Anyone able to double check some spring rate stuff?

saitrix

saitrix

saitrix

Soldato
Joined
2 Jun 2003
Posts
8,950
Location
Nottingham
Hi,

I am wanting new springs for my car. Now I am wanting to lower the car 1" and at the same time get stiffer springs. I have data for the stock springs and for the ones I would like.

Front springs,
Old spring rate = 139lbs
Old unsprung length = 14"
Old Sprung length = 10"

New spring rate = 250lbs

Now that will mean I want a sprung length of 9" to lower it 1" from the standard 10". But the new spring rate is higher so the difference between the unsprung and sprung length would be less. Am I correct saying it should be a linear relationship?

From there I found out the ratio between the 139lbs and 250lbs, which I got 0.556. So from there I just times the difference between the sprung and unsprung length (4") to get me how much longer the new 250lbs springs should be when unsprung. I got an answer of 11.25" rounding to the nearest quarter inch.

This all sound correct to people?
 
Sounds right to me.

/EDIT On second thoughts spring rate would be a function of spring length so have have you got a rate for the new spring without knowing how long it is?
 
The spring stiffness would be proprtional to the overall length of the spring when uncoiled (Which you can work it out from the number of coils and the diameter of the coiled spring), the coil thickness and of course the material.

I guess if whoever is making the spring already knows how many coils they'll be using then the actual spring length wouldn't affect stiffness.

Have I confused you yet cos I'm doing a good job of confusing myself!
 
First bit has reminded me of A level Physics now. :p

Hmm thats true, I would be able to get them to sort out the maths part of it if I just tell them I want the sprung length 1" shorter. Afterall they can then play round with number of coils and thickness etc a lot easier than me.
 
139lbs is shorthand for 139lbs per inch. So for every 139lbs of weight, the spring compresses by an inch. That means the cornerweight is about 4*139 = 556lb.

If you want a 250lb spring that's 9" when compressed, it needs to be 9+ (556/250) inches... about 11.24 uncompressed.

BTW 250lbs is almost certainly way too stiff. You also don't want to use the same for both front and back.
 
Why would you say 250 is too stiff? I know people using 400-450lbs on track days...

And yes I am getting 175 on the rear.
 
I'm running 190lb springs on the front of mine and we decided that because his is about 25% heavier than mine at the front that 250lbs would be fine. 250lb sounded too much to me at first but factor in the extra weight and it's not so bad.
 
You must log in or register to reply here.
Back
Top Bottom