spring rate help for 3.5 touring ;)

[gareth]

hi. i'm new to this forum but have had a good read around it and am impressed so far :bow:

i have a 325i touring with a 3.5 M30 conersion and i'm having a lot of fun with getting the correct suspension setup for it, understandably as it's a fairly oddball weight distribution!

currently it's about -40mm at the rear and -70mm at the front. a lot of the front drop is due to the engine and i also believe the unkonown springs are for a 4 pot touring as they're so soft! when braking, it hits the bottom out stops on the front! it also has koni adjustable shocks at present.

as the front is too low and too soft, i am thinking of doing a coilover conversion so i can fine tune easily but have trouble finding what spring rates to start off with.

does anyone know what rates the bmw springs are?

does anyone have and advice on spring rates for the heavier M30 at a -50mm ish drop?

cheers in advance!

[JRKOUPE]

common set up I think w/ GC coil overs is 350/500 up to 500/650 ish for track work..... f/r. You could do a 375/400 or similiar. Call GC in Ca USA.......

[gareth]

cheers mate.

do you know how these compare to the oem bmw spring rates?

[matt]

You should find out the actual weight of the front end, that will help you select the proper spring rate. Then get coilovers for the front, so you can set the ride height wherever you want.

[gareth]

is there a rule of thumb for spring rate V axle weight that i can follow?

i have a rough idea on the weights of the car in relation to a standard saloon. the touring model (i don't think you got it over there did you?) weighs about 100kg (220lb) more than the saloon but this weight is located over / behind the rear axle so makes a neglegable difference to the front axle weight. (most suspension kits use the same front springs for a saloon, cabriolet and touring). the M30 conversion adds about 60kg (130lb) over the front axle.

the all up weight of a similar converted 3.5 M30 touring i know of is about 1280kg (2800lb)

[matt]

There are rules of thumb... I believe you generally want a wheel rate (not spring rate) between .6 and .8 times the corner weight on a wheel for a high performance car. I really think coilovers are the way to go for you.

I am not 100% sure on that number though... it relates to a factor called natural frequency, which depends on the sprung mass of the car and the spring rate. Street cars generally have a natural frequency around 1hz, race cars are higher.

[1991 318is]

Spring Rates.

E30 spring rates

Stock M3 -> 140 -> 250
Dinan Sport -> 172 -> 300
H&R Race -> 315 -> 570
H&R Sport -> 185 -> 380
Eibach Pro-kit -> 102 -> 277
Eibach Race -> 160 -> 445
Ireland Stage 3 -> 315 -> 570
Tokico Kit - > 165 - > 265
Landshark Coil over - > 350 - > 450
Cosmo Coil Over - > 275 - > 375

I currently have 165# fronts, Bilstein Touring shocks, and a very stiff bar. Also 17" wheels with stiff side wall tires.The car corners flat yet is still compliant and has a close to OEM ride. It is lowered 1.5" front and 1.75" rear. Start with tires and wheels. Tires also have a spring rate. My car is way too soft with 14" weaves. I've been through three different setups to get one I like and that includes 2 different coil over spring sets. Forget coil overs unless you are tracking the car. They are for:
1) adjustable ride height
2) quick spring changes
3) setting corner weights
To put them on a street car and then not change anything for thousands of miles is a waste.

[matt]

E30 spring rates

Stock M3 -> 140 -> 250
Dinan Sport -> 172 -> 300
H&R Race -> 315 -> 570
H&R Sport -> 185 -> 380
Eibach Pro-kit -> 102 -> 277
Eibach Race -> 160 -> 445
Ireland Stage 3 -> 315 -> 570
Tokico Kit - > 165 - > 265
Landshark Coil over - > 350 - > 450
Cosmo Coil Over - > 275 - > 375

I currently have 165# fronts, Bilstein Touring shocks, and a very stiff bar. Also 17" wheels with stiff side wall tires.The car corners flat yet is still compliant and has a close to OEM ride. It is lowered 1.5" front and 1.75" rear. Start with tires and wheels. Tires also have a spring rate. My car is way too soft with 14" weaves. I've been through three different setups to get one I like and that includes 2 different coil over spring sets. Forget coil overs unless you are tracking the car. They are for:
1) adjustable ride height
2) quick spring changes
3) setting corner weights
To put them on a street car and then not change anything for thousands of miles is a waste.

Come on man... pay attention.

He has a freaking M30 in the car, which is a touring. If he tries to run any of those off the shelf springs (desiged for M20 powered coupes and sedans) he'll be bottoming out all over the place unless he puts some bigass spacers under the springs, and even then they'll be much too soft for the extra weight.

Coilovers will allow him to get the spring rates AND ride height he needs.