Spring rate for rear coilovers
 

Hello guys,

I would like to install coilovers on the Front and the Rear of my SB. On the rear I've got 944 S2 aluminium trailling arms and want to remove the torsion bars to install uniball links. Knowing that the SB will be powered by a 2.4 T4 engine what will be the right spring rate for the rear coilovers ? (mainly street usage but want to go also on the track)

I'm interesed into Bilstein 944 escort cup rear coilover or the Koni cup ones

Thx,

Peter

hello obiwan and what about QA1?

less expensive than bilstein and koni..

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

Hi F4P, I think we "know" each other from an other germanlook forum :D
I'm looking closely your progress on SB high quality parts production. (I'll buy from you the uniball link for my 944 aluminium arms but I'm also very interested in your camber plates, front coilover studies. I think I'll also contact to discuss about callipers brackets to adapt my 996 callipers onto the beetle)


QA1 is also an alternative coilover solution. I' haven’t any feedback on QA1 quality. In addition I expect an issue between coilover body and the 944 arms. QA1 coilover are "generic" and the lower mount hole is cantered and not misaligned as the Bilstein is.

I've also contacted Intrax in Netherlands to have some information on their RAS & 1K2 coilovers.

Bilstein Escort cup rear coilover
with helper spring: $875
w/o helper spring: $745

Koni cup coilovers (w/o helper spring) : $735

Lee Arnold (LAperformance) is said to make very nice rear coil overs specific for 944 alu arms in bugs with adjustable shocks for 300 pounds or s/th?
I would probably use those if I'd go additionally coil-over on the rear.

With the Uniball set up you will be loading the top bolt fixing on the dampers to >700lbs that puts a considerable bending moment on the 12mm bolt and a substantial twist on the forged arm so it is vital that the forces are redistributed back into the chassis. A full 5 bar Kafer Cup brace is the solution for this.

As regards to the spring rate you can compare the current wisdom on torsion bar selection - 23.5mm dia for mainly street and 25.5mm dia for mainly track with the equivalent wheel rate and then convert that back to the coil spring rate. It all revolves around lever arm lengths. The torsion bar conversions are available on the web - just Google. Coil springs in especially 2 1/4" dia are readily and cheaply available so it is feasible to try some different poundages around the starting point even to the extent of taking corner weights into account.

When you have decided on the spring rate talk to the damper suppliers about appropriate dampers.

Clive

I ordered my coil-overs today, if using with steel arms make sure you get the smaller diameter springs (1.9") or they may foul on the trailing arm. I bought pro-techs and have in the past, much better than previous AVO and GAZ from my personal experience in quality and value for money (they are all ally bodied and retail at £80+VAT each but easy to get money off). They also build them to your exact spec for the standard price, today I was offered options on length, bushing type and size (rubber/spherical) as well as rod diameter 10/16mm

http://www.protechshocks.co.uk

I'll receive them on tuesday :)

you can use search QA1 in this forum you have feed back on it ;)

it's 50% less than escort cup .

Thanks guys for your feedback. It's really appreciated. :)

Regarding rear axle reinforcement, I've got the Eyeball engineering torque bar.
http://eyeball-engineering.net/TorqueBar.html

Intrax answered to my questions. They can build specific coilovers based on 944 ones on my desires. Nevertheless the price is too HUGE for me (2350€). A bilstein cup set is about $2000.

UK Pro-Tech coilovers can also be a good alternative regarding the price of an equivalent set. Especially if it possible to select coil overs with off-set lower eye mounts.

Great link Rich, I will lock it away in the brain cell for future reference!

The Eyeball Eng. torque bar is not suitable IMO for a full coil over conversion. The unit should be no less than a 5-bar Kafer Cup brace set that fully triagulates the the top fixing position for the coilover unit. The EE torque bar won't prevent the top fixing from moving upwards or sideways. The EE torque bar may resist the frame horns from moving downwards under torque but the bar is under a point load bending moment that also has a bolted joint at its maximum bending moment position. It all goes against the very basic principles of structural engineering although you can make anything strong enough if you add enough weight.

Offset eye mounts on dampers should be avoided at all costs since they will impart a side load to the internal seals that will fail all too quickly.

Clive

Thanks Clive for your explanations on the EE torque bar limitations. I'll sell it & look for a 5-bar Kafer Cup brace set.

Regarding your "general" comment on the offset eye mounts, I understand your point of view but I'm also confident in Bilstein engineers that designed the escort cup set with the an offset eye mounts for the rear coiloovers. Until now I've not seen such complaints or issue on this bilstein set on sevrals Porsche 944 dedicated forums.

wow! How can they make them that cheap? :eek:
Can you expect a quality shock for this amount of money plus an all alu body and spring??
Do you have any long term experience with them Rich?

I used pro-tech dampers on my Phantom GTR, they were good and held up to the elements well. I can't remember the exact history but I think the Pro-tech owner was with one of the other companies and then left and set up on his own. The fact that they are relatively small and build to order to your exact specs cheaper than any competitors in ally rather than steel means they win hands down for me...

I used Gaz on my GTM Libra, they corroded quickly and the damping rate was poorly matched, owners often had to do a DIY calibration side to side to get them close to matching, not able to rely on just doing same number of clicks on each side! The bushes tended to destroy themselves after a year or so too...

I had AVO on my speedster, these also suffered from corrosion.

Hopefully they will arrive today and I'll get some pics up tonight... If you can't wait, look in my friend's sti speedster replica build blog, link is on my site...

Can't argue with that! Good find!

I don't doubt Bilstein's expertise on this one its just that it kinda goes against the grain. The dampers that Bilstein supply are presumably set for 2.5" coils? with the threaded section that goes low down on the damper body so the potential for the coil to foul the arm is pretty high. If you use Rich's 1.9" coils and carefully select the spring length then there is a better chance of acheiving the clearence.

It occurs to me that offsetting the eyelet to get better clearance is just the same as putting a spacer on the bolt and using the standard eyelet. Assuming that the bolt is long enough, the load position and hence the BM will be the same for both the offset eyelet and the standard eyelet with spacer, although the angle at which the damper operates will be less vertical than normal but should be still within the ability of the bushes/bearings.

Clive

Here are the pro-techs, more pics on my build thread...
http://www.ricola.co.uk/images/cabrio/protechs.jpg

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)

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..

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 ;-)

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

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

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

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...

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

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%

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

Clive

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

Thanks a lot Clive.

Calculated values are the base of my suspension setup workshop :D

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

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

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:o (maybe a bad conversion from N/mm?)
My apologies to all at least I am comforted that you read my drivel.;)

Clive

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

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

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

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 :D
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.

...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

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:eekno::eekno:

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?

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