Springs Rates for Racing... Again

[sopwith21]

Last fall I had a new suspension put on my 260z including 350# springs in front and 250# in rear, adjustable Tokico shocks and Proshock coilover kits, and a locked rear end. When I was running in GT-2 (and could use racing slicks) the car handled pretty well. It would go where it was pointed and was a fast car.

This year I switched to Production Superstock class which requires DOT tires. Man, did that change everything! I've used both Toyos and Hoosier DOT slicks. No matter what I do, the car pushes like a dog on DOT tires. I've adjusted the coilovers to compensate and maxed out my strut settings. Of course that didn't make the front grip any better, but it did loosen up the rear considerably. The result was that the car didn't have any grip at EITHER end when we were done.

Soooooo.... some of you had suggested that the heavier springs should be in the rear and the softer springs in front. It worked better the other way around with slicks, but with DOT's it appears that I need to make the change. Something MUST be done to make the car turn, and softer springs on the front would - theoretically - do the trick.

Any thoughts? Reversing the springs should free the car up so much that I'll probably be struggling to get bite in the rear tires... a nice change from pushing all over the track. Then we can start working with sway bars, coilovers and struts to get some traction in the back end.

What sayeth the Z car faithful?

[thehelix112]

Errr.. I think making the front springs softer will only be an issue over bumps, if you're wanting to adjust roll stiffness (because you think you are overloading the front outside during cornering), just loosen the front bar off a notch or two.

When you say maxed out strut settings, what does that mean? You are running a locked rear end (as am I), so you will need some toe-out to get better turn in, and a more rear-roll biased car (less front bar, more rear bar) to combat the understeer feel. That combined with as much castor as you can poke a stick at, and appropriate camber should be ok: as in whatever the track/pyrometer tells you.

More details as to your settings at the moment would help with the advice.

Dave

[260DET]

Always an interesting situation, the dreaded Z understeer. And sometimes it does not take much to minimumise it or at least get good turn in which is half the battle although a locked diff does not help things. Get a Torsen type if you can, they are brilliant.

As Dave indicated, front toe out is a must. Other than that its a matter of playing around although my preference woud be spring rates ~350 all round, light rear bar ~18mm, moderate front bar ~25mm.

Having said that, others who have more experience may have a different opinion. Is the car caged? Body rigidity is relevant.

[preith]

The spring rates issue really seems to be a can of worms. You don't hear it too much anymore, but especially in IT there was the east vs west coast set-ups. The east coast guys liked stiff spring rates, stiffer up front, with small bars, and the west coast the exact opposite.

Here's a VERY good thread on the topic:

http://zdriver.com/forum/showthread.php?t=6075

From what he's written, your numbers seem to be in the ballpark, even with the stiffer up front, but you might still want to try a slightly stiffer rear spring.

Basically I have a west coast setup on my car, which I run 225/50/15 Kumhos on. I must have gotten lucky with the setup, because right out of the box it was pretty good, and it doesn't push much. I'm at the 250 front/275 rear numbers which he scoffs at, but it feels real good to me. I'm running a 1.125 front bar and a 7/8 rear. He mentioned chassis distortion, but my car is more like a production class chassis, which much more rigid than an IT car.

It seems like the toe setting is critical on these cars. Right now I'm at about 1/32 in on the front, but many guys run a slight toe out, which he describes as well.

Also, this will bake your noodle. I remember a discussion on HybridZ a while back about NOT running the same spring rates on all 4 corners. I was talking with a local autocrosser, Mike Tews, who also co-drives John Anderson's Z at the soloII nationals. This car is very competetive. If I remember correctly, he said they were using 300lb on all 4.

[sopwith21]

I've got no camber plates, so I have virtually no camber/caster adjustment. I don't know the gauge of my sway bars but I can try to find out. When I say that we "maxed out" the struts, I mean that we have no further adjustment available (they adjust from the softest setting of "1" to the stiffest setting of "5").

We're running 1/16th of toe out to help the car turn into the corners. The car has a full cage as well, plus additional roll bars running back to the frame near the rear strut mounts and forward of the firewall to add rigidity.

I'm anxious to read the thread Preith suggested. Right now the only real answer I can think of is to reverse the springs, with the 350's in back and the 250's up front. That would surely help pin down the front tires. Any other thoughts?

[preith]

I really think part of your problem may lie with the difference in spring rates. Most setups I see don't vary more than 50 lbs, and yours are by 100. Also, I don't know what kind of struts you're using (illuminas??), but in the thread he mentions Illumina's don't react well to any spring rates over 275, and I've heard this elsewhere. But you say you've ran the car like that with the slicks, so who knows...

If you do in fact have smaller diameter bars, I'd try the rates that he has on his car, or in other words increase your rear rates by 50#.

As a stop-gap measure, you can slot the strut towers for camber adjustment.

[tube80z]

The spring rates issue really seems to be a can of worms. You don't hear it too much anymore, but especially in IT there was the east vs west coast set-ups. The east coast guys liked stiff spring rates, stiffer up front, with small bars, and the west coast the exact opposite.

Here's a VERY good thread on the topic:

http://zdriver.com/forum/showthread.php?t=6075

From what he's written, your numbers seem to be in the ballpark, even with the stiffer up front, but you might still want to try a slightly stiffer rear spring.

Basically I have a west coast setup on my car, which I run 225/50/15 Kumhos on. I must have gotten lucky with the setup, because right out of the box it was pretty good, and it doesn't push much. I'm at the 250 front/275 rear numbers which he scoffs at, but it feels real good to me. I'm running a 1.125 front bar and a 7/8 rear. He mentioned chassis distortion, but my car is more like a production class chassis, which much more rigid than an IT car.

Also, this will bake your noodle. I remember a discussion on HybridZ a while back about NOT running the same spring rates on all 4 corners. I was talking with a local autocrosser, Mike Tews, who also co-drives John Anderson's Z at the soloII nationals. This car is very competetive. If I remember correctly, he said they were using 300lb on all 4.

There are a couple of ways to get in the ballpark with springs. One is to use a multiplier of corner weight. For DOT R tires the multiplier is usually .5 to .7. On hybridZ I gave more detail on how to do this. Or you can do a frequency analysis and pick springs that way. In this method you need to know the speed your travelling in a corner, wheelbase, etc. The rates are picked so that you don't have a harmonic that upsets the other end, which could explain how a west coast versus east coast setup could be made to work.

Katman had a point that is starting to become more well know. The concept is called the roll gradient. Basically you pick bars and springs so that it minimizes the amount of torsional twist the car is asked to absorb (where it becomes an undamped spring).

My autox car runs a soft setup of 425 front 400 rear. A friend found with his goodyear radial slicks that 550 at all four corners works great. Let the car tell you what it needs. There's no need to fight it.

Cary

[preith]

There are a couple of ways to get in the ballpark with springs. One is to use a multiplier of corner weight. For DOT R tires the multiplier is usually .5 to .7. On hybridZ I gave more detail on how to do this. Or you can do a frequency analysis and pick springs that way. In this method you need to know the speed your travelling in a corner, wheelbase, etc. The rates are picked so that you don't have a harmonic that upsets the other end, which could explain how a west coast versus east coast setup could be made to work.

Katman had a point that is starting to become more well know. The concept is called the roll gradient. Basically you pick bars and springs so that it minimizes the amount of torsional twist the car is asked to absorb (where it becomes an undamped spring).

My autox car runs a soft setup of 425 front 400 rear. A friend found with his goodyear radial slicks that 550 at all four corners works great. Let the car tell you what it needs. There's no need to fight it.

Cary

Hey Cary, thanks for the input. I guess I quickly forget to ask the z community "suspension expert" to join in on the conversation.

According to your equation for DOT R's, with a nominal weight of 2150, this would put the spring rates betweeen about 270# and 375#, which very close to the two setups which katman refers to, interesting.

[tube80z]

According to your equation for DOT R's, with a nominal weight of 2150, this would put the spring rates betweeen about 270# and 375#, which very close to the two setups which katman refers to, interesting.

I'm not an expert on this by any means. It's just when you start trying to build a car from scratch your forced to learn a lot of things, which has been good fun for me.

The multiplier for spring rates was something I learned three years ago. I took a race engineering class from ICP and they shared this. They recommended a rate of 1 to 1.3 for cars running on bias ply slicks. It was a considerable leap of faith for me to jump to a rate of 1. I've since gone all the way to 1.5 trying to find different setups that work. The biggest issue is finding a set of shocks that can deal with this, which is how I ended up going with the AD ground control stuff.

Another really useful thing I learned in this class was the concept of magic numbers. You can measure a number of things on your car and use these to find different spring/bar combos that are equivalent. It's pretty freaky to actually see this work in real life.

One things I forgot to mention on the above post was to make jumps in 20 percent increments. This will quickly tell you if the change was good or bad. Then you can go half way again to hone in on the rate you want.

Cary

[preith]

I'm not an expert on this by any means.

I find it much easier to listen to someone who is obviously very knowledgable, but is quick to state that they don't know everything.

The biggest issue is finding a set of shocks that can deal with this, which is how I ended up going with the AD ground control stuff.

I was going to mention the same thing. I'll probably get some heat for this, but one of the reasons I chose to stay with the softer springs was because I'm using Illuminas and don't want to fork out dough for those AD's as of yet. $1600 is not a cheap proposition. I know, I know, stay out of racing if you don't want to spend money, but I think for now at least I've made the right choice. I need much more seat time before I start tweaking the suspension more; 5 race weekends just doesnt' cut it.

[terry]

Hello, I'm new to this site. I too am looking to figure out what spring rates and sway bars to use. I have a 240z I have run in SCCA ITS class. Car with fat driver is aprox. 2570 lbs. Looking for set-up info, formulas, etc.. Very curious about the multipling factor regarding corner weights. I am also looking for limited slip diffs insted of using my welded diffs. Thanks

[260DET]

To the original query, if the R tyres are radials, Toyo's are, 3-4 degrees negative front camber will help front end grip. The cheap way to get that sort of camber is to have the struts bent, some suspension places do that.

IIRC cross ply tyres do not require near as much camber to work properly.