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Thread: Coil over spring length and rates

  1. #1
    Join Date
    Apr 2004
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    Boston
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    1979 323i BMW. 2003 GTI 20 V Turbo

    Coil over spring length and rates

    Guys,

    I have been sourcing the parts to make Coil Overs for my 323i.
    Any idea what the Length of the coil over springs should be?
    Are 7 Inch ones ideal?

    What spring rates are people using??
    I was thinking 350 front 300 rear.....
    "..Horsepower is a measure of work done over time, or the rate at which work is done."




    http://www.cardomain.com/ride/579694/1


  2. #2
    Join Date
    Sep 2007
    Location
    Seattle, WA USA
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    264
    My Cars
    82 320i E21, 95 Porsche 911 Carrera, 95 MR2 NA
    Front = 400lbs and 6inch
    Rear = 350lbs and 7inch

    Front are higher due to M20 and wanting to track the car from time to time.
    Marco = 82 320i Hennarot - 95 Porsche 911 Carrera - 95 Red MR2 NA Hardtop

  3. #3
    Join Date
    Apr 2004
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    1979 323i BMW. 2003 GTI 20 V Turbo
    Quote Originally Posted by Marquinho View Post
    Front = 400lbs and 6inch
    Rear = 350lbs and 7inch

    Front are higher due to M20 and wanting to track the car from time to time.
    Why are the fronts one inch shorter than the rears?
    "..Horsepower is a measure of work done over time, or the rate at which work is done."




    http://www.cardomain.com/ride/579694/1


  4. #4
    Join Date
    Aug 2006
    Location
    Michigan, USA
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    1,844
    My Cars
    E21, 04 STI, LS6 Sonoma
    I got 7's for both. 400/350. Reason i got the same lengths is so i could swap them if need be. 400# is right around the limit for billie sports i've heard. I havent had much driving on this setup, but the few times i drove it it felt stiff (good :-)). I am not DDing this car either, if i was it would probably be 350/300. Eibach springs arent all that expensive and you've got a lot of options.

  5. #5
    Join Date
    Sep 2007
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    Seattle, WA USA
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    264
    My Cars
    82 320i E21, 95 Porsche 911 Carrera, 95 MR2 NA
    Quote Originally Posted by jjgbmw323 View Post
    Why are the fronts one inch shorter than the rears?
    It allows for more flexibility when lowering the front - with 7" it seems to always too close to the end of the threaded sleeve. Plus, I don't like the look of raised front suspension compared to the rear. Actually, look at the picture of my car below - it is almost all the way down with 7" springs...
    Marco = 82 320i Hennarot - 95 Porsche 911 Carrera - 95 Red MR2 NA Hardtop

  6. #6
    Join Date
    Nov 2007
    Location
    Colorado Springs, CO
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    253
    My Cars
    '83 320is PROJECT
    if you do a search there was a really good thread on this a while back.

  7. #7
    Join Date
    Oct 2004
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    North Idaho, sometimes
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    No BMW's currently
    Length depends on how you are mounting the sleeves. If you just put them on the existing spring perches you will have to use shorter springs which has it's drawbacks. Shorter springs coil bind easier on compression (severe bumps) and loose contact with either end when the suspension goes into droop (coming off a bump) causing severe slamming when the suspension comes back down and the spring makes contact again with the spring perches.

    Consider this, an 8" spring made from .5" dia. wire has only 4" of travel before it coil binds so the spring rate had BETTER be quite high. 400lb springs will compress about 2" at rest with a m20 powered car leaving only 2 inches of travel before the spring is coil bound and you have no more travel available-this works on a smooth race track but your tires will skip across the bumps on an even slightly rough road causing the car to skitter toward the outside of the corner-been there, done that and didn't like it at all.

    When you consider that the regular Eibach springs are about 100lb in the front and about 95/180 progressive in the rear and they handle quite well I truly wonder why so many people choose such stiff springs.

    It will be a while before I get my '82 on the road so I can't give direct feedback on how my choices will work so stay tuned but this is what I have in mind.

    My '82 will be driven very often and I want to keep my kidneys in their original locations so I will be using 220lb front springs 12" long mounted to collars welded to the tubes to position them roughly in the middle of the suspension travel with the car at the desired ride height so I will have some adjustability up and down from there. These springs will be compressed about 3" at normal ride height allowing some droop before they come away from the perches and have plenty of travel for compression.

    Rears will be 200lb 10"

    There are basically two theories regarding spring rates.

    One side thinks using stiff springs is the way to go to set the chassis up and get your desired roll rate and uses lighter sway bars to fine tune the balance of the car. This is probably the easiest way to set up a chassis if you have an adjustable rear sway bar. This works well on smooth surfaces but as noted above it can cause problems on rough surfaces.

    The other side thinks softer springs are the way to go using larger sway bars to set the roll rate of the chassis. This works well on both smooth and rougher surfaces as the soft springs allow the tires to keep in contact with the pavement better.

    Shock valving is important to make any suspension work correctly.

    ken

  8. #8
    Join Date
    Apr 2004
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    Boston
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    1979 323i BMW. 2003 GTI 20 V Turbo
    Quote Originally Posted by kdanielson View Post
    Length depends on how you are mounting the sleeves. If you just put them on the existing spring perches you will have to use shorter springs which has it's drawbacks. Shorter springs coil bind easier on compression (severe bumps) and loose contact with either end when the suspension goes into droop (coming off a bump) causing severe slamming when the suspension comes back down and the spring makes contact again with the spring perches.

    Consider this, an 8" spring made from .5" dia. wire has only 4" of travel before it coil binds so the spring rate had BETTER be quite high. 400lb springs will compress about 2" at rest with a m20 powered car leaving only 2 inches of travel before the spring is coil bound and you have no more travel available-this works on a smooth race track but your tires will skip across the bumps on an even slightly rough road causing the car to skitter toward the outside of the corner-been there, done that and didn't like it at all.

    When you consider that the regular Eibach springs are about 100lb in the front and about 95/180 progressive in the rear and they handle quite well I truly wonder why so many people choose such stiff springs.

    It will be a while before I get my '82 on the road so I can't give direct feedback on how my choices will work so stay tuned but this is what I have in mind.

    My '82 will be driven very often and I want to keep my kidneys in their original locations so I will be using 220lb front springs 12" long mounted to collars welded to the tubes to position them roughly in the middle of the suspension travel with the car at the desired ride height so I will have some adjustability up and down from there. These springs will be compressed about 3" at normal ride height allowing some droop before they come away from the perches and have plenty of travel for compression.

    Rears will be 200lb 10"

    There are basically two theories regarding spring rates.

    One side thinks using stiff springs is the way to go to set the chassis up and get your desired roll rate and uses lighter sway bars to fine tune the balance of the car. This is probably the easiest way to set up a chassis if you have an adjustable rear sway bar. This works well on smooth surfaces but as noted above it can cause problems on rough surfaces.

    The other side thinks softer springs are the way to go using larger sway bars to set the roll rate of the chassis. This works well on both smooth and rougher surfaces as the soft springs allow the tires to keep in contact with the pavement better.

    Shock valving is important to make any suspension work correctly.

    ken


    Ken very interesting.
    I have my 323i set up on Racing Dynamic Springs and Bilstein Sports with BMP front and rear bars; I feel that the Racing Dynamic Springs are not as high spring rate as I would like.


    My current plans are to run my Dinan rear adjustable subframe, and some version of coil overs. My sway bars are stock I do have a Dinan front sway bar but i have heard that it will not work on my older style control arms..so my sways are stock at the moment. I do have the alpina lower bar that i will be having installed. Also, I have fully threaded bodies in the front that are left overs from a UK motorsport company and I am going to get them welded into the strut housings. The rears I am using TEP threaded collars.

    So I have no idea what length of spring to use. I was thinking 7 inches and some some for sale on the AFCO website.
    "..Horsepower is a measure of work done over time, or the rate at which work is done."




    http://www.cardomain.com/ride/579694/1


  9. #9
    Join Date
    Apr 2005
    Location
    Corona California
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    4,313
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    79 320/6 81320i 65malibu
    As Ken said ..... (no need to repeat)

    I will add that the first thing you have to decide is "What am I building this car for?" Same as when you build a motor.

    So:
    1) If it is an all out track only car you need to go in one direction
    2) If you want a track/Auto-x car then you need the ability to change springs easy because one set-up will not work really good for both.
    3) If you want an auto-x/street car then another set-up will be needed
    4) Almost forgot - what are you doing to the weight of the car? You need to get this done and finished before you start wasting money on springs and shocks. Every time you take weight off, add weight or move weight, you change the needed spring rates.

    We are still playing with the track car - right now it is TOO stiff very tail happy - so I am going to go to 425 springs in the front and 350 springs in the rear next time out for auto-x.

    I am going with 8" springs front and back so I can change them around, and going along with what Ken was saying about coil bind. On the Race Car 4 to 5 inches of total travel, adding compression after ride height till coil bind plus droop from ride height, will be more then enough travel.

    I am not even thinking of working on the shock set up until we finish working on the weight of the car, settle on spring weight and ride height.

    Now on the 80iS I have 350lb springs in the front and 250lb springs in the back. It is a daily driver and Auto-x car, it is a little tooth jarring on the freeway for long trips, but we do not use it for long trips much, and it works well at the track - not too tail happy with the 205's up front and the 225's in the back - so I'm leaving it as is.

    I'll give you up dates as we get the race car sorted out.

    Always FUN TO DRIVE - Build Thread & Tech info - 79 320/6 track car build thread -- Videos of track car -Adam in car Auto-x video - Start-up video - 4/2011 Adam's TOP BMW time San Diego BMWCCA - 4-5-15 Dyno break-in run new M20B25 - Exhaust Thread - Link

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