airiscool

As an aside to my previous spring rate vs damping thread https://rennlist.com/forums/racing-and-drivers-education-forum/425858-damper-adjustments-vs-spring-rates.html, I've been giving lots of thought to the front to rear spring rate split on my car. This thread is less about naming an ideal spring rate for the chassis and more about why everyone seems to do it differently to acheive the same end result.

It would be very helpful to know the motion ratios for the front and rear suspension designs. My car has a weight distribution of approximately 39% front and 61% rear which should indicate the need for much stiffer springs on the rear compared with the front but a large difference in motion ratio, or suspension leverage, could throw this out the window.

For the purpose of this discussion let's assume the said 964 is equipped with adjustable RS anti-roll bars.

The only valid Porsche factory suspension specification that I can find for comparison is for the 1992 RS which had 308lb front and 508lb rear which gives us a split of 1.65.

Information that I have found on this forum shows popular 964 race cars using 700lb front and 900lb rear (1.29) and even as close as 900lb front and 1000lb rear (1.11). One Porsche race shop even suggested using the same rates front and rear in combination with double-adjustable dampers.

The popular Germany tuner Manthey Racing, who knows a thing or two about speed on the Nordschleife, uses a KW track kit that has 340lb front and 680lb rear (2) or a KW street kit with 170lb front and 450lb rear (2.65). The H&R RSS kit has a similar split of 400lb front and 720lb rear (1.8). FWIW, the KW kit for the 997 GT3 is 285lb front and 910lb rear (3.19), although I appreciate this is an entirely different vehicle.

So what can we learn from all of this? Do the Germans favor a large front to rear split while the North Americans like to keep the rates closer? Does a car with only 39% of it's weight on the front axle front springs approaching the same stiffness as the rear of the car?

Please chime in with your experiences and knowledge.

Tom W

It's a matter of driver preference, what shocks you are running, what your alignment and corner balance are, and what tires you are running. There are a lot of options and there is no single set of springs (or f/r ratio) that works for all the possible combinations of car setup and the track you are running on.

Some people run MoTeC or other data systems to closely examine the suspension behavior and tune as required for a particular track. The low-tech way is to try different springs that will get close to the desired handling and tire temps with the sways disconnected. Then attach the sways and tune further (and change if required).

You ask for a simple answer where one does not exist. You also can't have a 39/41% weight split as you are missing 20% of your car - I assume you meant 39/61?

Geoffrey

The 964 has a front motion ratio of <1 and the rear has a motion ratio of > 1. Different sway bar combinations will also change the spring rates that you run. The 964 Cup runs progressive springs that are 200-600 in the front and 240-800 in the rear. The RSR ran a similar spit in the 1200-1400 range. My RSR had understeer with 600/700 and is happy with 600/650. I've driven 964s with 900/800 and the car understeered and had odd handling characteristics. It has been my experience that the split required is not that great with the 964.

Larry Herman

It's my turn to watch. I screwed things up enough in the shock thread.

airiscool

Tom and friends,

Yes my typing mistake shed 600lbs from my car

I am not asking for one simple answer, rather I am trying to determine why the approaches taken by tuners and racers are so different on both sides of the pond.

Porsche, H&R and KW all favor using much stiffer rear springs (large split) while the Americans keep it much tighter.

Has anyone tried an aftermarket 964 suspension with a large split, i.e. 340lb front and 680lb rear? With the proper sway bar and camber set up this should be a nice neutral car.

airiscool

Can you say how close to 1:1 the front and rear are?

Your car is 1.08 towards the rear and the "odd" car was .90 towards the front which is a large difference, plus it is a much stiffer set up.

Even the new GT3 has front springs in the 2XXlb range and it handles pretty well

Geoffrey

The GT3 requires heavy springs because the rear of the car has so much toe steer from the suspension, it toes in when the suspension is rebounded from ride height and toes out when the suspension is compressed. To make these cars handle well, you need to limit the amount of wheel travel and that means heavy springs. You don't want the rear steering the car.

Ag02M5

Do you think the same holds true for the turbo or is there a reason for a larger split?

Geoffrey

The turbo has more weight on the back and the suspension needs to be altered to affect that. They require more spring rate in the rear to help control the car.

Gasser

I just went to Chris C. and had him pick out my springs/shocks etc... and coordinated all my suspension upgrades here locally and I can't be happier. At our local 1.8 mile track, I dropped my times by 3 seconds my first time out on it and I have not started fine tuning anything yet other than a simple sway change. I can't even tell you my spring rates, I just wanted Chris to take care of it and I am happy!!!