Topic: e36 cut springs

First post ever. Total rookie team

Anybody have experience on cutting springs for an e36 ? I gutted ours and its about 3" taller. Cutting some junk yard springs maybe? like something harder and lower. Just a starting point would help. 
Thanks, Bill

Re: e36 cut springs

I've not done them on a BMW, but coil springs should be coil springs, unless they are coil overs.

Whenever you cut a coil off of a spring, you not only make it shorter, but you raise the spring rate.  Just do not do too much at once.  I know on our Mustang one coil was just about perfect in the front and back, but you may wish to start with just taking off half a coil to see how much that gets you.

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Re: e36 cut springs

Similar situation, we initially clamped our springs for the first race to get a feel for the level of lower we could do.  Then we cut the rears and replaced the fronts with a higher rate, shorter spring.

I am by no means an expert but I believe racinrob's explanation of the change in rate is not 100% accurate.  The rate itself does not change if it is a straight rate spring but the "effect" rate changes because the number of coils involved in the sag/extension...so if you cut just enough coil to effective have the sagged suspension where you want it (close to stock ride height)....it would most likely feel very similar to "stock" but will bottom more quickly.

Re: e36 cut springs

How to pick a spring off the ground and calculate it's rate:

k = Gd^4 / 8D^3N

d = wire size (inches)
D = Mean Diameter (inches)
N = Number of active coils (always less than the total number of coils) 
D / d = Index correction
G = Shear Modulus of Material
k = Spring Constant

You'll notice that "number of active coils" is indeed a variable.
Cutting "dead" coils off...ones that are compresssed and touching with the weight of the body on it will not affect spring rate.  Anything further than this will stiffen the spring considerably...go slow.

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Re: e36 cut springs

The ones I've cut (non-bmw), have all ended up with 1 to 1 1/2 active coils cut (assuming its not the prius type spring that has 3 active coils total).  You want to sneak up on it.  Measure the floor to top of wheel opening to quantify what you are doing.  I'd cut 5/8 to 3/4 active spring, put it together, bounce the car and measure again.  It probably won't move much.  Then start cutting in 1/4 increments.  You cut a little, no change, cut a little, no change and then all of a sudden each small cut translates directly in reduced height.  At some point you stop needing to compress the spring on a mcpherson strut type so it starts to go pretty quick.  Cutting the spring will stiffen the suspension and in most cases buy you some negative camber.

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Re: e36 cut springs

Just to be clear, this is BEFORE or AFTER you put the cage in?

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7 (edited by Hamsa9 2016-02-12 10:40 AM)

Re: e36 cut springs

Guildenstern wrote:

Just to be clear, this is BEFORE or AFTER you put the cage in?

This is an important thing to note.  I would hold off cutting springs or working suspension issues until the car is caged, any engine swaps/fuel cells installs/ and additional bodywork and theme done, and with cool-box in.  All of these installs can bring you right back to only ~400lbs from stock weight, and could effect the f/r weight balance and roll center anyway.

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Re: e36 cut springs

ironwillie wrote:

First post ever. Total rookie team

Anybody have experience on cutting springs for an e36 ? I gutted ours and its about 3" taller. Cutting some junk yard springs maybe? like something harder and lower. Just a starting point would help. 
Thanks, Bill

I don't think you'll be able to do it. We tried on our e36 and it was impossible, the spring ends are tapered to fit the perches and as soon as you cut them they no longer rest/fit properly. I'm sure it's been done but it definitely didn't seem safe and it occurred to me while looking at it up on stands that you could potentially knock the spring out of place if you hit a curb or something while the suspension was unloaded.

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9 (edited by ross2004 2016-02-15 03:25 PM)

Re: e36 cut springs

You just need to make a spring pocket I believe for the rear to sit in. Pictures are out there.

Re: e36 cut springs

Spring pockets are your friends. smile

All the info you seek is on the Internet.  It has all been done.  Cutting springs works.

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11 (edited by Red Lemons Racing 2016-02-16 08:52 AM)

Re: e36 cut springs

Ours lifted up about 3 inches as well.  We cut out one lower section and had no issues,but  we eventually converted to coilovers that we got used

I do suggest you look for a used set of Ground Control's they are very easy to use and build.