Motorcycle Cornering: Forces Acting on a Stable Bike

[Adrian Baker]

The forces acting on a cornering motorcyle are pretty complicated - particularly so the mechanism of counter steering whereby you turn the front wheel the opposite way to the one you wish to travel in (and vice versa when you want it to sit up).

However, when the bike is STABLE in a long sweeping bend with no change in speed or lean angle, what forces are then acting to keep it in its circular path?

The front wheel here has little (no?) effect whatsoever I think, as, if the road drops away, and you have the throttle well open, the bike continues in a curved path, quite happily, with the front wheel off the ground.

 

[ramollari]

Well, there is the combined effect of having an angle with the vertical, and the turning of the wheel. When the curve is long, these two factors are so slight that they may be unobservable. When the front wheel is risen there will have to be a slight angle, there's no other way.

 

[rcgldr]

The forces acting on a cornering motorcyle are pretty complicated - particularly so the mechanism of counter steering whereby you turn the front wheel the opposite way to the one you wish to travel in (and vice versa when you want it to sit up).

Counter-steering is used to change the lean angle. A bit of counter-steering is needed to maintain a lean angle because the self-correction in the geometry (trail). At high speeds, the self-correction is dampened out by gyroscopic forces, requiring counter steering to straighten up. At slower speeds, you can just relax on the handle bars and the bike will straighten up on it's own.

The front wheel here has little (no?) effect whatsoever I think, as, if the road drops away, and you have the throttle well open, the bike continues in a curved path, quite happily, with the front wheel off the ground.

Assuming that you're not a maximum lean where the bike would just slide out, doing wheelies while cornering increases the conrering radius severely. It's possible to turn while doing a wheelie, just like driving an unicycle, but this is a very low g-force turn.

You can visit http://www.uponone.com, which includes a video of a guy doing some cool slow speed wheelies including 360 turns.

 

[Adrian Baker]

... A bit of counter-steering is needed to maintain a lean angle because the self-correction in the geometry (trail). At high speeds, the self-correction is dampened out by gyroscopic forces, requiring counter steering to straighten up.

That makes sense, thanks. Much research seems to be on turning into and out of bends, but not on the stability you experience midcorner. It is also hard to judge where and if you are countersteering when in a long sweeping corner. Something opposes your weight pulling the bike down though - particularly so when you think how far sidewards bike racers can lean.

On fast twisty bends, the force you need to apply to the bars on my Ducati to turn in quick is really high, but mid corner it feels like you could take your hands of the bars.

 

[Adrian Baker]

I just found an excellent website that seems to explain it all. Well worth a look:

http://pdmec4.mecc.unipd.it/~cos/DINAMOTO/indexmoto.html

The posters (in Italian and English) are very nice.

 

[rcgldr]

A bit of counter-steering is needed to maintain a lean angle

Of course, if you hang off to the inside of the bike while turning, it may be enough to overcome the self-correction, and this would affect what steering input would be required to maintain a lean.