aben8057

I have decided to upgrade my suspension on my M474 optioned 951. I decided to go the inexpensive route by purchasing the following:

Front:
Koni Sport Inserts
Ground Control ride height kit along
400# Hypercoil Springs
Stock sway bar (upgrading to M030)
New Delrin bushings

Rear
Currently have M030 sway bar
Delrin bushings
QA-1 coilovers
Keep torsion bars
Spring???

The question I have is with the rear spring rate. I decided to keep the torsion bars and not delete them. With that said, what would be a balanced spring rate that I should research?

The car is a daily driver however, it sees about 10 DE’s per year. I also don’t mind the rough ride on the road and want to balance the rears with the 400# fronts.
Any feedback appreciated.

Kurt R

I'd run something around 200lbs. Maybe a little more

You're going to have to reindex your torsion bars. Here's a good site
Rear Spring Rates vs. Effective Rates - 924, 944, 968

aben8057

Kurt,

Great site thanx for the feedback. I noticed it didnt mention reindexing the torsion bar on the site. I am assuming if you leave the stock t-bar index measurement and calculate you desired rate, reindexing is not necessary.

Kurt R

You'll have to reindex unless you like the offroad look.

Let's assume you have a perfectly balanced 2800 lb car. 700 lbs on each corner. The stock rear torsion bars are effectively 126 lbs / in, so they are compressed 700/126 = ~5.5 inches. Now let's up the spring rate to 350 lbs, The same 700 lb load will only compress the rear 2 inches, so the car will sit 3.5 inches higher.

But wait, you say, I'll adjust the coilovers so that they just barely touch at stock ride height. Sure, that will look good, but the springs will rattle over every bump, and you'll have a really weird spring rate curve as the coilover unloads and loads. If you had a car with downforce you might even want to try that, but I wouldn't recommend it on a 944.

KLR

You'll need approximately 200 lbs effective rate added to the rear if you keep the stock 23.5mm torsion bars. If you use 375 lb rear springs, your rear spring rate would be 337 lbs, which is identical to a 30mm torsion bar. The 30mm torsion bar is a very common combo with a 400 lb front spring, although you could go a bit higher in the rear.

As noted above, you will definitely need the rear t-bars reindexed at those spring rates. You can get away without reindexing on QA1s only if you add a minimal rear spring -- perhaps 80 or 100 lbs. The rear eccentric height adjustment will not come close to adjusting the ride height for rear springs that match 400 lb fronts.

Kurt R

My brain's effective IQ seems to be reduced this afternoon. KLR is right.

M758

If you plan to re-index going removing the stock t-bars for 30 mm t-bars is 10 minutues more work. Then you don't even need the rear coil overs.

aben8057

Joe,

I have a set of hypercoil 375# springs which I was going to use however, I am trying to avoid reindexing because it looks like a PITA to do.

M758

In order to get enough rear spring rate to balnace the front you WILL need to re-index.

There is no way to get the proper rear ride height with 3x stock spring rates unless you reindex the t-bar.

ryoji

Is there a short free length spring?

Z-man

I have 400lb springs up front, and a 31mm (hollow) torsion bar in the rear. I prefer that approach vs. doing a "helper-spring and reindexed torsion bar" combo. With both, you have two factors that need to be dialed in. Plus you'd have to get to the torsion bars to reindex them, so might as well replace them with a beefier bar and forget about the helpers...

That said, I just re-read your post, and you already have the coil-overs for the rear. In that case, be very careful with going too 'big' in the back - you may get too much oversteer, making the car a big harder to catch.

My $0.42,
-Z

Potomac-Greg

Let me muddy the speculation with a real-world experience. I have 400# up front (Bilstein Escort Cup) and 250# Escort Cup in the back as "helper" springs. In order to achieve an original rear ride height, I first dropped the existing eccentric bolt adjustment to its lowest setting (dropping ride height 0.5" to 0.75") and then used the rear coilovers to jack it back up to original height. That allows the helpers to bear a load. I've had no issues with rattling in the back, and I have not observed any odd effective rates. I suppose that, theoretically, the inside rear coilover is unweighted in a hard/fast turn, but if it is, there is no apparent handling problem. I have two long track days on the car and I love it. It works so well on these springs that, so far, I have not upgraded my sway bars.

aben8057

Z-man..your $.42 cents is always welcome.

Greg...How (if any) is the oversteer? In other words, is it managable on the track to the point that you are able to control it without any hesitation?

Riff

Not to say that what you did will not work, but the original rear ride height was not optimal as the original US setup had some forward rake to it. When I redid my suspension last fall, I went 350lb fronts and a 30mm rear t-bar. I also took this as the opportunity to level the suspension. My car now sits level at the doorsills.

Good luck!

Potomac-Greg

I fully expected understeer for two reasons. First, my track tires (Toyo RA1s)are staggered (235s up front and 255s in back). Second, I went with a combined rear spring rate (t-bars plus coilovers) that is probably no more than (and probably less than) the 400# fronts. Conventional wisdom says this is an understeering setup.

Turns out the car is very neutral. If I want to oversteer, I can do it with the gas pedal (by lifting off the gas during a turn), or by trail braking. I am very happy with the setup right now.

In truth, the reason I went with coilover helper springs was so that I could do it myself and not have to pay the labor to mess with the torsion bars. To remove and index torsion bars, you need to drop the entire rear suspension; and to do that, you need to disassemble a bunch of other things (exhaust system, rear driveshafts). It was way over my head, so I took the easy way out. I saved a TON of time and was able to do it all myself in a few hours. If I was a better mechanic (or felt like paying for one), I'd have just gone with a 30mm rear t-bar.

The only objective reference I have is the picture below. This is a hard left (follows a hard right) running flat out (lots of lateral G). (Summit Point, Turn 8). It's hard to make car-to-car comparisons because drivers vary, but overall the car does laps quicker than cars with 50-100 more horsepower.