Swing-arm Eccentrics - Sportbike Addicts

BurnCycle · 08-17-2006, 05:50 PM

Ah yes… In a never ended attempt to make you wonder why I do the things I do I’ve gone and done it again. This time I’ve installed adjustable swing-arm eccentrics that allow me to run the pivot +/- 2mm from stock. Based on feed back from another rider that's used them I’ve installed them in the +2mm position.

Remember, it is not to ask why, but why not!

***Cakes206*** · 08-17-2006, 06:11 PM

Hey Bern, you're always doin some wacky stuff to your bikes...what will this help with?

IrocRob · 08-17-2006, 06:33 PM

Changing the swingarm pivot point changes the amount
of load that is placed on the rear suspension under acceleration.
Remember that as the engine is pulling the
chain forward, the rear spring/damper will extend.

BurnCycle · 08-17-2006, 07:42 PM

Quote:

Originally Posted by IrocRob

Changing the swingarm pivot point changes the amount
of load that is placed on the rear suspension under acceleration.
Remember that as the engine is pulling the
chain forward, the rear spring/damper will compress.

Correct... This should help with driving out of a corner.

I'm a bit in the dark with this and information is a little sketchy at best when it comes to changing the pivot and its results on a TLR. Time to play test pilot to see how it works with my set up. Basic principle is the same for all pivot changes but it is set up specific and what work for one may have a different result for another.

Total time to change out the pivot was just over an hour so making changes isn't that hard, but I can't see being able to do it at the track.

***Cakes206*** · 08-17-2006, 08:26 PM

Will this affect the rear shock's extension?

BurnCycle · 08-17-2006, 08:48 PM

No. Remember that the TLR sits on an arm that is connected to the swing arm via a link. In this pic you can see the arm and linkage. Now imagine the pivot being raised and how the linkage compensates for the change without changing the shocks extension.

CBRBob · 08-17-2006, 11:17 PM

Quote:

Originally Posted by IrocRob

Changing the swingarm pivot point changes the amount
of load that is placed on the rear suspension under acceleration.
Remember that as the engine is pulling the
chain forward, the rear spring/damper will compress.

Under accelereation the swingarm on 99% of bikes EXTEND, not compress. This is different than 4 wheeled vehicles.

IrocRob · 08-18-2006, 12:23 AM

Yes - Bob is correct - Depending on the relationship
of the countershaft sprocket, the rear wheel sprocket,
and the swingarm pivot point. If your swingarm pivot
point is "above" the line between the two sprockets,
(as all sportbikes that I'm aware of are) then the rear
spring/damper will extend under acceleration, and not compress.

ffejtable · 08-18-2006, 07:48 AM

I am confused... I thought the spring/damper/suspension will compress but the chain expands/extends? damn geeks!

BurnCycle · 08-18-2006, 07:55 AM

You’re both correct due to the different aspects of motorcycle suspension and geometry. Rob, you are correct with the initial statement about the load on the rear suspension during acceleration due to weight transfer. Unchecked this force would cause the rear to compress but as Bob noted it doesn’t. The trick is to utilize the anti-squat principles of the pivot location and swingarm angle. It’s anti-squat we are dealing with after all; something that’s countering the natural tendency to squat under acceleration with weight transfer. Too much of that weight transfer exiting a turn and you can find yourself running wide in turns (raise you hand if you’ve never heard of a TLR running wide… Didn’t think so). Increasing the angle such as I have done increases the portion of forward thrust that acts to lift the back of the bike.

To test the theory grab the front brakes and start to release the clutch. Yes, the rear end is lifting. The force against the pivot is trying to raise the back. As a practical example lay a hand flat on your desk and get a pen or pencil. Put one end of the pen on the desk the other end on your hand (see the relation to swing-arm angle). Push the pen toward your hand and the angle will increase. Like the pen, the greater the angle of the swingarm, the more force is transferred into lift.

BTW… Some cars will do it to. Mainly those with rear-wheel drive (solid axle) that utilize a torque arm. The Buick Grand National comes to mind.