Hi I have a 951 race car that weights 2470 lbs. My spring rates are way off 650 in front and 450 in rear with t/b delete. Is there a reason why people run more spring rate in the rear of the car? Thinking About Running 800 to 900lbs in front and so what would be a good rate for the rear Thank You.

 

Because the effective spring rate in the front is much higher then the rear. You rear effective rate is probably in the 250 range with the T-bars deleted. Also, the 951 tends to understeer a bit but yours must be pretty bad with that set-up.

 

800 - 900# front? Jesus.

Have you done anything to measure shock travel to make certain you need that hard of a spring?

 

Coil springs on the rear of a 924/944/968 are approximately 56% effective, due to the location they are at from the fulcrum point. Basically, multiply the coilspring rate by .56 and you'll get the effective rear wheel rate. That means your 450# rear spring equates to a ~252# wheel rate in the rear. Supposedly the fronts are 90% effective, though I am not 100% positive about that. That would mean your fronts work out to a 585# wheel rate. That is a HUGE bias towards the front of the car, and has to be causing some INSANE understeer. Heck, I am running 400# fronts (360# wheel rate) and stock TB's with 200# helpers [(200x.56) + 126 = 238# rear wheel rate). I experience a TON of understeer, and that's just spirited street driving.

The most important question is what are you using your car for? Does it see any street use? I can't attest to how stiff a full track car should go, as I have seen some insanely high rates on a few 944's (I believe in the 1200#+ range) and it really depends on the type of racing, track surface, etc. Something around an ~800-850# (800# = 448# wheel rate, 850# = 476#)spring in the rear should help remove alot of the understeer and provide for much better handling. Though, it also depends on what size sways you have, as those can also affect the under/oversteer nature of the car as well. Hopefully someone with more track experience and rear coilovers will chime in, as I myself don't have much experience with higher spring rates. The math behind it is easily understandable if you read some of the 944 suspension FAQ's floating around the net.

On a final note, props on running manly spring rates! Hehe. At least you aren't running weak 250# or lower rates. Hope you don't have osteoporosis. j/k

 

On track cars, a common setup is to run approximately 15% more actual spring in the front than the rear with torsion bars removed. I know many track cars running similar setups and they don't understeer. Additionally, I just put in 3 track days with my new setup with this ratio and I can tell you definitively that understeer is not a problem.

 

A lot of the serious club guys are running unbalanced set ups with the fronts exceeding the rears (even before factoring in the effective rates).

These rates were arrived at by testing as opposed to theory – my guess is that they have stiffened up the spring rate to the point that the additional front spring rate is just increasing turn in response because the car is not leaning.

I have not tried the unbalanced approach yet but many have - and its usually the fast guys!

Chris White

 

I was speaking more from a street car/street tire perspective. If you don't mind my asking, Chris, what rates do you run? I'm thinking of going with either a 300 or 325# rear spring in conjunction with the stock TB's. Though, I'm also thinking about just ditching the rear coilovers (968 m030's) and doing 29 or 30mm rear torsion bars as there will ultimately be more room for tire clearance and less to fiddle around with.

 

Finster, your current spring rates sounds right to me, have you driven it on the track yet? Also try posting in the racing forum. Good luck

 

15% is normal, but what he is wanting to run is not.

Typically run a front bias in a balanced car for stable braking, essentially maintaining the alignment rather than creating excessive bump in the suspension which reduces braking effectiveness (result of camber bump) and a nervous car (result of excessive toe curve propagation).

You need to evaluate how much anti-dive geometry is present in the 951 and address the front rate as such. Some car have prevelent anti-dive (such as a Sentra SE-R) which reduces the amount of front spring bias required to a small degree.

 

I run 350 front and 550 rear in the street car. Nice balance for street driving. Good on the track but I would stiffen up the front more for a track only.

Chris White

 

Yikes, 800-900 front is going to be pretty stiff for a car that light unless you're running at only billiard table-smooth tracks. I could see that high of a spring rate if you are going to use Koni racing shocks, which tend to require a stiffer spring than Moton, JRZ, Bilstein, etc.

Several people here have data for stock-weight cars but I don't think many on this board have tried and tested spring rates for lightened cars. Try asking on the Racing forum as a few more people there have experience prepping lightened 944s.

 

I run 600 lb. front and 700 lb. rear on my track car. Car weight is 2370 lb. I have been happy with this set up.
Kevin