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One of the most deadly mistakes you can make is called doing a highside. At slow speeds this usually results in little or no damage to the bike or the rider. Even at higher speeds, given that the rider is wearing appropriate protective clothing, most damage is restricted to the bike. In either case, these are known as doing a lowside - meaning that the rider exits the bike by going in the direction of the fall: down. Obviously, doing a highside means that you exit the bike by being thrown up and over the high side of the bike. That, in itself, is not particularly deadly, but it happens that the bike usually follows the rider into the air and then it comes back down, often on top of him. What causes it and what can you do to prevent it from happening? To begin with, a highside starts when you use so much rear brake pressure that you lock your rear wheel. Actually, the front wheel will turn in the direction of the slide by itself - your job is merely to let it. But now he can make a mistake that can cost him his life - he can release the rear brake. Let's look at what is happening at the instant his rear brake locks up causing his rear wheel to begin to slide and the instant that he releases pressure on the rear brake. Note that the back tire is always 'scuffing' a little as it tries to get into the same direction pointed to by the front tire. Now at this instant the rear brake locks and the rear wheel loses a significant amount of its traction (at least 20%). The driver now allows the front wheel to turn in the direction of the slide. Meanwhile, the rear end continues to slide and is still moving FASTER than the front end at this instant. The bike is trying to 'lay down' because with the rear-wheel no longer spinning you have lost its gyroscopic effect and, thus, attitude stability for about 80% of the bike and will do so if nothing else happens quickly. But the rider, realizing that his rear end is sliding completely out of control, decides to release the pressure on the rear brake to try to drive out of the situation. When he does so the rear tire, which is being dragged forward as well as to the side, is suddenly able to start turning again. At the same time, because full traction has been regained, the sliding movement of the rear end of the bike comes to an abrupt end. Since the bottom of the rear wheel has stopped sliding, (all stopping forces are at the contact patch), clearly a torque is developed. The result is that the bike is violently twisted in the direction of the earlier slide. The front wheel actually helps this twisting action because it has a bearing in its axle and the bike merely rotates using that bearing as an axis. Naturally, the driver will be thrown in the same direction as the bike is twisted. The mistake, of course, was releasing the pressure on the rear brake. Said differently, if you are in a situation where the rear wheel is sliding out from under you, despite having turned the front wheel in the direction of the slide, then the safest course of action is to RIDE THE BIKE INTO THE GROUND - do a lowside. If the bike is moving in a straight line, particularly if the bike has any form of integrated braking, and the rear wheel brake locks resulting in a skid, it is still possible to do a highside, but the odds of doing so are far less than when in a curve the faster you are moving, and the greater the camber (slope) of the road, the higher the odds. Still, the best decision the rider can make is to NOT RELEASE the rear brake if it is locked to try to insure that a highside does not result. Abruptly releasing the front brake when the rear wheel is locked and skidding can also cause a highside because it will increase rear wheel weight and, therefore, traction. Nevertheless, the only possible way to 'ride out' of this situation is to get the front end of the bike to go faster than the rear in the direction of the skid. Thus, a gentle relaxation of the front brake is a reasonable action to take. Can a highside occur if you do not release the rear brake pressure at all? If you have ever witnessed a 'straight line' highside accident you will remember that the skid mark was a straight line until the very end at which point it became a 'J'. This, then, is approximately when the bike stops its skid and violently snaps into the air. Having seen that a rear end skid requires that you gently relax front brake pressure and maintain rear brake pressure in hopes that the front wheel can be coaxed into catching up with the rear one (slow more slowly), what should you do if the front wheel begins to skid instead of the rear one? Gently release the front brake and maintain the rear one! Thus, you do not have to make a decision based on which tire is skidding. So, above I said that if you have a choice you should ride the bike into the ground rather than do a highside. I also said that the dynamics will almost certainly result in a highside even if you do what is corrective - turning into the slide and feathering the front brake. It means that as soon as you know the attempt you are making is not going to work, CLIMB ON THE FRONT BRAKE! If you have ever seen the results of a highside, you should kiss the ground that you have the ability to stop it by laying your bike down. I have been asked why aggressively using the front brake will cause a lowside rather than making a highside happen sooner. This is because by applying front brake you cause weight transfer that further relieves the rear wheel traction which, in turn, both reduces the odds of a highside and slows the bike faster. Some people have argued that if you can release the rear brake quickly enough after it locks you can avoid a highside and regain control of your bike. During a skid your tire is not rotating at the same speed as the bike is moving and so you scrub off some rubber but you are still fundamentally in control of your bike - that is, the tire is still pointing in the direction of bike movement during a skid. During a slide, however, the bike is FALLING OVER and the rear wheel is moving laterally - to the side - and you are no longer in control of your bike. If you release the rear brake during a skid you will feel a modest 'jerk' as the rear wheel regains traction and you continue on - UNDER CONTROL. If you release the rear brake while in a slide regaining control is far from assured as the 'jerk' becomes a very severe 'jolt', or worse, a high-side. So, the advice to not release the rear brake when it is locked refers to the situation where a SLIDE HAS BEGUN. For almost everybody this means NEVER RELEASE A LOCKED REAR BRAKE because a slide begins VERY QUICKLY in the real world and most people cannot react quickly enough or even recognize that the rear tire is sliding - it is foolish in the extreme to pretend that you are the exception and can catch it before that slide has begun.
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