The chassis I am building at the moment came with 600lb rear springs and 450lb fronts. It was going to be used purely for racing and I know that the XTR2 uses 600lb spring alround when it is racing. These are far to stiff for everday use.
What size springs are the mid engined cars running?

it will vary widelly depending on weight and the angle of the shock..... my understanding is that the more acute the shock angle in relation to the wheel movement the higher the spring rates need to be.

Just an idea of what people are using would point me roughly in the right direction. I can always fine tune the rates once the car is on the road.

e mail me if you want a spreadsheet for calculation of suspension travel and spring rate, from first principles, as described by Allan Staniforth.

Cheers

fred WB

As people have said, the lever ratio will have a big influence- this is affected by both the shock angle and the position of the pick up point aloing the wishboine (if mounted on a wishbone!).

I'm currently tinkering with spring rates on my mid-engined Sylva Mojo- there is some info here:

http://homepage.ntlworld.com/neil.everett/post_sva/shocks.htm

...including the rates I started with and what I've moved to now.

And if you want to get really techie, this spreadsheet may be of use:

http://homepage.ntlworld.com/neil.everett/documents/mojo%20corner%20weights.xls

Hope that is useful...

Neil

[Edited on 26/10/05 by nre]

Wheezy,
It is impossible to say what spring rates you need wothout knowing:
Installation motion ratios - suspension mounting points.
Static wheight distribution - corner weights
Dynamic aero loads due to lift/downforce

And most important is tire data - at very list you need cornering force vs normal load.

This will let you calculate (aproximate) rates. I would suggest that you start at steady state cornering event - calculating resultant wheel loads and so under/oversteer characteristics of your setup. Remember that you should calculate dynamic load distribution not only from elastic weight transfer (springs, arb's etc.) but also geometric portion of weight transfer resulting from dynamic coordinates of your instant centers.
Than when you have accepatable behavoir in steady state cornering (state that car will rarely if ever see in reality). I would suggest that you study it in a generic cornering event.
Max straight line braking
Reducing braking increasing steering input
No braking (allmost), max steering
Throttle pick up, reducing steering
More throttle, less steering
Etc.

This way you will be able to study (again very roughly) US/OS characteristics during that generic cornering event.

And now what rates do you need? I would suggest that for the start you calculate it as frequencies - you'll need your wheel rate and corner weight. Than it will depend on tire characteristics, suspension kinematics, aerodynamic requirements etc., etc.

In VERY general way I would suggest that for the street you need ride frequencies in the 1.5 to 1.75 hz in the front & about 10% higher in the rear. For track days you will need 2 to 2.5 hz in the front & higher in the rear.

Also assuming that your car is XTR - your calculations are going to be inacurate for simple reason - its chassis is very flexible and all the above mentioned calculations assume rigid chassis. So in reality your dynamic weight distribution will not match that calculated. Still you should be able to arrive at reasonably drivable setup - not good to win races thought.

Hope this helps.

Thanks
Ted

PS: All this will be very aproximate - as there's also a miriad of things that should be included - camber effects, diff effects, suspension bits compliance etc., etc, etc.

[Edited on 26/10/05 by Tudor (Ted) Miron]

quote:

---

e mail me if you want a spreadsheet for calculation of suspension travel and spring rate, from first principles, as described by Allan Staniforth.

---

rates

Looks nice Fred, would you email it to me?
nevaracing at mail.ru (remove spaces)

Thanks
Ted

Cheers for that guys. Plenty of useful info to get my head around.
I suppose even with a text book set up it still comes down to driver preference and what best suits his driving style.

Fred, Ill drop you an e-mail for that spreadsheet.

I'd fit what you have and see how they feel.... then borrow springs of different rates to get were you want.

for instance I've got some 350 front MK ones that I'm going to replace with 225ones... so I guess there must be other people out there doing the same.

Quoting Tudor Ted:-
In VERY general way I would suggest that for the street you need ride frequencies in the 1.5 to 1.75 hz in the front & about 10% higher in the rear. For track days you will need 2 to 2.5 hz in the front & higher in the rear.

Here's my suggestion based on TT's "right ball park" figures-

taking teds figures for hz you have nominal values of 1.625 for the road and 2.25 for the track. this difference is in hz, not in spring lbs per inch!

The equivalent difference in lbs/inch is about 2:1, so whatever it was for the track, halve it for the road.

So if your car originally used 600lbs all round for racing try 300lbs all round for the road. The anti roll bars will also need to be reduced in stiffness by half. If they are solid bars then reduce the diameter by 16%.

The problem with this approach is that when equipped with road tyres, running at road ground clearance and with road suspension movements the car may handle somewhat differently to how it did on the track. It does however give you a simple starting point based on what you already know.