Right now I have steady-state understeer (I think its worse on left turns). I don't know if I should crank valving, increase rear preload to raise rear spring rate, or something I don't currently understand.[/QUOTE]
I assume you already corner balanced? You may have a bit of "wedge"(>50% weight on the LR and RF corners). It may be a simple issue that you need more -camber on the front end.
There is a shitload of variables, so I have some questions:
- Have you verified your tire temps as measured outside/middle/inside immediately after hot lapping? You want to see slightly higher temps on the inside of the tires(especially on the fronts).
- Do you have adjustable sway bars?
- What shocks and spring rates are you running?
I am NO suspension guru, but I have picked a lot of minds over the years, so here are my thoughts on the subject:
If you increase the preload it will make the car sit higher and increase shock travel while static. To compensate for a higher sitting car you should then shorten the shock body(if you have adjustable body length shocks).
Preloading the springs is HUGE in the offroad and rally car world, but I am not sure how many road racers or autoxers do it. I have not experimented with it yet, but the optimal goal for every vehicle is to maximize traction, so it makes a lot of sense that the off road cars that catch a lot of air and hit tons of dips and bumps would want to use spring preloading coupled with SOFT springs to optimize the traction.
That said, people that are running FM's AFCO suspension seem to be putting down some pretty good lap times, so maybe there is something to be said for running softer springs, but having shocks with TONS of travel(coupled with spring preload to keep the static ride height).
I like the suspensions that use helper springs of decent stiffness) - this makes preloading the shocks irrelevant(unless the helper springs are not stiff enough, like under 100ft/lbs).
The results to spring preload is:
1. Less preload your suspension will have less droop and down travel.
2. More preload will add more droop and suspension down travel.
The compression/rebound damping of the shocks have TONS to do with how well they react over big bumps/dips, so spring rate + shock damping need to be taken into consideration if you are contemplating adding preload to the springs.
I played with some shocks years ago that did not have adjustable body length, but they did have adjustable ride height via spring preload. The spring rates were quite low so in order to get a decent ride height I had to add a **** TON of preload(like 15 full turns on each collar) which ended up causing the springs to coil bind under hard compression. It sucked.
On a track or autox miata I would NEVER go below 375/250 front/rear spring rates, unless you have fairly stiff helper springs on the coilovers.
[QUOTE=hustler;594160]I still don't know when to adjust spring pre-load vs. ride height.