comparing methods of stopping

I've been searching youtube for a comparison video and haven't had any luck. I'd like to see a side by side road test of someone with freewheel and front & rear brake, a fixie skidding, and a fixie backpedaling with a front brake. Have all three side by side going 15-20mph, then all hit their respective "brakes" at the same time.

I would guess that the skid stop would take the longest, but the other two would be about the same. Anyone know of such a comparison or willing to do the test?

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Permalink Reply by Jessica on November 7, 2008 at 9:05am
I'm thinking about this from a physics perspective...and let me put a disclaimer that I'm a chemistry person.

If the ground is dry, wouldn't the skidder stop first?? Or would his wheel break the friction coeffiecient (meu)? I don't think dry it would matter...it's whoever has the most friction wins.

Ahh shit. I forgot the bike doesn't skid up front too. NEEEEVER mind. TGIF.
Permalink Reply by GabeW (not the other Gabe) on November 7, 2008 at 9:18am
I know that the way ABS works on a car is by pulsing to prevent skidding. if you skid you've lost traction (friction) but if you maintain traction and slow the wheel, you stop faster and in more control ???

I also think having an old beater with center pull brakes vs a new ride with modern campy/sram/shimano components would be good to see compared as well
Permalink Reply by Jessica on November 7, 2008 at 9:22am
I've got avid shorty 6 cantis. I LOOOVE them. :)

ABS only works wet/icey. This I know for SURE. Skidding does cause loss of control when it is slick, but skidding (all four wheels) as opposed to pulsing stops faster on dry pavement. :) I'm a car girl too.
Permalink Reply by GabeW (not the other Gabe) on November 7, 2008 at 10:00am
um, the ABS on my WRX will kick on in the dry if I stand on it hard enough. And in every F1 race I've watched, they argue over traction control and it's effect on lap times (driver going fast vs car computer making the driver go fast by preventing him from locking up the tires going into a corner and not spinning them on the way out).

But back to bikes, I still want to see the three methods side by side - like they do on Top Gear: just put them on a road together and have them try to do the same thing at the same time to see which does it best.

Jessica said:
I've got avid shorty 6 cantis. I LOOOVE them. :)
ABS only works wet/icey. This I know for SURE. Skidding does cause loss of control when it is slick, but skidding (all four wheels) as opposed to pulsing stops faster on dry pavement. :) I'm a car girl too.
Permalink Reply by vxla on November 7, 2008 at 10:26am
Make sure you also do it in wet, as well as dry, weather.

I'd say that you shouldn't really compare skidding on a fixed-gear; instead, compare skipping as that is the most efficient way to stop, brakeless, on fixed (in the absence of a handbrake).
Permalink Reply by Ali on November 7, 2008 at 11:22am
OK, As a former Formula driver and an engineering student I feel, I have to put an end to this misconception anytime I see it. There are 2 types of friction Static and Kinetic. Static coefficient of friction is always greater than Kinetic coefficient of friction and this is where people make one of the most common misconceptions about braking efficiently...This is just fancy way of saying that given a metal block on a wooden table; one will need to apply more force to get it moving, but once it starts moving less force is required to keep the block moving at a constant velocity...

A car tire(and a bike tire as well) needs to be stationary compared to the surface, so one can apply more force to decelerate. Locking the wheels changes your friction coefficient from static to kinetic and we just discussed that static friction is greater than kinetic. So simply put the most efficient braking is achieved when tires still have positive contact with the road while you are applying just enough force to keep them from locking-up.

And might as well put an end to another misconception about ABS too...All an ABS consists of is sensors on wheels, and a central computer that senses the motion of those wheels compared to your relative speed...ABS kicks in when the tires lock-up(Sensors sense the wheels are no longer moving) and it lightly eases of the braking to restore the rotation and then applies a little more pressure again...This leads to well known pulsating effect of ABS. It is not a faster way of stopping but simply a way for us humans to keep the car in control. Technically a perfect ABS system will have realtime information from the road conditions(surface type, surface conditions; snow, ice, wet) & vehicle conditions(weight, tire temprature, tire composition, tire pressure, disc temperature & so on); using this information it would calculate in realtime what the maximum braking g's would be and keep the car decelerating at that rate.

-Ali

PS: Sorry about the technical nature but feel free to ask questions...

Jessica said:
I've got avid shorty 6 cantis. I LOOOVE them. :)
ABS only works wet/icey. This I know for SURE. Skidding does cause loss of control when it is slick, but skidding (all four wheels) as opposed to pulsing stops faster on dry pavement. :) I'm a car girl too.
Permalink Reply by GabeW (not the other Gabe) on November 7, 2008 at 11:55am
right on, and we're getting to that season.. I'd give the nudge to the fixies in the wet. But only seeing is believing.

And Ali, thanks for giving me physics classroom flashbacks

vxla said:
Make sure you also do it in wet, as well as dry, weather.

I'd say that you shouldn't really compare skidding on a fixed-gear; instead, compare skipping as that is the most efficient way to stop, brakeless, on fixed (in the absence of a handbrake).
Permalink Reply by 2slowtrek on November 8, 2008 at 3:01am
it's all about surface area, if you drop the bike and skid you will stop the fastest bacause you have the most surface area stoping power, or crashing into a stationary object- that will get you stopped fast.

Thanks Ali you had it right. Max resistance just before breaking traction will stop the fastest in any condition.

As for the test, who has a flip flop hub and front/rear brakes? I have sidewalk chalk!
Permalink Reply by Tommie on November 8, 2008 at 3:22pm
There are some variables to take care of:

1) rim surface
2) brake pad compound
3) rider weight
4) tire compound
5) road surface
6) speed
7) cantilever, v-brake, caliper, disc brake
8) heat.
9) weight position when stopping. (if you're way forward on a skid you'll skid farther.)

8 seems weird but if one were to be using carbon rims and carbon brakes on a really hot day there's a really good chance that they would grip so well that the ride would endo. In fact, IIRC, someone in the tour died this way a few years ago. No doubt, that combo would win over anything else.

My .02 is that discs would stop first, then v-brake, then good cantilever, then caliper then u-brake, then skid. But this is assuming that one doesn't lock up the wheels when braking--
Permalink Reply by vxla on November 8, 2008 at 4:36pm
Again, skipping, not skidding. There's a big difference between the two.
Permalink Reply by Ali on November 8, 2008 at 5:16pm
Brakes are the fastest way to stop a bike...The End.
Permalink Reply by vxla on November 8, 2008 at 6:52pm
Ali said:
Brakes are the fastest way to stop a bike...The End.
Hand brakes.

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