Z3 M Coupe & M Roadster Boot Floor Cracks

Just like the E46 3-series, the Z3 M also suffers with the Rear Axle Carrier Panel cracking.
(If you haven't heard of our huge success with the E46 chassis please read more here)

Most people, including BMW, know this area as the Boot Floor so for ease we'll join them and refer to this area as 'Boot Floor'.

Problem:
As much as us Zed lovers hate to admit it, the design of the BMW Z3 Boot Floor was inferior and can suffer with spot weld failure and eventually stress cracks in the steel panel.

The torsional load coming through the Propshaft and turning 90 degrees out to the Driveshafts has immense force which is directed to a singular Diff Bush which is on the LEFT of the Boot Floor.

[PICTURE]

This singular Diff Bush/Mount has to withstand loading (throttle applied) and unloading (coasting) which are very opposite forces.
Over time this loading/unloading nature acts upon the sheet metal of the Boot Floor where spot welds are holding panels together.
Eventually, spot welds on and around the Diff Mount Bracket and especially the Left Boot Floor and Left Diff Crossmember area are at risk.
They can no longer keep these repetitive compression and tension forces at bay and the sheet metal fatigues.
This ultimately leads to certain spot welds breaking (known as spot weld failure) which then amplifies the situation as the neighbouring spot welds have to do more work and carry more load which of course means they prematurely fail and it becomes a domino effect.

Simply put, the sheet metal of the Boot Floor and Diff Crossmember/Bracket cannot withstand the rotational stresses placed upon it from the drivetrain.

Consequences:
Once a few spot welds have failed, the Diff Crossmember (and bracket) sag and drop on average by a few millimetres.
Although this doesn't sound much, it's already quite a problem as now the Diff Crossmember is dragging down the flimsy Boot Floor and causing it to be out of shape as well as alignment.
Also, the lack of structural security for the Diff Crossmember causes the loading/unloading moment to become even more pronounced and ultimately violent.
This results in an undesirable driving dynamic for the occupants and the drivetrain underneath them.
It can sometimes be apparent a problem is lurking due to unwanted noises in the form of metallic creaking, or Diff Bush failure resulting in a knocking noise, or perhaps a less than smooth ride, or even an 'axle-tramping' effect on spirited drives.
In drastic cases the left side of the Diff Crossmember can not only tear but quite literally rip itself away from the Boot Floor and left Chassis Leg area.
Unfortunately, once 1 spot weld starts to fail, others will follow so if you're concerned it's best to get an inspection carried out sooner rather than later.

Options:
Up until recently there have been only 2 real options:
Genuine BMW Boot Floor Panel (£3416.75 in 2023 just for the part, before any labour).

Randy Forbes Kit (

Randy is a legend in the Z3 world. A seriously nice guy who contributes to the community and came up with the first and proper viable Z3 Boot Floor reinforcement solution.
Over the past 20 years he's personally repaired & reinforced over 100x Z3 M Boot Floors and sold hundreds more kits around the world (including to us).
We've used Randy's kit several times and think it's great.
We have nothing against either the Genuine BMW Boot Floor Panel or the Randy Forbes Kit, we're simply here to share our solution and add another option for owners to consider.

"Just weld it"
As much as we'd all love to be able to 'just weld the cracks' it isn't that simple.
We've seen many Z3's (and E46's) brought to our workshop where someone has attempted or believed to have fixed the issue by 'just welding it'.
But weld repairing alone is not a long term solution as it doesn't address the inherent design flaw.

The Z3 Boot Floor is weak and flimsy 

(see our video clip below)

It need a substantial reinforcement process to not only hold up the already fatigued panel but ensure it doesn't happen again.
Welding the cracks alone will also introduce further risks of failure due to the fair amount of tension the panel pieces will be in (due to the fatigue).
All metal pieces need to be in a relaxed state when welding occurs to avoid repeat failure from stress forces acting around natural heat affected zones.
To do that a complete Load Relief solution needs to be introduced which is where our method comes in........

Redish Solution: