Subframe Bushing Help

[travisp412]

Hi everyone,

So I will be getting my subframe reinforced in the spring (Reddish plates) and wanted some advise on what subframe bushings to go with. I've pretty much decided to stay with OE bushings for the diff (I dont want to deal with the additional NVH) and have narrowed it down to the two options for the subframe:

1. CMP solid bushings (I've read due to the added stress the solid bushings cause, you have to reinforce the top side as well. Is this necessary?)
2. AKG 95A Bushings.

What would you guys recommend or are there any other options I should look at? The car will primarily be used as a street car.

Thanks for the help guys!

[phlip]

I've done a couple dozen chasis reinforce/bushing jobs at the shop I work at.
We use AKG 95A almost exclusively in subframe and diff, as well as trailing arm when not using bearings in
trailing arm. I have not used CMP products but I've been told they make nice stuff.
Never had any issues with AKG bushings.

[ZHPizza]

I feel like we know enough now to confidently say that tying the subframe mounts to the chassis rail via topside reinforcement is critical, far more so than plates on the bottom as it actually addresses the root cause of failures by redirecting the load path. Some topside options are easier than others:

- CMP/SME hidden/Vincebar require welding
- Vincebar epoxy is still a lot of work, but no welding
- SME/Mason/Partshopmax braces are the easiest to install with some simple drilling/hole-sawing, but cost you some trunk space


I went down this path throughout 2020. At one point I had 3 x braces, a Vincebar, and two sets of subframe mounts in the garage. I ended up using solid mounts, some homemade chassis rail brackets, and an SME X brace. If I could go back and do it again, I would just install the AKG poly mounts and SME X brace. Would have saved a lot of time/headaches to get the result I wanted: stiffer rear chassis + protected RACP without added NVH



I still have some of the parts I didn't use if you need them: another SME X Brace, Partshopmax brace, AKG 95a subframe mounts, maybe more?

[r4dr]

ZHPizza What's your reasoning on why you would go with AKG poly mounts vs. solid mounts if you could do it again? Just out of curiosity.

[dilmorecg]

I just had mine reinforced with underside plates and I went with solid subframe bushings and stock rubber everywhere else. I used Turner solid bushings mainly because I got them on sale about a year ago. I did a fair amount of research on the old forum and decided on this setup because the consensus was no NVH with solid subframe bushings. I cant remember 100% but I believe the new M3's have solid subframe mounts. I cant actually give you my opinion on the feel yet since I just had it done and haven't had much seat time since then. IIRC - with solid there is no flex so less pulling on the mounting points.

Like ZHPizza says the most recent research says topside is also required to fully fix the issue. I have a vert which has more metal to begin with so I just went with underside plates for now. CMP has a lot of info on their site in the tech section and down the road id probably just add their RACP to chassis plates.

[enjoy_m3]

You have to reinforce the topside regardless of what bushings you use for the subframe. The issue is the whole RACP and the stress fractures are both on the bottom and top. You only transfer additional stress to the topside if you only do the bottom reinforcement. Look up the reddish videos on youtube and they do a complete explanation of the RACP failure on the e46 chassis.

I've reinforced the bottom and used AKG 95 bushings on the subframe and still found cracks around the wheel well and trunk area and had to drop the subframe again for a vincebar and repair the popped spot welds and stitch weld around the trunk and wheel well areas. I've since switched to solid subframe bushings and have no noticeable NVH increase.

[LSBNick]

I used AKG solid subframe bushings, SDW monoball RTABs, and OE Diff bushings. No complaints so far. Also used the AKG subframe kit b

[Syfon]

OP FWIW I have the CMP solid raised subframe bushings in mine. I replaced all the rear suspension bushings when I did the rear end; everything is OE except the rtabs (powerflex yellow i think) subframe bushings. NVH increase was marginal, and I can hear a little diff whine if i put it in neutral and coast. I don't hear anything while driving though; my exhaust is too loud lol.

[ZHPizza]

ZHPizza What's your reasoning on why you would go with AKG poly mounts vs. solid mounts if you could do it again? Just out of curiosity.

I elected not to add plates, so I wanted to maximize the contact area between the subframe bushings and RACP to spread the stress loading. It seemed like a lot of the solid bushings just copied the shape of the OE bushings, while AKG's designs really spread the load. In the end I went with solid bushings from Partshopmax for even more contact area and the resulting subframe height was a bit wonky with the aluminum bracket on my ZHP. They said they never test fit them on a 330 and weren't helpful with finding a solution. I also get some clunking in the cabin from gear changes now but I think that's mostly due to the helical diff that I installed at the same time.

[travisp412]

OP FWIW I have the CMP solid raised subframe bushings in mine. I replaced all the rear suspension bushings when I did the rear end; everything is OE except the rtabs (powerflex yellow i think) subframe bushings. NVH increase was marginal, and I can hear a little diff whine if i put it in neutral and coast. I don't hear anything while driving though; my exhaust is too loud lol.

That's perfect! My exhaust is pretty loud too, so pretty sure any slight noise would be drowned out lol

[Tbonem3]

As far as nvh is concerned, you get more from diff choice than subframe

[George Hill]

We see this question a lot of what did YOU use, but I think the question should be what did you use and WHY? I think we find a lot of people use X part because someone they hold to a high regard (whether deserving or not) said use this part.

[oceansize]

Agree with George. I don't comment at all on the technical suspension comparisons or really technical engine internals because I don't have the required experience and breadth of knowledge to contribute. I mean unless I'm pushing the car to the limits, or close to it, which means going to a track I'm not sure how I could. Even then I would need to experiment with many different setups (like a few here have).

The only mod that I feel I can safely consider of universal benefit to everyone is solid RTAB and subframe bushings. There really isn't any downside (assuming proper reinforcement), other than maybe the tiniest bit of noise, because they make the car feel more planted and responsive. I had poly and IMO even poly doesn't provide what solid does.

The other crucial piece that everyone can benefit from is simply keeping all your ball joints, bushings, couplers, csb/guibo, and engine/tranny mounts fresh.

[Good]

I love my CMP solid Diff/Subframe bushings. Car feels great, and sounds like the cabin of a GT3 Cup car.

[Slideways]

Does anyone have a good amount of experience (particularly someone sensitive to NVH levels on a street car) with the following setup: quiet exhaust or stock, OE diff bushings, and solid subframe bushings?