Spring rate for rear coilovers

[ricola]

Here are the pro-techs, more pics on my build thread...

[Wally]

Very purdy indeed!

What spring rate did you end up with and how will you set your torsion bars? (like i.e. 1 or 2 splines lower)

[ricola]

I set up the spring plate so it was level with the rebound stop on the chassis, this is with stock swing axle (ie shorter) torsion bars, not sure what that equates to in splines drop... The springs are 125lb/in. Without the coilovers, the stock bumpstops were just about to touch the chassis with the engine/trans in so with most of the weight at the back (no doors, roof or interior though). Will wait to see what adjustment it will all need once the car is built up again and if I need to adjust the spring rate, fine tuning should be easy enough with the coil-overs..

[Wally]

Good reasoning; I was figuring similarly low springs too, as in between 100 and 200 lbs when you're using the torsion bars as well. Curieus how it turns-out once your driving her ;-)

[evilC]

Does anyone have any corner weights for the Super? I was thinking to calculate some 'ideal' torsion bar/spring loads. This won't be a definitive answer but just to see a comparison between the empirical result and the seat of the pants one.

Clive

[Steve C]

Originally Posted by evilC

Does anyone have any corner weights for the Super? I was thinking to calculate some 'ideal' torsion bar/spring loads. This won't be a definitive answer but just to see a comparison between the empirical result and the seat of the pants one.

Clive

Hi

Could that be worked out with the front to rear weight split?

Steve

[Sandeep]

I was doing some calcs on the rear spring rate for my coilover installation and recall a value of 360 lbs/in.

2200 lbs with driver / fuel, 62 % rear bias, Max G of 1.5, 4" rear travel. I am not sure if this is in the ball park or not as I recall Pete with the '52 split having a rear rate of ~ 600 lbs/in, but the car has not been driven yet.

Sandeep

[Wally]

I was doing some calcs on the rear spring rate for my coilover installation and recall a value of 360 lbs/in.

Originally Posted by Wally

The 944 cup racers use 300-400 lbs for rear set-ups...
That would be a start I guess (emphasis on 'guess')

So, I wasn't too far off Sandeep?
4"wheel travel seems quite a lot though (from stand still to max compression). I think 2 or 3 would be more likely?

[ricola]

To add one into the mix, I used 300lb/in on my old speedster replica with 915 and EJ20 turbo, no torsion bars and that was a good stiffness for street use...

[evilC]

Originally Posted by Steve C

Hi

Could that be worked out with the front to rear weight split?

Steve

Yes, it could but I was never confident about the numbers. From the information that is available from various sources the actual values differ enough to throw doubt on the potential answers especially when you factor in the suspension frequencies for both road and track. Having actual corner weights would take the first variable/approximation out of the equation.

Clive

[Humble]

Here's the corner weights from my 1303 race car when it was still stock. Full tank, with spare, no driver.

Total 1931lbs.

LF 394 RF 409
LR 582 RR 546

Front 803 41.6%
Left 976 50.5%
Cross 991 51.3%

[evilC]

Thanks, I will do the measurements and calcs over the w/e. It'll be interesting to see the results.

Clive

[evilC]

Right! I have used Humbles corner weight and calculated the theoretical spring rates for the following spec:

Super Beetle Macpherson strut front and IRS rear with a sprung weight of 401.5lb at each front corner and 564lb at each rear corner. The lever arm at the front is 1:1 and at the rear is 1:1.277
For a quick road car the spring frequency would normally be between 80 and 100 CPM and for an unaerodynamically assisted race car would be 100 - 120 CPM
This gives the following results:
Road car:
Front
80CPM = 72lb/in spring rate
100CPM = 113lb/in spring rate
Rear
80CPM = 131lb/in coilover spring rate (no torsion bar)
100CPM = 204lb/in coilover spring rate (no torsion bar)

Comp Car
100CPM rates as above
120CPM front = 163lb/in
120CPM rear = 294lb/in (no torsion bar)

The above rates should be amended so that there is a 10 - 15% difference between the front and the rear spring frequency to prevent uncomfortable pitching that will occur if the spring frequencies are too close together (sympathetic resonance) with the fronts being softer. Please note that these rates are the theoretical ones that will only be a starting point for selecting the final settings, much of which will be governed by the feel and handling as well as the grip.

For comparison a standard 1303 super has the following equivalents:
Front spring rate + 80lb/in (83CPM)
Rear notional spring rate of the TB at the damper position (coil over position) = 186lb/in (95CPM)

Intuitively, the rears on the calculation are less than I would have expected although the normal recognised torsion bar upgrade to 23.5mm dia gives an equivalent spring rate of 216lb/in and a CPM of 102.

Hope these figures are of interest.

Clive

[Obiwan]

Thanks a lot Clive.

Calculated values are the base of my suspension setup workshop

Nevertheless I've got an additional question: How helper springs rate should be taken into account ? If it had to be...

[evilC]

The helper springs are there essentially to retain the main spring against the spring seat particularly where the main spring is stiff and has limited travel. Also the helper spring adds an element of dual rate and on the road softens the initial take up for both comfort and to get the damper moving so as not to rely on its high frequency/short stroke characteristics that seem to go off so quickly on all dampers.
The smallest common helper spring is 50lb/in that on the rates quoted above would be significant (~ 50% main rate) on the front and noticeable on the rear (16%-40% main rate). For the front I would consider them as an initial comfort setting, whereas on the rear the helper spring will lock up more quickly to handling advantage. Also, because of the light weight they will allow greater suspension travel before lock-up making the front seem initially too soft and uncontrollable on a comp car. In summary I thing helpers would be worthwhile front and rear on a daily driver/occasional comp car but only on the rear for a more focused comp car.

Thats my sixpenny worth and I would be keen to hear further thoughts.

Clive