Spring rates! A sudden enlightenment!

To give you an idea why I was surprised:
My old 240SX had really soft and comfortable F 4.5kg/mm and R 5.5kg/mm.
Those were so soft that I had problems with sudden momentum changes in chicanes. Plus the ride was pretty smooth on the road.
I should mention that I had to toe in on the front to make up for the turn-in understeer that came from having softer springs in the front and also staggered tires.
I sold the car before I upgraded to a spring and shock combo of F 8.0kg/mm and R 6.0kg/mm, which were still considered "mild" by racing standards for the 240SX.

4.5kg/mm = 252lbs/in
5.5kg/mm = 308lbs/in

8.0kg/mm = 448lbs/in
6.0kg/mm = 336lbs/in

I understand that there are springs other than Tokico that come in these ranges. I also know that these can be combined with Koni shocks for the combo I'm looking for.
However, I want to know if you really need this stiffness for a 240Z. (I am looking to match the cornering ability I achieved with my previous car.)
What are the rates used by those who regularly race their cars? Autocrossers? Club drivers? Weekend racers?



Post Edited (Nov 7, 3:04am)

You have to look not only at the spring rates, but the motion ratio. Remember, these are strut cars, so the springs are nearly a 1:1 ratio. Many cars have a motion ratio between .5 and .7, meaning that the spring is closer in toward the inner pivot on the control arm {MR=(D1/D2)^2 where D1 is the distance from the spring to the inner pivot and D2 is the distance from the lower ball joint to the inner pivot}. Add that to the weight of the 240 and you don't need super high spring rates. I don't know off hand what ratio the 240SX is, but that car is a double A arm suspension if I'm not mistaken. I'm pretty sure it is nowhere near 1:1. The Tokico springs you mention are virtually identical in rate to most aftermarket Z springs for street cars. Your comment about lower standards makes no sense. Springs rates are not arbitrary numbers. They are designed for a specific car and driving condition. Anybody can make high spring rates - that doesn't make them any "better" than a lower rate. I just looked up the 2002 Porsche 911 and the spring rates are 146lb/in front and 203 lb/in in the rear. This is obviously a capable car, yet has very soft spring rates. For pure track cars, the rates will be significantly higher than a street car. That is why virtually every street car converted to pure track use will get higher rate springs.

> and BTW, the AE86 I'm sure has a weaker,
> more flexible chassis than the 240z.

I doubt it. Remember, the 240Z was designed in the late 1960s and has a pretty flexible front.

> you can either go for stiff springs, and not as big of sway
> bars, or softer springs, and stronger sway bars. do
> most Z people just go for the latter? is there a
> reason behind this? maybe, something specific about this
> car that makes it work better?

The fast 240Zs (the ones that run close to or beyond the lap records for their class) run stiff springs and soft anti-roll bars. They also run very sophisticated shocks. I ran Penske 8760 triples adjustables on the Rusty Old Datsun with 275 front and 300 rear springs, 23mm front and 19mm rear anti-roll bars. That car basically ran SCCA GT2 lap times on DOT-R tires.

Comparing spring rate numbers from one chassis to another is foolish. Unless both vehicles have the same suspension type (on both ends), roll centers, CG, sprung weight, unsprung weight, etc. then you're comparing apples to oranges.

- John