What springs for drag racing turbo e36?

[chikinhed]

What rear springs are you using in your turbo e36 for drag racing? Height and spring rate? With or without ride height adjusters?

[someguy2800]

Stiff springs and raise the ride height as high as possible to correct the instant center. The more you lower the ride height the less anti squat the rear suspension geometry has.

[TheJuggernaut]

I'm no drag racer but I would personally play with softer springs and work with the low speed damping. I would think it would give you better traction out of the hole with more compliance and control the rate of weight transfer with the dampers.

[Robocop]

does the e36 have the same problem as the e30 when it squats it gets infinite negitive camber?
if so, u want some super stiff springs in the rear and if u really want to get wild
raise the mounting point on the control arm as high as it can go.

on the e30 it probably much easier tho e36 looks like it will need a new cross bar section.

https://pazi88.kuvat.fi/kuvat/Projektikuvat/e30%20xxx/2019/20190216_172919.jpg?img=medium


my personal experience with traction is raising the rear helped a ton AND putting in 75# stiffer springs basically gave me all the traction i need.
before with 100 less hp, lowered, spring kit it was a burn out machine because the crazy camber changes just from the act of it squatting.

serious raise the car as high as you can tolerate it will change everything.

[chikinhed]

I have 5.5” 800 lb rear springs with height adjusters. I was thinking a 7.0” 900 lb spring with the height adjusters. I just want to confirm if that will be high enough to give it some anti squat. I wonder what’s in the back of the CES e36 M3 or what did MikeR use?

[PEI330Ci]

I'm no drag racer but I would personally play with softer springs and work with the low speed damping. I would think it would give you better traction out of the hole with more compliance and control the rate of weight transfer with the dampers.

Sometimes it's a bit counter-intuitive.

For example, on the Drag Radial cars like X275, the cars actually target rear suspension EXPANSION rather than squat. Look at one of those cars at the 330ft, and you would think the back end was going to lift into the air! It all depends on the suspension geometry and the tire being used. In general, an IRS car wants to squat before it moves forward, where a 4 link car will look like it moves forward before it squats.

The other thing is that with drag slicks, you aren't always looking for more traction. A lot of times you are trying to get the tire to slip to help keep it's shape in a good place, to prevent uncontrolled release of energy. Wheelie bars are used to tune the rate of slip on launch, not primarily to keep the front end down. If you are using the bars to keep the front end down, you are missing the point.

A stiffer spring doesn't sound like the right thing to do, but in reality, it often is exactly what is needed. The only way to find out though is to test....and for that you'll want to have a range of springs. From my experience with the Pro BMW road-race programs, you can do 100lb/in spring changes, and tune smaller amounts with canister pressure. (Remote reservoirs) Most Pro drivers have a hard time feeling a spring change of 50lbs FYI.

The more power you have applied, the more important the quality of the damper is. Most "street" type dampers aren't designed for the kinds of low-speed loads that drag racing on a slick creates.

[someguy2800]

People often mistakenly think that an IRS car squatting is a good thing because it’s transferring weight, but in actuallity with a trailing arm setup like the e36 rear suspension what is happening is that the hub is going above the trailing arm mount at the hit and now the suspension is actually lifting the tires up due to the trailing arm angle. Once the suspension compresses the toe and camber go nuts and the car starts to try to steer from the rear. You have to keep the back end up for the suspension to work properly. Ride height is the only way to change the anti squat, and since the trailing arms are relatively short, small changes in ride height will make a big difference to the angle.

As Adam touched on as well the tire type makes a big difference. Bias ply slicks need slot of slip percentage to get best acceleration. They continue to accelerate faster up to like a 17% slip rate. A drag radial however will not tolerate wheel spin. Once you spin a drag radial the pass is over, they don’t regain their grip again till they cool down. So it’s essential to hit the tire hard into the track to keep it stuck.

[PEI330Ci]

People often mistakenly think that an IRS car squatting is a good thing because it’s transferring weight, but in actuallity with a trailing arm setup like the e36 rear suspension what is happening is that the hub is going above the trailing arm mount at the hit and now the suspension is actually lifting the tires up due to the trailing arm angle. Once the suspension compresses the toe and camber go nuts and the car starts to try to steer from the rear. You have to keep the back end up for the suspension to work properly. Ride height is the only way to change the anti squat, and since the trailing arms are relatively short, small changes in ride height will make a big difference to the angle.

As Adam touched on as well the tire type makes a big difference. Bias ply slicks need slot of slip percentage to get best acceleration. They continue to accelerate faster up to like a 17% slip rate. A drag radial however will not tolerate wheel spin. Once you spin a drag radial the pass is over, they don’t regain their grip again till they cool down. So it’s essential to hit the tire hard into the track to keep it stuck.

Great explanation on the geometry.

I've heard even as high as 25% slip....but that is probably combination specific, and very much related to wheel speed vs vehicle speed.

The best way that I understand the drag radial situation is that the rear suspension is a lever arm pushing the tire into the ground, and that you are trying to get the chassis to balance over that lever point so that it doesn't tilt forward or backward under acceleration. (Stays level) So you can have the entire car being pushed upwards parallel to the ground, and making the car less pitch sensitive increases the sweet spot from a chassis tuning perspective.

Ever notice how most X275 cars don't have a wheelie bar? I think the guys that do put one on the car have it as a safety device. (Instead of using an IMU to measure rate of pitch, with a power reductions strategy to prevent the car from flipping over down-track)

One thing I always have to remind myself is that the tire/wheel moves faster than the chassis. We think we are adjusting the suspension to get the chassis to sit a certain way, but in reality we are trying to get the tire to sit a certain way, and the chassis reacts to that.

[TheJuggernaut]

People often mistakenly think that an IRS car squatting is a good thing because it’s transferring weight, but in actuallity with a trailing arm setup like the e36 rear suspension what is happening is that the hub is going above the trailing arm mount at the hit and now the suspension is actually lifting the tires up due to the trailing arm angle. Once the suspension compresses the toe and camber go nuts and the car starts to try to steer from the rear. You have to keep the back end up for the suspension to work properly. Ride height is the only way to change the anti squat, and since the trailing arms are relatively short, small changes in ride height will make a big difference to the angle.

As Adam touched on as well the tire type makes a big difference. Bias ply slicks need slot of slip percentage to get best acceleration. They continue to accelerate faster up to like a 17% slip rate. A drag radial however will not tolerate wheel spin. Once you spin a drag radial the pass is over, they don’t regain their grip again till they cool down. So it’s essential to hit the tire hard into the track to keep it stuck.

Again I'll try to minimize my participation due to lack of drag racing experience but I want to clarify that I'm talking about the rate of weight transfer, not squat. I appreciate the leverage and geometry issues associated. But what I'm suggesting is to control said squat through damping, not spring. Increasing spring rate increases your ride frequency a fixed amount and there's nothing you can do about it. So if the rate if weight transfer is too much for your tire, which I imagine is an especially important factor on radial tire cars, it's just going to break loose and then possibly wheel hop. With softer springs and stiff low speed damping, you can adjust the rate of weight transfer without having excess squat, and retain compliance over any bumps, especially after the hole shot when the force being applied to the tires quickly fades (but your spring rate doesn't). Again I'm probably missing the big picture since it's foreign to me but the general rule is if you want grip you have to keep the rate of acceleration/jerk on the tire in check, which is damping. And while that is obviously constrained by other factors which override everything at a certain point, such as suspension geometry and roll centers, all I'm saying is I would sneak up on it from the other end, not go stiff as fuck but try to keep the springs as soft as possible while being able to control the motion with the dampers. And if that ends up at a 2000lb/in spring, so be it. But I wouldn't start there.

[chikinhed]

The IRS Corvette guys use stiff springs to limit changes in toe/camber. I have non remote dual adjust dampers and I’ve tried a bunch of different setting combos with no luck. It squats and hops regardless. I’ll have to jack the back of the car to see how much it compresses the 800 lb springs and see how much I need to raise it to get the trailing arm to have a downward angle. I was just hoping that someone had a height/spring rate that has been working for them as a starting point.

[Kevin325i]

For the longest time I've been the opposite of Brad for having no issues with wheel hop despite similar tires, suspension, power etc etc. Last year I went to a little bit softer spring and had horrible hop issues. Before I chalked it up to the new more responsive turbo and shitty roads I went back to the stiffer springs and hop went away. Summary below.


F: 550 lb/in, R: 750 Lb/in, 28mm solid front bar to control arm, no rear bar- no wheel hop, easy to put power down on corner exit.
Ride height F: 12.5 in, R: 11.5 in

F: 450 lb/in, R: 650 Lb/in, 28mm solid front bar to control arm, no rear bar- lots of wheel hop, damper changes were no help
Ride height F: 13 in, R: 12 in

F: 450 lb/in, R: 750 Lb/in, 28mm solid front bar to strut, no rear bar- no more hop
Ride height F: 13.25 in, R: 12.25 in

My car has a coilover off the shock in the rear so there were changes to helper springs as well. I converted the above numbers to be an equivalent spring mounted at the upper control arm.



What I noticed was that any changes I made did nothing if the rear spring was unloaded at full droop. I would try a longer main spring in a similar rate since you already have some pretty high rate springs back there. You might also want to sneak a zip tie around the damper rod to see what range you're stroking the shock and make sure it's not hitting the bump stops.

[TheJuggernaut]

The IRS Corvette guys use stiff springs to limit changes in toe/camber. I have non remote dual adjust dampers and I’ve tried a bunch of different setting combos with no luck. It squats and hops regardless. I’ll have to jack the back of the car to see how much it compresses the 800 lb springs and see how much I need to raise it to get the trailing arm to have a downward angle. I was just hoping that someone had a height/spring rate that has been working for them as a starting point.

By double I assume you mean high speed compression and rebound? If so, that won't help, you need a low speed compression adjuster. If you crank that bitch all the way up, it turns your car into a no suspension car at low shaft speeds, so there will be no squat of any kind.

BTW where are your springs located? Over the damper or in the stock location? The stock location of the E36 isn't a much higher motion ratio than the E30 and for that, 800lb/in isn't especially stiff (I think it's like a .65 ratio in the rear?). For instance, I had 1100 lb/in in the back on my car and had nice grip in the back but it was too soft for my liking so I'm going to 1600.

[MikeE36]

Put a camera on your rear suspension, make a pass, and then review the footage. Keep raising the car until it stops squatting. I run a 7" 8k rear spring, and my car does not squat at all, it just goes forward.

Here is the camera angle you need for example. This was before I dialed in a lot more anti-squat, and the car got substantially (I can't stress this enough without full capslock) faster in drift. Forward bite increased substantially, and drift suspension tuning is becoming more and more similar to drag race suspension ideology every day as we constantly try to find more bite on a given tire.



Mike

[Robocop]

the car got substantially (I can't stress this enough without full capslock) faster in drift. Mike

experiencing it for yourself is a really stunning moment.
not talking about chikinhead but just any street dude with some h&r street garbage going to some stiff rear and raising it 2''
it's phenomenal. i bet you with street springs at full squat you go to like -7* or -10* back there. Turning your 235s into 195s.

[PEI330Ci]

I also have 8" tall front spring, that are I think 180 lb/inch? I was planning on trying them on the black car...but that program ran out of time.....

[Gmfwd]

As others have said keep the rear up for anti squat. I add a touch of positive camber and check the contact patch on launches to dial it in to where im happy with it. I also drift so I cant go super stiff, so some squat is unavoidable. But I can at least make sure my camber is just about zero when it is squatted out.

[chikinhed]

How much toe change per change in ride height is typical?

[Gmfwd]

How much toe change per change in ride height is typical?

That depends on the ride height. The angle of the rear upper/lower control arm dictate that. When the ride height is high (control arms angled down towards the hub), the car gains toe in on squat. On a low car it gains toe out on squat.

The closer the arms are to level, the less toe change. Ive seen some free suspension modeling software out there. It would be pretty easy to simulate e36 rear geometry and play around with it.

You can move the subframe up or down to alter the toe change characteristics. It would be really cool to move the trailing arm pivot point upwards but its not easy to do.

[Robocop]

you would think there's some youtube guy doing slammed 1/4 times vs raised up.