Front Brake Usage
Subject: 9.17 Front Brake Usage
From: John Forester <jforester@cup.portal.com>
I have dealt for many years with the problem of explaining front brake use, both to students and to courtrooms, and I have reached some conclusions, both about the facts and about the superstitions.
The question was also asked about British law and front brakes. I'll answer that first because it is easier. British law requires brakes on both wheels, but it accepts that a fixed gear provides the required braking action on the rear wheel. I think that the requirement was based on reliability, not on deceleration. That is, if the front brake fails, the fixed-gear cyclist can still come to a stop.
In my house (in California) we have three track-racing bikes converted to road use by adding brakes. Two have only front brakes while the third has two brakes. We have had no trouble at all, and we ride them over mild hills. The front-brake-only system won't meet the normal U.S. state traffic law requirement of being able to skid one wheel, because that was written for coaster-braked bikes, but it actually provides twice the deceleration of a rear-wheel-braked bike and nobody, so far as I know, has ever been prosecuted for using such a setup.
The superstitions about front brake use are numerous. The most prevalent appears to be that using the front brake without using the rear brake, or failing to start using the rear brake before using the front brake, will flip the cyclist. The other side of that superstition is that using the rear brake will prevent flipping the bicycle, regardless of how hard the front brake is applied.
The truth is that regardless of how hard the rear brake is applied, or whether it is applied at all, the sole determinant (aside from matters such as bicycle geometry, weight and weight distribution of cyclist and load, that can't practically be changed while moving) of whether the bicycle will be flipped is the strength of application of the front brake. As the deceleration to produce flip is approached, the weight on the rear wheel decreases to zero, so that the rear wheel cannot produce any deceleration; with no application of the rear brake it rolls freely, with any application at all it skids at a force approaching zero. With typical bicycle geometry, a brake application to attempt to produce a deceleration greater than 0.67 g will flip the bicycle. (Those who advocate the cyclist moving his butt off and behind the saddle to change the weight distribution achieve a very small increase in this.)
A typical story is that of a doctor who, now living in the higher- priced hilly suburbs, purchased a new bicycle after having cycled to med school on the flats for years. His first ride was from the bike shop over some minor hills and then up the 15% grade to his house. His second ride was down that 15% grade. Unfortunately, the rear brake was adjusted so that it produced, with the lever to the handlebar, a 0.15 g deceleration. The braking system would meet the federal requirements of 0.5 g deceleration with less than 40 pounds grip on the levers, because the front brake has to do the majority of the work and at 0.5 g there is insufficient weight on the rear wheel to allow much more rear brake force than would produce 0.1 g deceleration. (The U.S. regulation allows bicycles with no gear higher than 60 inches to have only a rear-wheel brake that provides only 0.27 g deceleration.) I don't say that the rear brake adjustment of the bicycle in the accident was correct, because if the front brake fails then the rear brake alone should be able to skid the rear wheel, which occurs at about 0.3 g deceleration. The doctor starts down the hill, coasting to develop speed and then discovering that he can't slow down to a stop using the rear brake alone. That is because the maximum deceleration produced by the rear brake equalled, almost exactly, the slope of the hill. He rolls down at constant speed with the rear brake lever to the handlebar and the front brake not in use at all. He is afraid to apply the front brake because he fears that this will flip him, but he is coming closer and closer to a curve, after which is a stop sign. At the curve he panics and applies the front brake hard, generating a force greater than 0.67 g deceleration and therefore flipping himself. Had he applied the front brake with only a force to produce 0.1 g deceleration, even 100 feet before the curve, he would have been safe, but in his panic he caused precisely the type of accident that he feared. He thought that he had a good case, sued everybody, and lost. This is the type of superstition that interferes with the cycling of many people.
My standard instruction for people who fear using the front brake is the same instruction for teaching any person to brake properly. Tell them to apply both brakes simultaneously, but with the front brake 3 times harder than the rear brake. Start by accelerating to road speed and stopping with a gentle application. Then do it again with a harder application, but keeping the same 3 to 1 ratio. Then again, harder still, until they feel the rear wheel start to skid. When the rear wheel skids with 1/4 of the total braking force applied to it, that shows that the weight distribution has now progressed as far to the front wheel as the average cyclist should go. By repeated practice they learn how hard this is, and attain confidence in their ability to stop as rapidly as is reasonable without any significant risk.