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Lateral Acceleration
Amplifies a 'twitch' into a 'jolt' or highside

By: James R. Davis

There seems to be a common understanding that because traction may be regained after releasing the rear brake during a locked rear-brake skid/slide a highside can occur and for that reason one should never release that rear brake once it has locked. But that understanding seems to include a belief that regained traction when sliding is necessary to turn a locked rear-brake into a killer. The regaining of traction is simply NOT necessary to convert a slide into a high-side though when that happens a highside is certainly more likely.

First, some fundamentals... during a slide or skid it is NOT true that traction is absent - it is merely reduced from what it would be had the tire not been sliding or skidding. There remains often as much as 80% of the original traction at such times.

So that everyone is clear as to terminology... a skid is in the direction of bike travel while a slide has a lateral component (to the side.)

If you lose then regain traction during a skid, so what? A brief hesitation but NO 'twitch' that tries to unseat you at minimum or throw you over the side at worst. But if there is a slide involved - meaning the bike's rear-end is pointing in any direction other than the direction of travel, then the regaining of traction always involves a 'twitch' - the greater the angular difference between tire travel and direction the bike is pointing, the greater that 'twitch' is.

Now, and only as to the rear tire... regaining traction while being out of alignment not only means that there will be increased 'drag' which tries to slow that tire's movement (in the direction of bike travel), but to the extent that the rear tire is spinning as a result of having released its brake there will be an instantly applied acceleration in the direction the tire is pointing (which can be as great as 90 degrees different from the direction the bike is moving.)

Please note: EVEN IF TRACTION IS NOT REGAINED A HIGH-SIDE CAN RESULT FROM RELEASING THE REAR BRAKE. That, because as I said above, there is still a high level of traction that exists even in a sliding tire so that when its ability to rotate is restored and it begins to spin (even modestly) that drag will contain a vectored component which translates into a partially lateral (to the side) acceleration. Should you do the unthinkably stupid behavior of releasing the clutch as well as the rear brake and thus allow engine power to reach the wheel/ground, up to about 80% of that power will be translated into a substantial lateral acceleration to the extent the tire is not pointing in the direction of travel.

ANY lateral acceleration of the rear wheel amplifies that 'twitch' into a 'jolt' at minimum and a high-side at worst. In the following graphic you see the effect of releasing the rear brake after the bike has already begun a slide. A lateral acceleration is inescapable (though this graphic suggests a mere 'twitch' I assure you that if the bike was this far out of alignment that 'twitch' would be a SEVERE 'jolt' or even a highside.) If you then did the unthinkable, released the clutch and applied power, you would increase that lateral acceleration to a point that virtually guarantees a highside.

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

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