CMP Subframe Reinforcement Completed

[Tonggi]

It’s just perforated hardboard wrapped in alcantara. Used cardboard to get the shape right first…it was a quick arts and crafts project to cover up the bottom portion of the rear seat. PM me if interested and I can send additional pictures.

message sent. car looks great!

[TexaZ3]

Top job - looks awesome too.

[BMWE46M3]

Hey guys, Thought I'd check out the forum and saw I had a notification bringing me here. It's a Friday night and I've had a couple beers but I'll see what I can offer.



From memory the Z4 chassis doesn't have a RACP and where it bolts to the chassis is two tube like extensions protruding downwards. I've never seen the underside of a Z4 in person and am reflecting on old images I saw what was probably years ago now.

What I do know for certain is that E9X M3's are hugely different to E46's. About double the size and installed from above not below. There's no protrusions from the bush and the core sits flush against the chassis with a locating pin rather than on a stud with a hex. I picked up an E92 M3 very recently and was literally working on updating my subframe bush design for the chassis today. Fun fact, it looks like I can raise it a few mm like I have with E30/36/46's.

Regarding the worn out bushing creating an impact against the chassis and harder material reducing dampening and increasing peak force under impulsive forces, in short, both are correct.

harder bushings obviously wont flex so just like stiffer shocks, are harsher on the chassis.

The worn bushing argument applies to just stock bushings and technically uses the same principle. The rubber becomes so weak and flexes so easily that it basically does nothing to slow down the momentum of the moving subframe resulting in the steel protrusion in the bush to ram/hammer into the underside of the chassis. This is worse than solid bushing as you have no dampening however, you're also giving the subframe a chance to build momentum before colliding while a solid bush braces thing rigidly.
The contacting surface are of the protrusions is also a lot less than the hex so you have an eve greater peak force on a smaller area thus creating a greater surface pressure.

I've seen some cars locally with Turner's old style of reinforcement installed which from memory is about 1.5mm that have deformed from worn rubber bushings hammering into the chassis. On this topic, I just spent my weekend fixing a car with only 116k km (aprox 70k miles) that had just Turner plates put in and need re-reinforcing and the subframe bushings were so worn that there were visible cracks in the rubber almost the whole way around the rubber core. It had also been hammering the chassis so hard the paint had come off the plate.

The theory the cracks and deterioration will only accelerate applies to both the chassis and the rubber bushings.

The reason the protrusion on the stock subframe bushings is an issue is that there's an air gap between the sheet metal you see underneath and the female threaded body inside the RACP. If this air gap was not present the surface would be laminated thicker resulting in a greater shear strength which would reduce flex and thus the rate of fatigue.
Even in that scenario I would change the bushings as the rubber would eventually wear and become a problem.

So let me ask this question - if the E46 M3 coupe came from the factory with solid bushings, not rubber, would there still be issues in the RACP?

[poss]

So let me ask this question - if the E46 M3 coupe came from the factory with solid bushings, not rubber, would there still be issues in the RACP?

This excerpt from CMP's website appears to answer your question (yes - the RACP will be even more likely to fail with solid bushings unless the design flaw is addressed with topside reinforcement)

https://cmpautoengineering.com/pages...-the-pros-cons

"The cracking and failure that occurs in the RACP is a result of fatigue. A structure is generally design to a ‘fatigue life’ which means the predicted period of time till fatigue will occur. Some things can be designed to an infinite fatigue life by keeping designed stresses below the fatigue limit however, often fail due to other variables such as material imperfection etc. This fatigue life is dependent on the severity of abuse (stress) and the amount of cycles it occurs.

By reducing the dampening of impulsive forces you increase the peak stress acting on the chassis thus reducing the fatigue life. Meaning, failure is both more likely to occur and occur sooner.

This is essentially the only negative to solid mounting the subframe and as mentioned above, only bears any significance on chassis that suffer existing fatigue issues such as the E46. Most cars do not suffer from fatigue even with ridiculous increases in power and thus opting for solid subframe bushes has no negative and only improves the drivability and handling of the car.

To combat this negative for the E46 chassis, the structural rigidity must be increased to withstand the increased peak stresses to the point that such fatigue related failure is eliminated. For E36's this can be achieved with just underside plates however, E46's have to go several steps further to achieve peace of mind. This is why we encourage these bushes be paired with at the least some additional topside chassis reinforcement such as our Stage 1.5 chassis rail plates.

Without going too off topic, plates provide localised rigidity at the subframe mounts however, not throughout the structure in other areas also within the load path distributing the same stresses. So failure is inevitable beyond the plates and that small addition makes a significant change to the load path as well as a good difference in chassis rigidity without adding significant weight to the chassis or obstructing the boot.

The reason some of the CMP RACP reinforcement kits are on the more substantial side is because they intend to not only correct the design flaws to prevent failure occurring but continue to do so even after such modifications have been made or substantial increases in power."

[heinzboehmer]

So let me ask this question - if the E46 M3 coupe came from the factory with solid bushings, not rubber, would there still be issues in the RACP?

Yeah. I think Vince's sketches are the easiest way to understand why the panel fails. The cutaway makes it very clear that the load path from subframe mounts to chassis rails goes entirely through thin sheet metal, hence the fatiguing and cracking.

https://nam3forum.com/forums/forum/main-forum/e46-2001-2006/118430-subframe-dropped-and-cracks-found?p=128852#post128852

[poss]

Yeah. I think Vince's sketches are the easiest way to understand why the panel fails. The cutaway makes it very clear that the load path from subframe mounts to chassis rails goes entirely through thin sheet metal, hence the fatiguing and cracking.

https://nam3forum.com/forums/forum/m...852#post128852

Yup. Solid subframe bushings will make the area of the RACP where the subframe is mounted more rigid, but this will actually result in more stress being transferred via those thin sections of sheet metal indirectly mounting the RACP to the chassis rails via the wheel arches (as per Vince's sketches) - accelerating metal fatigue.