My understading is that the effective rate at the rear is 0.6*spring rate. So you should have with standard torsion bar an effective rate:137+0.6*250=287. This will perfectly mached to the 300 front spring rate.

 

I hope so, as it would avoid a difficult procedure for me, but I think that Z-Man is right. Jason at Paragon did mention that I should seriously consider re-indexing the torsion bar.

It'll all come out at the track, I'm sure.

 

Hi Zoltan,

Your above post with the Paragon link is extremely helpful, as I didn't know anything about effective rates prior to reading it. I just have a question concerning sway bars & strut bars, now that you've mentioned them - how much more or less do they generally effect the spring rates? Or is it more of a "fine tuning" sort of thing like adjusting stiffness on Koni struts?

I'm curious now to figure out what my own front/rear effective spring rates are. I'm running a 968 non-m030 stock suspension with added m030 front and rear bars (rear set on the stiffest setting), upper KLA strut brace, and a Brey-Krause underbrace that connects the control arms together.

 

No, I think you are looking at the desired rate.

Going with 287 lb rear, you have closer to 95 percent the effective rate of the front springs, not what most people believe is better (somewhere around 75 percent).

eohrnberger: why didn't you just put a lighter helper spring in the back? If you go with a 125-150 lb spring, you shouldn't have to re-index the torsionbar - you could tune the balance via sway bar adjustments.

-Z-man.

 

Well, it's like this:

I talked to Jason at Paragon to get an idea of what was possible and what was recommended (my Chief DE Driving Instructor, who races a 944 turbo, strongly recommended a coil over solution).

At that time, I was unaware of the torsion re-indexing issue, nor how much of a PITA is was. You'd think that Paragon would kinda take that into account when recommending a solution, wouldn't you? (Don't get me wrong, I'm not complaining, just observing).

This is probably my fault for not educating myself sufficiently before ordering the parts, but I was in a bit of some time pressure in that I needed to get the parts ordered and in so that work could being and be complete within the time frame that I thought was needed (this was off again, as it took a lot less than I thought it was going to). That, and I was all excited to get the upgrade .

All around, it was a real educational experience, and most things are with this car. I guess to sum it all up: 20/20 hind sight!

But no matter. I'm confident that I'll sort this out yet.

 

Sway bars definately make a difference in the handling characteristics of your car. But I don't think they figure into the overall spring rate of your car. That said, you definately need to consider your sway bars when tuning your car - not just fine tuning.

My take on sway bars for our cars is this: I believe a +1 rear swaybar upgrade is most effective. With a 944S2/951/968, keep the stock front swaybar, and upgrade the rear to a 968 M030 19mm 3-way adjustable. If you go with a stiffer bar upfront (like the M030 front), then upgrade the rear to either a Turbo Cup sway bar (impossible to find), or a Welt 22mm adjustable swaybar. The reason for this logic is this: the Porsche engineers knowingly dialed in a good dose of understeer in our cars, to make them safer for the average driver. For track and AX, you want to dial some of that understeer out, and the easiest way to do this is to stiffen the rear of the car. Swaybar upgrades are by far the most cost effetive start. (Note: a good second upgrade is to run the same size tire at all four corners - in my case, I run 245-45/16's on 8Jx16 968 wheels on the track)

Strut bars and your underbrace BK unit stiffens the front of the car, which typically will lead to more understeer. Did the front of your car wash out alot at last Sunday's autocross? I suspect it did. If that's the case, a quick remedy for you would be to upgrade your rear swaybar to the 19mm adjustable unit, for starters.
You probably have 25.5mm torsion bars - which equates to 175 lbs effective for the rear of the car. I believe your front springs are in the neighborhood of 200-225 lbs. Check the tech specs for a 968 on either Paragon's website or Pelican's.

Hope this helps, and again, as a disclaimer - this is all stuff I've read, researched, and others have shown me. I did a fair amount of research before upgrading my car. My current setup seems to work very well for me and my driving style - but as always, your mileage may vary, and there are different approaches to suspension tuning.

Happy driving,
-Z-man.

 

That is correct. You see various numbers for "effective" rates for front and rear. They are generally useful to get ballpart numbers. Typically you can use around 90% effective front and about 55-60% effective for the rear coil spring. The torsion bar rates are given as effective wheel rates, so they are used as is.

So if you have 300s in front (300 x .9 = 270) and 250 in the rear with S2 24mm T-bars (250 x .55 = 137 + 137 = 274). So you have a front to rear eff. spring rate ratio of 270/274 = 0.99. My guess is the car will slightly oversteer at the limit.

With that technique, I found while playing with my track cars (around 500 lb/in front rates), that anything up to about a 1.2-1.3 ratio will tend to oversteer, the 1.3-1.5 is somewhat neutral, and 1.5 and beyond will be understeer.

>>>EDIT(add): where z-man talks about the rear being 75% of the front rate: that equates to a 1.33 front to rear eff. ratio, so sounds pretty similar to the spring rate/balance that I found. <<<<

But here is the other kicker, it seems that the "optimum" front to rear ratio changes as the spring rates increase. At lower/stock spring rates, the neutral handling cars will be around 1. At much higher spring rates (500 plus), neutral handling is closer to 1.4 and up.

In the end, any of this is really only useful as a ballpark rule of thumb, nothing absolute about it. Its really just a little mathmatical trick to guess at handling characteristics when buying springs. Most importantly it really comes down to how the car handles when you drive it (because of all the other factors: sway bars, wheel/tire size, track or autocross, driver style, engine output/torque, etc).

As far as reindexing torsion bars when adding a helper spring: the most common method is to set the torsion bars so they are close to neutral when the car is sitting at ride height. The weight of the car will be held up by the coil springs. Is the car currently sitting high in the back end? Or do you have the coilover perches adjusted so there is no weight on the springs, and the car just sits on the torsion bars? If so, you probably will get some spring popping noises when you drive the car.

 

Thanks Zoltan,

Actually, at the Sunday AutoX I always felt like the backend of the car was about to slip out - but this may be because I do take corners *a bit* faster than I probably should and I do get very throttle-happy halfway through the turn . I'm going to start hunting the paragon website for answers. Thanks again!

-chris

 

B I N G O - read, heed, obey. Car handling is artful. Eye of the beholder, and all that jive.

For those who drive different tracks, you know this all too well. How many tight corners where you really have to be on the brakes? More than one? Scared that a little too much plow will waste your fronts for the rest of the tight course? How many long sweepers that just beg for that pinch too much of oversteer to really ruin your day? How's that outside rear doing after a 30 minute stint... soft like candle wax? Brake bias? What, you mean I can actually control the double-apex and not just hope it will happen?

Makes you wish there were a book...

How many folks can set up a car by the math alone and be confident it will work? When you call, and they say "this and that", then they've done it many times before... sans calculator.

IMHO, of course

 

Skip, could we keep this on topic? Is there a #250-300 front 968 spring that will maintain the car's stock clearance?

 

My bad, Kevin... switching to topical:

I may not be filling in all the blanks correctly, but I guess "968 towers" means that you have 968 strut assemblies? And, you found out that the 968/944S2 (smaller, shorter) do not share the same springs as all other 944/951 (larger, longer)? You want to keep stock ride height because you're concerned about ground clearance, you exclusively street drive but you like a little sporty spice now and then? If so, then...

It's already been mentioned that you can use the Adjustable Ride Height Kit (ARHK) to convert your setup to 2.5" ID coil-overs. Given the correct free spring length (10-12") for a rate of 200 or 250 lbs/in, you could maintain stock height... or, even higher. Anything above 250 and you really must address the rear end.

For a fixed-height spring, there's really nothing that will work. Our 968/944S2 Hypercoils are exactly like the H&R units in that they run approximately 1" lower than stock. 944/951 Weltmeister/Eibach springs have an optional 1" spacer that effectively negates the lowering - unfortunately, there's nothing similar for the 968/944S2.

If you were really committed, I bet Hypercoil would make you a custom set.

All this said, I'd like to talk you into lowering. Dang TUV and DOT makes our cars look a little awkward sometimes. Taking a queue from what's right for performance, and from what Porsche does with non-volume cars, lowering 20-30mm is perfectly acceptable from all angles. The geometry still works, the arches support it, tires won't rub, and so on. In my opinion, the car looks mo'betta 1" lower. As you probably already know, the 968/944S2 lowering spring is right around 220 lbs/in. Stock was around 160 lbs/in for the 968 (heavier engine).

Now on to the fears of adding excessive understeer. Horsepucky... kind of. On the street, obeying most of the laws, you'll likely never notice the additional tendencies to plow through a corner. On the track and at autocross, surely - no argument there. On the street, it's a bit more fun to have some extra stiffness up front. It helps reduce braking and cornering dive substantially... along with the Konis, of course.

So, go with the ARHK for adjustability if you change your mind, or, bite the bullet and go low(er).

Clear as mud?

 

Skip,
Well said. I wholeheartedly agree. I probably didn't clarify that the numbers I threw out there are good starting points. Add a bit of personal driving style, various tracks, sprinkle with some fairy dust, and add judicial amounts of black magic. THEN you MAY have the car setup the way you wish.

Start with a baseline (Like make the front 75% spring rate vs. the front), and then adjust to suit the application (AX? DE? CR? Tight courses, fast courses) as well as the driver's style (Agressive? Smooth? Likes a touch of understeer? Likes a bit of oversteer?...etc).

Oh, and I think the tuning of our cars is not only a black magic art, but also a never ending pursuit - there will always be something that needs tweaking...

-Z-man.

 

Well, ran the new suspension at DE today. First two runs were in the rain, and the last two were in the dry.

During the first two sessions, when it was raining and wet, the car was a handful to handle. The back end just would not stay back there and it was difficult conditions to learn how the new setup would handle.

However, during the last two sessions, when the track had dried off, things were much better for traction. But the car is so different now from the stock suspension, that it's like driving two different cars. It's much flatter and firmer, but it also seems much more sensitive to un-smooth driver input. The mistakes and un-smoothness that the stock suspension had absorbed and smoothed out without a whimper, this setup now seems to amplify and be sensitive to and would throw you off line.

While I may be able to push the car harder in the corners, I did push as much as I felt comfortable, and did not notice that the car was tail happy. Right now, I would hazard a guess that in the dry, the fronts and the rears appear to have about the same amount of grip. In the wet, trying to corner on the track, the tail would step out at seemingly nothing. But then, I didn't do this to drive on wet racing tracks either.

Many thanks to Jason at Paragon products for his advice on this configuration. So far, no torsion bar re-indexing needed, and it seems to perform very well.