Rear springs to suit weight jackers?

[broke]

Finally getting onto my suspension, have my front coilovers ordered and rear weight jackers installed. I bought the DP Racing kit that uses the standard diameter spring. My question is what length spring do I need? I see some people saying 6", some saying 9" etc.

I'm running 14" wheels and want the car to be pretty low, but I'd like a bit of adjustment up or down too if possible. Also what spring rate does everyone recommend? I'm planning on buying one long offroad type spring and cutting it in half to make two springs of twice the rate with half the length as most people seem to do. Thanks.

[datzenmike]

Rear spring rate should match the front. Because of cantilever design control arms, the rear spring must be 3.8 times what you really need. For example: for a 100 pound per inch rate at the axle, you would need a 380 pound per inch spring rate. Cutting a spring shorter makes it stiffer. A 400 pound per inch spring cut in half would make two 800 pound per inch springs half the length.

The stock spring rate for the 510 is about 98 pounds per inch. Everyone is different and with different needs. Generally a spring rate between 175 and 225 seems to be the norm. In extreme cases rates over 300 are common. Best to take a ride in someone else's car and see what you like. I think that having a softer spring and wanting something stiffer is better than stiffer and wishing it was softer if you drive a lot. Perhaps stiffer if you don't drive very much or far. I can't tell you what you would like. A stiffer spring will settle less than a stronger rate spring so spring rate alone is not how to set the ride height in the rear nor is the spring length. The ideal spring will be slightly short so you can add rubber ring shims to raise the body to the ride height you like.

There is a math formula using multiplying and division applied to three measurements of a coil spring that will give you the spring rate. You would need to accurately measure the number of active moving coils, (coils not touching each other) the diameter of the coil (center to center) and the thickness of the wire. The fewer the coils the stiffer the rate. The smaller the diameter of the coil the stiffer the rate and the thicker the wire the stiffer the rate.

[funwithmonkeys]

As Mike said you should correctly match the rates. I would go with 6" if you want to be low. I have 300lb in front and 1100 in the rear. Regardless of what you go for make sure to match the shocks to them.

[datzenmike]

The 510 suspension, like all production vehicles, is a compromise to appeal to the average driver comfort in order to sell the most of them. If you are above average and want a better handling car, lowering and firming up and reducing the bad characteristics of the suspension is a given. I urge you to exercise caution. 300 is a bit much for most. It's kind of on the high side and should be considered only if you are capable of using it or can put up with the stiffness. Nothing worse than modifying the enjoyment out of your ride.

[broke]

I would go with 6" if you want to be low.

Thanks, that's more or less the answer I was looking for.

[bertvorgon]

Do make sure, that you have ample travel before you hit any bump stops.

Sudden loss of compliance can put you off the road.

[funwithmonkeys]

Keith with the weight jackets you remove the bump stops. If you run out of spring it is not pleasant.

[bertvorgon]

Yah, coil bind not good either! Been there once!

[broke]

DP Racing sells the kit to suit either standard diameter or 2.5" springs.

NZ's regulations are very strict/retarded so I specifically ordered the standard diameter ones, because you can keep the standard bump stops with those, the 2.5" ones you can't, and also welding the rear arms would mean they need to be sent away for crack testing.

[broke]

Yah, coil bind not good either! Been there once!

Funny you mention that, I saw a few pics here of an arm that broke from coil bind due to removed bump stops. That was a real eye-opener!

In my opinion it's another argument for converting to true coilovers in the rear, however that's not allowed where I am, hence the weight jackers instead...

[bertvorgon]

As our rear control arms are actually easy to bend, I made up a "doubler" plate that we welded across the whole top surface of the arm. I have a picture somewhere but you get the idea, dramatically increases the stiffness of the arm and makes an off road excursion a bit more survivable for the rear arms.