Race car spring rates, advice needed!
- Thread starter Mac944
- Start date
Neil Haughey
New member
Ed when I had a good look over my front suspension in the garage I used the following guidance
http://eibach.com/america/en/motorsport/products/suspension-worksheet
The figure of 0.94 sounds like it assumes a motion ratio if 1:1 for our strut suspension (cos 20 = 0.94), an assumption which is only valid I believe for a strut setup with zero scrub. Having said that I can't remember now where I got the 0.8 from which sounds to low.
The 0.54 is a typo as it should be 0.56 for the rear as it comes from here;
https://www.paragon-products.com/kb_results.asp?ID=58
The guts of that article has been there for as long as I can remember (more than 10 years). Many years ago they had some stuff about it which I may have read on rennlist back then that the number came from advice given by Porsche motorsport to support turbo racers back in the day.
http://eibach.com/america/en/motorsport/products/suspension-worksheet
The figure of 0.94 sounds like it assumes a motion ratio if 1:1 for our strut suspension (cos 20 = 0.94), an assumption which is only valid I believe for a strut setup with zero scrub. Having said that I can't remember now where I got the 0.8 from which sounds to low.
The 0.54 is a typo as it should be 0.56 for the rear as it comes from here;
https://www.paragon-products.com/kb_results.asp?ID=58
The guts of that article has been there for as long as I can remember (more than 10 years). Many years ago they had some stuff about it which I may have read on rennlist back then that the number came from advice given by Porsche motorsport to support turbo racers back in the day.
Neil Haughey
New member
This bit is interesting Mac, can you shed any light on the cams as I have been looking for many years for a more aggressive cam solution. That and a non-forged bottom end is what holds the S2 back, the former because it prevents the car from making much power lower down and the latter because its needed to get the revs out of the engine safely.
Neil Haughey
New member
I should have added that the front effective rate calc is largely irrelevant as one compares a front number to a rear number then if say you want to go up 200 on the front use that paragon products calculation to work the change required in the rear rate to result in the same balance (assuming one is looking for the same spring rate balance). In reality it is likely one would make the front relatively stiffer than the back as you up in spring rate for several reasons.
Eric_Oz_S2
New member
The info on paragons website is incorrect. The wheel rate is motion ratio squared which for the rear is 0.65^2 = 0.42. If you use the racers edge shock mounts which have the standoff to increase the lever arm on the trailing mount, the wheel rate is even lower.
Mac944
New member
The cam reprofile is very mild and to be honest I have no idea if it's made any difference as I didn't test the original profile with other updates. My objective was to retain the hydraulic lifters (for now) to try and keep costs down.
I believe it's possible to get custom cams from the US which are much wilder but they require changing lifters and valve springs and to see any real benefit the head will need quite a bit of machining etc, so for me it was just too expensive.
Mac944
New member
I suppose most 944 are built to race across the various club series including Porsche Club which operates a minimum weight limit so I guess most guys don't cut too much away because it's unnecessary but there's a lot of parts and panels that can be swapped for lighter materials or deleted completely.
I don't think I've gone too mad but I think it's possible to get below 1000kg with a bit more nip and tuck [
]Neil Haughey
New member
The augment cam is for an 8v, I have never seen one for the 16v engines.
Eric when we were building the car I asked my brother to try and measure the rear motion ratio by carefully jacking the hub, measuring how much one of the wheel studs moves and then measuring how much the spring plate on the coilover moves. He never gave me an accurate measurement as it is really difficult but I settled on the Paragon number as he was telling me its about 3/4. I would have liked to have done it again myself but its not easy. It also changes if one fits spacers and due to the banana arm location the movement changes the ratio as the arm goes up and down i.e. its non-linear.
I can remember this stuff being argued about endlessly many years ago on Rennlist. I will say one thing though I have learned, one shouldn't worry about exacting these measurements or calculations as the characteristics of the dampers you will be using makes such a huge difference. The Gaz shocks and it seems the earlier Ledas as well both show on the shock dyno loads of low speed bump and rebound on the rear but very little on the front struts, this is a fundamental reason why we all seem to end up using relatively much stiffer front springs than rear when compared to some other solutions people have used over the years. Ultimately no substitute for track testing and development.
Eric when we were building the car I asked my brother to try and measure the rear motion ratio by carefully jacking the hub, measuring how much one of the wheel studs moves and then measuring how much the spring plate on the coilover moves. He never gave me an accurate measurement as it is really difficult but I settled on the Paragon number as he was telling me its about 3/4. I would have liked to have done it again myself but its not easy. It also changes if one fits spacers and due to the banana arm location the movement changes the ratio as the arm goes up and down i.e. its non-linear.
I can remember this stuff being argued about endlessly many years ago on Rennlist. I will say one thing though I have learned, one shouldn't worry about exacting these measurements or calculations as the characteristics of the dampers you will be using makes such a huge difference. The Gaz shocks and it seems the earlier Ledas as well both show on the shock dyno loads of low speed bump and rebound on the rear but very little on the front struts, this is a fundamental reason why we all seem to end up using relatively much stiffer front springs than rear when compared to some other solutions people have used over the years. Ultimately no substitute for track testing and development.
Neil Haughey
New member
I would prefer it if all of us regardless of where the cars raced operated to reasonable minimum weights. Once one is cutting metal out of the body shell you start compromising safety. It would be tempting for example to remove the under bumper structure on the front that the connects the two chassis legs but I haven't and won't as its part of the cars crash structure as are the inner wings and rear wings and inners behind the axle. These are the reasons behind the numbers PCGB used for our cars as it just happens to be the race weight more or less for a dipped and caged car with heavy spec fibreglass front wings, bonnet and bumper ergo no compromises made for safety. It actually even has words in the regs about the fibreglass panels having to be similar weight to original which really means stiff like a steel panel rather than full on race spec lightweight and flimsy.
Neil Haughey
New member
That tallies with something I said earlier about the front getting relatively stiffer compared to the back the harder one goes with the overall setup. This seems to be the same regardless of car layout as well, race 911s seem to go like that in particular. On our cars there are some good reasons why it is a good idea, we always want more traction which is counter to stiffening the rear but at the same time we also want as sharp a front end as possible. 800 lb/inch on the front is pretty extreme with list 1B tyres but that is how hard some guys have gone, mine with 600 does feel a little soft to be honest in fast corners but in the past the car had hilariously bad handling in the wet or cold/damp which I don't want to make any worse. Its a bit of compromise at the end of the day drawn wherever a driver is comfortable, I am pretty comfortable with the setup I have now.
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