Highsiding

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.] 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 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. 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.)

Borrowed from The Master Strategy Group

Submitted by Tom Scully
Chapter Educator