Subframe Bushes for RACP Reinforcement?

[R60BBA]

When I drove the car home from the shop which was about a 1.5 hr drive, I noticed the rear end definitely felt more stable and less harsh over road imperfections. I believe the worn bushings actually started to transfer some nvh but I didn't notice until I had the subframe bushings done and started driving the car home. Turn in was a little more responsive and car felt more planted to the road.


Well there was actually no issue with the reinforcement of the plates on the underside of the body. I think most e46 owners believe that "subframe reinforcement" is just the weld in plates on the underside. I've tracked the car a few times, on 265 ad08r tires, which is a street tire and only on KW V2 coilovers which is a relative soft suspension setup. The crack developed on the wheel well side on the bottom where there is seam sealer. It's a very small crack that you probably won't notice but I do my own maintenance on the car and would typically give the car a look over before a track day. This was when I started doing a lot of research and found out that you definitely need to reinforce the trunkside in some way. There are some shops who will reinforce the top side by welding it. Which may be enough, but I can confirm the plates alone is only 50% of the reinforcement.

Ahh I misread what you said (I thought you were implying the ride was better with the stock worn bushings!). Thanks for the clarification.

Fully aware on the issue and as mentioned will be installing a Vincebar + Gussets Kit.

[enjoy_m3]

Already answered in post 3.




I agree with this (bar the solid bushing point). Some questions:

1. When you say you noticed a better ride with the stock subframe bushes over the AKG Black 95A bushes, could you please elaborate?
2. You mentioned your car cracked again despite installing underside plates. Who did the work and are they willing to rectify it for you?

I don't think you can compare the new BMW design which bolts the subframe to the RACP to using solid bushes on the E46, due to the new subframe design itself having a much wider surface area to disperse load.




Thanks for confirming, a roll cage should further solve this 'cracking' problem for you too.

When I drove the car home from the shop which was about a 1.5 hr drive, I noticed the rear end definitely felt more stable and less harsh over road imperfections. I believe the worn bushings actually started to transfer some nvh but I didn't notice until I had the subframe bushings done and started driving the car home. Turn in was a little more responsive and car felt more planted to the road.

Well there was actually no issue with the reinforcement of the plates on the underside of the body. I think most e46 owners believe that "subframe reinforcement" is just the weld in plates on the underside.
I've tracked the car a few times, on 265 ad08r tires, which is a street tire and only on KW V2 coilovers which is a relative soft suspension setup. The crack developed on the wheel well side on the bottom where there is seam sealer. It's a very small crack that you probably won't notice but I do my own maintenance on the car and would typically give the car a look over before a track day. This was when I started doing a lot of research and found out that you definitely need to reinforce the trunkside in some way. There are some shops who will reinforce the top side by welding it. Which may be enough, but I can confirm the plates alone is only 50% of the reinforcement.

Here's 1 video of their subframe reinforcement, it's definitely worth watching along with the reddish motorsport videos. You'll learn a lot about the failures of the rear e46 racp. Truth be told, this is such a huge hassle to deal with. But I've rebuilt my motor already. I do enjoy driving the car, especially on the track but would like to keep it streetable so I guess there's no turning back now. Lol.

this video shows why they fail at the 5 min mark

https://www.youtube.com/watch?v=ZR5mn5iN4vY&t=419s

this video shows more on the repair

https://www.youtube.com/watch?v=zji7OU6yRKY&t=215s

You can see that they already reinforced years ago with just plates and they still cracked, granted the car was swaped with a v10 but this may apply to e46 m3 owners with high hp forced induction. Stock s54 may not have the same kind of failure but this video and my experience confirmed it will fail, if not in the racp, then in the seams of the back side of the e46 if not reinforced properly on both top and bottom.

[stash1]

Went w/the red (75D) AKG's (because I just couldn't commit to solids) about 4 yrs./15k mi ago when I did the rear sf/suspension, and couldn't be any happier...really firmed up the rear as everyone has been mentioning. Almost no perceived increase in NVH. I just inspected them the other day while some other maint. work, and they still look brand new.

[BMWahba]

I'm installing Vincebar + Gusset kit with CMP raised bushings. No worries here about aluminum bushings causing more fatigue.

[Apex_80]

FWIW I went with solid aluminum variant from TMS. They are a really nice piece and after reinforcement I think aluminum is the answer. Remember these subframe mounts crack because of torsion acting on a bad load path, not because of the pressure of the bushing on the chassis. With the added plates especially I see no reason why an aluminum piece would cause more cracking. It is worth noting that I experienced no increase in NVH from them the car just feels tighter.

[Altaran]

You sir have hit the nail on the head.

I think I'm going to go AKG Black 95A, which should be appropriate for my usage. Their bushes are also made of a military grade compound (exclusive to them) which has a memory function and so should not deform like how some Powerflex ones have been reported.

However, may I ask whether you have had any issues with your RACP (i.e. cracks/popped spot welds re-occuring) since you installed the AKG 75Ds?

I have not had any issues with the RACP, but as mentioned I am using reinforcement plates. 95 works great on the subframe and diff, just stay away from the motor and tranny mounts unless you really want race car NVH.

Going forward I do plan to use a top side reinforcement as well, but this is more due to the fact I switched to rear coilovers so I want to spread the load on the factory rear shock towers which now have to carry spring load as well. But in any case it does not hurt.

[R60BBA]

Why wouldn't you just do a topside reinforcement

Already answered in post 3.

I have akg 90a bushings with PSDesign plates. I noticed a much better ride from stock worn bushings and did not add NVH. I have very little NVH due to 75a differential bushings.

Based on my experience and research, underside plates does not solve the RACP issue unless the trunk side is also reinforced. Driftworks documented the issue on their v10 m3 explaining why adding plates alone only makes it worse. Of course, I wasn't aware and though. You will only transfer the stress to other areas and the spot welds will start comming apart.

I've tracked my car a few times and I noticed the seam sealer cracking on 1 side of my wheel well.

I'm now re-doing my rear end with solid subframe bushings and adding a vince bar.

Subframe bushings do not add NVH. The Differential bushings and transmission bushings do.

So make sure you reinforce the trunk side also. When the upper trunk side and bottom is reinforced, then you should be good.

I would argue that you dont want flex in the rear subframe mount to the body, if you look at the f80 m3, they have the rear subframe solid mounted to the body, for increased rigidity and reduced flex without harsh nvh. It's unfortunate that the e46 has this issue, its a real pain and hassle to take care of carefully.

If you watch reddish motorsport videos on youtube, you'll get an idea of areas to look for where cracks develop and spot welds start comming apart, it's not just around the rear subframe mounting points. Good luck!

I agree with this (bar the solid bushing point). Some questions:

1. When you say you noticed a better ride with the stock subframe bushes over the AKG Black 95A bushes, could you please elaborate?
2. You mentioned your car cracked again despite installing underside plates. Who did the work and are they willing to rectify it for you?

I don't think you can compare the new BMW design which bolts the subframe to the RACP to using solid bushes on the E46, due to the new subframe design itself having a much wider surface area to disperse load.

I have been running them about 5000miles so far... At the moment those 5000miles have been entirely street miles but my car has been built around the fact it will transition from street to street/track and then fully track... so as a result all parts have been selected for it to be perfectly drive-able on the street. As the only change from street to street/track will be CF Roof and Cobra Nogaros with quickfit harness - and then the transition from that to track will only be full cage and air bag deactivation or potential removal depending on how permanent the track status is all the parts currently installed on the car will stay... Id say I have taken my track focus just a whisker further then what Obioban has in his build as his focus stayed a bit more on street than mine whereas I was happier to make more compromises - althought I see no compromises with the CMP solid subframe bushings and monoball RTABs and only improvements (IF you have done both top and bottom RACP reinforcement...)

Thanks for confirming, a roll cage should further solve this 'cracking' problem for you too.

[STAATS]

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 have been running them about 5000miles so far... At the moment those 5000miles have been entirely street miles but my car has been built around the fact it will transition from street to street/track and then fully track... so as a result all parts have been selected for it to be perfectly drive-able on the street. As the only change from street to street/track will be CF Roof and Cobra Nogaros with quickfit harness - and then the transition from that to track will only be full cage and air bag deactivation or potential removal depending on how permanent the track status is all the parts currently installed on the car will stay... Id say I have taken my track focus just a whisker further then what Obioban has in his build as his focus stayed a bit more on street than mine whereas I was happier to make more compromises - althought I see no compromises with the CMP solid subframe bushings and monoball RTABs and only improvements (IF you have done both top and bottom RACP reinforcement...)

[cozmo kraemer]

I did mine many years ago, and so there are much better solutions on the market now. That being said, I have had no issues in 180k miles.

I had done the BMW epoxy foam to the top of the rear two mounts (4 total tubes). I got paranoid when many other people reported failures and so I pulled the rear subframe, did Turner plates (there were no Reddish plates at the time), and the AKG black poly bushings for the fact that they increased the surface area of the mount to the RACP. I felt like that would reduce the concentration of force that caused the mounts to crack. It all seems to be working.

If doing it now, I would do solid bushings, Reddish plates and the Vincebar (or similar).

[enjoy_m3]

I have akg 90a bushings with PSDesign plates. I noticed a much better ride from stock worn bushings and did not add NVH. I have very little NVH due to 75a differential bushings. Based on my experience and research, underside plates does not solve the RACP issue unless the trunk side is also reinforced. Driftworks documented the issue on their v10 m3 explaining why adding plates alone only makes it worse. Of course, I wasn't aware and thought plates were enough. You will only transfer the stress to other areas and the spot welds will start comming apart. I've tracked my car a few times and I noticed the seam sealer cracking on 1 side of my wheel well. I'm now re-doing my rear end with solid subframe bushings and adding a vince bar. Subframe bushings do not add NVH. The Differential bushings and transmission bushings do. So make sure you reinforce the trunk side also. When the upper trunk side and bottom is reinforced, then you should be good. I would argue that you dont want flex in the rear subframe mount to the body, if you look at the f80 m3, they have the rear subframe solid mounted to the body, for increased rigidity and reduced flex without harsh nvh. It's unfortunate that the e46 has this issue, its a real pain and hassle to take care of carefully. If you watch reddish motorsport videos on youtube, you'll get an idea of areas to look for where cracks develop and spot welds start comming apart, it's not just around the rear subframe mounting points. Good luck!

[oceansize]

You sir have hit the nail on the head.

I think I'm going to go AKG Black 95A, which should be appropriate for my usage. Their bushes are also made of a military grade compound (exclusive to them) which has a memory function and so should not deform like how some Powerflex ones have been reported.

However, may I ask whether you have had any issues with your RACP (i.e. cracks/popped spot welds re-occuring) since you installed the AKG 75Ds?

Why wouldn't you just do a topside reinforcement which would then allow you to run solids and be done with any concern about whether or not you are going to have issues in the future?

[R60BBA]

I would not go with completely solid aluminium bushings, since that would increase the stress on subframe and chassis quite a bit. Having a little flex in the bushings helps.

You sir have hit the nail on the head.

I think I'm going to go AKG Black 95A, which should be appropriate for my usage. Their bushes are also made of a military grade compound (exclusive to them) which has a memory function and so should not deform like how some Powerflex ones have been reported.

However, may I ask whether you have had any issues with your RACP (i.e. cracks/popped spot welds re-occuring) since you installed the AKG 75Ds?

[Altaran]

I have been running the red AKGs in subrame and diff for the last 5 years. They were installed at the same time the subframe mounts were welded and reinforced with plates.

NVH increases a little bit, you can also hear the diff a bit more than with stock. But it is not nearly as bad as the NVH from using poly motor and tranny mounts.

I would not go with completely solid aluminium bushings, since that would increase the stress on subframe and chassis quite a bit. Having a little flex in the bushings helps. Though compared to stock before it really firms up the rear end.

Race guys use solit metal "bushings", but they get away with it since those cars have cages welded in to distribute the load.

[oceansize]

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.

None but I'm in a vert that doesn't see track use, just a fair amount of spirited driving. I definitely do not baby the car on TN back roads At this point I'm really interested in the comparison between poly and solid as I'm moving from fresh OE RTABS (2500 miles) with limiters and fresh poly to the solid setup.

[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.