Spring rate for rear coilovers

[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

[ricola]

I never weighed my speedster but found that 300lb springs were good for it...

[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.

[owdlvr]

...pretty much the exact reason I posted before ordering springs. Based on this thread it "makes sense" but in my head it just seems so way completely off :P

"paging Clive to the red courtesy phone...Clive to the red courtesy phone."

:-)

-Dave

[spannermanager]

There are some very high spring pressures being discussed here, something is not right, i ran 150lb front/ 300lb rear on my 850 kg 1303 with f/r arb's and 8" slicks. i always run the lowest spring value to give me the balance i want, this leaves the dampers with a good adjustment reserve and margins for fine tuning on the days conditions. I cant see how you've arrived at 400 at the front Wally, it must understeer big time, even for the rear thats a lot of spring, even with a G50 and a T4, hope this helps anyway

[Wally]

Nope, essentially no oversteer (also to my own surprise) and only lower speed understeer if I push it on the track.
It seems the measurement might be different?
I use AVO front springs as on this page:
http://www.avouk.com/Downloads.htm
Those springs are rated from 0-1600 lbs, so 400 seems not out of range for an axle without an engine on it?

[evilC]

Originally Posted by owdlvr

...pretty much the exact reason I posted before ordering springs. Based on this thread it "makes sense" but in my head it just seems so way completely off :P

"paging Clive to the red courtesy phone...Clive to the red courtesy phone."

:-)

-Dave

Sorry for the delay folks - been busy.

Adding my sixpennyworth. I have Spax PSX progressives on the front of the '03 that I estimate are 100 - 130lb/in over the travel distance. On the back I have 23.5 P944 rear torsion bars that give around 10% extra stiffness over stock. Having run the motor for around 6 months now and 6,000mls on our country roads around here the spring rates are hard for normal road use but when pushed feel about right - I would hate to up the rates much more especially at the back. Looking back at the figures these rates concur with fast road car spec, which fortunately is where I expected to be.
The roads around here are very bumpy and I do have an issue with the exhaust bottoming out so I am pondering whether to increase the rear ride height. The wheels do follow the bumps really well especially the faster you go, which is very helpful as it only has a cooking 1600tp so is driven with a brick on the throttle.
The front end sticks uncannily well and when I push very hard the back begins to hop that I suspect is either a anti-roll bar or bump stop issue but that is the only time it unsticks. I initially dropped the roll centre of the front and you can certainly feel the initial bite on turn in but I want to soften the action so I will try the original roll centre over the hols.
All together I have I think produced a road rally car that is a softer suspended setting than tarmac rally and almost a gravel spec with a lowered stance. My interim conclusion is that its got the right spring/damper rates but some of the settings need playing with.

It would be wrong to criticise any other set up that is very different as it is all about getting what works for you in the arena you play in.

Clive

[Wally]

Originally Posted by evilC

It would be wrong to criticise any other set up that is very different as it is all about getting what works for you in the arena you play in.

Clive

I agree, but you know how it is...people want exact answers

It still wonders me how a 944 with the engine in the front runs the 23,5mm stock in the rear and how that can be an acceptable upgrade for a sporty beetle with the rear engine?

In line with your very true above remark: I run the 25,5mm 944 M030 (optional) torsion bars and still think they are too soft.

[owdlvr]

...but guuuuys, I wanted a hard number!! ;-)

I knew well before I started on this project that I was going to be buying a variety of springs. Ideally the first set ends up somewhere in the middle, so I'd have a good base to start from. If you're far too soft, or far too hard, on the first set of springs, it's really tough to figure out how much further in one direction you should move. Not to mention the fact that even if one of you had "the perfect gravel setup" using XXXlb up front and XXXlb in the rear, chances are I would put them in my car and decide it was all wrong anyways! It's as much about feel as it is armchair math...though laptimes (when available) should be the ultimate deciding factor.

Taking both Clive and Wally's suggestions above I ended up going with a 300lb spring as my starting point in the rear. Based on Clive's math these should probably work out well for my road setup. If they're too stiff, they might have use in a track setup. If they're too soft, as Wally may suspect, then they may work well for either my gravel or snow setup. And hey, if they're completely wrong for everything...that's what Ebay is for.

-Dave

[owdlvr]

It still wonders me how a 944 with the engine in the front runs the 23,5mm stock in the rear and how that can be an acceptable upgrade for a sporty beetle with the rear engine?

A 944 weighs in at around 2700-2900lbs, with nearly 50/50 weight distribution.
A 1303 weighs in at about 1950lbs, with about 40/60 weight distribution.

So, roughly 1400lbs static weight on the 944 rear vs. 1140lbs static weight on the Beetle rear.

-Dave

[Wally]

Originally Posted by owdlvr

A 944 weighs in at around 2700-2900lbs, with nearly 50/50 weight distribution.
A 1303 weighs in at about 1950lbs, with about 40/60 weight distribution.

So, roughly 1400lbs static weight on the 944 rear vs. 1140lbs static weight on the Beetle rear.

-Dave

Never calculated it through like that, but if your numbers are somewhat accurate, thats a good point and would explain a lot
Mine's a 'little' heavier, but your point is well taken!
Thanks!

[evilC]

Originally Posted by owdlvr

A 944 weighs in at around 2700-2900lbs, with nearly 50/50 weight distribution.
A 1303 weighs in at about 1950lbs, with about 40/60 weight distribution.

So, roughly 1400lbs static weight on the 944 rear vs. 1140lbs static weight on the Beetle rear.

-Dave

Dave, if my memory is correct Porsche's 911 2.0ltr rally cars ran 23.5mm (standard type3) rear TBs for gravel as an 'upgrade'

Clive