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      10-24-2015, 11:58 AM   #1
shirtpants_
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Spring rate questions

Hey guys, I've been looking into aftermarket springs and dampers for my '11 335i m-sport. I know this is m3post, but our suspensions are pretty similar, and aftermarket spring rates between the m3 and 335 should be pretty similar as well.(Plus I figured you m3 guys might be a little more interested in optimal handling)
Bottom line, I think theres something wrong with most aftermarket spring rates in relation to the front to rear ratio

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:

Quote:
Originally Posted by Orb View Post
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 from the factory, 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

Last edited by shirtpants_; 10-24-2015 at 12:04 PM.
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      03-30-2016, 08:01 AM   #2
burnrbr
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Bumping this old thread because I have the same question around front and rear balance based on aftermarket springs versus stock. I just upgraded to Swift Spec R springs in my M3 E90 ZCP and the rear feels loose compared to the front for initial turn in.
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      03-30-2016, 07:56 PM   #3
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Thread got more action on e90post: here's the link: http://www.e90post.com/forums/showthread.php?t=1187724


After a lot more research, it seems like the best thing to do is to go as stiff as possible in the front, and deal with the F/R imbalance with sway bars or damper settings (if there is one).

Due to the front mac strut geometry, once you lower the car, the lower control arm starts to point further up(measuring from the subframe to the spindle) than at stock height.

This means that during cornering, as the strut compresses on the outside wheel, you actually lose camber. At stock height your camber gain follows an arc. It easiest to understand if you think of it as a U shape. It's my understanding that on the stock car you get some camber gain under compression, but once you pass over the optimal angle (the bottom of the U) you start to lose camber. Once you lower the car past a certain point, you will only lose camber when you corner. At the end of the day, you want the angle formed between the LCA and the strut to be less than 90 degrees for camber gain.
That's probably why some people are going for over 4 degrees of negative camber on the track, and still wearing the outside of the tire much more than the inside
Lowering the car also messes up the roll centers as well. The only option we have to correct both of these is bimmerworld's 1,100 dollar kit that replaces the UCA, LCA, and tie rod ends with spherical joints with rod ends.

This is why you want to go as stiff as possible in the front, to limit travel so you only experience the small amount of camber gain our cars are capable of.

The attached picture should help explain
Attached Images
 

Last edited by shirtpants_; 03-30-2016 at 08:03 PM.
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