Suspension vs. Power Mods

[mikeo]

Spook,
It is my belief that if you don't plan on regularly doing Driving Events, autocrosses, or track days coilovers are probably more than needed for good street handling. My E36 M3 had an increasingly more serious suspension path, from stock to Ground Control coilovers with all the extra bits. The best for street performance, in retrospect, was in the middle of that progression. H&R OE Sport springs (which are apparently no longer available) only lowered the car ~1/2" but when coupled with Koni Sport single adjustable struts/shocks, and UUC SwayBarbarian bars (on softest settiings) gave the M3 fantastic but still supple street handling. It took Hoosier r-comps at autocrosses to show the weaknesses of that setup (too soft spring rates for race tire grip).

If you choose to mod the suspension I would suggest

1) Have a clear picture of how you intend to use the car and what handling goals you want to attain

2) Talk extensively to TC Kline (or Pete), Terry Fair at Vorshlag (AST), or your favorite KW dealer about #1 above and the best way to get there. Be very careful about selecting spring rates for coilovers.

[mikeo]

About the 135i springs

According to TC Kline, who put 135 springs on the test bench, the rates were:

Approximately
Front 120 lbs/in
Rear 340 lbs/in

In my opinion, these OE spring rates are designed to increase rear load carrying capacity and induce understeer handling characteristics. It's my guess that 280 -300 lb/in front springs would pretty much eliminate understeer without changing anything else, for the street.

When I spoke to TC Kline, which was before Charles1 had his c/o system installed, he reported that they had just tested a 135i on track with 400 lb/in front springs and 300 lb/in rear on their Koni double adjustable c/o with great results--and the ride was not too stiff. Of course, "too stiff" is subjective and may be fine for TC who builds and drives BMW race cars, but not for you or me. I will say 400/300 is relatively soft in my experience.

[Spook]

Is blasting up 14 then over to up the Pecos river for fly fishing a clear handling requirement? Smiles.. thanks Mike. I get the message that I'll have to work a bit harder to get a good balanced street setup. I look forward to talking to those folks.

-Andy/Spook

[mikeo]

I was remiss in talking about spring rates without dicussing the importance of 'motion ratio' and how that impacts the 'wheel spring rate', which is what really matters. Most recent BMWs have a front motion ratio of about 1:1, while the rear ratio is more like 1:.5. or when the rear wheel moves up 1" the spring compresses 1/2" due to its inboard location relative to the wheel centerline. This obviously impacts the spring rate the wheel/tire experiences and helps explain why rear spring rates are usually higher than the front. In other words, the wheel acts as a lever on the spring reducing its effective spring rate by the time the rate reaches the wheel.

[adc]

Originally Posted by mikeo

The best for street performance, in retrospect, was in the middle of that progression.

+1. Best street setup I've had was the OEM ZHP package with UUC sways set medium in front, stiff in rear and a little extra camber (by removing the strut locating pins).

At the track on street tires it was just shy of perfect. Things would probably would have been different on race tires, but I never got to that stage.

That's why I have such high hopes for the BMW Performance suspension, perhaps coupled with a good set of sways.

[Charles1]

Still loving my TC Kline suspension setup. They are very helpful ppl.