Temporary Rear Subframe Fix?

[Znewbie]

I have a Z3 Roadster that has started showing signs of subframe failure. I've noticed a few spot welds that have broken and my subframe is hanging very slightly. My pinky finger cant even fit in the space between the subframe and trunk floor so it's just started to come down.

I'm well aware of the proper way of fixing this problem, however I'd like to enjoy my car for the summer and have it properly repaired when it's off the road anyways in the winter.

I was thinking about re-welding the broken spot welds to prevent any further damage and to strengthen it a little for a few months, but I'm not sure if this would help at all? Has anyone ever done this before? I know someone who can weld so it wouldn't cost me anything except maybe lunch!

Also, I tried to jack the subframe back in place so it connects with the trunk floor, but it won't budge and the car just starts raising up. Is there a way to get it back in place?

[Wertles]

Any “fix” you do now only makes the proper fix WAY more difficult (read expensive). I understand the desire to put it off and enjoy the car now but as I tell my kids...Do it once, do it right, don’t do it again.

[Znewbie]

Any “fix” you do now only makes the proper fix WAY more difficult (read expensive).

How would re-welding the spot welds that have broken make the actual fix more difficult? I'm assuming that would need to be done anyways when it's repaired properly. Genuine question.

I'm mainly concerned with how to reconnect the subframe with the trunk floor.

[z3forlife]

You might want to post some pics. This could already be very serious

[khammack]

A good shop can do the work in a week or so. If you wait, you won't enjoy your car because you will be worrying about it. Fix it now and then you can enjoy your car for the rest of the summer. There is no way to really fix it temporally.

[danomite]

...their is no temporary subframe fix. You either fix it, or make it worse. (*enter Randy Forbes)

[Venom13132]

Knock on wood, but I have been driving my car for 2 years with some welds starting to fail. None have torn or anything. But there is no temporary fix. There is only drive till it fails or fix it. I avoid bumps in the road as much as possible and don't drive aggressively and I haven't seen any change in my subframe. If you cant fix it now keep an eye on it and if you see it getting worse then its time to stop driving the car till you can fix it.

[Mr Bingley]

The Sub-frame issue with our cars is actually misnamed, it has very little to do with the subframe. It is more of a differential mount/cross brace/trunk floor issue. To be precise, the subframe does not connect to the trunk floor at all, and therefore cannot be reattached to it. The subframe attaches to the uni-body via the two subframe mounts, and the differential attaches to the subframe via mounting bolts atop the differential. The differential mount connects the offset ear of the differential cover to a bracket that is welded to a brace that runs across the car (side to side) and which in turn is welded to the trunk floor and the frame rails. It is these welds--between and affixing the cross brace to the trunk floor and frame rails--that fail:



The other failure point is the bracket itself through which the differential mount is bolted, which stress cracks and ultimately tears loose:



If you are observing welds breaking free in the trunk floor, what you are seeing is evidence that the the brace under the trunk floor is tearing free of the car. It is likely that more welds are broken than you can casually observe by visual inspection of the trunk floor, or than you can easily access to re-weld. Further, the type of welding your friend will employ is called plug welding, while the original welds are spot-welded. Plug welds are much harder to remove than spot welds, and will require more extensive damage to the surrounding metal when removed. If you are planning on doing the job properly in the near future, the installer will not thank you for having to deal with plug welds in the tear down process, and may have to fabricate and weld in replacement patches. I am guessing it will be more expensive on that account.

Regardless, you do not have to be concerned with reattaching the subframe to the trunk floor, or that you can insert your pinky between them. Your concern should be focused on the welds that secure the cross brace to the trunk floor, and the bracket that the differential mount is bolted through. Re-welding the broken welds you can identify is a tempting temporary resolution, but will make things worse in the end and is therefore contraindicated.

[CMM3]

Best solution is to send your car to Randy Forbes. It will come back looking bone stock with the problem fixed and value added to your car.

[BimmerBreaker]

I'm also pretty sure re-welding those welds is not part of the correct repair. The entire section is cut out, and new parts welded in. As I understand it - welding those holes as a temporary measure can cause the forces and stress to migrate to a different part of the system, causing at best a more difficult "correct" repair down the road and at worse a much worse type of catastrophic failure than would have normally been experienced.

[ProductUser]

I drove my roadster with several popped welds and a 3" crack in the crossmember for a few years. However, I did eventually install the RF kit as the one-and-only repair for this issue.

[me77]

Some people have had success with drilling out the broken welds (you don't need the special drill bit, the welds on the car are really soft) and putting a screw through the hole. It's a common fix mentioned in German Z3 forums.

Screw head
Washer
Sheet metal car
Washer
Nut

Example picture: https://www.z3-roadster-forum.de/for...8204&mode=view
(taken from: Schweisspunkte Differentialhalter rausgerissen)

[Vintage42]

Some people have had success with drilling out the broken welds... and putting a screw through the hole...

A bolted repair that didn't last:
RSF Bolting Failure.jpg

[Vintage42]

The Sub-frame issue with our cars is actually misnamed, it has very little to do with the subframe. It is more of a differential mount/cross brace/trunk floor issue. To be precise, the subframe does not connect to the trunk floor at all, and therefore cannot be reattached to it...

Here is a photo that shows the true rear subframe which could also be called the suspension and differential carrier. This subframe is connected to the body at each side and at the differential. The connection at the differential is a mount on a channel (also called a subframe) that is spot welded across the bottom of the trunk floor.

RSF Diff Carrier 1.jpg

The differential mount transmits acceleration and deceleration forces from the drive train into the channel and tries to tear it loose from the spot welds. In addition, as the two rubber bushings at each side of the subframe grow old and soft, their play allows sideways movement of the whole subframe during cornering. This side to side force is transferred to the differential mount and into the channel.

So the channel across the bottom of the trunk, so called the subframe, is twisted back and forth and pulled side to side by the differential mount. Eventually the mount and the channel can tear loose.

[BimmerBreaker]

Some people have had success with drilling out the broken welds (you don't need the special drill bit, the welds on the car are really soft) and putting a screw through the hole. It's a common fix mentioned in German Z3 forums.

Screw head
Washer
Sheet metal car
Washer
Nut

Example picture: https://www.z3-roadster-forum.de/for...8204&mode=view
(taken from: Schweisspunkte Differentialhalter rausgerissen)

Thats literally the worst thing you could do

[danomite]

thats literally the worst thing you could do

this!!!!! ^^^^^

do not attempt to "repair" it with screws!!!! You will destroy the entire trunk floor!!!

[zielritter]

Here is a photo that shows the true rear subframe which could also be called the suspension and differential carrier. This subframe is connected to the body at each side and at the differential. The connection at the differential is a mount on a channel (also called a subframe) that is spot welded across the bottom of the trunk floor.

RSF Diff Carrier 1.jpg

The differential mount transmits acceleration and deceleration forces from the drive train into the channel and tries to tear it loose from the spot welds. In addition, as the two rubber bushings at each side of the subframe grow old and soft, their play allows sideways movement of the whole subframe during cornering. This side to side force is transferred to the differential mount and into the channel.

So the channel across the bottom of the trunk, so called the subframe, is twisted back and forth and pulled side to side by the differential mount. Eventually the mount and the channel can tear loose.

I realize I'm raising a zombie, but wanted to say thanks for giving the best, most concise explanation I've read about how this issue actually arises. I'm looking to get mine reinforced in the near future. Going to start with stiffer subframe bushings and a welded reinforcement kit. I'm not a racer, but i do have an aggressively lowered suspension setup. Thanks again.

[325bob]

There's nothing wrong with bolting in reinforcing washers, have done it often with success, screws aren't bolts, a coupla big washers on each side would expand the torn spot welds area of stress by a considerable amount, and will be my plan if I ever see a problem, none yet with big sticky tires and 50k of autocrossing,135K total..

[BimmerBreaker]

Where's Randy F when you need him?

[JustinHalverson]

I took an approach similar to that proposed by OP. My spot weld failures were not as severe as he has described. Two key parts of my repair, which has proven successful for almost 5 years and 15,000 miles of semi-spirited driving, was the addition of IE rear subframe bushings (RSFB), and the help of a nuclear qualified welder. My logic was that the stress on the trunk has to be the result of torquing motion allowed by the relatively soft RSFB, and the poly RSFB's would transfer more of that torque to their mounting points thereby reducing the forces on the trunk welds. I thoroughly cleaned up the area around the failed spot welds, drilled holes between the failed spot welds to allow for additional plug welds, and then I had my highly qualified welder friend plug weld both the failed areas and the newly drilled holes.

Since the repair, I check the welds religiously, and have not seen a hint of failure. Honestly however, I wish I had bitten the bullet and bought the kit and had my welder friend weld it in instead of going with what is possibly a temporary fix. Once I decided to go with replacement RSFB's, the labor involved to add Randy's kit would not have been that much more.

[ProductUser]

Where's Randy F when you need him?

I'm sure Randy F. is tired of posting evidence around how ad-hoc fixes just don't work and the consequences of those ad-hoc fixes over time.

[Randy Forbes]

Where's Randy F when you need him?

I'm sure Randy F. is tired of posting evidence around how ad-hoc fixes just don't work and the consequences of those ad-hoc fixes over time.

You read my mind; I mean TO THE LETTER!

I AM tired, and people are still going to do whatever they want. For the people that drilling MORE HOLES in an already too weak panel for works, that's just great__go buy a lottery ticket before your luck runs out.

For all the cars that a band-aid won't be enough, it will double the cost when somebody takes a more thorough approach and treats the root cause, versus merely addressing the symptoms.

I've pretty much stopped doing the installations myself__fifteen (15) years, and over a hundred cars (103-104?) was long enough__but I'll try to keep the trunkfloor/differential mount reinforcement package available for as long as I can. I placed a restocking order for the laser-cut & press-brake bent materials a month ago and they haven't shown up yet; there was only a single kit left on the shelf yesterday, and I received a PayPal notice of new funds this morning, so might be dry for another week or so.

I see my cardiologist on Thursday to set up some work__I need the same valve job Mick Jagger just got__fingers crossed! The cardio doc drives a new Alpina B6, but I can't help but think my odds for success would be better if he had an M Coupe with a few broken spot-welds...

[khammack]

Good luck on your upcoming upgrade Randy!

[Mr Bingley]

I see my cardiologist on Thursday to set up some work__I need the same valve job Mick Jagger just got__fingers crossed! The cardio doc drives a new Alpina B6, but I can't help but think my odds for success would be better if he had an M Coupe with a few broken spot-welds...

Good luck Randy--not that you need it, it's a pretty common procedure these days with a really high success rate.

[EGS]

My 86 year old father had the 'valve job' and was home the same day. No setbacks and he's doing fine. Hope your procedure goes smoothly!