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Weight and Mass
They are NOT the same!

By: James R. Davis


It appears that the concept of Center of Gravity (CG) is not as well understood as I had imagined when constructing my earlier articles. I, for example, regularly receive e-mails in which the sender insists that standing on the pegs lowers the CG of the bike. Indeed, it has been reported by several that Keith Code makes that very claim in one of his books. Rubbish!

The CG of a motorcycle (by itself) is an imaginary point from which the bike could be suspended without that bike changing attitude in any direction as a result. That is, it is the absolute center of mass of that motorcycle.

When a rider is added to the bike then there will be THREE CG points: One for the motorcycle itself (which will be IDENTICAL to where the bikes CG was without the rider), one for the rider, and a COMBINED bike/rider CG. The combined CG will ALWAYS be located somewhere between the other two CGs.

So what? That seems perfectly obvious to even the least educated reader, Im sure. Yet I continue to receive those e-mails and many people absolutely believe what Keith Code and others have said that standing on the pegs will lower the bikes CG.

Lets look at that belief and see if we can figure out where the problem lies. First, lets figure out which CG is alleged to be lowered by standing on the pegs. Certainly its NOT the CG of the rider as it is clear that when his body is raised so, too, will that bodys CG. Since there has been no change to the mass of the motorcycle itself as a result of the rider standing on the pegs, it cannot be true the bikes CG has changed position whatever. That leaves only the combined (or composite) bike/rider CG. Since no mass is being lowered and some mass is being raised, it must be true that the combined bike/rider CG rises as a result of standing on the pegs.

Yet, again, the belief is a strong one and not just because an expert authority says so. Many people simply do not realize that CG is a function of mass, and not weight. The argument goes something like this: If you shift your weight from the seat to the pegs you are lowering where that weight attaches to the motorcycle. Thus, the combined CG must go down. Again, rubbish!

Lets demonstrate the effect it has on the CG. When you brake and, thus, decelerate, you cause a weight transfer from the rear wheel to the front wheel of the bike. You can observe that because you can see the front shocks compress when you brake. Clearly there is more weight on the front during braking than when not braking. But NO MASS is transferred to the front during braking. The CG of the bike, the rider, and the combined bike/rider will not move from where it was when the bike was at rest. This, because CG is a function of mass, not weight.

(Yes, when braking the CG of the bike will lower modestly because the bike will get lower when the shock shortens that is, the mass of the bike will get lower and as a result of that the CG will lower. If the location of the CG were a function of weight then the instant a stoppie began, even before the wheelbase shortened, the combined bike/rider CG would be directly above the front tire contact patch which it obviously is not.)

So lets look at what actually happens when a rider stands on the pegs.



The top graphic shows where the three CGs are located when the rider is firmly planted in his seat. H1 is the height of the combined bike/rider CG. The lower graphic shows what happens with those three CGs when the rider stands on his pegs. H2 is the resulting height of the combined rider/bike CG. The bikes CG (the lowest of the three) has not changed in any way. The riders CG (the highest of the three) has risen.

So now that we have demonstrated that NO CG lowers as a result of standing on the pegs lets talk about the practical effects of standing on the pegs. By standing on the pegs you are essentially disconnecting the riders body from the bike so that the rider and the bike can change positions independently from each other. That is, you could lean the bike to the left while the rider leans to the right and the combined rider/bike CG would remain in exactly the same place it was prior to that lean. That would allow you to lean the bike deeper into a turn and, thus, turn more sharply, than it could if you were fully seated on the bike. Some would argue that you are able to lean more into a turn because you have lowered the bikes CG. That, as we have shown above, is simply not true. Furthermore, you CANNOT change the lean angle of the combined rider/bike without changing either the speed of the bike or the radius of the turn as those two quantities ABSOLUTELY determine the lean angle regardless of the height of the CG.

Copyright 1992 - 2021 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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