v-lux wrote:The way ive always understood suspension (from downhill mountain bikes) is...
Spring rate - governed by the overall weight on the spring. So vehicle + load (water, fuel, people, other tat) The correct spring puts the shock just above halfway through its travel, this measurement is referred to as 'Sag'
(There is precise adjustment on a good MTB shock as they have a threaded adjuster to fine tune the sag)
Compression damping - This is governed by the weight of the oil and the settings of valving inside the shock. The heavier the oil, the higher the rate of compression damping - meaning its slower through the stroke of the shock and harder to make the shock bottom out and vice versa.
(high speed compression is whats used when fork hits a big bump ie the shock is moving through its travel quickly and ramps up towards the end of travel) (low speed compression is for when its only moving a little over small bumps and is very supple to allow the wheel to track over the ground)
Rebound damping - This is the rate at which the spring rebound is dampened. The correct amount of rebound damping should bring the shock back from full compression to the point of 'sag' quickly but without any bouncing.
There are other factors that come into play of course. The weight of the wheel (unsprung weight) has a big effect on things but we can probably assume that most of us have fairly similar weight wheels (except Russel of course).
The spring rate is probably mostly governed by the weight of the vehicle and its contents rather than any other factor. Unless you have a rock crawler which is never going to drive fast enough to need anything but huge amounts of squish to keep in line with the outrageous amounts of articulation.
Most vehicle shocks ive come across don't have the full range of damping adjustments, usually only compression damping. Some allow you to fine tune the spring rate with a threaded collar. No idea what sort of rebound adjustments they have, most probably just set at a value.
So to conclude,
Unless you've made your van heavier or regularly carry heavy loads then the stock spring is most probably the correct spring rate.
If you want more ground clearance then the spring needs spacers added, not replacing with a stiffer spring. (but watch the sag!)
If your van bounces around all over the place and your on standard springs then you probably need new shocks/altered damping.
Phew, glad i got that off my chest.... anyone got any thoughts?
Great post Al
jed
