Probably a simple answer but, I have Gaz shocks with 9" springs, I need softer ones on the back. Most second hand ones seem to be 8", so I assume most people use 8" springs. The threaded bit on the shocks will cope with 8" springs but should I stay with the longer ones or go to 8" springs? What difference to ride and suspension?

There will be no difference in ride comfort at all (as in spring rate). If you go for an 8 inch spring, you may not be able to get the same ride height you had before though, since the new spring is an inch shorter.

As long as you have sufficient thread in either direction, I think a 9 inch spring with a lower spring rate will be your best bet.

You may or may not be ok with an 8 inch though, but since you're going for a softer spring (which will compress more under a given load), 9 inch is best.

You will probably be ok with 8 inch springs, but you need to established that the shocks won't bottom out by having an inch less initial travel. Or something! That's what Matt at Procomp told me a while back anyway.

This is what Matt said:

"Just make sure that when there is the drivers weight ( or equivelent ) on board that the rear dampers still have at least 30mm of travel left before they would hit the rubber bump stop. Make sure that the bump stop is pushed all the way to the top when checking the available travel."

Hi

You want 30mm off DROOP travel before you top the damper out IE fully extended. And the rest of the travel in compression.

If you have a 9" spring fitted now and no ride height issues then you still require a 9" spring length but a softer poundage for more compliance / comfy ride.

Cheers Matt