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What spring lbs to use for Coil over for rear suspension?
Northpole - 10/1/15 at 05:36 PM

I have Triumph Spitfire Mk3 and am modifying the Rear Suspension to Coil overs.

How ever I am not sure what spring lbs I should go for. I use the car for race/fun and road.

I did some search and found two owners of Spitfire who are using 200 lbs and 250 lbs (for GT6)

Can anyone help? Preferable the ones who are racing, but any good advice appreciated.

Sam_68 - 10/1/15 at 05:54 PM

Unless you can find someone with specific knowledge of the Spitfire coilover conversion:

1) Work out the existing wheel rate by measuring the static deflection and the rear corner weights.
2) Calculate the leverage ratio of your proposed coil spring conversion.
3) Use the above figures to calculate the equivalent rate for your new coil springs.

If you're planning on going racing, then I'd suggest that as a starting point you should keep the same ratio of wheel rates front:rear... so if you stiffen the rear wheel rates from standard, you need to stiffen the fronts by a corresponding amount, too.

Bear in mind that the Spitfire chassis is just a ladder frame - not terribly stiff - so if you increase the spring rates too much, all you'll do is move the deflection from the springs (which are damped) to the chassis (which is not!).

Northpole - 10/1/15 at 10:15 PM

quote:
Originally posted by Sam_68
Unless you can find someone with specific knowledge of the Spitfire coilover conversion:

1) Work out the existing wheel rate by measuring the static deflection and the rear corner weights.
2) Calculate the leverage ratio of your proposed coil spring conversion.
3) Use the above figures to calculate the equivalent rate for your new coil springs.

If you're planning on going racing, then I'd suggest that as a starting point you should keep the same ratio of wheel rates front:rear... so if you stiffen the rear wheel rates from standard, you need to stiffen the fronts by a corresponding amount, too.

Bear in mind that the Spitfire chassis is just a ladder frame - not terribly stiff - so if you increase the spring rates too much, all you'll do is move the deflection from the springs (which are damped) to the chassis (which is not!).


Thanks Sam for your reply. I have consider these factors, but sometimes the theory is not what we get, and in my case i think I will have to relay on the experience from the owners of this type of a car, from race tracks. i was lucky because. i found one who has done this very same modification on his car.
So to give the information to others the spring rate will be appr 180 lbs to 225 lbs, depending on what the driver is looking for, street or race use.