Charon
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Locking the front wheel causes the bike to drop pretty quickly. Locking the rear usually causes the rear to slide out to one side or the other, often to the right since the road is crowned that way. If you are already in a turn when the rear locks, it will slide to the outside of the turn. Once the bike is out-of-line, there are three possibilities. One, the rider might be able to recover. Two, the slide continues and the bike drops to the pavement. This is a low-side crash. Three, the rear wheel regains traction, either because the rider releases the brake or because it hits something like the curb. When it does the bike will violently flip over the other way, tossing the rider into the air. This is a high-side crash.
Motorcycle brakes can be treacherous. Consider a rider who usually uses the rear brake, and "saves the front brake for emergencies." This rider steps on the rear brake, just to the verge of lockup. He realizes this isn't going to do the job. Now he applies the front brake. The front brake application causes a nose dive, but more importantly it unloads the rear wheel because of weight transfer. This rear wheel unloading instantly causes the rear wheel to lock, and takes the rider by surprise. Remember, he is already trying to stop, and he is unlikely to associate rear wheel lockup with front brake application. Odd as it might seem, cruisers are a little less vulnerable to the problem because of their long wheelbase.
Some of these things are easy to demonstrate to yourself. Having properly dressed for the chance of a fall, go find a dirt road somewhere. Dirt, because the forces required are lower (tires slide much more easily on dirt than pavement). You are deliberately going to play with locking brakes, and sliding on dirt doesn't damage tires as does sliding on pavement. And, should you go down, neither you nor the bike will suffer as much (probably). Accelerate to modest speed, say 20 - 25 mph. Squeeze the clutch, to eliminate engine interactions. For the sake of discussion, we will start with the front brake. Start squeezing it. Get a "feel" for how much braking you can do, preferably using only one or two fingers on the lever. You will have to accelerate between the experiments. Eventually, you will exceed traction limits and the front wheel will lock. When it does, it will immediately turn one way or the other because of the bike's geometry. RELEASE THE BRAKE IMMEDIATELY, or you will go down. You are learning what a front end lockup feels like, and you won't like it much. Now to the rear. Repeat the experiments, gradually increasing brake pressure until the wheel locks. Because you are going straight, the rear wheel lockup won't seem very dramatic, and you will probably be able to keep the bike fairly straight. If by chance it does get very far out of line, keep it locked until you stop. Again, get a feel for how much braking you can apply. Now, the fun experiment. Get going and apply about as much rear brake as you can without locking the wheel. Then apply the front brake and observe that the rear locks up.
Sorry for the length of the post, but this is something that I do not believe is covered in most motorcycle training courses. By the way, it wouldn't hurt to take a friend along, just in case something goes wrong.
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