I drive an E30 that has become strictly a track car. I am not racing W2W, but I spend a considerable time on the track ... I used to have a Ground Control / Koni s/a, suspension setup but this year I am changing to a AST system.

It wont be the full Race suspension, but similar to what VORSHLAG in the US offers as SS2 (4200). I am getting my setup from Holland (through LMP Motorsport, Ottawa) and the front struts will be totally rebuild by AST and for the rear I will get a real coil-over instead of the seperate spring on the RTA.

Funny thing now about the spring rate. AST (Holland) is saying that their usual setup they sell in Europe and recommend for my car is softer in the rear (70kg/cm for coil-over) then in the front (110 - 130 kg/cm). Strangley though, the "Package" options that VORSHLAG offers in the US are in the usual range, front softer then the rear (GTS = fr/rr ~650/750) and that's what I had on my old GC.

So what would you think is better? I always thought it was funny that the heavy front of the car gets the softer springs, .... but I dont know much ... soooo, any recommendations are welcome.

AST will sell me of course any spring combination that I want. BTW, my car was very well balanced with my old setup, no particular under/oversteer issues.

Klaus

Interesting position. I guess my first question I would ask myself if I were in your shoes, which I was, would be...Am I happy with the way the car handles? And if not, what would I want it to do better? Once I figured that out, then I would go after what would make my car do what I wanted to do. When I went through this process, I determined that I was for the most part very satisfied with the handling dynamics of my car I wanted something that was harder but didn't throw alot of change into how the car actually handled. I decided that the best option for me was to upgrade to upgrade the shocks in the front & rear along with 23mm (frt) & 30mm (rr) torsion bars. This basically makes the same balance as the stock setting only a much stiffer ride, which I'm completely fine with.

I guess if you just stay focused on what you want your car to do pretty much determines what parts you ultimately end up with. Good luck.

I was discussing this issue with Louis P
Spring rate relation front/rear will affect the balance of your car mid turn ( steady state )
The 2 other factors that might affect the balance of your car mid turn is the % weight front/rear to a lesser extent and the relative size of your front/rear sway bars and their adjustment position if applicable.
So depending on your sway bars size relation front/rear, different spring package can achieve a neutral car.
Understeer/oversteer situation at turn-in or track-out are corrected using dampers rebound and differentiel lock-up % respectively

Klaus,

I'm not sure exactly how the e30 is, but on e36 if you switch from OE mounted springs (on the control arm) to a true coil over, you expect the rear spring rate to be about 0.6 of what it was. This is due to the actual wheel rate instead of the spring rate. So, if you ran a 1000# spring, switching to coil-overs, you'd now be running a 600#. Did you account for this difference when talking to AST?

Klaus

Totally agree with Arek
On a coil over rear conversion at the shop, we went from 800# to 400# springs to achieve neutral behavior on a E36
The actual rate of the rear spring will vary a lot depending on the suspension pick-up point you choose

Thanks a lot for the explanation, this makes a lot of sense.

So the rear will only be "theoratically" softer, but in reality would be about the same as before.
Last year I installed my OEM swaybar again as I always had issues with the inside rear wheel spinning in tight corners (even with a customized LSD). For classifying reasons I could not disconnect the aftermarket swaybar and I was looking for losing PIP points to get into a lower class. It turned out that by putting back my OEM swaybar, I had no more wheelspinn problems whatsoever.

As classification is not such an issue for me this year, I am thinking now that with the "softer" rear springs I could be installing the Suspension Technique stronger and adjustable swaybars again .... or would I be looking for problems with wheelspin again?
The adjustability range of the new shocks and the ST swaybar is larger, so I would have to do a lot of testing to find the best combination.
Is it worth going through the trouble?

Hi Klaus

If you were running springs in their stock locations, then the rear spring rate should be 150% of the front spring rate. Per exemple, my e30 M3 runs "soft" 425# at the front and 650# on the back. Fine tuning is done with the swaybar where one notch stiffer or softer makes a lot of difference exiting a corner.

The rate of the rear springs will be affected depending on if they are in their stock location or as coil-over as the leverage is not the same. If you intend to use stiff rear swaybars, then be carefull. THis makes the suspensio less independent and doesn't allow to apply power until the rear end is (relatively) flat on the ground.

John, what Lee is saying, is that the key element in "inside wheel spin" (= having both wheel on the ground) is the swaybar.
Softer "suspension" will not lead to having both rear wheels on the ground.

When I bought my car it had front and rear Suspension Technique swaybars installed and I slowly upped my spring rates over the years from about 300# to over 700#, but always had issue with inside rear wheel spin. The rear bar is adjustable and I set it to max soft, but it did not solve the issue. The next step was disconnecting it, but as I needed to find preparation points to drop my car into a lower class, I thought of installing the REAL soft OEM swaybar. Dropped a class .... and I got rid of the inside rear wheel spin last year. Disconnecting the rear swaybar is for some car just as much an improvement (Pip point) as in other cars having bigger swaybars (RWD vs FWD)

As I am still at the very beginning to understand this Suspension voodoo stuff, there is still lots unclear. So I am wondering what advantages do I give up by not having an adjustable big rear swaybar. Should I just stay with the OEM swaybar ??

Basically, it is the springs that keep the car from "falling" on the ground. In a corner, the shell is subjected to some roll because of the centrifugal force and weight transfer over the car's roll center. The outside springs are pushing back to keep the car from "falling" on its side. But depending on spring rate and roll center, the car will still have more or less body roll. Now, what happens with the swaybar is that while the outside rear wheel (per exemple) tends to move up (closer to the fender lip), the swaybar follows the outside wheel, and transfer the movement to the inside wheel. And in an effort to lift that wheel it allows the inside part of the shell to stay down, but it also reduces the pressure of the inside wheel against the ground. So, when applying power out of a corner, because only the outside wheel has full grip while the inside wheel has less than full grip, the back end will slip when applying throttle too early. Just wait untilthe car is flat, which is not fast...

Now, if the swaybar transfered less upward movement to the inner wheel (by using a smaller swaybar), then the inner wheel will have more pressure on the ground, and will be able to transfer more power without slipping. More grip results in earlier application of the power. And higher speed at the end of the straight.

Thanks Lee, with the expereince in my car setup I understand that part.

.... but this leads me now to the question, why would anyone go with a stiffer rear swaybar in a RWD car?
Is there another benefit from it?

drbergeron said:
"So depending on your sway bars size relation front/rear, different spring package can achieve a neutral car."

Is there a rule fo thumb?

thanks for the help

".... but this leads me now to the question, why would anyone go with a stiffer rear swaybar in a RWD car?"

The answer might be because people want the comfort of soft springs and think that stiffening the rear end with a stiff sway bar will do the job of tuning toward oversteer. It will. But it will also bring all sorts of problems like running with a rear live axle...

If no swaybar will result in maximum rear traction as both wheels are totally independent from each other (after all, isn't that what we do when we track the car in the rain and need traction?), then adding a swaybar will more or less result in less traction in corners and will also help tune understeer/oversteer by reducing the traction/pressure on the ground.

I currently run 425#fr and 650#rr and just purchased a set of 700#rr so that I can use a stock rear swaybar, while still tuning toward oversteer (I like a happy tail to help turn-in on tight tracks like Calabogie.

To answer your question about the usefulness of a big rear swaybar in a RWD, we have to go back at the basics.

The reason why a race car ( compared to a road car ) has stiffer springs, bigger swaybars and a lower stance is because we want to limit the energy lost by the car rolling side to side, up and down or back and forth. All the energy lost by these movements reduces the forward momentum.

The downside to it is all these  reduce the amount of grip available

Suspension set-up is an art of compromise 

The sway bars are designed to limit roll for a given spring rate. So they permit the use of softer springs in street car while limiting roll ( comfort ). I a race car you can achieve the same by using harder springs. The main reason why we use these bars in racing is for fine tuning the oversteer/understeer balance of the car while fully loaded ( mid turn ).
Some of these bars are adjustable from the cockpit to help adjust balance when the tires wear during a race. Sometimes these bars are use because for example, we want softer rear springs to enhance the " power down " ability of a high HP car at track out, but that would make the car push mid-turn, so we use a soft front bar and a tight rear bar to compensate  . Some race car classic set-up ( group N ) don't use rear sway bars because they are of no use with the high spring rate ratio ( rear to front )

Interesting subject, hope this will help you get a hang of it



 

In addition to energy loss concerns, there are the change of geometry concerns. The less the suspension moves, the less the geometry changes. If your car has toe changes under compression (bump steer), then minimizing suspension movement minimizes the problem.

Another problem with soft springs is their characteristic low frequency which can make them hard to control. You have to consciously get the car to take a set before you really corner hard, you have to think about when the springs are going to bounce back when you go through an S, and if you get a little sideways, you have to include the springs in your timing calculations. All that energy storage can make it a lot harder to drive the car. Hard springs make the car snappy, twitchier, less forgiving, but easier to correct, a more direct feel, less feeling of wallowing.