Best Bushings

[Sid_E46]

What's the reasoning behind the solid subframe bushing recommendations? Is it to reduce any possible movement in the subframe to theoretically prevent floor cracking? I feel like poly is a good choice for NVH and was curious if there was something I wasn't understanding.

That is true, by preventing the RACP to flex as much as it would with oe bushings, it also prevents cracks to a certain extent. Definitely would pair them with subframe reinforcement. I had poly subframe bushing before I went with PSM solid and there is 0 difference in NVH.

Even BMW has caught onto the solid subframe bushings movement. My f10 550 has rubber bushings for the subframe but my f10 M5 has solid bushings.

[pynacl]

That is true, by preventing the RACP to flex as much as it would with oe bushings, it also prevents cracks to a certain extent. Definitely would pair them with subframe reinforcement. I had poly subframe bushing before I went with PSM solid and there is 0 difference in NVH.

Even BMW has caught onto the solid subframe bushings movement. My f10 550 has rubber bushings for the subframe but my f10 M5 has solid bushings.

That's the consensus I suspected. But, I feel like this is actually having the opposite effect if you think about it in terms of physics.

The force acting on the RACP is dependent on the rate of acceleration of the subframe. For the force to be reduced, the rate of acceleration needs to be reduced by creating deformation/damping (rubber/poly will act as the damping agent). By using solid subframe bushings, you are reducing the dampening of impulsive forces and increasing the peak stress that directly acts on the chassis thus reducing the fatigue life. Meaning failure is more likely to occur sooner.

IMO this isn't really a huge deal on most cars, but with the E46 thin sheet metal I think it would accelerate the failure prone RACP without a bottom AND topside reinforcement kit, like the mason engineering x brace or the CMP topside chassis rail plates.

Am I wrong in my thinking??

[DoubleSidedTape]

Am I wrong in my thinking??

That’s pretty much in line with what CMP states: https://cmpautoengineering.com/pages...-the-pros-cons

[pynacl]

That’s pretty much in line with what CMP states: https://cmpautoengineering.com/pages...-the-pros-cons

Yup, it's paraphrased from CMP. Felt like they would be a credible source of information and the explanation makes sense. Although they do make money selling reinforcement kits so it's nice to see if anyone has a contrasting opinion.

[Sid_E46]

I can see that solid subframe bushings would make the RACP more prone to complete failure such as tearing from the stitch welds in the wheel wells without any top side reinforcement. What I believe is that they prevent cracking at the mount points due to less flex in the bushing itself.

[pynacl]

I can see that solid subframe bushings would make the RACP more prone to complete failure such as tearing from the stitch welds in the wheel wells without any top side reinforcement. What I believe is that they prevent cracking at the mount points due to less flex in the bushing itself.

That could be, I genuinely don't fully understand the way stress works on the floor with the weird two piece design.

In simplest terms in my mind:
Sheer stress = weight + acceleration of subframe movement

Since you can't really reduce the weight, the only variable you have to play with is acceleration. In this case a rubber or poly bushing would slow the acceleration via deformation and therefor reduce the sheer stress. I do think though that a worn bushing could cause some free play, which actually increases the acceleration and would be much worse than a solid bushing. So that's why I thought poly > rubber for a standard non top-side reinforced car. Solid has many performance benefits, but having a downside that directly impacts a known failure point seems a bit risky.

Suppose much of it is speculation and and not really any observable differences in real world applications. Just interesting to consider!

[mcfreid]

The other factor is the area over which the force is applied. A deforming rubber bushing can cause uneven pressure against the RACP such that the pressure applied to the RACP increases due to the smaller impact area. A solid bushing won't have this issue since the pressure is applied evenly across its surface.

[jet_dogg]

Has anyone had a re-failure of the panel whilst having solids installed?

[pynacl]

I've heard of a few instances of the floor re-failing after a bottom side reinforcement, but I think it's still pretty rare. No clue on the percentage of failures involving solid mounts, that would be interesting to see for sure. I'm sure the differences are negligible in real use, I just wanted to provide a counter to "poly is never a good choice for anything". In the use case of a non top-side reinforced E46 (likely a majority), poly seems to be the best balance between performance, longevity and stress on the floor.

[Arith2]

I've heard of a few instances of the floor re-failing after a bottom side reinforcement, but I think it's still pretty rare. No clue on the percentage of failures involving solid mounts, that would be interesting to see for sure. I'm sure the differences are negligible in real use, I just wanted to provide a counter to "poly is never a good choice for anything". In the use case of a non top-side reinforced E46 (likely a majority), poly seems to be the best balance between performance, longevity and stress on the floor.

Polys have torn in the past where no solid bushing has failed nor OEM bushings within a reasonable service life. The flexing is going to happen because the subframe is not actually a part of the floor structure but separate connected through flexible bearing. Even force will absolutely be applied and while it may not exceed parameters short term, the steel clearly fatigues. With solids, the subframe is part of the floor reinforcement and now the diff is using the diff bushings to apply the force to the subframe which, more evenly, distributes it through the body.

[pynacl]

From my understanding solid subframe bushings will never make it part of the floor, since the floor is two pieces. Without a top side to go with these solid bushings, the forces will transfer to tearing the two pieces apart. I was also thinking that since the subframe assembly also houses upper control arms, rear trailing arms, brakes etc, there are quite a few other forces acting on it besides just the diff. I'm wondering if solid bushings have the potential to cause more harm than good on these cars that are not properly reinforced on both sides.

[Cronenberged]

Updated. Input has been really good and I hope this helps anyone in the future who might have this question just click for one source.

[Schnout]

Whatever you do, be very cautious about using RevShift bushings.
Installing diff bushings is no minor project. My E46 M3 with less than 130,000 miles needed a bushing refresh, so I installed RevShift polyurethane differential bushings.
After only 1,000 miles, the RevShift bushing experienced a catastrophic failure. The polyurethane literally extruded out of the bushing carrier around the bolt, causing the differential to twist relative to the subframe. This caused the driveshaft/differential interface flange to impact the tunnel. Upon removal of the failed RevShift bushings, I was astonished to discover that the "polyurethane" material had the elasticity of a rubber band. Don't be fooled by the "reviews" on Revshift site. They are rigged. Within less than one hour of my posting the appropriately negative review on their site, my honest review was removed. What remains in their review section is only three very suspicious 5-star reviews. I thought I got a bad batch of bushings, but now I know that RevShift fundamentally has no integrity.

[gashby]

Has anyone gone poly diff bushings and gone back to oem?

[9kracing]

Has anyone gone poly diff bushings and gone back to oem?

I'm in the process of doing this.

Right now my car is having the CMP front brace welded in, and the DMG RACP brace installed in the rear.

I had poly bushing for subframe and diff, and now I'm going solid subframe and OE diff.