Spring rates

[k77-widow maker]

What's the ideal set up for a front engined car, when it comes to springs / shocks.

My logic says that the sprngs on the front need to be a slightly higher rate than the ones on the rear. Is this correct???

[joospeed]

well it's the rate at the wheel that's the important bit, but essentially yes. but that's a general rule that will most surely have it's exceptions , one bit decider on spring rates is driver preference of course.

[GreenV8S]

Depends on the weight distribution. Relative roll stiffness front/rear is the most important thing to get right. In general you want the undriven wheels to do more than their share of the weight transfer in cornering, and the driven wheels less than their share. So for fwd front softer in roll. But this is relative to the weight distribution front/rear, and you haven't said what that is.

>> Edited by GreenV8S on Wednesday 23 February 16:00

[HiRich]

Check out this page:
Springs

There are three elements to consider:

Design Load
Typically a 3g bump before you get a metal-to-metal situation (e.g. suspension arm hits the chassis).

Spring frequency & Ride
Which includes spring stiffness and sprung weight carried. Lower frequencies generally mean a more comfortable ride. The rear frequency should always be higher than the front, otherwise it creates a seasickness-like rocking.

Handling
No fixed theory here. It depends on the quality of the geometry, the surfaces you expect to drive over, where your Centre of Gravity is, how much roll and harshness you are willing to accept.

[edc]

How do you factor in cars with anti-roll bars front and/or rear?

[HiRich]

You don't (sort of). It depends on whether ride quality is important or not (effectively - is it road or race?)

If handling is primary, you (might) start by looking at the theroetical bit, studying your geometry, and defining acceptable levels of roll/true camber.

You then have several available means of cotrolling tyre camber (which is effectively what you want to do):
- Spring stiffness
- Damper stiffness
- Roll bars
You can use all of them in different amounts, and there is no absolute ideal. Different conditions, and different driving styles will lead to different solutions. As an example Yvan Muller (softer set up) and James Thompson (stiffer) are about as fast as each other in the same basic car.

The whole thing's a bit of a black art. Theory only gets you so far. You then need experience from the engineer and good feedback from the driver (and lots of scope for adjustment) to finalise setup.

Back to the original question, spring frequency is dependent on the square root of stiffness over sprung mass. A typical FWD car is likely to have a forward weight bias, so is likely to need a stiffer spring.

[denisb]

A thorny subject.

You could work out the frequencies as a starting point but even the best books give a massive range to work with (25-50%) so it really is just a pointer. Instead of needing to guess what spring rates you need you just have to guess what frequency you need!

You could copy someone else with a similar spec car, preferably who is winning races.

The only hard figures I have heard quoted for a race winning FWD car were for a Toyota Corolla GT running about 900Kg and on normal road tyres. It had 400lb fronts and 500lb rears.

I know that XR2's run stiffer at the rear in order to reduce understeer/invoke oversteer.

Someone like LEDA would be able to give you a very good starting point having supplied so many cars.