RPG951S

Looking for some suggestions for a rear spring rate.

Front Setup is:
TurboS M030 shocks, converted to 2.5" coilovers. Rebuilt and revalved by Truechoice. 425lbs front springs. Stock TS sway.

Rear Setup:
Stock TurboS t-bar (25.5), KLA-style aluminum coilovers (valved for 300lbs+ springs). Stock TS sway , Spring Rate(?)

Look for a good spring rate for the rear to match the front. Rather err on the side of caution (i.e. mild understeer vs. mild oversteer) than bravado (don't want the tail to step out at Mosport turn 2!).

Recommendations?

Thanks!

Oddjob

To be on the side of neutral to understeer, probably somewhere in the 150 to 200 lb/in rear helper springs.

Also dependent on what rear whl/tire sizes are you running.

Dave E

Motion ratios are .90 for front and .56 for rear.

M758

Well

425 front assuming .9 motion ratio for = 382.5lbs/in.

In most cases a good ratio is to run about the same spring rate front and rear.

Stock turbo s was 150 to 170 lbs/in front and the 25.5 mm tbar.
So effective was 135 to 153 lbs/in and 25.5 mm tbar lead to 175 lbs/in

Front to rear ratio was .77 to .87

Therefore Lets look for a .87 ratio. I believe that this works well for moderate spring rates and with a stock sway bars.

With the 383 lbs/in front effective would mean 440lbs/in effective rear spring rate. Since the t-bars already provide a 175 lbs/in rate 265 lbs/in effective is what you need. To get a 265 lbs/in rear effective you need a 474 lbs/in coil over. So best to add in a 450 lbs coilover to the back. It should be ok and place to start. You own driveing preference may dict take other things. Rates will be 383 front and 428 rear. (0.89)

Personally I run with a 350lbs springs with 30 mm torsion bars.

That gives me an effecive rates of 315 and 335 or a ratio of 0.94. I use weltmeister sway bars to fine tune the car.

The odd thing is that for a base 944 the set-up was and effective 126/126 front and rear. For me the .94 ratio is good. I do find it odd that stock turbo S is a 0.87. Seems that they added oversteer from this spring set-up, but that may be counteracted by impact of sway bars.

Another thing to consider was tire size. 944 NA's came with 215/60's front and rear. Turbo's had 205/55 front and 225/50 rear (Turbo S 225/ 245). Normaly that difference in tire size means understeer, but a minor suspension change would mean could reverse that.

Hope this helps!

You can also check here for more information.

944-Suspension Link

Geo

I'm having a bit of trouble following that site. I don't see where the .65 effective rate comes from.

What I do know is we are talking about compound springs and it's not as simple as adding up the spring rates. It requires a more complicated formula to get to the effective spring rate. A quick search hasn't turned it up, but I know I've posted it here before. It's anything but intuitive.

Adam Richman

Is this (the front) a measurement you took? I have 1.42:1/1.77:1 from a good source - would like to know if there is other information or why there may be other information out there (also if there is any difference on the front M/R between early and late offset LCAs).

Thanks,
Adam

RedlineMan

Hey Raagi;

First off, just keep your foot in it and Turn 2 will be child's play!



My car is full coilover. I previously had 400F/450R. This gives 252 effective rear. The car was wonderfully balanced and could be driven VERY aggressively with total confidence. I now have 500 front, but have not had the opportunity to test it in ideal warm weather conditions to see if the balance remains, or if it will now default to understeer during spirited cornering.

Since you already have the 175 T-bar back there, I would shoot for about a 150 coil to give you an extra 84 effective and bring the total to approximately 259. You can indeed try and up the rear rate to creep closer to the front rate, but you also have to really be on your game to manage the tail. As you state, you are not interested in the rear steer thing, so I might shoot for what I offered. I would not go more than 200 on the "helper."

You will be at a disadvantage in tuning with the OE sway bars, and I'm not a fan of "hybrid" shock setups, but I know for a fact that the spring combo of 400F/450R (252 effective) works very well.

I sell, install, tune, and use this stuff (LEDA, Hypercoil, Tarret, etc.), so I do have some experience here.

Geo

Again, this is wrong. You cannot add the rates of two springs to come up with the final spring rate.

Rick

This is a great discussion as I'm trying to rationalize if I have the right setup. Over the winter, I had my race shop install Moton Clubsports & Kokeln adj swaybars (front & rear).
They put on 700# springs in the front and 200# springs in the rear.
I've only had the car on the track twice since the install - handling was very neurtal with just a touch of understeer. There was a fair amount of body roll which we'll be taking a closer look at this coming weekend at Mid-Ohio. John from my race shop will be there helping me sort it out.
Any comments?

Geo

Here is a dual spring rate calculator:

http://www.proshocks.com/calcs/2spgrate.htm

I'm still trying to find the formula again.

Oddjob

George, using a helper spring in conjunction with a torsion bar is not the same as stacking two coil springs on top of each other, so the dual spring rate calculator is not applicable for this situation.

Rick, Im assuming you have torsion bars still in back? But unless they are about 30mm, its seems you will have quite a bit of understeer at the limit. 700/200 is quite a difference front to rear.

Something to consider: the Cup/Escort cars used a progressive front spring of about 410lbs/in, with 25.5mm T-bars in back with around a 270lbs/in helper spring. But from what I have seen, this setup tends to oversteer more than some drivers like.

I agree with John Hajny, startout with somewhere between 150 and 200 lb/in helpers, leaning more towards the 150 if you prefer a hint of understeer.

smokey

Check the Paragon website tech section. They have the correct formulas and tables for calculating the effective spring rate of a torsion bar plus coil-over at the rear of a 944.

RedlineMan

Hey;

Absent the magical formula that seems to have disappeared and is stubbornly in hiding, I will stick with my numbers. They may not be correct in the reality of the mathematics, but I know for a fact that the physical combination works very well. Good enough for me. Until we have that Silver Bullet, we can't do much else, eh?

Yet another reason I would not have T-bars... Math!

The closer you move toward having the front and rear spring rates the same, the harder the car will be to manage. For instance, a 650/650 setup will be really quick, but you have to drive the car VERY HARD to get enough wheel pressure to keep the rear end in place. Definitely advanced driver territory!

Sort of like a downforce car. They are tricky because in the first 60% of driving, they are like anything else. In the next 20% they are deadly because you have exceeded tire grip, but not developed significant downforce. Only in the last 20% do they really come into their own. Only talented few can access that last 20%!

I can't imagine how you can have a lot of roll with 700lb springs. Of course any car that weighs near stock will have SOME roll, and only when you get around 2000lbs GVW does it really flatten out, but 700lb-ers should keep it pretty flat. I would guess that this setup would give some pretty heavy understeer if driven aggressively. I would not have chosen those spring rates at all. Just my SLTHO (somewhat less than humble!).

Adam Richman

Rick, not knowing the valving on those struts/shocks, the effectiveness of the swaybars, the tire stagger, the wheel sizes, if there is/is not a lsd, where/how the cage ties into the car, its somewhat guesswork but I think the fronts sound right and the rears are a little light. If you can get a set of 300# and 350# rear helpers (assuming you have the torsion bars in the car still and you aren't running a lot of rear sway bar), I would think you will be close to your happy hunting ground.

On my street/track car, I run 400fr/400rr w/ factory t-bars (t-bars don't engage for about 1-2 inches - effectively progressive rear) w/ a set of front and rear sway bars from an 8v (what's that 21mm fr/14mm rr ?? my S didn't come w/ rear sway), KONI Yellow fronts, KONI 3012 rears (a little over half stiff on compression, can't recall rebound), 225s on 7" fronts, 245s on 9.5" rears and a welded in roll-bar, 1/16th toe out front, as close as we could get to 0 toe rear (right rear arm doesn't want to help out - bassard!), a bit over 2.5° front, sub 2° rear camber and as much front caster as we could get and an assortment of OE bushings with varying ages (many are actually new..ish ) and ride height is pretty level front to rear (eyeball wise) and 14ish cm. from the bolt under the front CA to the ground IIRC. Its (to me) extremely easy to drive on the track (but two years of racing a CRX helped that - didn't used to think so). Who knows, might be a complete POS by someone else's standards. And with the new lsd, who knows, might be a horrific POS by my standards too No way I'd race the car like this but for getting groceries and going to the track, it works pretty well and man is it fun in the snow (well, a light North Carolina snow).

RPG951S

Thanks for all the help guys!

I've actually already got a set of 140lbs springs for the rears (they came with the coilovers), so I think I'll start with them. I was concered that with the fronts at 425, the 140 rears would be woefully inadaquete, and I be turning my car into an understeering pig
I think I'll stick with this setup for the first couple events and feel it out.

Got a followup question:

What's the theory, or the best recommended practice in terms of 'setting' the rear preload?
The reason I ask is because the PO of the car had reindex the torsion bars (which was nice of him!), and set them quite low (the fronts were also lowered - actually too low for Zone1 specs). Putting on the coil-overs, it was very easy to get some preload into them (in fact, even at full drop, their's quite a bit of load on the springs.). I actually use the coilovers to bring the tail 'up' a inch or so, to match the slightly raise front.

The 'problem' I have is that the first 2" or seems to be ENTIRELY suspened by the spring (which at 140lbs is very soft), then it seems to stiffen up quickly after that.

Have I done this incorrectly? Should I set the rear coils so that they sit 'exactly' at the 'engagement' point of the torsion bars? Is that even possible? Is the torsion bar rate linear or progressive?

Thanks Again!