Calculating spring length - Mercedes Benz SLK Forum

SLK R170 General Discussion SLK 200, SLK 230K, SLK 320, SLK32 AMG

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#1 Old 11-23-2013
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Calculating spring length

I've just had new springs on front and back of the car (R170) and the front is too high.

Does anyone know exactly how to calculate how much to cut off the front springs to achieve the exact ride height I want?

I don't think it's as simple as cutting off e.g. 1cm of spring height to reduce ride height by 1cm.

My guess is :

Spring length in car with wheels on ground = x cm

No of coils on spring = y

Height per coil (z) = x / y (cm / coil)

c = desired reduction in ride height

Coils to remove = c / z


Before anyone worries that the springs won't sit in their seats correctly because they should have a flattish base, which would be lost by cutting off coils, the previous and current springs do not have flattened bases and sit very will within their seats.
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#2 Old 11-23-2013
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before you modify your springs, i highly recommend you contact the company that produced the spring and find out if there are any dead coils in the spring. normally when springs are made, the manufacturer makes them with exactly the number of coils that are needed to have the specified spring rate. modifying the spring could result in changing the spring rate giving you terrible ride quality. also, when and if you modify the spring, it must be done a certain way as to not compromise the metal and again ruin the spring rate. i suggest that if you want to go lower, or want to be able to control your ride height down to the cm, do it the right way, and get adjustable coilovers, yes they can be much more expensive, but it'll be doing it the right way, and you'll get the respect you deserve
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#3 Old 11-24-2013
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Is it worth checking with the garage you brought them from/ had them fitted by first?

I had new front springs fitted as the old ones were broken- I think the flats you mentioned- new ones were identical to my old ones in ride height and the whiz wheels just clear the wheel arch

Can't say I noticed any difference in the ride or steering between the new and old springs
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#4 Old 11-24-2013
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Springs

AJ had new springs on my Lotus years ago and it looked and was high. However the springs were new unused and needed a few days to settle which they did.
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#5 Old 11-25-2013
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I'll give it a few more days to see if they settle, then I'm going to get the angle grinder out if they're still too high.
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#6 Old 11-26-2013
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How high do you mean by too high? MB uses pads/seats of various thicknesses to adjust the final stance of the car. Supposedly 8mm, 13mm, 18mm & 23mm in thickness. (There should be 0, 1, 2, or 3 tabs on the side of the shim to indicate which one.) That could be a bit over 1/2" at the max differential. (I've seen part numbers indicating this, but have never verified it myself.)

Speaking as an engineer, I cringe when I see the "TV" mechanics "tourching" a spring. Generally it means that you have the wrong springs.
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#7 Old 11-28-2013
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Originally Posted by lilred320SLK View Post
How high do you mean by too high? MB uses pads/seats of various thicknesses to adjust the final stance of the car. Supposedly 8mm, 13mm, 18mm & 23mm in thickness. (There should be 0, 1, 2, or 3 tabs on the side of the shim to indicate which one.) That could be a bit over 1/2" at the max differential. (I've seen part numbers indicating this, but have never verified it myself.)

Speaking as an engineer, I cringe when I see the "TV" mechanics "tourching" a spring. Generally it means that you have the wrong springs.
Thank you. I will contact MB dealer and find out about thinner shims. Will get my mechanic to take springs out and check size of existing ones first.

The ride height is too high by approx 2" - height difference from wheel centre to top of arch from the back, where the wheels fills the arch beautifully, and the front where the wheel looks completely wrong in the arch - if you see what I mean.

The rear matches another SLK I measured a few days ago where both front and rear wheels fill the arches just right, with more or less equal spacing from front, rear and top of wheel to the arch.

Last edited by ajberrow; 11-28-2013 at 05:58 AM. Reason: Added details
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#8 Old 11-28-2013
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AJ - might it be worth a quick trip down to Stuttgart garage in Ponty (up the hill next to the station) and ask them to take a look. You could always ask as though you are looking for a quote for the work.

Steve
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#9 Old 11-28-2013
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We build air suspansion for cars.
Usually the rate is 1/2.
If you want to drop your car 2cm you should shorten the compressed spring lengrh araound 1 cm.

Now this is the tricky part:
When you take the spring out it is going to be longer than while the load of the car on it. Therefore you should carefully calculate to achive the length that I gave.

Also spring usally settle after using around 1000kms about 1cm lower than first installed. But if you buy springs from KW they won't settle at all. Quality is important in this case. Take this into account as well.

Good luck.
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#10 Old 11-29-2013
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Quote:
Originally Posted by ajberrow View Post
. . .The ride height is too high by approx 2" - height difference from wheel centre to top of arch from the back, where the wheels fills the arch beautifully, and the front where the wheel looks completely wrong in the arch - if you see what I mean. . . .
2" is a lot for some form of casual error. I'm happy with mine so I haven't done much in that regard other than look. I did once drop and stiffen an 81 Mustang I had. I'm not sure if they were Eibachs, or a private version, but when my mechanics got the car up, they had to call me to make sure that I had the right springs. The new ones were like 4" shorter than stock!! But these were stiffer and progressive! The car did sit 1" or so lower, as advertised, and much stiffer. I didn't have any ride/rub issues.

The main troubles I have with cutting springs . . . 1) Since you are not changing the rate (K value) you will now bottom out the car on bumps you didn't have issues with previously. 2) If you did not cold cut the spring, the material around where you hot cut it (or just used the heat to let the spring settle) will no longer have the same characteristics as original. 3) normally the end of the springs are wound tight or flat ground so they sit nicely in the supporting structure. If you cut those coils off, that will no longer be the case.

Did someone perhaps try to lower the vehicle previously and only got half way? Mine sits at 25.00" and 25.75" f/r. (Ground to arch over the center of the wheel. OEM everything.)
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