[shirtpants_]

Hey guys, I've been looking into getting aftermarket springs and dampers for my '11 335i m-sport. It looks like the spring rates for my car stock are:
Front: 145 lb/in
Rear: 460 lb/in
I've been doing some reading, and one thread in particular has caught my eye.
http://www.e90post.com/forums/showthread.php?t=85206

About 1/3 of the way down the page Orb starts explaining that the spring rate advertised is not the actual wheel spring rate we are getting on our cars.
I'm quoting him here:

Originally Posted by Orb

The e9x rear suspension has a very high motion ratio so to make any scenes of the spring rate we have to do a few simple calculation so it is comparable. The end result of the calculation will give you the wheel spring rate and this is what your tires see and what you feel.

The motion ratio for the car:

• Front: 0.96^2
• Rear: 0.563^2

Calculation for wheel rates:

Coil spring rate * Motion ratio = Wheel spring rate

Stock 335i example.....

E.g. 145lb/in * 0.96^2 = 134 lb/in
E.g. 460 lb/in * 0.563^2 = 146 lb/in

Spring rates:

Stock 135i:

• Front: 120 lb/in
• Rear : 350 lb/in

Stock 335i e92:

• Front: 145 lb/in
• Rear: 460 lb/in

Stock M3 e92:

• Front: 160 lb/in
• Rear: 550 lb/in


Orb’s 335i:

• Front: 392 lb/in; Wheel spring rate: 361 lb/in
• Rear: 1008 lb/in; Wheel spring rate: 318 lb/in

To scale the OEM suspension frequencies you can use this formula to get a rear spring rate.

(New front spring rate/OEM front spring rate) * OEM rear spring rate = new rear spring rate

E.g. (265 lb/in /145 lb/in) * 460 lb/in = 840 lb/in

If you do the calculation you will see the rear spring is really soft in these performance setups. Soft ass end means more understeer! Kind of makes one wonder if anyone is telling the truth that thier car is all that great!

Orb

(I'd encourage you to read the whole thread, If Orb is right, there's lots of good information throughout)


Based on these calculations, most aftermarket manufacturers spring rates are way off, making the front much stiffer than the rear. This should result in increased understeer, especially with an upgraded front swaybar

So, are the rates I'm seeing people running like Front: 500, Rear: 800, just completely wrong, and they just don't know any better, or are Orb's calculations off?

Example:
(M3 KW Clubsport
Front: 508 lb/in wheel spring rate: 468 lb/in
Rear: 800 lb/in wheel spring rate: 254 lb/in) Example from Mr. 5's sticky- my calculations

It's pretty commonly known that BMW set up our cars for understeer, so why would they set up the rear springs so much stiffer than the front if Orb is wrong? Why would all the aftermarket companies set up their spring rates to be much stiffer in the front than the rear in comparison to the stock Front to Rear ratio, which should promote understeer from the factory?

If you've got any input on this, please share

[ajsalida]

No his calcs are correct, that is a great thread. Problem with the rears being soft relative to stiffer than stock front springs is mainly you sometimes simply can't get rates as high as theory would suggest with those motion ratios and space/structural issues in back. And the 5-link rear starts to misbehave at high spring rates, some find they have to disconnect rear bar or go much softer. Using M3 as comparison is somewhat misleading as M3 has true LSD and e9x non-M does not, in addition to wider track and other differences.

But also McP strut front end does not understeer more with stiffer springs/roll rates, up to a point. That is because the front geometry is such that softer means tire will lean away from the pavement under side loads and lots of roll, so more roll stiffness keeps contact patch optimal. Also less travel in front esp when lowered means you want front stiff as is reasonable. Then you just live with the limitation on rear spring rates and perhaps sub optimal wheel freq F/R.

That is not to say there aren't some weird rates out there with aftermarket springs. But a more relevant quantity than wheel rates is natural freq., which they go into later in the thread. It is not as far off as just spring rates alone appear since you're doing sqrt(k/m) and not just k, and m is sprung corner weight.

[shirtpants_]

Originally Posted by ajsalida

No his calcs are correct, that is a great thread. Problem with the rears being soft relative to stiffer than stock front springs is mainly you sometimes simply can't get rates as high as theory would suggest with those motion ratios and space/structural issues in back. And the 5-link rear starts to misbehave at high spring rates, some find they have to disconnect rear bar or go much softer. Using M3 as comparison is somewhat misleading as M3 has true LSD and e9x non-M does not, in addition to wider track and other differences.

But also McP strut front end does not understeer more with stiffer springs/roll rates, up to a point. That is because the front geometry is such that softer means tire will lean away from the pavement under side loads and lots of roll, so more roll stiffness keeps contact patch optimal. Also less travel in front esp when lowered means you want front stiff as is reasonable. Then you just live with the limitation on rear spring rates and perhaps sub optimal wheel freq F/R.

That is not to say there aren't some weird rates out there with aftermarket springs. But a more relevant quantity than wheel rates is natural freq., which they go into later in the thread. It is not as far off as just spring rates alone appear since you're doing sqrt(k/m) and not just k, and m is sprung corner weight.

Gotcha, I was doing some reading in another thread (here: http://www.e90post.com/forums/showthread.php?t=719649) and they were pretty much saying the same thing. Stock suspension location in the rear basically tries to rip the subframe from the car the higher spring rate you go. They discuss converting the rear to a true coilover, but it looks very costly, and it looks like you would have to do some serious reinforcement in other places. Plus from what I understand, the McPherson struts on our cars like to lose camber with significant body roll, so limiting front side to side motion with stiffer front springs would help with that.
Thanks for the input.

[feuer]

Originally Posted by shirtpants_

Gotcha, I was doing some reading in another thread (here: http://www.e90post.com/forums/showthread.php?t=719649) and they were pretty much saying the same thing. Stock suspension location in the rear basically tries to rip the subframe from the car the higher spring rate you go. They discuss converting the rear to a true coilover, but it looks very costly, and it looks like you would have to do some serious reinforcement in other places. Plus from what I understand, the McPherson struts on our cars like to lose camber with significant body roll, so limiting front side to side motion with stiffer front springs would help with that.
Thanks for the input.

Correct, stiff rear springs could damage the floor which was very common on e36 and e46. The e90 is better and doesn't suffer from this issue. Tuned 335i and 335d make a lot of torque and if this was an issue like on the older chassis there will thread all over the forum. I've not heard of even a single flool failure on e90.
To run true coil over in the rear is not very expensive, or at least not more expensive than current coil over setups, just the parts.
What will make it difficult and costly is reinforcing the rear wheel well where the strut mount will go.
Regarding loosing camber on the front due to the McPherson strut design more negative camber will help more than anything, Including stiff springs and roll bars.
If you run stock camber but stiff springs on the front end then yes the body will not roll but it will roll the tire, it will skid and understeer.
Others might disagree but from what I seen on this chassis more negative front camber help more than stiff springs.

[ajsalida]

Originally Posted by feuer

Correct, stiff rear springs could damage the floor which was very common on e36 and e46. The e90 is better and doesn't suffer from this issue. Tuned 335i and 335d make a lot of torque and if this was an issue like on the older chassis there will thread all over the forum. I've not heard of even a single flool failure on e90.
To run true coil over in the rear is not very expensive, or at least not more expensive than current coil over setups, just the parts.
What will make it difficult and costly is reinforcing the rear wheel well where the strut mount will go.
Regarding loosing camber on the front due to the McPherson strut design more negative camber will help more than anything, Including stiff springs and roll bars.
If you run stock camber but stiff springs on the front end then yes the body will not roll but it will roll the tire, it will skid and understeer.
Others might disagree but from what I seen on this chassis more negative front camber help more than stiff springs.

Yeah I do not agree with what they were doing as far as super stiff front springs, just explaining the thinking behind it. Anybody going that stiff already should have maxxed out camber, solid bushings, etc. I would never want super stiff springs without more camber, and then never want super stiff springs unless for a track-only car. And maybe not even then. I can't imagine how uncomfortable it would be on the street.

However I can attest to the fact that a stiffer front bar does not add understeer, at least on a XI, with only pins removed for more camber, maybe -1.5 deg total each side. I have UUC front and 22.5 mm M3 rear, very nice and neutral, very slight bias to the rear. I do not track the car though, or push it super hard on the street. With the stock bars, the front just plowed like a pig. Weight transfer + camber change front would just wash out. Then I added the rear M3 bar, far too much snap oversteer on initial turn in, I mean stupid amount. You'd straighten it out, try to give it some gas, front washed out again. Really a handful though kind of fun. For a day or two.

Added the front UUC and very nice balance. This is with Koni yellows and eibach pro-kit springs, which are relatively soft for lowering springs, close to RWD ZSP rates.

As for switching over to full coil-overs in the rear, pretty much you're talking full welded cage with it tied into the rear shock mounts, plates welded in, etc. That shock tower area can no way sustain springs loads without massive reinforcement. Again appropriate for track only cars after you've done everything else and looking for that last bit.

[shirtpants_]

Thanks guys, this has cleared up alot.
So I Basically just need to decide how far I'm willing to go/how much money do I really want to spend.
My car is a DD, and wont see more than probably 1 or 2 track days a year at this point, so a rear conversion to
coilover and all the headaches that come with it, is out of the question.

I'm liking Ground Control's kit. It comes with camber/caster plates, adjustable dampers,
and they'll let you pick the spring rate. That's probably what ill end up doing.
I'll probably only lower the car about 1/2" and go with something around 180-200 lb/in front, and around 550 lb/in rear.
Thanks

PS. It's my understanding that when you lower the car, you lower the center of gravity, but at the same time, you lower the roll center even more. This increases the roll couple distance, actually resulting in more body roll(if spring rates stay the same). If someone made some kind of roll center adjuster, or control arm spacing (I'm not entirely sure how you would adjust this on our cars, still trying to figure it out.) Then we would get a higher roll center that is closer to the cars center of gravity, resulting in a smaller roll couple, ultimately reducing the body roll. From what I understand, this means we could run softer front spring and softer front swaybars, while still reducing body roll. Of course, this probably doesn't matter on a street car, and if you're building a track car, you can afford to have custom parts fabricated. GC makes a roll center adjuster kit for the e36(http://www.ground-control-store.com/.../II=939/CA=232), but as far as I know, no company makes something like this for our cars.

[feuer]

Originally Posted by shirtpants_

I'll probably go with something around 180-200 lb/in front, and around 550 lb/in rear.

Those are similar to stock rates.

[shirtpants_]

Originally Posted by feuer

Those are similar to stock rates.

I don't want anything that's gonna rattle my teeth on the way to work in the morning. Are those rates still too soft, even for daily driving? I'm not entirely sure how much of a difference 10 or 50 lb/inch will make. what I came up with was about a 25% increase of the wheel rates over stock.

[feuer]

Originally Posted by shirtpants_

I don't want anything that's gonna rattle my teeth on the way to work in the morning. Are those rates still too soft, even for daily driving? I'm not entirely sure how much of a difference 10 or 50 lb/inch will make. what I came up with was about a 25% increase of the wheel rates over stock.

Not much considering the 335i is heavy car.