"Reduce chain torque to the rear wheel"... means?

[ecumike]

I was just looking through my Penske Shock Technical Manual, and it says that raising ride height will "Reduce chain torque to the rear wheel", while lowering ride height, will increase it.

Wha?

Is that good? bad? does it matter?

Anyone?

[Frank_Angel]

There's not much you can really do about chain torque, it's a fact of life with chain driven motorcycles. Chain torque messes with the rear suspension and is what can cause hopping of the rear wheel under hard positive acceleration.

In case you don't quite know what it is, it's the action of the chain pulling on the rear sprocket. All things equal, the swingarm would rise under this force, causing the bike to squat. This is because the swingarm pivot point is higher than the rear axle, and torque applied along the line of the top of the chain will attempt to place the countershaft, swingarm pivot and rear axle in a straight line. However, since the rear wheel is usually well tracted to the pavement, the force instead pushes up on the frame of the bike through the swingarm pivot. If you pay close attention to a bike that's accelerating very hard you'll see that that the rear of the bike actually rises. That's due to chain torque.

If you've ever ridden a dirt bike you probably have used chain torque to your advantage. Think about how much better a bike handles when landing a jump when you add some throttle just before landing. What you've done is stiffen your rear suspension by using the chain torque to your advantage.

[MELK-MAN]

There's not much you can really do about chain torque, it's a fact of life with chain driven motorcycles. Chain torque messes with the rear suspension and is what can cause hopping of the rear wheel under hard positive acceleration.

In case you don't quite know what it is, it's the action of the chain pulling on the rear sprocket. All things equal, the swingarm would rise under this force, causing the bike to squat. This is because the swingarm pivot point is higher than the rear axle, and torque applied along the line of the top of the chain will attempt to place the countershaft, swingarm pivot and rear axle in a straight line. However, since the rear wheel is usually well tracted to the pavement, the force instead pushes up on the frame of the bike through the swingarm pivot. If you pay close attention to a bike that's accelerating very hard you'll see that that the rear of the bike actually rises. That's due to chain torque.

If you've ever ridden a dirt bike you probably have used chain torque to your advantage. Think about how much better a bike handles when landing a jump when you add some throttle just before landing. What you've done is stiffen your rear suspension by using the chain torque to your advantage.

couldnt have said that better myself.. riding dirt bikes really drives this home. going through woopdedoos..you have to have the rear wheel driving. everyone thinks the rear would squat on a street bike when gassed, but like Keith Code says, put the front of the bike against a brick wall and put it in gear, let the clutch out with some gas and watch the rear end rise.

[ecumike]

Cool, thanks for the 411.
Yea, I've read TotW II and understand about the rear rising under acceleration. Makes sense and great expl. Frank!

So as I now understand it, there's nothing to do/note here about this, and it's just telling me a 'fact'.

[Frank_Angel]

Yep, Greg's right. Watch the rear end of a bike doing a burnout or on the dyno.

As far as doing anything, no. Just be aware of the effect of the change in rear ride height with respect to chain torque, because it will impact the ability of your rear suspension to work. More torque, stiffer acting rear suspension. Other than the knowledge of how the change in geometry can impact suspension in a non-obvious manner, there is nothing you can do about it.

That said, not many club racers change rear ride height that often, i.e., different setup for different tracks. If you do change the ride height, just remember that you may want to consider adjustment of the rear shock to compensate for the change in chain torque. I think that's what Penske is trying to convey in their docs.

[tshort]

Or you could just try this little experiment next time you're on your bike:  put the front brake on (make sure you're not on anything loose or slippery), put the bike in gear, let the clutch out a tad, and add a tiny bit of throttle.  This should make the rear rise - somewhat counterintuitive.

[ecumike]

No, this is a 'first install' I just got my Penske and put it in yesterday w/ about half of max ride height.

Was just curious if this really meant anything to me or any effect on my setup. Being that I haven't checked/set the sag or anything yet.. that's a 'no'.  :)