I've seen lots of talk about spring rates, and how to calculate them for a given spring, but can't figure out what I'd do with them once I know what they are.

It'll be something to do with weight on the axle, vs... travel? pre-load?

Seems to me I should figure out what spring rate I want, then get springs made to suit. But how do I figure out what I want?

Thanks...

Springs are holding the car up. So the first thing is to know the weight of the car and the front/rear weight bias.

The wheel have a mechanical leverage over the springs so the "wheel rate" is lower than the spring rate by a square of the leverage ratio. So you need to know this as well.

At ride height your selected springs stored load will be equal to the load the corner of the exerts on it. So you have one known point of interest.

The cars ride ehight should be known and the displacement of the shock should be calculated at full bump. At full bump your spring should provide enough load to withstand a 5g bump ie hitting a kerb the chassis won't bottom out. So your corner weight becomes 5 times the staic weight and your fully compressed spring should exert a load at the wheel equal to the bump load.

With this you should be able to calculate the required spring.

Or do it the easy way and a book Locost needs 275 lb per inch front spring and 175 rears. Look at the shock dimensions on my website and you'll have enough information to back calculate what you require for a similar car with a different suspension layout.

www.gtscougar.freeserve.co.uk

Kind regards,

Darren

Thanks Darren,

Using a 5g bump helps it all make sense.

Cheers,
Greg H

Darren,

As a matter of interest, what engine did you use in the chassis that had those spring rates?

The reason for asking is that I bought springs early on, to the book values. I believe these were 200 front, 180 rear. The rears seem OK and everything sits nicely, but I already have 1.5" preload on the fronts and it's still not up to the proper height! I'm using a X-flow, which isn't a heavy car engine.

My shocks are slightly more upright than in the book because the top mounts are on the side of the tube, rather than underneath (braced, I hasten to add). This means that the springs don't need to be quite so strong, but I suspect that I'm going to need at least 250 lbs/ft.

Any guidance appreciated...

David

Those springs are for a X-flow engined book car.

Kind regards,

Darren