DTM Stlye Rear completed on Z3M Coupe

[CMM3]

After a full season I found that my rear suspension was mechanically loose. The rear of my car has solid aluminum sub frame and diff bushings along with adjustable posi-lock RTAB's both on camber and toe with Delrin bushings. The cause was two fold; 1. The Delrin bushings were cracked and the inside diameter of the metal sleeve inside the bushings had grown in diameter causing the trailing arms to have play on the sub frame. I've also found that the end plates for the RTAB bushings that secure the bolts were getting sloppy on the bolts and liable to strip. This is due to the fact that the end plates are only about 3-4 mm thick and only have a few threads to catch the bolt. Another issue is that the camber I could dial into the rear was limited to about -2.5 degrees. I run about -4 in the front and have been told by a couple of folks who raced Z3 coupes competitively that the only way to get a Z3 coupe to really hook up was to dial in a lot of rear camber close to or equal to the front. So I found some used trailing arms and a sub frame to work from scratch and kept my original set up in the event it didn't work out.

There is a nationally renowned fabricator a few miles from my house who builds top fuel dragsters. The shop owner told me that if one of the rear wheels of a dragster moves at all it will cause a loss of control and he was dealing with 6000 HP at the wheels with a lot more traction. So yes he should be able to come up with a solution. I brought my rear suspension to him and explained my issues and desire to add helm joints to the trailing arms, find a way to get increased camber and make the toe and camber easily adjustable.

After thinking about together we decided to use the same posi-lock style adjusters but with both inner and outer orientated vertical. This enable a greater range of camber adjustment by allow the inner attach point of the trailing arm to be lowered and the outer raised. In addition the inner mounting bracket point would be notched to allow the helm joint to go lower for increased camber. The toe adjustment is now accomplished by a large bolt with a metal rod. The helm joint on the inner now threads into the large bolt/sleeve and there is another bolt on the opposite end to lock it in place. The rod fits inside a the sleeve now part of the trailing arm. He told me that he's used this simple configuration to get add adjust ability on the dragsters he's built for years and never had one come loose. Pretty slick. The brackets where also reinforced to prevent the metal from tearing at the brackets and both the shock bolt and sway bar link mounting bolt locations were reinforced. I thought it was a good idea to reinforce the shock mounting bolt location since I run a true coil-over in the rear and this point now carries all the load with no inner springs. Since I no longer use inner springs I had the perch plates removed.

After the modification to the trailing arms and sub frame where done I test fitted the sub frame and found it had shrunk by about 3/8 of an inch due to the heat from welding it. I brought it back to the shop and he was able to reheat on the other side of where the heavy welding was done and bring in back into alignment. I then had the trailing arms and sub frame sandblasted and powder coated, installed new wheel bearing for good measure along with parking brake shoes and hardware.

Here is what a stock sub frame looks like, the 3rd shows the shock bolt mount






Here is mine modified: Note how the bracket are now closer together providing a solid mating surface for the helm joints;



The helm joints are high heavy duty 3/4 with PTFE races. The hardware is now all mil-spec 1/2 inch and the end plates have added material provided additional threads for the bolts. Moving the end plates of the brackets closer together also provides additional clearance so I could add a jam nut for added security of the bolts





Showing both inner and outer adjusters in vertical with a jam nut on the outer:





Toe is now adjusted here at the inner attach point of the trailing arms, by first loosening the rear bolt then turning the large nut which allows the helm joint to thread in and out on the assembly:



Reinforcements on the back of the adjuster brackets:





Reinforcement to the shock bolt:




Now all I have to due is get this installed back in the car, have it corner balanced and aligned and I should be good to go with no more issues.

[Sharocks]

Dan this looks awesome! Will be sending you a PM with some more questions soon! Cheers!

[Remotion]

You can buy similar parts ready made from Millway.

[CMM3]

You can buy similar parts ready made from Millway.

I'd be very surprised if they offered anything for a Z3M. The trailing arms and sub frame are unique to Z3M coupes and roadsters and there is not a whole lot of demand. Besides shipping parts this heavy from Europe would cost a small fortune.

[BimmerBreaker]

Awesome work!

And yeah I am 99% sure you cant buy any aftermarket trailing arms for Z3M's

[19Spec3]

Drool. How is access to this stuff once on the car? I have the toe/camber adjustment kit from Garagistic and it's awful to adjust on the car. Access is very very tight.

[CMM3]

Access to the nuts and bolts on the adjuster brackets is somewhat improved since the brackets are closer together. This provides some more room especially at the side where the jam nuts are located. With the inner leg of trailing arm set at about the same height as where I previously had it = -2.5 degrees of camber it should be easy to move the outer leg up as needed to get the additional negative camber I'm after.

The large nuts locking the helm joints on the outer legs of the trailing arms should never need to be adjusted as I set them to be the same length as stock. Toe is adjusted at the inner legs now with access to the very large nut and rear most bolt on the assembly easy to access. So the only location that could be tight is access to the helm joint locking nut on the inner legs as the trailing arms angle down a bit with the suspension in full drop which might make it hard to get a wrench straight on them. But with the suspension in normal position I think it will be no problem. Worst case is that the bolt holding the coil over might need to be removed to swing the trailing arms up a bit to gain access. I test fit the exhaust and it's completely out of the way.

For those who might be looking to replace the dog bone rubber sway bar links on there Z3M I found these helm joint style ones made for an e30 front at Ireland engineering and they are the same length The studs that inserts into the trailing arm is slightly larger so the hole need to drilled out and the stud that fits into the sway bar is slightly smaller. I had some material welded onto the end of my sway bars to make them longer to add another adjustment ( 4 holes instead of 3 ) to go softer if needed and at the same time had all the holes filled and re-drilled so it every thing now is tight. Had the sway bar re powder coated after.

With solid aluminum sub frame / diff bushings, the modifications to the trailing arms / toe & camber adjustments and the sway bar links there is now no rubber or poly anywhere in my rear suspension. It's now all solid and can only move in places and in the manner it is suppose to with no potential to bind, which is in practice ideal for a race car.

I just got the sub frame and trailing arms installed in the car over the weekend, although the sub frame is not all the way secured yet as I'm waiting to pick up some new diff mounting bolts the pictures below provide a pretty good idea of the access;

Access to the outer height adjuster for camber should be no problem. I've already raise it up a bit over the stock height as a guess so I may now may actually have too much negative camber and the outer can easily be lowered if this is the case or raised if I should need more.



The inner height adjuster shouldn't need to be accessed but it's no worse than stock



The large lock nuts for the helm joints shouldn't be a problem once the trailing arms move up a bit with weight on them.



Sway bar link shown here;

[CMM3]

Drool. How is access to this stuff once on the car? I have the toe/camber adjustment kit from Garagistic and it's awful to adjust on the car. Access is very very tight.

BTW the delrin bushings and sleeve issue I describe where with Gargastic RTAB bushings. The delrin they use must be lower quality and compared to the others the wall thickness of the sleeves that insert into the bushing is about 1/2 as thick as others I've used and I suspect of softer lower quality steel. The white ones shown in the pictures of the stock sub frame are from Condor and the sleeves appeared to be made of an alloy and are much thicker wall and tighter tolerance fit to the bolts than the what I now think are cheaply made Gargastic ones. Needless to say I'm no longer a fan of Gargastic products and will never purchase anything from them again.

[ryeland]

I dont suppose that you'd be interested in having several of these made and selling them? I know at least 2 others, besides myself, that would be very interested in buying. I researched these years ago but simply dont have the time to try and duplicate them on my own.

[CMM3]

I dont suppose that you'd be interested in having several of these made and selling them? I know at least 2 others, besides myself, that would be very interested in buying. I researched these years ago but simply dont have the time to try and duplicate them on my own.

The biggest challenge in doing this would be getting the parts needed to do the modification. The sub frame and trailing arms are unique to the z3M's and pretty hard to come by and costly. This means that these would have to be taken off a car and supplied so there would be significant down time. Also the shop that did this is not in business of producing parts and they did this as a one off project. All and all not something that could be done in practice. it would also be very expensive to duplicate with all the labor involved putting the cost too high that anyone would want to buy one in the end.

[CMM3]

Well I had the car aligned and corner balanced and the rear suspension worked out as I hoped it would. When I did the initial assembly I had arbitrarily set the camber with what I thought would be more negative than I needed and it ended up a -5 and -6 degrees. So I now have more range of camber adjustment than I'll ever need. The toe adjustment was actually pretty close and same here with pretty much unlimited range of adjustment. Getting the camber dialed in was a little tedious just because it was so far out to begin with and it took some time to zero in on it. The toe adjustment turned out to be a snap once we figured out that after loosening the rear locking bolt turning the large adjuster nuts didn't do anything until the rear locking bolt was tightened back up. What was happening is that the large nut was just spinning on the helm joint threads but not moving the trailing arms until the lock bolt was snugged back up. Once we figured this out it was really easy to adjust the toe. All and all it works well. I now have -4.2 degrees of camber in the front -3.8 in the rear with 1/16 total toe out front 1/8 total toe in rear. I lowed the front spring rates from 750lbs to 650lbs and the rear springs stayed the same at 350 lbs but I have true coil-overs in rear. I also lowered it just a bit more by about 1/2" front and rear. The car without driver now weighs 2635 lbs with a full tank of gas 2551 lbs empty.

Quite a few changes so I'm really looking forward to getting back on the track and seeing the results.

[BimmerBreaker]

I know a guy who collects and repairs Z3M arms. I've talked to him a little about stuff like this, I don't think he has the means to do it, but he has a ton of cheap Z3M arms at his disposal. If someone had the means to convert arms like this for people, I'd be in and gladly share this guy's info

[Randy Forbes]

I know a guy who collects and repairs Z3M arms. I've talked to him a little about stuff like this, I don't think he has the means to do it, but he has a ton of cheap Z3M arms at his disposal. If someone had the means to convert arms like this for people, I'd be in and gladly share this guy's info

[BimmerBreaker]

Could have put an arm on the other side too to check pass side arms on the same assembly Nice jig though, you gonna start whipping up some DTM style arms for the group? Trade for M Rain seats?

[Randy Forbes]

Could have put an arm on the other side too to check pass side arms on the same assembly Nice jig though, you gonna start whipping up some DTM style arms for the group? Trade for M Rain seats?

Maybe when I retire, no time to do it now! Glad I made those up when I had the chance

Start with known good parts__brand new, in this case__to make up the fixtures



Completed and painted to preserve, LH and RH trailing arm test-jigs (requires hub & bearing fitted) and suspension carrier (built to suit M versions, but works as go/no go for Z3s too).



I'll use them again for the car I'm currently working on__RH toe-in excessive__to know which way to bias the toe/camber kit installation.

[CMM3]

I know a guy who collects and repairs Z3M arms. I've talked to him a little about stuff like this, I don't think he has the means to do it, but he has a ton of cheap Z3M arms at his disposal. If someone had the means to convert arms like this for people, I'd be in and gladly share this guy's info

I'd be very surprised if he has any arms and/or sub frames for the Z3 M's hanging around they are both different that the non-M's which are more readily available but still hard to come but. But the "M" sub frames and trailing arms are pretty much non-existent used. I looked everywhere for both and was lucky to find the one arm and a sub frame at Purassian and another arm at a bone yard in TX. Anyone who might have these parts now realizes how scarce they are and are asking top dollar for them. And with the Z3M's gone collectible and people salvaging any that are worth it, they are only going to get rarer.
I think they are still available new but your talking big money.

[BimmerBreaker]

I'm well aware M and non-M arms are different, I met him when he contacted me about some of my M arms I had for sale He has quite a few, some have been repaired, all are straight, cheaper than most places...

[CMM3]

I'm well aware M and non-M arms are different, I met him when he contacted me about some of my M arms I had for sale He has quite a few, some have been repaired, all are straight, cheaper than most places...

I think the same person contacted me on ebay to let me know that he was out of stock and one of his customers was interested in the sub frame I'm selling there. You may want to check with him again on what he actually might have.

[JS154]

Did you fab these up yourself, and what did you use for a jig? Did you change the arms to 11 degree sweep while you were at it, or just convert to heim joints only?

[CMM3]

Did you fab these up yourself, and what did you use for a jig? Did you change the arms to 11 degree sweep while you were at it, or just convert to heim joints only?

As mentioned in the original post I had a local shop do the fabrication, no jig was used.

I did not change the arms, just converted to helm joints.
Not sure what you mean by going to an 11 degree sweep?

Got my first real track day and without pushing as hard I was seeing about a 1 second improvement over previous times on older tires. The rear is definitely much more solid both in corners and braking and overall the car felt like it was easier to drive.
It was well worth the effort.