Originally Posted by
ItsJustMe
I think it's possible that off-road braking is significantly different than road braking. In off-road braking, you could easily exceed the traction of the front wheel and cause it to start skidding; at that point you've certainly lost your most effective braking; sliding friction is a fraction of static friction. With a loose surface, you absolutely want to use both brakes and de-emphasize the front brake, because you need square inches on the ground to provide traction to brake with.
The numbers I was citing comes from Bicycling Science (referenced elsewhere) and apply to road conditions. Mountain bike conditions were used as an example of the most extreme braking situation I can think of. A sliding front wheel is just as disastrous on- or off-road. It is a situation that few, even with superior bike handling skills, can recover from.
Your comment (emphasized above) could be used for dry pavement as well. That is the absolute best braking situation you can find yourself in. Both wheels are at the maximum of adhesion and your brakes are operating at their maximum efficiency. Remove one from the equation...by not applying it...and you reduce the overall braking capacity of the system. Why would you do that?
Originally Posted by
ItsJustMe
The same would be true on any surface where the front wheel's traction was not sufficient when braking to lift the rear wheel from the ground; gravel roads, wet pavement, snow, ice, etc. In fact, on ice and loose gravel or sand, it can be very dangerous to use the front brake at all. However, once you get to the point where you've got so much traction on the front wheel that you can flip the bike over it, I don't really get how the physics could support anything but maximum braking on the front wheel, just to the point where there's hardly any contact on the rear wheel. If I'm braking 90% front, and 10% rear, and I increase the front braking power a bit so it's 100%/0%, and I still have not started skidding the front tire, I don't believe that I'm slowing down SLOWER than I was before I increased the braking power on the front.
First, I'm not saying that you should use your brakes in a 90/10 or any other kind of ratio. The maximum deceleration a bike can achieve without going over the front wheel, according to Bicycling Science, is around 0.5g to 0.56g with a seated rider. 90% of that deceleration comes from the front wheel. But 10% comes from the back wheel. Remove the rear wheel from the equation and the overall maximum deceleration has to decrease.
The maximum deceleration point of the system isn't when the rear wheel lifts off the ground and the rear wheel starts to skid. The maximum is reached when both wheels are contributing the most to the overall deceleration. That's when both wheels are in contact with the ground. That holds if the pavement is dry, wet, icy, or nonexistent - i.e. dirt
Originally Posted by
ItsJustMe
It certainly is not a good idea to go all the way 100%/0% in almost any situation though, since you'd have almost no control left. But if I'm anywhere near that, I'm going to let go my rear brake, because the rear wheel would skid with almost any brake pressure when that much weight had shifted forwards, and I do NOT want either of my wheels skidding in emergency situations; skidding = loss of control and much less traction.
That's why mountain bikers release pressure on the
front brake. Pitch over is the point where the front wheel is about to stop, i.e. skid. Releasing pressure on the
front brake transfers weight back to the rear wheel, puts it back in contact with the ground and increases the overall deceleration. In other words, you stop faster.
Weight shifts also effect this weight transfer...and is, again, something that mountain bikers learn early on. A tandem, for example, has a maximum deceleration around 0.8g. All of the extra deceleration ability comes from the extra weight over the rear wheel. Moving your center of gravity back and down while braking, can increase the rear wheel's ability to stay in contact with the ground and provide more braking power. It works very well in high angle descents on a mountain bike and it's much more effective on level pavement.
Originally Posted by
ItsJustMe
I am not insisting that I'm right, but I can't visualize how the physics would support the partial-rear-brake balance as stopping faster. I'd like to see the study, or if someone wants to draw up the force vectors of the two situations, I'd be very interested.
Think of it this way: Two riders using identical force in identical situations braking under maximum deceleration. Neither rider is allowed to skid the rear wheel and, for safety sake
, both wheels remain on the ground. Rider 1 uses only the front brake. His deceleration is around 0.5g. Rider 2 uses both brakes. His deceleration is going to be 0.5g on the front wheel
and 10% of the deceleration of the front wheel on the back wheel or, overall, 0.55g. Not a huge amount of deceleration but Rider 2 will stop a little sooner than Rider 1.
Now let's let the rear wheel come off the ground. Rider 1's deceleration is the same, 0.5g. Rider 2's deceleration goes from 0.55g to 0.5g (neglecting the slight amount of friction from the sliding tire
). Their deceleration is now the same and the stopping distance will be the same. If both riders, with the brakes applied, shift their weight rearward, the tires will come back in contact with the ground and Rider 2's deceleration will increase. Rider 1's will remain the same.
Now try releasing pressure on the front brake. Rider 1's bike will come back in contact with the ground but his overall deceleration will remain close to the same, minus what amount he had to give up to keep from doing a pitchover. Rider 2's bike will come back in contact with the ground minus the deceleration needed to prevent a pitchover
but his rear wheel is providing deceleration again and his deceleration is higher than Rider 1's. He'll stop sooner.
I can't think of a situation where losing the rear brake's contribution, albeit minimal
, to the overall deceleration of the bike is a detriment. As long as the wheel is rolling, the braking power it adds is significant enough that I'll use both brakes in
all situations. I do adjust for extremely slick conditions with more bias towards the back, however
