It doesn't matter that the rear wheel is doing nothing. The braking >> force on the front wheel has been increased by as much as braking >> force on the rear wheel has been decreased. > >Either the rear wheel starts coming around on you like the rear end of a >car or the motion changes into roation of the frame and rider about the >front wheel. The maximum-effort sustained braking on a bicycle has zero load on the rear wheel. The rear will not come around unless you lock it (by using the rear brake). At that point any further braking by the front will start to flip the bike. However, it is possible do better than that because you don't need to brake indefinitely; once you've stopped, the rear wheel will come back down if you haven't tipped it too far. >> Passenger cars do not shift weight forward enough during maximum braking to >> unweight the rear wheels, so they still need rear brakes for a maximum >> effort stop. > >On they do become unweighted enough that the rear of the car starts >coming around and/or the rear lifts up on the suspension. I'll wager >that if the rear end did not have a suspension but was solidly mounted >like a bicycle's wheels we'd see lift of the wheels off the ground. The >suspension travel and spring displacement simply takes up the motion. Sure, the rear lifts up; that's what load transfer is all about. If the rear starts coming around, it's because the rear brakes locked, which is all about brake proportioning. But there's still weight on the rear. I'm pretty sure this is not because of the suspension; the suspension only has an effect on load transfer during the transition from non-braking to braking, not the equilibrium reached during constant-effort braking, except in as much as the CG of the car is slightly changed by the suspension's motion. -- There's no such thing as a free lunch, but certain accounting practices can result in a fully-depreciated one.
From: Matthew T. Russotto on 15 Jul 2008 14:13 In article <P6SdnWBta8EynOXVnZ2dnUVZ_hjinZ2d(a)comcast.com>, Brent P <tetraethylleadREMOVETHIS(a)yahoo.com> wrote: > >Ok... > >I decided to think... what's a way to settle this... I started googling >for motorcycle testing. I found this: > >http://www.motorcyclecruiser.com/streetsurvival/advanced_braking/index.html > >Myth #1: >Use Only the Front Brake > ><...> In the deleted section: "Unless you are braking so hard that the rear wheel of your motorcycle is in the air, you can shorten your stopping distance with proper application of the rear brake." That "unless" is the important part. A maximum-braking-force stop will unweight the rear wheel. If you aren't braking that hard, it isn't a maximum-braking-force stop. -- There's no such thing as a free lunch, but certain accounting practices can result in a fully-depreciated one.
From: DanKMTB on 15 Jul 2008 15:04
On Jul 15, 1:44 pm, "Paul Hovnanian P.E." <p...(a)hovnanian.com> wrote: > "DanK...(a)gmail.com" wrote: > > [snip] > > > Without question. And if we allow over-the-bar tumbles (which still > > won?t equal a face plant, at least with me riding), I guarantee I can > > out stop most cars, > > [snip] > > Doubtful. Your ability to stop is limited by the amount of braking > torque you can develop at the front wheel before you go over the > handlebars. This is due to the height of your CG relative to wheelbase. > Bicycles have higher CGs than SUVs relative to wheelbase. Once having gone over the bars, there's a whole lot more mass in contact with the ground. You will stop shorter. Locking the front wheel at speed will demonstrate this quite abrubtly. > Cars, although they suffer from the same weight transfer effect, are > able to develop the maximum braking torque allowed by the tires without > suffering from this phenomena. In fact, cars maintain some weight on > their rear wheels even at maximum braking, producing a braking force > that does not contribute to the forward weight transfer torque. > > In fact, I'm somewhat suspicious of your cycling technique. When I ride, > I apply as much braking force as possible to the rear wheel without > locking it for panic stops, staying off the front brake to prevent > unloading the rear. That makes your stopping technique extremely suspect. Theres no way that rear brake only will cause the fastest possible stop. Any experienced cyclist will tell you that rear-brake only is a foolish way to achieve an emergency stop, and it certainly is not the fastest way to stop. |