reza

What is the correct way to adjust spring preload on most coilover system that allows separate spring preload and ride height adjustment?

Should it be just snug to the spring.
Should it be loose?
Should it be compressing the spring by an inch or so?

What are the thoughts?

Application will be on regular autocross and track car.
Currently experiencing inside rear wheel not touching the ground in big sweeper.

DamonB

Typically you don't need (or want) much spring preload. For most aftermarket springs with ground and closed ends a 1/4" or less of preload is more than plenty. The preload of the spring must be overcome before the suspension can move into bump (if the spring is so short that at full droop it leaves the seats it's impossible to preload it). Too much preload and as the chassis rolls and shifts weight from one side to the other the car won't settle as smoothly.

Typically if lifting the inside rear tire there is too much roll stiffness in the rear; either too much spring (hard to accomplish) or too much bar (easy to accomplish) compared to the front. Does the rear get loose? When exactly does it get loose?

If you really do have a shock/spring problem:

It's also possible that the rear shocks are setup too short and don't have enough droop travel, lifting the inside rear tire as the car rolls. If you have a seperate adjustment for shock length try lengthening the shock and then re-adjusting the ride height. Lengthening the shock while keeping ride height the same will trade some bump travel for more droop travel.

Take a look at this pic:



If you lengthen the shock body and simultaneously lower the ride height with the spring perch you're moving the shock shaft and piston deeper into the shock body when the car is at rest. This will allow more droop travel of the suspension (assuming you have enough stroke in the shock to begin with that the piston won't bottom at full bump). If you can't lengthen the rears try shortening the fronts to limit their droop.

Where exactly in the corner? Only with brakes on? Only with power on? Only at steady state under constant throttle?

rynberg

Reza, Zeal recommends two turns past the point of solid contact between the spring and the seat. This works out to be very little pre-load, maybe 1/8".

reza

Rynberg, that is not zeal suggestion but Rishie's. :P He told me the same thing on the JIC, when he was pushing JIC like crazy. Hehe...

Damon,

There is a long sweeper in our recent autocross, almost 150degree increasing radius sweeper. So as soon as I enter the sweeper, I am on throtle, tire sticks very well, rear stay put, the car accelerates thru the sweeper. In the middle of the sweeper, I can hear the rear spinning, and engine RPM went up, then came back down as it accelerates.
I can't prove it in pictures that the tire is off the ground, but that is what it feels like.

I have both adjustment on my setup. RB rear swaybar set at mid stiffness.
I worry that the shocks don't have enough droop. But if I jack the car up, there is about good 6-8 inch drop.

If I drive the piston deeper into the bottom mount, the car will be lower. To bring up the ride height back using spring perch, has to be with preloading the spring. Because in unloaded condition the spring already just snug on the spring perch


Reza

DamonB

Sounds like too much rear bar. You apparently have enough tire that the car doesn't go instantly loose when the inner wheel lifts but the problem is due to too much rear bar. The increasing radius probably masked the car getting squirrely as the tire lifted as well.

You have a stock diff? The Torsen works great unless one wheel comes completely off the ground. When that happens all power goes to the unladen wheel just like in an open diff. If the RPM zinged up around the corner and then you heard the inside tire spin as it touched back down and RPM stabilized again you were certainly lifting it. The car didn't go loose because once a drive wheel lifted no power was being put down. Too much rear bar. Soften the rear bar and if you need more rear roll stiffness go up on the springs instead.

I think your shocks are just fine.

reza

Thank you Damon, I will try loosening the rear bar.
Springs are stiff enough I thought. 690 front and 550 rear.

Someone had suggested stiffer front bar. I had my Tripoint swaybar in front set to hole 2 and 3 from soft. Would this be effective?

Carl Byck

If my TII did that I'd also lighten the rear bar. GL, Carl

DamonB

I run mine at the third (middle) hole on all but the slipperiest (wet) courses. I find if I go softer the front end is more forgiving and can be pitched in harder but the power doesn't go down as well at exit.

reza

Lets track back to the preload.
With no preload, the only load on spring at static is the car weight. e.g. 690# spring will go down slightly more than an inch for car with 700# corner weight. So that is the preload at static condition.
If we preload the spring by 0.5inch, then at static, it will just go down additional 0.5inch. Is this thought correct?
Then what would happen at dynamic state, where the car is turning, one side loaded, and another unloaded. Along with swaybar in effect?

DamonB

okay

Mostly yes.

The problem occurs as the car rolls or lightly loaded tires absorb bumps. Imagine the car leaned over hard in a turn with the inside tires lightly loaded. As the car exits the turn and begins to unroll weight begins transfering back onto the inside tires. If the springs have too much preload the chassis can't unroll until enough weight shifts back to the inside to overwhelm the preload and allow the inside shocks to travel back up into bump. So the chassis wants to settle but it's "stuck" in the rolled over attitude for an instant until enough weight shifts to overwhelm the preload. This means the chassis attitude is not allowed to respond hand in hand with the amount of weight transfer.

Imagine the same turn again but with the inside tires encountering small bumps as well. The inside is lightly loaded due to weight transfer so it will take quite a bump to overwhelm the preload and allow the shock to travel into bump. If the bump isn't big enough to overwhelm the preload then the car rolls over even farther as the inside lifts and rides over the bump and at the same time the shock can do no work in dealing with the bump since it isn't moving. At best the bumps will become harsher. At worst the suspension becomes incapable of dealing with them because compliance becomes too harsh and the shocks aren't allowed to work.

It's easy to insist that this isn't a real problem because you drop the car off the jack and see the springs compress even if heavily preloaded so you think no big deal. When the car is at speed though weight is constantly transfering around and that is where problems start. With too much preload you're removing compliance from the suspension and at the same time not letting the shocks work as well as they could.

Springs are displacement sensitive. The farther you displace them the stiffer they become. Shocks are velocity sensitive. The faster you displace them the stiffer they become. Anytime shock velocity equals zero the shock is doing absolutely nothing. Too much preload can force the car to absorb bumps without use of the shocks. Bad!