Suspension-chassis question.

**cowpoos** · 22nd February 2009, 15:02

Why is it that motorbikes don't have the output shaft [front drive sprocket] at the centre of the swingarm piviot point???

Does this not make sense in regard to anti squat and swingarm pumping issuse under hard excelleration???

**greenman** · 22nd February 2009, 15:14

My understanding is that the relationship between the swingarm pivot and the drive shaft is dependant on the engine power and the weight of the rider and bike. so that under acceration the forces trying to make the bike rotate about the rear wheel are resisted by the offset between output shaft and swingarm pivot causing an equal force stopping this rotation.
It all gets rather complicated, but having a bit of squat or rise under power is benefical to having constant chain tension. It's a feedback thing.....

**Kickaha** · 22nd February 2009, 15:17

Originally Posted by cowpoos

Why is it that motorbikes don't have the output shaft [front drive sprocket] at the centre of the swingarm piviot point???

Does this not make sense in regard to anti squat and swingarm pumping issuse under hard excelleration???

Some aftermarket frame manufacturers have built frames exactly as you describe,Spondon I think were one that did and I think Bimota were another

looks like it was a while ago SB2 days

The Bimota also held its fork legs at a different angle to the steering head (28 degrees the forks, 24 the head) to reduce the change in trail under braking. Bold engineering was equally in evidence at the rear of the chassis, where the Bimota was among the first roadbikes to use a single-shock rear suspension system. The swingarm was a long, box-section steel structure that curved outward to pivot concentric with the final drive sprocket, maintaining constant chain tension. Fork yokes, foot controls, and rear brake caliper carrier were machined from aircraft-grade aluminum alloy.

**pete376403** · 22nd February 2009, 15:29

have seen pics of a frame (English IIRC) that had a double sprocket that ran on a bearing on the swingarm pvot. One short chain went from the gearbox sprocket to one side of the duplexed swingarm sprocket. The second chain went from the other side of the duplex to the back wheel. Main intent was to remove the pull of the chain from the suspension , but an added benefit would be to allow a wider back tyre (by not requiring the rear wheel sprocket to align with the gearbox sprocket.

THe new BMW off road bike has the swingarm pivot outboard of the gearbox sprocket.

**James Deuce** · 22nd February 2009, 15:31

Originally Posted by pete376403

THe new BMW off road bike has the swingarm pivot outboard of the gearbox sprocket.

Swingarm removal to replace the front sprocket. {shudder}

**cowpoos** · 22nd February 2009, 15:44

Originally Posted by greenman

My understanding is that the relationship between the swingarm pivot and the drive shaft is dependant on the engine power and the weight of the rider and bike. so that under acceration the forces trying to make the bike rotate about the rear wheel are resisted by the offset between output shaft and swingarm pivot causing an equal force stopping this rotation.
It all gets rather complicated, but having a bit of squat or rise under power is benefical to having constant chain tension. It's a feedback thing.....

My take on this is far more consistant loading of the rear wheel under acceleration...at any throttle opening?? making setup for superbike class bikes easier/better...so your not comprimising handling in other areas as much as they currently do.

What your discribing is...what actually happens and one of the effects of what currently that tuners try to get a net benifit from.

**Motu** · 22nd February 2009, 16:17

Originally Posted by James Deuce

Swingarm removal to replace the front sprocket. {shudder}

So you haven't had to remove the gearbox sprocket on any pre unit British bike?

Sprocket on the swingarm pivot centre has been attempted many times before...with success.But is considered not worth the expense and complication for the little benifit gained.

**Ozzy27** · 22nd February 2009, 16:48

Originally Posted by cowpoos

My take on this is far more consistant loading of the rear wheel under acceleration...at any throttle opening?? making setup for superbike class bikes easier/better...so your not comprimising handling in other areas as much as they currently do.

What your discribing is...what actually happens and one of the effects of what currently that tuners try to get a net benifit from.

Have a look at this site www.tonyfoale.com and have a look in the articles section To dive or not? there is a bit on rear ends. Also download the user manual for his software and have a read you will understand a bit more.
His book is well worth the money and you understanding of motorcycle chassis dymanics will be improved greatly.

**Mental Trousers** · 22nd February 2009, 18:39

Some squat is a good thing. It gets weight on the back wheel.

If the centre of the sprocket is also the centre of the swingarm you can get wheel spin because the weight isn't transferring early enough. Having the sprocket in front of the swing arm means the torque created levers the weight backwards at the same time it's putting power down. But having the sprocket/swingarm centres aligned means you have to dial squat into the suspension, but for that to happen the back wheel needs grip on the road. If it doesn't have that grip to start with, it wheel spins, you get no squat and you just keep spinning the wheel.

If gave you Tony Foal's book btw.

**Ozzy27** · 22nd February 2009, 19:06

Originally Posted by Mental-Trousers

Some squat is a good thing. It gets weight on the back wheel.

If the centre of the sprocket is also the centre of the swingarm you can get wheel spin because the weight isn't transferring early enough. Having the sprocket in front of the swing arm means the torque created levers the weight backwards at the same time it's putting power down. But having the sprocket/swingarm centres aligned means you have to dial squat into the suspension, but for that to happen the back wheel needs grip on the road. If it doesn't have that grip to start with, it wheel spins, you get no squat and you just keep spinning the wheel.

If gave you Tony Foal's book btw.

????

Not quite correct. Weight Transfer has nothing to do with the relationship between counter shaft sprocket and swing arm pivot. But torque reaction does!

**Mental Trousers** · 22nd February 2009, 19:20

Guess I didn't quite explain it correctly.

Given a triangle (top of the front sprocket to top of the rear sprocket to swing arm centre making the chain on the top the longest side of the triangle) when you power on the torque reaction tries to shorten the longest side of the triangle, transferring weight to the backwheel.

Does that sound better?? I know what I'm trying to say but having trouble saying it haha.

**Ozzy27** · 22nd February 2009, 19:39

Originally Posted by Mental-Trousers

Guess I didn't quite explain it correctly.

Given a triangle (top of the front sprocket to top of the rear sprocket to swing arm centre making the chain on the top the longest side of the triangle) when you power on the torque reaction tries to shorten the longest side of the triangle, transferring weight to the backwheel.

Does that sound better?? I know what I'm trying to say but having trouble saying it haha.

I think you are talking about torque squat or anti squat that most modern sport bikes have.
When you look at a data trace of rear suspension the suspension hardly squats at all on throttle application and by the time the throttle is fully open the rear suspension is almost fully topped out making full use of the top out spring softening the wheel rate giving better traction.
There is a lot written about torque antisquat and how to tune it for best traction. There is a whole chapter in tony Foales book on it and 2 in Aprilla's suspension principles Book.

**cowpoos** · 22nd February 2009, 20:29

Originally Posted by Ozzy27

When you look at a data trace of rear suspension the suspension hardly squats at all on throttle application and by the time the throttle is fully open the rear suspension is almost fully topped out making full use of the top out spring softening the wheel rate giving better traction.
.

That depends on the amount of tyre slip a rider creates though doesn't it...a slower rider with less slip would have more squat? tyre slip alows the swing arm to extend?

**Ozzy27** · 22nd February 2009, 21:06

Originally Posted by cowpoos

That depends on the amount of tyre slip a rider creates though doesn't it...a slower rider with less slip would have more squat? tyre slip alows the swing arm to extend?

The antisquat torque is a resultant about the tire contact patch. If the anti squat torque is greater than the torque the tire can trasmit to the ground (tire sliding) the resultant antisquat torque will be less. (Not extending the suspension as quickly). Which will mean more weight on to the rear tire as the weight transfer caused by acceleration is not countered by the antisquat.

Too much for my head on a sunday night.