Spring rate for rear coilovers

[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

[Wally]

Clive,
I have AVO springs on the front now as you may know (or not) and I ordered the helper springs seperately. Those are way less then 50 lbs. I think more like 10-15 lbs and only help to do what you described firstly: retain the main spring onto the top perch at sudden full rebound.

I would have liked them a little more stiff than that, but its all they have in eitehr 2 or 4" length.. I guess AVO knows what is best, so I don;t worry about it too much

Walter

[evilC]

Originally Posted by Wally

Clive,
I have AVO springs on the front now as you may know (or not) and I ordered the helper springs seperately. Those are way less then 50 lbs. I think more like 10-15 lbs and only help to do what you described firstly: retain the main spring onto the top perch at sudden full rebound.

I would have liked them a little more stiff than that, but its all they have in eitehr 2 or 4" length.. I guess AVO knows what is best, so I don;t worry about it too much

Walter

Ah! You've spotted the deliberate (not) mistake. If I had done my research properly I would have found helper springs at 4lb/in. I cannot say why but I have the poundage of helper springs fixated at 50lb/in (maybe a bad conversion from N/mm?)
My apologies to all at least I am comforted that you read my drivel.

Clive

[SuperRSi]

I am trying to figure out rates for my Meyers Manx STi and figure the front will weigh about 560 pounds and the rear 840 pounds. Any ideas on spring rates? Ricola does this approximate your speedster weight? Are the Kerscher units KW Motorsport shocks?

Thanks

[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.

----

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

[Wally]

Originally Posted by owdlvr

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

Well...I dunno how to compare springs, but I think the above is 'way off'?
I had 300lbs springs ( I assume they are 300lbs/inch?) on the front and that was firm but still very drivable.
Maybe UK lbs are lighter then US lbs?

I now run 400 lbs (uk lbs) on the front (!) and it still compresses (much to my own surprise), drives 'reasonable' hard on the street and is very good on the track
It costed me two extra sets of springs to find this out, so take your advantage
So, my reasoning further was that if 400lbs is good on the track for the front, about 550lbs would be equally good on the rear without torsion bars. Haven't got rear springs yet though ($$).

I do have a slightly heavier car then most and moved quit some weight to the front, so YMMV here as well.