Rear coilover shock option - QA1

E28 technical advice asked and given! Troubleshooting, modifications and more.
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iamcreepingdeath
Posts: 442
Joined: Dec 28, 2008 10:03 PM
Location: Mayer, MN

Rear coilover shock option - QA1

Post by iamcreepingdeath »

The background story: I have built my own homebrew coilover suspension for my E28 (Koni yellow adjustables and Eibach 7" 400 lb/in front, and Bilstein yellow sport and Eibach 8" 350 lb/in rear), and the rear bilsteins aren't adjustable, and just don't have enough damping to handle the firm ride I am looking for. So therefore, I started searching for rear shock options. As we all know, koni yellow adjustables aren't available for the E28, so I needed to think outside the box.

After looking at several companies that make E28 specific coilovers (for example BC Racing, but they offered a 6-8 week lead time, and shocks from Taiwan), I landed on QA1 single adjustable coilover shocks: https://www.qa1.net/automotive/suspensi ... ver-shocks

These shocks have a great reputation with American car enthusiasts and are made in America.

From the drop down menus on the site I selected single adjustable, bushing mount, and 13" compressed, 19.5" extended. This is based on measurements I took from the rear suspension at my desired ride height. I found this shock for about $200 a piece on Summit Racing: https://www.summitracing.com/parts/qa1-ds702

You will also need this stud top shock mount: https://www.summitracing.com/parts/qa1-ss110sdm

And prob a good idea to get the wrench set: https://www.summitracing.com/parts/qa1-t114w

As it turns out, the QA1 stud top mount works perfectly with the stock E28 mount.

Image
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The next issue is getting the right sleeve insert for the lower shock mount bushing. Some sleeves are provided, but with the wrong ID.

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What you want is a sleeve that is 3/4" OD, 9/16" ID (the correct size for the factory M14 bolt), and 1.25" long. I ordered these from Mcmaster: https://www.mcmaster.com/2868T139/

Alternately, if you are handy, you can just drill out the provided sleeves with a 9/16" drill bit, although ideally you want to do this with a lathe.

Here is my method of bolting the shock to the trailing arm:

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Those are all standard 1/2" washers (which happen to fit quite perfectly on the M14 bolt).

The next thing I noticed is that with my 8" long spring, the spring seat on the QA1 didn't thread up the shock quite enough. Here is a comparison of the Bilstein w lower spring seat located where I had it set on the car vs the QA1 shock. You can see the threads on the QA1 shock body don't go high enough.

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To try to illustrate this better, here is a pic w the lower spring seat on the QA1 placed where it needs to be. Notice it is above the threads and not engaged on the shock.

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Another issue is the top spring seat on the QA1. It is the type that slips onto the shock from the "side" so to speak (I don't have a good pic), it has a side cutout so you can slip the top spring seat onto the spring from the side without having to remove the top shock mount. What this means is that at full droop, you don't want any spring slack, because the top seat can bounce right out of there.

The solution for both of these issues is helper springs. I picked up a set of helper springs and seats from Summit here:
https://www.summitracing.com/parts/hyc-cs100
https://www.summitracing.com/parts/eib-spacer250

The helper springs add spring stack length so that the lower mount can engage fully on the shock body threads, and also takes up any spring slack at full droop. Here it is all put together (stock E28 top shock mount not installed):

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You can also see in the above pic the flat steel ring that is supposed to go between the spring and lower spring seat (I installed it later), but this helps the lower seat spin for ride height adjustment. It is a good idea to put some heavy grease between the steel "washer" and the lower spring seat. QA1 also offers actual thrust bearings for this same reason, to make it even easier to adjust ride height under full spring load.

I put the car on the ground and adjusted the height to where I wanted it, and dialed in the damping (9 out of 18 clicks, so half way), and seems to ride great so far! Definitely holds steady at 120 mph!! :shock:
Last edited by iamcreepingdeath on May 18, 2022 8:39 PM, edited 4 times in total.
LeiseyJr
Posts: 1525
Joined: Dec 22, 2013 10:11 PM
Location: Houston,Tx

Re: Rear coilover shock option - QA1

Post by LeiseyJr »

Very cool!
I retrofitted Porsche 914 Rear shocks to mine, when those head out. This will be good route to go.
adam_poll
Posts: 206
Joined: Jul 23, 2014 8:19 PM
Location: Kitchener, Ontario, Canada

Re: Rear coilover shock option - QA1

Post by adam_poll »

Nice job!

Those should hopefully match up well with the Koni's in the front and it's awesome they mounted up pretty easily. Hopefully they have enough adjustment to control the springs well, it looks like your ride height should be pretty close to the bilsteins with the helpers locked out.
iamcreepingdeath
Posts: 442
Joined: Dec 28, 2008 10:03 PM
Location: Mayer, MN

Re: Rear coilover shock option - QA1

Post by iamcreepingdeath »

adam_poll wrote: May 18, 2022 10:29 AM Nice job!

Those should hopefully match up well with the Koni's in the front and it's awesome they mounted up pretty easily. Hopefully they have enough adjustment to control the springs well, it looks like your ride height should be pretty close to the bilsteins with the helpers locked out.
The shock adjustment has 18 settings, which range from fairly soft to fully cured concrete, so that should be sufficient for these springs.
Shawn D.
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Re: Rear coilover shock option - QA1

Post by Shawn D. »

Great work!
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