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-arschloch-
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Discussion Starter #1
This thread got me thinking:

http://forums.mtbr.com/showthread.php?t=41192

First, let me qualify my post by saying that I think skidding on any erosion sensitive or politically sensitive trail is pretty stoopid.

HOWEVER, what is up with the wholesale condemnation of locking-up the rear wheel? What do you all do in racing situations? Until someone comes up with a better ABS system for bikes, the best way to drop the most speed in the smallest amount of time is to lock 'em up. I think I remember reading back in the day about 'one-zero-braking' or 'binary-braking.' :confused: Yes, I understand and apply the concept of modulation, but there are still situations where locking-up the rear wheel is faster AND smoother.

Another problem is drifting! Is drifting so much better for the trail than skidding? With a set of low pressure DH tires it's possible to leave veritable trenches in the ground (not mud) with a hard drift. Just rolling downhill on tacky soil or even on the fire roads, if I turn around, I can see clear tracks from my Minion DH tires. Should I stop riding because I leave tracks on the trail?

My point is that I agree that abusing the trails with wanton skidding is pretty lousy for everyone, but I guess the comment that got me in the other thread was:

skinny-tire said:
LOL, you try skidding on our trails kiddo, and you have a mouth full of rocks.
To me that statement seems over the top! :eek: Skinny-tire, are trail issues and trails so super sensitive in your area?

Btw, I realize it's a dead horse, I just didn't realize that locking-up my rear wheel occaisionally makes me a mtn. bike pariah.
 

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I race quite a bit and almost never lock up my back wheel. The only time I skid is because I went off a short drop off, the fork compressed raising the back wheel a bit resulting in a loss of traction. That's not nearly the same as locking it up. Locking up your wheel means you have zero traction, none at all. I remember reading somewhere about going as fast as possable you need to learn to use your brakes.
So, anyway, how do you think that skidding will make you stop the fastest? If skidding worked so well then why do so many cars come with anti-lock brakes? We need someone with a physics background to leave some good information. I can only speak from experiance.
 

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false assumption

the best way to drop the most speed in the smallest amount of time is to lock 'em up

You are incorrect. Once the rear wheel starts skidding, the coefficient of friction, or grip, or whatever it's called -- ain't an engineer here -- goes way down. That's why kids can lay down big long skids, because there isn't enough grip between the tire and the surface to stop them any quicker. If your false assumption was true, only very short skid marks would be possible.
It's exactly the opposite of why drag racers do everything in their power to AVOID burning rubber. When their tires are spinning relative to the pavement, they are wasting a lot of power, vs modulating the clutch or whatever so they get maximum grip just below the spinning threshhold.
The best way to drop speed quickly is to modulate your brake so the wheel is just below the point of skidding, but still turning. That way, the knobby rubber is doing its job.
To do that, you grab a lot of front brake and use the rear brake as a sort of weight shift detector. When you're grabbing so much front brake that the rear wheel starts to lift off on the way to an eventual endo, the lightness of the rear wheel will allow it to start skidding. At that point, what you do is let up a little on the FRONT brake, which shifts more weight back to the rear so the rear is once again gripping without slipping. Once you master this technique you can stop in the shortest possible distance. Any skidding just increases the time and distance needed to stop..
 

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-arschloch-
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Discussion Starter #5 (Edited)
Yes, I am aware of what 'physics says' about the difference between static and dynamic coefficients of friction. However, much anecdotal (naturally, my own) evidence suggests to me that skidding is faster for stopping. The biggest example is the following:

Back in the day before servo-electronic antilock braking became normal in cars students were taught that the fastest way to make an emergency stop was......... yup, you guessed it: locking-up all four wheels. (Granted this example is a little bit of stretch, because of the weight transfer involved in locking-up the front and rear wheels of a car versus locking the rear of a bike is quite a bit different and has a HUGE effect on braking efficiency...)

I think the point comes down to whether or not we can modulate the braking as well as a computer controlled ABS system. We cannot modulate as well. It would be very interesting to see a head to head comparison of braking times from 20mph for the following techniques:

1. Unmodulated lock-up,
2. Fully modulated stop (0% lock up),
3. And some combo of 1 and 2...essentially a crude attempt for us to reproduce what an ABS system does. (EDIT: Essentially threshold braking as described by bulC -damned reading comprehension skills- except that I think that taking it to the point of starting to skid and then releasing is faster and easier.)

My money goes to number 3 being the fastest for the majority of the riding population.

Btw, thanks for the civil responses on an ugly topic. ;)
 

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bulC, nice description of threshold braking! Definitely the fastest way to stop given the conditions.

dd.''

.........................................................................
 

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-arschloch-
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Discussion Starter #7
hmm. OK, in straight line stops skidding is most likely not the ideal choice. I am not sure how many people can actually really use 'threshhold braking' to its fullest potential.

I realize I am grasping here, but I really believe that are situations where I am faster through certain obstacles or terrain when selectively locking-up the rear wheel.

How about high speed switchbacks then? I am pretty sure that on downhill switchbacks if I initiate a slide by skidding I am faster through the corner than if I modulate. READ THIS: I would never-ever-ever do that except on a closed race course.
 

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Maybe this will help. Of course, given the situation, it appears the are multiple right answers. ;) In fact, Chuffer might be correct in the right off-road conditions.

dd..''
 

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Dirt is a different dynamic. Comparing static and dynamic coefficients of friction of a car on pavement is not an apples to apples comparison to driving on dirt. I'm not much of a mtb-er, but I've got plenty of experience driving, on road and off. If what you say were true, off road cars would be going slower around a corner by pitching the back end out and slding around. On road cars don't do that, because on road, drifting or sliding of any sort is slower, but on dirt, it's faster.

Disclaimer: I don't agree with the idea that locking up your brakes is the fastest way to stop. Tears up your tires too..
 

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Kinda like sprint cars (unlimited class) doing ~150 mph through the corners sideways on one wheel. ;) Granted this is an extreme case. :eek:

d./
 

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One more comment, especially with respect to autos.
Once you are in a skid you don't have much in the way of control.
You are basically broken loose bumping along until you recover or you stop.
When brakeing below the skid you have much more control.
 

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archer said:
One more comment, especially with respect to autos.
Once you are in a skid you don't have much in the way of control.
You are basically broken loose bumping along until you recover or you stop.
When brakeing below the skid you have much more control.
Depends on the conditions. See this.

dd..''
 

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It's about showing up.
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What a great reference!

The analysis concerning lock-up was an eye opener and supports an intuative understanding which I was unable to explain. The build up of material in front of the tire on dirt (they use snow) actually increases stopping. Therefore, the build up of material on the outside of a tire in a turn on dirt(drifting) increases grip. This doesn't happen on pavement.
This doesn't mean that we can't try to be careful on trails and keep skidding to a minimum. All you have to do is look at the bottom of Big Trees in Joaquin Miller to see what skidding does.
 

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chuffer said:
Back in the day before servo-electronic antilock braking became normal in cars students were taught that the fastest way to make an emergency stop was......... yup, you guessed it: locking-up all four wheels.
you are following a false premise.
the days before abs was not that long ago. fastest way to make an emergency stop is irrefutably not to lock up the wheels. i bet whoever taught the student thinks that putting super premium in an old car improves gas mileage, too. not only will the stopping distance be longer, no friction = no traction = no turning out of the way. proper method would have to be heel & toe, brake, and, if necessary, steer :D
 

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Agreed. Although skidding may afford you a quicker stop off-road, it should be avoided in leu of the damage it causes. Of course, that brings us back to what chuffer was asking about. ;)

dd..''
 

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redfooj said:
you are following a false premise.
the days before abs was not that long ago. fastest way to make an emergency stop is irrefutably not to lock up the wheels. i bet whoever taught the student thinks that putting super premium in an old car improves gas mileage, too. not only will the stopping distance be longer, no friction = no traction = no turning out of the way. proper method would have to be heel & toe, brake, and, if necessary, steer :D
Except on gravel, snow and loose ground where skidding is actually a quicker stop according to the link provided in an earlier post.

dd.'''
 

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i can fix your bike so the rear rotor/brake doesn't lock up.
 
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