Subframe Bushes for RACP Reinforcement?

[R60BBA]

The M3 will be going in for RACP reinforcement soon and I thought it would be helpful to hear some thoughts on subframe bushes.

Out of the following, which would you pick and why?

• AKG Motorsport Black (95A) - £260
• AKG Motorsport Red (75D) - £260
• Condor Speed Shop UHMW (68D) - £230
• Powerflex Black (95A) - £209
• Powerflex Yellow/Purple (70A [top] / 80A [bottom]) - £170
• SuperPro SPF4328K (80A - 90A) - £173

Car is mainly for fast road use with three to four track days per annum.

Cost isn’t an issue but am always up for a sweet deal.

N.B. that I spoke with Redish Motorsport about this late last year who recommended Powerflex Yellow/Purple. However I have heard cases of them mushrooming and not being stiff enough. I am also aware that they had to be redesigned in 2017, but their press release does not confirm whether it was to rectify this or not.

Companies info:

• AKG Motorsport, trading since 1985, offer lifetime warranty on parts, based in the US
• Condor Speed Shop, trading since 2007, lifetime warranty on parts, based in the US
• Powerflex, trading since 1996, offer lifetime warranty on parts, based in the UK
• SuperPro, trading since 1983, lifetime warranty on parts, based in AU.

Materials Chart:

SOFT - OE rubber, 70A, 80A, 90A, 95A, 60D, 75D, aluminium - HARD

Diff bushes will be replaced with OE bushes (inc. new diff cover) to keep NVH minimal so don’t worry about those.

[Albino09]

Instead of any of those, you should go solid subframe bushings. Since you're plating/reinforcing the floor pan, I'd recommend the CMP parts out of Australia. They've sized the new bushings to correct the suspension geometry caused by the plates. The increase in NVH from a stiff poly to a solid is minimal, and the primary generator of vibration is from the drivetrain. Since you've already stated you're doing rubber diff bushings, this would be the perfect combination (solid subframe, OEM diff). This is what I plan to do when I reinforce my subframe.

CMP Auto Engineering

[R60BBA]

Instead of any of those, you should go solid subframe bushings. Since you're plating/reinforcing the floor pan, I'd recommend the CMP parts out of Australia. They've sized the new bushings to correct the suspension geometry caused by the plates. The increase in NVH from a stiff poly to a solid is minimal, and the primary generator of vibration is from the drivetrain. Since you've already stated you're doing rubber diff bushings, this would be the perfect combination (solid subframe, OEM diff). This is what I plan to do when I reinforce my subframe.

CMP Auto Engineering

Thanks for your input but I am not so keen on solid/aluminium subframe bushes as they are known for placing more stress on the mounting points (which is already a known weak point on the E46 M3).

Taken from CMP's website, cons of solid bushings:

The cons of the solid subframe bushes varies depending on the car however, we consider it fairly substantial given the issues the E46 chassis has with fatigue related cracking at the subframe mounts and the greater Rear Axle Carrier Panel (RACP).

The cracking and failure that occurs in the RACP is a result of fatigue.

By swapping a rubber bushing for something solid or bearing, you eliminate deformation/dampening and create a much shorter period of time for the rate of change to occur. As a result, the acceleration is much quicker thus creating a higher peak force.

Force on an area makes stress. A higher peak force results in a higher peak stress acting on the chassis.

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.

They then state in order to combat this negative they recommend owner's install their CMP Topside Beam Kit.

I will be installing the Vincebar + Gussets Kit but remain apprehensive about solid bushes for a road car nevertheless.

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

[Syfon]

Thanks for your input but I am not so keen on solid/aluminium subframe bushes as they are known for placing more stress on the mounting points (which is already a known weak point on the E46 M3).

Taken from CMP's website, cons of solid bushings:

The cons of the solid subframe bushes varies depending on the car however, we consider it fairly substantial given the issues the E46 chassis has with fatigue related cracking at the subframe mounts and the greater Rear Axle Carrier Panel (RACP).

The cracking and failure that occurs in the RACP is a result of fatigue.

By swapping a rubber bushing for something solid or bearing, you eliminate deformation/dampening and create a much shorter period of time for the rate of change to occur. As a result, the acceleration is much quicker thus creating a higher peak force.

Force on an area makes stress. A higher peak force results in a higher peak stress acting on the chassis.

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.

They then state in order to combat this negative they recommend owner's install their CMP Topside Beam Kit.

I will be installing the Vincebar + Gussets Kit but remain apprehensive about solid bushes for a road car nevertheless.

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

If you are reinforcing the top side of the mounting points, solid subframe bushes are not an issue. Reinforcement simply by adding plates is not a permanent solution.

I have the entire CMP kit in my car, including the 6 point race brace. NVH increase from oem subframe bushes to solid was negligible.

[R60BBA]

If you are reinforcing the top side of the mounting points, solid subframe bushes are not an issue. Reinforcement simply by adding plates is not a permanent solution.

I have the entire CMP kit in my car, including the 6 point race brace. NVH increase from oem subframe bushes to solid was negligible.

Yep the plan is to go Redish V2 Plate Reinforcement Kit + Vincebar w/ Gussets Kit.

Not concerned about NVH from subframe bushes as it is commonly known that differential bushes are the main culprit (for which I will be going OE with new diff cover as mentioned).

How long have you been running the CMP solid bushes? And what sort of use do you use your car for? (i.e. road, track, bit of both, track only etc).

[Syfon]

Yep the plan is to go Redish V2 Plate Reinforcement Kit + Vincebar w/ Gussets Kit.

Not concerned about NVH from subframe bushes as it is commonly known that differential bushes are the main culprit (for which I will be going OE with new diff cover as mentioned).

How long have you been running the CMP solid bushes? And what sort of use do you use your car for? (i.e. road, track, bit of both, track only etc).

About 18 months but only covered approx 2,000km as the motor spun a bearing mid last year. Car is a weekender and driven hard on the street. Rarely sees a track.

[oceansize]

No fault of cmp but I ordered bushings in April 20 and they still haven't left Australia. So if u are needing to do anything with any level of speed then don't order CMP.

[r4dr]

No fault of cmp but I ordered bushings in April 20 and they still haven't left Australia. So if u are needing to do anything with any level of speed then don't order CMP.

Worldwide shipping is a shitshow right now thanks to Covid, I wonder if any of the US parts companies would consider stocking CMP.

[ClevoCapri]

I did CMP plates and solid bushes with vince bar topside reinforcement may last year. Its my daily driver that I track every now and then. I also did the cmp monoball trailing arm bushes.
Very little if any noticable increase in nvh.
I would say I get more noise from tyres!

[STAATS]

Thanks for your input but I am not so keen on solid/aluminium subframe bushes as they are known for placing more stress on the mounting points (which is already a known weak point on the E46 M3).

Taken from CMP's website, cons of solid bushings:

The cons of the solid subframe bushes varies depending on the car however, we consider it fairly substantial given the issues the E46 chassis has with fatigue related cracking at the subframe mounts and the greater Rear Axle Carrier Panel (RACP).

The cracking and failure that occurs in the RACP is a result of fatigue.

By swapping a rubber bushing for something solid or bearing, you eliminate deformation/dampening and create a much shorter period of time for the rate of change to occur. As a result, the acceleration is much quicker thus creating a higher peak force.

Force on an area makes stress. A higher peak force results in a higher peak stress acting on the chassis.

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.

They then state in order to combat this negative they recommend owner's install their CMP Topside Beam Kit.

I will be installing the Vincebar + Gussets Kit but remain apprehensive about solid bushes for a road car nevertheless.

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

Something he doesnt really cover when he says they should be paired with the topside kit is that basically fatigue requires displacement which is also a explanation for why plates alone can never work... they stiffen up the direct area but still allow differential motion or displacement between the subframe mounts and say the wheel arch connection hence you can still end up with fatigue failures. his comments around the dampening are correct but if you install topside reinforcement and in essence make the load path between the subframe and the chassis rail direct and when you include solid bushes you massively reduce and arguably eliminate any differential displacement between the RACP and the rest of the chassis hence eliminating the mode required for fatigue to occur - you essentially move the potential point of failure the the pick up points on the subframe itself ie where the diff and trailing arms bolt up.

Arguably with topside reinforcement like the vincebar the solid bushings are a safer alternative as the reinforcement on the front points is not as direct as it is for the rear you are relying on the rear bar to take most of the force so with non solid bushings there is still displacement and potentially "hammer" effect on the front points which can be eliminated for solid bushings actually allowing the rear bar to pick up more of the slack. CMP has a bit of a better way of trying to pick up the fronts and vince has expanded his offering in that sense as well although vince tries to do this through a method of increasing the spread of the load so similar to plates but for the top and Cayn tries to do this through a beam back to the rear bar so he essentially sends the load to the rear bar... you can achieve a similar effect with the original vince bar but just using solid subframe bushings but instead of needing the topside bar from the front points the load is essentially transferred through the subframe to the rear bar.

Anyway without displacement testing rigs on these kits its hard to see how the theory plays out in real life so testing time will tell but in theory i would say that if you are installing plates only then yes you should stick with elastomeric bushings as the damping they provide helps reduce peak stress which reduces fatigue during displacement cycles - if you are installing a topside kit (then theoretically you have zero displacement - not fully but enough for this theory to hold in reality - engineering not science remember haha) would actually be better with solids than elastomeric in terms of assisting the overall reinforcement of the RACP.

Just my 2c

[Ceelooboob]

I went with AKG Motorsport Black (95A) when I did my subframe reinforcement about 3 years ago. I like that the rear-end feels a lot tighter after the upgrade, but in all fairness I went from 90k mile original rear suspension to completely refreshed. I also installed poly diff bushings and mounts all at the same time so NVH increased all around so I couldn't really tell you how much of it came from the sub frame mount bushings themselves. AKG seems like a reputable brand, their products have held up well for me over time and they were very helpful during the purchase process.

[STAATS]

I personally found I had reduced NVH going to Solid CMP Subframe bushings and Stock Diff bushings from PF Street Subframe Bushings and PF Street Diff Bushings.

[R60BBA]

I personally found I had reduced NVH going to Solid CMP Subframe bushings and Stock Diff bushings from PF Street Subframe Bushings and PF Street Diff Bushings.

How long have you been running the CMP solid bushes? And what sort of use do you use your car for? (i.e. road, track, bit of both, track only etc).

[oceansize]

How long have you been running the CMP solid bushes? And what sort of use do you use your car for? (i.e. road, track, bit of both, track only etc).

I had asked this question about bushings and the general recommendations for any type of driving was solid bushings for subframe and rtabs as neither of these contribute much in the way of NVH. I went with CMP for both. I'm removed my powerflex yellow which I only used for 5k to 6k miles. However, since my CMP bushings still haven't left Australia I can't comment on the difference moving from fresh poly to solid.

See my thread here. Check out post 4 and 14 in particular.

[R60BBA]

I had asked this question about bushings and the general recommendations for any type of driving was solid bushings for subframe and rtabs as neither of these contribute much in the way of NVH. I went with CMP for both. I'm removed my powerflex yellow which I only used for 5k to 6k miles. However, since my CMP bushings still haven't left Australia I can't comment on the difference moving from fresh poly to solid.

See my thread here. Check out post 4 and 14 in particular.

My concern isn't NVH.

In your 5,000 miles with the Powerflex Yellow/Purple bushes, did you have any issues? I.e. deformation, mushrooming of the bush etc.