Mathematical ponderings on FM spring rates...

Whilst re-assembling my suspension, 3bean seemed almost distraught when I re-attached my front sway bar on the softer setting... I gave the quick-simple answer "I want to try it."

I hadn't done quite all the math at the time, but...

Assuming Racing Beat isn't pulling our legs with their 28%/23% rate increase claim, the stock '02 suspension is:

162/118 f/r in lbs/inch.

FM springs are 342/228 lbs/inch f/r for an increase of 111%/93%.

With greater stiffness increase in the front than rear, the loose characteristics of my car should be addressed... So I should be able to run a softer front bar and have the same front/rear balance... We'll see come the end of February unless I get a wild hair and run up to Oklahoma for the beginning of February.

You engineering types may feel free to pick apart my off-hand analysis at will... I do realize that the spring rate comparison doesn't take into account changes in roll center, etc.

However, the car feels great. Steering response is phenomenal. bean will attest that I was fascinated by the lack of nose-dive under heavy braking...

And steering effort is much lighter, but that may be due signifcantly to the softer bar setting.

Yes, the lack of nose dive is awesome!

I still think, though, that you should re-torque all your suspension bolts in a week or so... just to make sure the suspension settles evenly.

And yes, I still think the stiffer bar would be nicer... but then again, I really dig a stiff ride. Awright... minds out of the gutters, you pervs! :evil:

Quote:

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Originally Posted by tailchaser

You engineering types may feel free to pick apart my off-hand analysis at will... I do realize that the spring rate comparison doesn't take into account changes in roll center, etc...

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If you have not seen this on the :batman: board.

An Excel spreadsheet with all the roll center and spring rates calculations you can think of. (Sway bar rates also)

I did not write this. Document credits author.

If you can understand this monster maybe we can have a class some day.

TC your assumptions seem correct to me. I haven't done a LOT of tuning with miatas, but I have on other similar cars and most follow this formulation.

The spreadsheet seems really cool but I already see one glaring problem. They use measured dimensions to calculte the torsional stiffness of the swaybars. I've done a lot of research and testing on this and it's almost never accurate. It's someitmes not even remotely close. The roll center stuff should be interesting though. I'll have to play with that some.