Spring rate for rear coilovers

[owdlvr]

Originally Posted by 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

I'm digging this thread up from the dead because I've got to purchase some rear springs for the car before Friday. (gotta have suspension if I'm going to bolt the body to the pan!)

I'm going to be setting up my 1302s for different conditions, each of which will require different spring rates for the rear of the car. I totally recognize that I'll be buying and trying different spring rates, but would ideally like to find something *close* to start off with. My car is Torsion Bar Delete, which means the coil over spring will be responsible for the entire weight of the vehicle. No corner weights are available, since it's just a Pan at the moment, so we will need to run with Humble's corner weights for now.

Here are the main setup conditions I will be looking to set the car up for:

Gravel Rally - For a gravel rally setup we'll theoretically be looking to set the car up for a "factory level" spring rate, but raised 2-3". One of the North American teams is running the 25mm SAW torsion bars with good success on Gravel, so perhaps a bit stiffer is in order.

Snow Rally - For a snow rally the theory is 2-3" higher but a softer spring rate then stock. The theory here is to allow some more weight transfer for traction.

Road Setup - I'll be lowering the car about 2" and ideally running a 'stiffer' setup then the stock spring rate. My '69, which was stock torsion bars, was quite nice but just a *bit* softer then I would have preferred.

Track Setup - Likely to be a bit lower, and certainly significantly stiffer as comfort isn't really an issue. This is the last setup I'll be looking to dial in for the car, and I would think by the time I get here I'd have enough testing in to pick a relatively close spring rate. Damping for this setup might also require a different shock, so for the purpose of this discussion we can probably ignore the track setup...for now.

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So, ignoring the front suspension for the moment (which is a problem in itself), and looking only to the rear. If I'm reading Clive's information correctly, a "stock setup" without torsion bars requires a rear coil over spring rate between 131lb/in to 204lb/in. While a Competition road/track car might go as high as 294lb/in.

Any of the rally or track guys in my area are all running 400-700++ lb springs on their subarus and hondas so I don't have an opportunity to borrow and test. I've fully accepted that I will be buying lots of springs over the next couple of years for testing, etc. At this point I just need a starting set to hold the car up and give me a baseline for testing.

Based on the information above a good starting point for me might be a 200lb/in spring. It should be on the slightly stiffer then stock side which *might* be ideal for either my road or gravel setup...but at the very least should be a good starting point for the road, gravel and snow setups. I think.

Can anyone see or suggest something that I'm missing, or perhaps offer some "been there done that"?

-Dave