Coilover spring question

[nfs13b]

Hello All,


First up, I know nothing about suspension....

I have put some SPAX adjustable coilovers in the rear of the mx5. (I had to slightly modify the mounting points..

I currently have some 250lb springs in the back and I think they are 11 inches long.

The car is really bouncy and I have the coilovers wound all the way down. the car isnt sitting high so I dont think the springs are too long, should I go for a lower rate spring?

Thanks

[CT]

What does wound all the way down mean? Down in setting such as bump/rebound or down as in spring perch height? Are they the yellow oil based SPAX units?

250lb would be great in a circuit car, too stiff for a drag car I would have though. I ran 260lb rear springs in my race car. If the shocks are down in bum/rebound setting, try stiffening them up a bit to stop it bouncing around.

[nfs13b]

Wound all the way down means the spring height. Yes they are the oil based SPAX units. I have played with the rebound settings too.

Is there some method to choosing the right spring height/rate? formula or something like that?

What is the stock rear spring rate?

[Matty]

There's a great spreadsheet available from the Fat Cat Motorsports website to compare spring/sway setups.

As a rough rule of thumb (NA) you should keep the front spring rate about 1.6 x the rear spring rate. Assuming you have OEM front springs, you have a higher rear rate than the front (~160), which is a recipe for exiting a corner backwards in a hurry...

(OEM rear rate is ~100lb)

You either need to drop the rear rate dramatically, or increase the front rate to about 400 lb, but the latter option will give you a very stiff ride on typical Aussie roads. Or do both (I'd say max about 200lb on the rear of a local road car).

The other factor to consider is how much free travel you have with the suspension would right down as you say. If you squash the bumpstops on every bump, that's akin to a massive increase in the spring rate, and an appalling ride quality.

I suggest measuring what you've got. Pull the shocks, remove the springs, re-install the shocks, and then compress the suspension with a jack until a) the bumpstops are just touching, and b) the bumpstop is fully compressed and just starts to lift the car off the stands...

Then set the ride height so that you've got a suitable amount of free travel (ie before you hit the bumpstops) - I'd suggest ~50mm.