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What makes a motorcycle stoppie rather than skid?
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BMW R1100GS
Honda CB-1

This diagram shows two motorcycles: a sportbike and a cruiser. They’re at the same scale (one pixel = 1 cm). The boxes represent the ratio between the weight of the motorcycle and the maximum braking force the front tire can create, here assumed to have a coefficient of friction of 85%. The boxes are taller because the bike can apply more vertical force to the road through its weight than it can apply horizontal force through braking. The diagonal line represents the combined weight and maximum braking vectors on the front wheel. The black and yellow circles represent my guess about where to bike’s center of gravity is with a rider. They could be off quite a bit in any direction, but they serve to illustrate the point.

Notice that the BMW’s CG is behind the diagonal line and the Honda’s is in front of it. Under braking, that CG will exert a “force” forward and down in parallel to the diagonal centered on the front tire. (The angle of the lines will be determined by how hard the motorcycle is braking. If it’s not braking at all, the lines will be vertical. Under acceleration, the diagonal will go in the other direction, towards the back of the bike.)

Since the inertial and braking forces are offset, they set up a torque. In these diagrams, that torque is counterclockwise. In the case of the BMW, however, that torque is not enough to overcome the momentum "torque" of the bike's CG. Since the Honda's momentum "torque" is less, it will stoppie.

 

I’m too lazy to go back and make more illustrations, but you can imagine similar boxes for the back wheels, but with the lines leaning forwards. These represent the combined weight and acceleration forces on the back wheel. In this case, the position of the CG and the line determine whether the bike will wheelie or burn rubber (assuming the engine can make enough torque). Since the Honda can wheelie and stoppie, I could even set some limits on where its CG can be. I haven’t done that. Consider it homework. :-)

Note: I have ignored the presence of the back wheel and its effects on braking. I’m assuming that both motorcycles are braking at maximum potential and am ignoring the effects of the back wheel’s contribution to braking. I’m assuming a frictional coefficient of 85%. Stickier tires would increase the stoppie effect. Reduced traction conditions would decrease the effect. The size, weight, and position of the rider and baggage would change the motorcycle’s center of gravity. This is not an engineering analysis of the specific motorcycles; I just used these images because I like them.

My apologies to owners of Honda CBR600f4s. There is apparently a rule that these motorcycles can only be photographed from the right side, so I had to flip the image around so it would match the R1200C. ;-)

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