Re: Important shock spring information for larger guys
I've just ridden a great track day after finally fitting the correct 17kg/mm (+11% stiffness approx over standard) spring for my heavy 240lb weight to my 929 which runs a GP Suspension (Dave Hodges, ex-Circuit One) kit revalved standard shock. This allowed me to set unloaded sag at about 5mm for a reasonable 30mm approx. loaded sag.
Complete magic and a lesson as to how sensitive these things are on track. My tyre chewing problem is gone (just a nice 'toothed' finish, no shredding or significant balling), no more squat problem at correct preload levels, the bike is much more comfortable on bumpy roads and the extra feel and predictability are allowing me to really drive hard and early off turns.
I'd harboured some suspicions that the GP revalve was possibly not allowing me into the right range of low speed damping, but no longer. It's an amazing feeling to be able to out drive most race 600s with aftermarket shocks out of a bumpy left hander at well over 100 mph at a high angle of lean and to just feel the suspension working away under you with no drama.
It's stable in performance as well despite what the shock houses would have you believe. I should also say that the revalve has now got a year and about 6,000 miles on it and the piston seal is still working as new. Plus it's held all its gas charge - the shock still almost locks down at max rebound damping with the spring off. The shock has a total of about 15000 miles on it.
I should have known better, but with my fairly limited track experience (I'm an ex-MXer) just did not realise how important the spring rate could be for tyre life. I was put off the trail a bit by the way many mags claim the spring is to stiff when the problem is really the much too high standard high speed compression damping.
The background. I'm reasonably quick by trackday standards but heavy and had found the need to run more preload than is ideal (0-1 mm unloaded sag at axle) on the standard spring to keep the thing turning under power.
I've in this situation consistently struggled with tyres getting hot and bothered on the 929. I switched to Rennsports last year from 010s but found even these suffered a bit. With careful adjustment of especially rebound damping I found it was not too bad but it was not ideal.
I now realise I was being forced to run too much rebound damping to control topping caused by the excess preload and that this was not right for the tyre.
I'd trouble getting a spring. Dan couldn't supply and it turned out in the end that despite what it says on their website that Race Tech don't do one any more - I'd trouble finding this out as they would not answer e-mails. Their spring rate calculator seems to be on the mark though. I eventually settled for a 17 kg/mm Eibach item from Lindemann. This was a little short and maybe 2 mm larger in diameter than was ideal but they very kindly supplied me with some aluminium spacing/centering rings.
I'd a bit of calculation to do to arrive at the correct spacer thickness without trial and error (it's a bit of a job to get the spring on and off using a car tyre spring compressor), but it came out on the button.