BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Converting a non-zsp to zsp suspension (335 stuff on a 328 whaaaaat?)

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      04-29-2015, 05:08 PM   #1
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Drives: 2007 e90 328i 6MT
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Converting a non-zsp to zsp suspension (335 stuff on a 328 whaaaaat?)

Hi Guys!

I just thought I would post up some of my findings from this past weekend about non-zsp vs zsp suspension.

Back story:
I have a non-zsp e90 328i 6MT, which is great, but not so great on the canyon roads, and it rides about as high as my old Jeep. (not really, but compared to some of the rides on here, it rides high). So I wanted to lower the car, and since I just turned over 100k miles on it, I figured it would be a great time to do a full suspension swap. I looked at coilovers, and lowering springs, and all the options, from koni yellows to a bilstein club kit. Problem was that post 100k, I don't really feel like putting $3k worth of suspension upgrades into my daily driver/thrasher. (Plus my two wheel hobbies seem to be stealing more from my fun fund lately)

So I searched around here, to see what people were doing, and I stumbled upon a set of front/rear dampers and springs from a RWD ZSP 2009 335i. All for $200! Best part was that this setup had just over 7500 miles on them. Basically new! So I bought them of course without much thought.....big mistake.

If I would have done proper research before hand, I would have found that BMW specifies different springs for MT and AT cars and 335 and 328 to account for the weight differences. Obviously, I did not, and ended up with the stiffest set of stock springs from bmw (335i AT on my 328i MT).....great.

BUT!!! This is where some enginerding came in. My initial assumption was that the ZSP dampers (front and rear) were probably shorter than stock non-zsp dampers (since they are different part numbers and more expensive than stock dampers). If this was the case then I might just possibly be able to achieve some kind of lowering. And for reference, I was looking for a moderate lowering, approx stock sport height (but that has now changed...read on).

Also when searching the forums I could not find any info on the damper length of the new shocks, or spring length. So after much back and forth deliberation, I said f it, I'll just rip out the old stuff and see whats up.

So I did, and here I am to share!

Digging Deeper:
When I found out about all the different spring rates, and lengths, I was pretty bummed, but hey I was only in this $200. But spring lengths and rates are a starting point, and are only half of the equation when it comes to the height at which your car sits. The preload on the spring (after it is compressed in the strut assembly) is what determines the ride height. The assembly of the strut (damper, perch, and spring) determine the initial preload on the spring (once installed) and then the weight of the car then determines the final height. So math....

I found on this incredible site:
ZSP springs (335i) D7 spec - 145lbs/in (front)
non-ZSP 328i C4 spec - 120lbs/in** (front)

**- this was an assumption, based on other BMW springs and could be wrong

So when I took out my old dampers and springs, I was able to measure them side by side with the 335i ZSP suspension. Here are the results:

So as you can see the dimensions are as follows:
-Damper travel: 3.75in
-D7 spring length: 12.5in
-D7 rate: 145lbs/in

-Damper travel: 4.25in
-C4 spring length: 14in
-C4 rate: 120lbs/in (assumed)

Also after even more research, I thought I might add some Supreme Power lowering perches to the equation, because why not. (Also thank you so much for all your help at Supreme! So sorry about all the messed up payment info though!)

So here is a quick and dirty on my calcs:
-The lower spring perch is about 1.5in lower than the damper seal (same on zsp and stock)
-The new upper SP perch is about 2.5in higher than the top of the usable damper travel (1.5in taller than stock)

Total spring height install on strut: 7.75in (3.75+2.5+1.5)

Stock strut spring length: 6.75in (4.25+1.5+1.0)

Since we have these two lengths we can find the preload on each strut prior to being installed on the car:
12.5in-7.75in = 4.75in of preload (145lbs/in*4.75in preload = 688.75lbs)
14.0in-6.75in = 7.25in of preload (120lbs/in*7.25in preload = 870.00lbs)

So essentially since the zsp shock was shorter, combined with the added perch reduced the preload on the spring. This is important because now we take into account the weight of the car.

Again an assumption: 3400lbs (2007 328i N52 6MT) only upside is that this particular setup is 50.5/49.5 when it comes to weight distribution. So for sake of easy calcs we'll assume 50%

This means that the front struts see about 1700lbs
zsp: 688.75lbs/strut (2 struts) = 1377.5lbs total preload (pre installation)
stock: 870lbs/strut (2 struts) = 1740lbs total preload (pre installation)

At this point I realized that I am probably off slightly on the spring rate of the C4 springs because when you set the car down, the spring obviously compress. But I knew I was close on the ZSP data.

Since there was a 322.5lb difference (1700-1377.5lbs) this would mean an extra 161.25lbs going into each strut, adding to the preload of the spring (read: lowering the car). 161.25lbs/(145lbs/in) = 1.12in more of preload lowering.

So this means that my car "should" sit about 1.12in lower into travel once it is on the ground. Not necessarily 1.12in lower than stock, just 1.12in past the static height of the strut assembly before the car is taken off jack stands.

for reference:
-Wheel gap on stock suspension: 55mm
-Wheel gap with new suspension: 45mm

So I did end up lowering the car 10mm (sorry for the unit switch). Which means that you can't tell at all haha. (front and rear lowered about 10mm each) As of now, I have decided to find some Eibach Pro Kit springs to properly set this car up.

The moral of this story is that ZSP dampers are shorter than stock dampers (and seem to have a stiffer tune, but it might just be the stiffer springs too, much less body roll). They will work better with shorter springs, and that is key. Also ZSP springs are shorter than stock springs and if you are smart and do the research, it is possible to get a "budget" sport lowering by going to a 335i setup on a 328. I did succeed in lowering the car, while keeping BMW dampers at all 4 corners. I tried to search for a list of all BMW springs and the associated rates, but to no avail. If someone has that, I think it would be a great resource.

Also I understand that I probably have a ton of rounding error, so sorry for that. But I hope you get the gist of it. Also I think this explains more of why your car will sit lower with SP perches by lengthening area where the spring sits on the strut.

I will update and add to this as I find out more info. But feel free to add to what I have here/your personal experiences.

Thank you! I'll be posting pictures as soon as I can
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Last edited by pbirsche90; 04-29-2015 at 05:22 PM. Reason: Adding Photos
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328i suspension, lowering perches, suspension, zsp

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