shiners780

How much can you change a spring rate, either +/-, before you have to have the corresponding shock revalved?

I have Leda coilover shocks with 525# springs. What is the range of spring rate I can use without revalving? Or is it best to have them revalved no matter how small the spring rate change?

I'll check with my supplier on Monday, but I wanted to start thinking about this now and they are not open on the weekend.

TIA!

SundayDriver

That depends on how adjustable they are (range) and where you currently have them set.

If the shocks are not adjustable, then they are almost certainly wrong already. The question becomes how wrong are they, are you moving towards better or worse, and what are you are able to detect and willing to put up with.

Some people (assuming they are perfect right now) would feel a shock problem if you changed spring rates by 5%. Others would not feel shock issues if you doubled or halved the rates. Most newer drivers like shocks that are far too firm - that provides better feedback and "feels' better, though it is slower.

Sorry to not give an answer you are looking for, but shocks are very much a feel thing and it is much a matter of what feels good to you and your ability to tune your car.

mikew968

It depends on how the shocks were valved in the first place. When Karl had mine built they were valved for a mild track set-up. My high end spring was 600f/800r. Beyond that and they need to be re-valved. Check with who ordered your's.


Mike

garrett376

Karl recommended that I re-valve my non-external reservior LEDA's on my 911 when I went from 300/450 springs to 525/800 springs. So I did, and they work great, and given the hard setting I use them on the track, it was a good idea to get them re-valved. If I had left them alone and just changed the spring rate, I think even at the max setting (21 or so clicks) it would not have been stiff enough, as I run them around 14-17 clicks at most tracks.

shiners780

Good info guys, thanks.

I 'think' I need a higher spring rate in the rear, but I'm certainly no setup expert. Here's what I've got, which is causing a fairly severe understeer issue:

24 point Leda adjustable shocks/struts, set at 5F/20R (1 soft, 24 firm)
Springs 575F/525R, torsion bars deleted
Welt sways 28F set soft/22R set stiff
Car height level front and rear, corner balanced almost perfect
All solid suspension bushings
Tires 235/40/17F on 8" wheels, 255/40/17R on 9.5" wheels

It seems to me that after adjusting the sways and shocks that the next logical step to dial out the push would be stiffer rear springs. It understeers bad on turn entry, moderate at mid turn and exit.

Here's some video from this past Wednesday. I didn't get any video showing my steering inputs, which would have been of more value to this discussion, but you can hear the tires fairly well. Don't know if this helps at all to demonstrate the understeering:

http://youtube.com/watch?v=sa0ml1UeXkc

trackjunky

What type of car? If it's a 944, you should first look at your alignment settings.

Let us know more about your car.

shiners780

1988 944 turbo, 2740# with half tank of fuel and driver.

Caster is maxed right around 3.5
Front toe at zero, rear toe in 3/64 total
Front camber is only at -2.0. I had it at -2.8 but the inside front tires were wearing quickly. Once changed to -2.0 the tire wear got better.
Rear camber -2.0

Any suggestions?

trackjunky

You need to be running -3.0 to -3.5 in the front. You will experience a push if you run the the same camber settings on all four corners. Not great for the street, but this is best for the track.

I run -3.1 front and -2.4 rear with zero to negative toe front and rear. Your might want to back your caster out a bit. Maybe 2.5 or 3.0.

Trust me, changing the alignment on these cars yield big changes.

shiners780

Thanks for the suggestions Bill.

When I had the front camber set at -2.8 the car understeered. In fact, when I changed the front camber from -2.8 to -2.0 (then adjusted the toe back to zero) I noticed very little change in handling. The only difference I found was that the front tires wore more evenly. So I'm not sure if going back to -3.0 would make much difference, maybe throw the fronts all the way in to -3.5 would make a more noticeable difference?

I think my front tire wear was more a result of the understeering issue, rather than too much negative camber. When in a turn, it feels like the inside edge of the inside tire is dragging. I figured this was causing the uneven tire wear.

My thought was to bump up the rear spring rates, and have the rear shocks revalved if necessary. Then adjust the front camber in to around -3.0, set the sways at full soft, set the shocks in the middle of their range, and see what happens.

Don't you think my rear spring rates are too light? I see you've got 50# more spring in the rear than front, which is opposite of my setup of 50# more spring in the front than the rear. Are your torsion bars providing any effective spring rates or have they been turned down? It seems like most serious track-duty 944's have more (actual) rear spring than front to the tune of 50-200# (with torsion bars deleted).

Nader Fotouhi

IIRC, the higher the rear spring rate produce more push (understeer). I have 25 or 50lbs lower rate in in the back and experience only slight mid-corner push at higher speed turns.

You should try changing the sways and shock setting first. I would move the F sway to middle if the backs are full stiff. It seems that the rear is a lot stiffer than front.

trackjunky

In my experience, taking camber out of the rear will stand the tire up and take mid corner grip away from the rear. Now, this is good so long as you are not giving up mechanical grip to compensate for a poor front set up. (use your tire wear and pyrometer to tell you what's best)

What I have found is that I dial in too much rear grip and it overpowers the front upon throttle application. I'll probably go 600# front with my 700# rears and get even better balance.

Too much caster results in weight jacking to the outside rear. Go talk to anyone running Karts and you'll see what can happen with a rigid chasis set up.

Sometimes, the set up is fine, but the driver and the line are what's messed up. But of course this has never happened to me.

Larry Herman

Keep in mind that there is a vector and a lever action in the rear mounting point of a 944 that affects the spring rate. The effective rate of a spring is actually .65 times it's lbs/in rating. Check out this thread to get a little more insite. I think you will find that your car is seriously undersprung in the rear, causing your understeer.

SundayDriver

Just curious - Is the rear motion ratio .65 or something like .8? What is the front motion ratio (I am pretty sure it is not 1.0)

FYI, the wheel rate (effective spring rate) is the spring rate times the motion ratio squared.

Larry Herman

Mark, I am not sure that I understand the terms "wheel rate" and "motion ratio". What I do know is that in the front, the spring presses directly down on the MacPherson strut, with allowances for the angle of the "king pin" inclination, and for the offset of the spindle from the strut. I am told that it is only a few precent. In the rear, the spring pushes down at a significant angle, and the point of attachment to the trailing arm is ahead of the centerline of the rear axle. So you lose effectiveness due to the angle of the spring/strut and you lose leverage due to the attachment point. That is in the neighborhood of 35%, hence the .65 multiplier.

You will have to tell me how this all relates to the wheel rate.

SundayDriver

Let me expand...

Motion ratio is the ratio of spring movement to wheel movement. If you have a solid axle and the spring/shock is straight up and down the motion ratio is 1.0. If you have a strut with a 45 degree angle the motion ratio (at that point) is .707. Motion ratios change as the suspension geometry changes.

A slight angle should not be neglected since you square the motion ratio to get wheel rate (I'll come back to that). Wheel rate is the equvalent spring rate if you had 1.0 motion ratio. IN your example, you understand that the rear rate is lower than the actual spring rate because things are at an angle and run through levers. That would provide a motion ration of .65 in your example. However, if the motion ratio is .65 and the spring rate is 600 lbs/inch then the wheel rate is 600 * .65 * .65. A motion ratio less than 1 needs to be accounted for due to squaring the number for wheel rate.

In all likelihood, the front motion ratio is less than .9 so the wheel rate is less than 80% of the spring rate. For tuning, you need to understand the ratio as it impacts what you are really doing with spring rates F/R.