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      07-10-2022, 03:21 PM   #1
wyshyvanuk
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Coil-Over

If you've had any experiences with rear coil-overs, with or without additional mount reinforcements, for your Z4 or Z4M, please please add a comment. OK to PM me to avoid the possible "embarrassment" of admitting to a coil-over without reinforcement, on an open forum
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      07-11-2022, 09:05 AM   #2
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I have heard of people running without reinforcement and it being okay. That said before upgrading to my current MCS suspension I talked to some well known race shops. All told me not to run a coilover rear without reinforcement so I stuck with the divorced spring/damper set up in the rear.

Looking at the front shock towers and the rear shock towers they are both just thin sheet metal though so I am don't know why you can run a coilover in the front but not the rear. Maybe the front tower sheet metal is thicker and designed to take the load vs the rear not being?
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      07-11-2022, 09:39 AM   #3
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Thank you, sir!! I've exchanged some emails with InTrax. Their position is, that for street only use (that's me), the coil-overs are OK, but I'd rather not depend on that margin. Those city streets and surrounding highways are dangerous ;~). Intrax can supply a split rear suspension, but I'm not sure if, by doing that, I'll lose some suspension wonderfulness. I'll try to get some additional assurances from InTrax.
Was MCS able to supply an acceptable split solution?
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      07-12-2022, 09:32 AM   #4
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Just re-enforce the rear shock mounts.. there are very simple or elaborate ways to do this..

These are from akg
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And run a rear strut bar, that will help to even out the loads and make the rear a tad stiffer too. I might even have one lying around somewhere I don't need any longer.

Or similar..

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Last edited by Vanne; 07-12-2022 at 09:55 AM..
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      07-12-2022, 01:44 PM   #5
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Thanks, Vanne!! Perfect.
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      07-12-2022, 04:18 PM   #6
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Quote:
Originally Posted by F360C View Post
Maybe the front tower sheet metal is thicker and designed to take the load vs the rear not being?
Front and rear shock tower sheet metal is the same, 1.3mm. Same thickness as on the e46 M3 and we have all heard the metal cracking horror stories about them.
For some reason there aren't many reports of Z4M's towers cracking.
I'm going to reinforce mine anyway while I have the car stripped down this winter.
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      07-12-2022, 05:17 PM   #7
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Interestingly, I just had a conversation with Ben at Roque Engineering. I was hoping he'd convince me that their Rear Shock Mount Reinforcement would be enough to run a coil-over, rather than the split system. He was not enthusiastic. He, in fact, recommended a welded reinforcement that also fit over the "shoulders" of the mounting point. Total fun.
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      07-12-2022, 06:58 PM   #8
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Quote:
Originally Posted by wyshyvanuk View Post
Interestingly, I just had a conversation with Ben at Roque Engineering. I was hoping he'd convince me that their Rear Shock Mount Reinforcement would be enough to run a coil-over, rather than the split system. He was not enthusiastic. He, in fact, recommended a welded reinforcement that also fit over the "shoulders" of the mounting point. Total fun.
You're asking a drug dealer whether he would recommend his drugs?
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      07-15-2022, 08:57 AM   #9
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Point taken, but, alas, I don't think that RE markets that type of reinforcement
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      07-15-2022, 11:37 AM   #10
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Quote:
Originally Posted by F360C View Post

Looking at the front shock towers and the rear shock towers they are both just thin sheet metal though so I am don't know why you can run a coilover in the front but not the rear.
Look at the difference in area on which the load is transferred.
the front top bearings might have a total area that is 4(?) times as large?
I don't think you can compensate for that with just loose platelets put inside the rear shock mounts.
Of course if you're going to weld a new structure it's a different story. But also then you have to know what you're doing, putting too much heat in the panels can weaken them as the original panels are cold pressed.
Even the front support domes without welding can crack in extreme circumstances, so I think the main question is: Why?
Why change the rear spring system? What benefit has the coilover spring compared to the spring between chassis and suspension arm? That last spring can also be changed out for a linear one or one with other characteristics. Witgh much less risk and work/cost.
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Last edited by GuidoK; 07-15-2022 at 11:44 AM..
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      07-15-2022, 12:26 PM   #11
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From a slightly edited email exchange with Donald Molenaar with InTrax.

Me:
"Do you consider the coil-over rear units to be superior to the split configuration? In other words, will I give up some capability by requiring the spilt system?"

Donald:
"The differences are:
1. Separate spring will be heavier due to extra leverage.
2. The separate spring is less easy to reach for adjustment, the coil over is easier to reach.
3. The separate spring we have in 2 spring rates, the coil over spring we have in many different spring rates
So no, not superior, [but] you win on weight, convenience and choice."

Valid points, but I don't see enough benefit to switch to the rear coil-over. I'm firmly, once again, in GuidoK's camp. "Platelets" = lol.

So, perhaps I'm doing just fine, bang for buck, with the Bilstein B6 system, and using the OEM rear springs. My car is exclusively driven on city streets and fast roads. No intention to lower the car....I can just about make it into my driveway now. I've read that the bump stops on the front B6 coil-overs should be shortened, and I could mess with the spring rates a bit, but at my level, I'd be just as likely to make things worse. BUT, if someone has a Goldilocks, street only, system upgrade, I'm all ears.
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      07-15-2022, 01:07 PM   #12
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Quote:
Originally Posted by wyshyvanuk View Post
From a slightly edited email exchange with Donald Molenaar with InTrax.
Interesting that you had contact with Donald Molenaar; he was quite a famous and succesful dutch racing driver; I didn't know he now worked at intrax.

Regarding the reach for adjustment: I don't think that is really a big difference on the Z4. The adjustment is easy reachable with the stock springs as well. (I don't really see one adjusting the spring tension with a hook spanner without taking the wheel off, and in both cases then the spring is right in front of you.)
I also wonder if a coilover spring is really significatly lighter, that differs I think with the way the specific spring is designed: for instance the stock front spring is also heavier than the linear spring on my KW front coilover, and they are both coilover springs for basically the same fitment. Just differently designed.
Mainly the progressive springs seem to be slightly heavier, whether they're on a coilover or between chassis and suspension arm.
In the end having a spring more on the outside requires a thinner spring but longer and one on the inside requires a thicker/beefier spring but shorter, to give the same compression force over a certain specified compression stroke lenght.
Another pro for the stock rear springs is that they are situated more to the center of the car and arguably lower, which are also plusses.
As for the differences in number of choices: that is of course brandspecific.

The only real benefit I see for a coilover spring in this specific case is that if you're changing shocks on a race weekend, with a coilover you take out the complete unit and taking the spring out is a separate procedure. If you want to follow that official procedure for swapping out springs, that involves removing the drive shafts from the diff flange. But with racing springs, I expect that you use shorter springs than the stock springs, and you might get away with swapping out springs without loosening the drive shafts, but that all depends on the lenght of the springs.

The reason why you have to do that (disconnecting the drive shafts at the diff flanges) is that the suspension arms can't be pushed down enough to create room for pulling the spring out.
You see this done by backyard amateur mechanics, putting a lot of pressure with their leg on the suspension system, stretching it down enough to get the spring out, but that either gives a lot of strain on the ball cage in the CV joint, or you deform the metal casing that holds the boots which can cause grease leaks (both result in potential damage, don't know which one happens first, I think the latter). This is why this is not the correct procedure and a real dealer/skilled mechanic (who knows his stuff) would never do this. And of course a race mechanic who does this has chosen the wrong profession...
It's cutting corners by ignorance.

But all in all, if you're changing shocks on race weekends, you're probably on a level that you're driving a dedicated race car, and at that point your monocoque will be heavily modified (full race cage etc etc) and surely beefing up the rear shock towers in that process by welding in all kinds of structural supports on those towers is probably already done for the cage.
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Last edited by GuidoK; 07-15-2022 at 02:04 PM..
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      07-16-2022, 11:43 PM   #13
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the biggest difference, which wont apply to most, is that using rear coil over is that you can set the running height much lower. On my setup the original height of the spring pertch would be touching the frame. which is also why you'd need to swap in different rear upper control arms/move the whole subframe... etc...

Pretty sure Donald has worked there for quite a while, i chatted to him at least a few years ago and he was then already at Intrax. He may even co-own it with Henk? I/we still havent finalized the front suspension as there are lost of measurements that need to be done after corner balance has been set.

Awesome guys to deal with though, def know thier stuff.
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Last edited by Vanne; 07-16-2022 at 11:56 PM..
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      07-17-2022, 04:50 PM   #14
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Thank you, all!!! You've defined the issues very well. All I need is the courage to decide what I actually want.
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      07-17-2022, 10:58 PM   #15
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I almost can't believe that ride height is an issue.
I mean with the normal system, there is enough room given by the stock spring setup to let the shocks hit the bumpstops. There's no lower point than that possible, unless you remove the bumpstops, which is definately not adviseable as that can cause some real damage.
Maybe if you physically alter the wheel arches and weld in a different arch or otherwise modify it....(but then you're already on the dedicated racecar path)
So for this discussion I think what's most important to know: are we dealing here with a 100% dedicated racecar or are we dealing with a streetcar?
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Last edited by GuidoK; 07-17-2022 at 11:08 PM..
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      07-18-2022, 07:06 AM   #16
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That's pretty much what was done to my car. Wheel arches completely rewelded/ up by about 2 inches. But yeah.. I guess ted will share what it will be used for.

Last edited by Vanne; 07-18-2022 at 07:16 AM..
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      07-18-2022, 08:49 AM   #17
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Thanks, again, gentlemen. In my case, the car will be used on poorly maintained Tucson city and suburban streets, and on the brisk monthly run with the local BMW club, across the relatively well-maintained Arizona highways. It's a far cry from race car requirements, but I do appreciate a well-sorted suspension. The current Bilstein B6s were an improvement over the OEM bits, and I was interested in determining if further improvements could be had. The B6s haven't lowered the car much at all, and ride height is crucial in that I'm now just able to clear my driveway bump. Bang-for-the-buck is of interest, but (and please don't think less of me) I'm not immune to the envy of fellow club members when exhibiting my new [insert your choice of brand] suspension system. In short, I'd like a very compliant but very controlled system.
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