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From: Matthew T. Russotto on 15 Jul 2008 15:08 In article <Xns9ADC807EB5567larrybud2002yahoocom(a)204.153.245.20>, Larrybud <larrybud2002(a)yahoo.com> wrote: >> 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). No, you haven't. -- There's no such thing as a free lunch, but certain accounting practices can result in a fully-depreciated one.
From: Brent P on 15 Jul 2008 16:41 On 2008-07-15, Matthew T. Russotto <russotto(a)grace.speakeasy.net> wrote: >>Not at all if you want to remain stable. > > The rear brake does not assist in stability in a maximum effort stop Define what you are calling maximum effort and what surface conditions are. I've had the rear wheel of a bicycle try to come around on me when the front was slowing faster than it was. >>>>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. That's why it's an 'either' statement. I suppose I could be unskilled at this front-brake-only maximium stop thing, but I get the shortest distance when I keep both wheels braking just above lock and on the ground. >>> 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 has to be a lot more movement to get the wheels off the ground when there are springs to stretch.
From: Brent P on 15 Jul 2008 16:46 On 2008-07-15, Matthew T. Russotto <russotto(a)grace.speakeasy.net> wrote: > 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." Of course you can't shorten the stopping distance with the rear brake when the rear wheel is in the air. That doesn't mean in my reading of it that one wheeled braking results in the shortest stop. Otherwise it wouldn't be called a myth. > 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. I thought we were after the shortest stopping distance without falling, not application of maximum brake force.
From: Matthew T. Russotto on 16 Jul 2008 16:36 In article <AJWdnZusU6YaluDVnZ2dnUVZ_j6dnZ2d(a)comcast.com>, Brent P <tetraethylleadREMOVETHIS(a)yahoo.com> wrote: >On 2008-07-15, Matthew T. Russotto <russotto(a)grace.speakeasy.net> wrote: > >>>Not at all if you want to remain stable. >> >> The rear brake does not assist in stability in a maximum effort stop > >Define what you are calling maximum effort and what surface conditions >are. I've had the rear wheel of a bicycle try to come around on me when >the front was slowing faster than it was. Maximum effort stop means a stop with as much braking force as possible. Implicitly the conditions we've been discussing are on dry pavement; on wet pavement you'd lock the fronts before unweighting the rears, so the situation is different. >> 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 has to be a lot more movement to get the wheels off the ground >when there are springs to stretch. I don't think the additional movement translates to much additional force, however. But that's just a intuitive guess. -- 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 16 Jul 2008 16:38
In article <AJWdnZqsU6YVkeDVnZ2dnUVZ_j6dnZ2d(a)comcast.com>, Brent P <tetraethylleadREMOVETHIS(a)yahoo.com> wrote: > >I thought we were after the shortest stopping distance without falling, >not application of maximum brake force. They should be the same thing, provided you don't actually flip the bike over. You can certainly stop a bike (motorized or otherwise) with the rear wheel in the air without actually flipping; that's a "stoppie". -- There's no such thing as a free lunch, but certain accounting practices can result in a fully-depreciated one. |