"Jacking-up" the rear end

[northwind]

Originally Posted by motorolly

Your right about the rates changing but it isn't worth worrying about, usually the linkage is changed to alter the rates.
When I raced mine it was 2" up at the back and 1.5" dropped at the front on a set of GSXR750 forks, the comparison in the handling was chalk and cheese.
Now as a supermoto its up so high the swingarm just touches the linkage on no load and the suspension still works fine! Its hard to compare as the handling is (great but) so different now!
Experiment, jack up the back of the bike, dog bones off, sit it at the height that feels good, measure the bolt centre and drill some steel bar of the right size, bolt on, test ride, make new ones as required. It doesn't take a lot of difference to in centres to raise the bike a lot.

I want to hear more about your bike But I'd not be as cavalier about the linkage rate. Yes, it's common to change linkages to alter the rates, but here you're changing the linkage to do something else and you have really no idea where it's going to go. I've run the same height with the shock lengthened and with short dogbones, and the two were noticably different- with the dogbones, it felt rather harsh, ragardless of what was done with the damping.

cheers johm. i'll compare those sizes with the ones i have currently. probably mill a couple as well, see how it goes.

Originally Posted by northwind

I want to hear more about your bike But I'd not be as cavalier about the linkage rate. Yes, it's common to change linkages to alter the rates, but here you're changing the linkage to do something else and you have really no idea where it's going to go. I've run the same height with the shock lengthened and with short dogbones, and the two were noticably different- with the dogbones, it felt rather harsh, ragardless of what was done with the damping.

Yep Northwind that is what I mean =) changing the tri-linkage (or shock length which will change the angles) will change the forces acting on your shock. Changing the position between the three pivots (or angle of the L) can radically change the ratios of movement between wheel and the shock, let along the dynamic ratios (the lever the swing arm uses to compress the shock gets shorter as the shock rises, hence harder to compress further and the rate 'rises'). However only changing the dogbones has a very minimal effect on the operation of the suspension, the shock and linkage are untouched. The angle of the dogbones will change a little and the lever that is the swing arm effectively gets shorter as the wheel base reduces (and longer as wheels moves up over bumps) so this is where the very slight change in suspension behaviour will come in.

Can't say I have ever noticed it as it is so small, the weight distribution and change in rake of the forks is much more significant.

I was thinking of milling up a new likage to go with them which would position everything to allow me to fit an XL600 shock I have, I need it a bit softer with more travel. Will be interesting to work out how the rates are affected. Interestingly KTM use a progressive spring rather than linkage, which presumably means damping is not affected as the wheel rises, just spring force.

I'll post some pics when the bike has progressed a little more, its just from the single track roads of Mull and the west coast where nothing could keep up =)

[northwind]

Originally Posted by motorolly

Can't say I have ever noticed it as it is so small, the weight distribution and change in rake of the forks is much more significant.

That's why I tried it both ways, just out of curiosity. I was pretty surprised.