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The Faster You Go
The Less You Weigh!

By: James R. Davis



Honest!

If you were to measure how much your motorcycle (and you) weigh at a dead stop and then while you are moving at, say, 80+ MPH, there would be a meaningful difference.

This happens because the faster you travel through the air, the stronger the turbulence and, thus, the lift that the relatively low pressure of turbulence creates. Turbulence below the bike is trivial by comparison to that above it.

And, if instead of sitting straight up you are leaned forward, your back creates an airfoil that creates even greater lift.

If the combined weight of you and the bike, when at a dead stop, is about 800 pounds, then when you are moving at speeds of 80+ MPH that combined weight will be closer to 760 pounds, or less. And the faster you go, the lower that total will be. For comparison purposes, a 1970 Chevelle wighed in at 3820 pounds standing still and was found to weigh 321 pounds less (8%) at 100 MPH. Your bike will probably not lose more than 6% of its weight at that speed (or at, for example, 80 MPH with a 20 MPH headwind) because it does not have quite the horizontal profile of an automobile.

You might think the lift would be evenly distributed fore and aft. Wrong. Far more of the lift is on the front wheel than the rear. And why that's important is that with a lowering of weight on the front end there is a corresponding reduction is traction and, thus, stabilizing force available from trail. Indeed, at very high speeds your motorcycle will feel unresponsive and 'light' in the front end - so much so that you will not trust it (nor should you) to hold onto the ground in your turns.

While there is always a net lift caused by airflow with speed, the effect on the rear end of your bike could well be to increase its weight. This, because just like the amount of weight transfer acceleration generates is determined by how high the center of gravity is as compared to the wheelbase, wind resistance creates a downforce in the rear proportional to how high the center of wind resistance is to that wheelbase.

[All wind resistance is above the ground while all forward motion forces are at ground level. That automatically creates a torque which tries to lift the front and lower the rear.]

Downforce on the rear wheel can be less than, equal to, or greater than the lift generated by turbulence as described above and that is largely determined by how high the center of wind resistance point is relative to the wheelbase.

The taller you sit, the more 'baggy' your clothing, the 'wider' you are, the bigger your windscreen, and the faster you move ... the lighter your front end will get, but the more likely the bike's rear end will get heavier rather than lighter.

Lest you think this is academic... the reason there are short (seemingly useless) windscreens on sportbikes is to diminish the turbulence BEHIND the rider so that there is not so much lift of the bike at speed that control is badly compromised. They are small so that they do not, at the same time, increase downforce from wind resistance that would lighten the front end even more. Tourers take note: your windscreen lightens your front end dramatically!

Given enough horsepower and proper gearing your maximum possible speed becomes limited to that which results in insufficient front wheel traction to steer the motorcycle.

Copyright © 1992 - 2018 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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