James R. DavisPeer Review:
Posted - 01/04/2006 : 12:48 PM
What happens and how to prevent it
By: James R. Davis
More often than not, making a mistake while riding a motorcycle leads to misfortune, usually not serious, but sometimes fatal. One of the most deadly mistakes you can make is called doing a highside.
When a bike is 'dumped', or 'laid down', it falls DOWN, gravity assisted, all the way to the ground and ends up on its side. 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. Not too many people survive such an encounter.
So how does a highside happen? 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. If you are in a curve, (or if you have also applied your front brake while going in a straight line, or if there is substantial road camber, or severely unbalanced loading of the motorcycle), this starts the rear end sliding/skewing away from the direction the bike had been moving because traction is diminished on the rear tire (it has become 'sliding friction' - about 80% of what it was just prior to the skid) and that tire has begun to MOVE FASTER (in the direction of bike movement) than the front tire (centrifugal force, among others, is having its way.) The automatic, and correct, driver response to this situation is to turn the front wheel in the direction of the slide. [Actually, the front wheel will turn in the direction of the slide by itself - your job is merely to let it.] Let me be clear about that - I do not mean that the front-end ACTUALLY is steered or turns toward the slide but that it will APPEAR to be doing so. Without steering input the front-end will continue to point in the direction of bike travel while the rear-end slides to the side which makes it look like the front-end is being steered in that direction - and your job is NOT TO FIGHT these dynamics. 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. Let's assume a rider is in a gentle turn at the time. (Riding in a straight line is exactly the same as soon as the rear wheel starts to skew to one side or the other of the front wheel track.) The bike is moving in the direction pointed to by the front tire at this instant. 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%). It begins to skew outward from the center of the curve.
The driver now allows the front wheel to turn in the direction of the slide. The direction of bike travel has thus changed. 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 connected with the roadway, the front-end's restoring force is no longer providing attitude stability] 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. This allows it to move in the forward direction much more easily than a moment before, and just as suddenly it regains traction (mind you, it lost only about 20% of its traction when it began to slide and it is picking up only that 20% or so of traction at this point.)
Whether the engine is driving the rear tire or not, because the bike is not simply 'dragging/scuffing' the rear tire forward with it (because the tire is now rotating), the bike begins to move faster (actually, is slowing more slowly) in the direction pointed to by the front tire. 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. And what next happens is the highside!
Whether the slide movement of the rear end is abruptly stopped because the rear wheel hits a curb, or because the tire has regained traction, the results are the same: centrifugal force, coupled with inertia, try to keep the center of gravity of the bike moving in the direction it was last traveling. 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. (i.e., do NOT release the pressure on the rear brake.)
Let me also add that there is one more thing that could have been done to avoid the highside described here: always straighten the bike BEFORE you aggressively use your brakes when in a curve!
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. (Note, however, that with any form of integrated braking, this is virtually hopeless because so long as the rear brake is applied the front brake is also being applied.) Increasing front brake pressure, on the other hand, will almost certainly result in immediately laying the bike down on the low side.
Can a highside occur if you do not release the rear brake pressure at all? You bet! 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'. What that shows is that the rider successfully managed to keep his front wheel pointed in the direction of the skid until he had turned his wheel to its limit (a 'stop' was reached.) When that happens, of course, he can no longer continue to turn into the skid and the direction the bike travels begins to abruptly change - the skid increases until it presents a 90 degree tire face in the direction the bike is moving, which happens to present the largest contact patch 'face' perpendicular to direction of travel and, thus, maximizes the odds that traction can be reestablished. 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? EXACTLY THE SAME THING! 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. The reaction is the same.
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. Is it hopeless? Must you do the highside? Not at all. It means that as soon as you know the attempt you are making is not going to work, CLIMB ON THE FRONT BRAKE! This will FORCE a lowside!!! (If you have any form of interlocked brakes you can also force a lowside by INCREASING rear-brake pressure because that increases front-brake pressure as well.)
Please, I do not want to get flamed for suggesting that you actively lowside your bike! 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. If you can do it, do it. If not, good luck to you anyway.
[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. I.e., it falls over (lowsides) sooner.]
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. This is TRUE! However, you should understand what that really means. There is a difference between a SKID and a SLIDE. 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.
Though it is often thought that a high-side can only occur while in a turn that is simply not true. This is a reasonably accurate graphic of a Highside accident personally witnessed by both Cash and myself.
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)