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Sliding Rear Tire - Brake Locked
Highside Likely

By: James R. Davis



Of all the articles that I have written here none has generated as much controversy and negative feedback as has the very first one - Highside Dynamics.

It seems that a large number of riders simply will not buy the fact that a sliding rear wheel, if traveling in a straight line, is a big deal. They have, after all, experienced a rear-wheel skid during their MSF classes and the bikes always tracked essentially a straight line as they came to a full stop. Their conclusion, therefore, is that a rear-wheel skid in a straight line is not particularly dangerous and that releasing the rear-brake is not particularly dangerous after such a skid has begun.

At the same time, these riders evidence virtual paranoia about the dangers of a front-wheel skid - straight line or not.

Permit me to point out a few things about the MSF rear-wheel skid exercise that makes it less than a good example of what happens in the real world.

  • You lock your rear-brake while traveling at speeds of about 20 MPH during the exercise.


  • You are NOT applying your front brake when you lock your rear one.


  • The MSF range is, invariably, flat - i.e., without slope or camber.


  • The exercise uses small bikes that do not have luggage compartments that, in the real world, are almost certainly unbalanced in terms of load.

So? So how long can you keep your bike standing straight up before you have to put your feet down when you come to a complete stop? About a second, right? At 20 miles per hour it takes about 1 second to come to a complete stop if you lock your rear brake. In other words, you do not have sufficient time during this exercise to experience the effects of having a lessening stabilizing effect of the restoring force generated by trail from such lower speed.

[At this point I should point out that, despite what you may believe, your balance is not a function of speed. The question I asked above is misleading. I should have asked, 'how long can you keep your bike straight up after your wheels stop turning?' Balance is affected primarily by the gyroscopic stabilization provided by your spinning rear wheel, not speed.]

What is significant about the fact that you do not apply your front-brake during the exercise? In the real world, during a panic stop you usually apply both brakes, not just the rear one. Why this is important is that if your front brakes are applied when your rear-wheel begins to skid, the rear wheel can begin to slow slower than does the front wheel. In such a case the rear wheel will move in the direction of slide faster than does the front wheel - it will 'catch up with' the front wheel. To do so it must skew to one side or the other from the track directly behind the front wheel.

In the real world our roadways are almost always cambered to facilitate the runoff of rain water. A sliding rear wheel usually skews (yaws) in the direction of that camber.

Whenever the rear wheel skews from a straight line, because of how the frame and front-end are connected together via your triple-tree (at an angle), the body of the bike must lean in the direction away from the skew. I.e., the bike will attempt to lay 'down' as the first step in a 'lowside'. [Actually, the leaning of the bike in the direction it is heaviest occurs FIRST followed by the skew.]

The fact that luggage is virtually never perfectly balanced means that the bike will tend to skew away from the heavier side of its load. The more out of balance the load is, the more certain the skew will be in the direction of the lighter side - and the faster that skewing will occur!

If ANY skewing has begun then your bike is in exactly the attitude it would be in if a rear-end slide had begun in a curve rather than in a straight line. Release of the rear-brake before skewing has advanced beyond perhaps 10 degrees results in a sharp torque to bike (and rider) as the rear-wheel regains traction and straightens out - usually not enough to throw the rider. But a skew greater than that leads to such a terrifically powerful torque if traction is regained that the rider and motorcycle can easily be thrown into the air - called a 'highside'.

For those of you that insist that the MSF would not allow this rear-brake locking exercise if it was as dangerous as I suggest, I will remind you that they have made the exercise 'optional' during the experienced rider course where you use your own motorcycle rather than the small ones provided during the beginner course - if you have any form of integrated braking. That is, if when you apply only your rear brake one of your front brakes is automatically applied, the exercise is optional for you. I STRONGLY suggest that you NOT do it! Obviously the MSF understands the dynamics involved. (That they permit it at all remains a mystery to me.)

Copyright © 1992 - 2019 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
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