F8X M3/M4 CF Drive shaft mod

[Altaran]

Sigh. I wonder where I'll end up with subframe bushings raising it up, but subframe reinforcement plates lowering it down. Should net pretty close to stock location…

Similar to where I am, I also have the same combo.

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[Obioban]

Sigh. I wonder where I’ll end up with subframe bushings raising it up, but subframe reinforcement plates lowering it down. Should net pretty close to stock location…

[Altaran]

So you're saying it does clear with the CMP main beam mounts that move it upwards?

Do you think it would be an issue with the standard diff mounts and CMP main beam mounts?

Yes, there's no issue with contact on the top side of the tunnel.

It should be fine with the standard diff mounts. The rubber portion in them is actually fairly small so the diff won't move a lot.

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[Altaran]

Woah. That's a significant shift. Which poly diff bushings were you running? I have AKG 95A in mine and would like to avoid running solid diff bushings so I can enjoy a freeway drive without a crying diff, but seeing how close that is I'm having second thoughts.

I had the AKG 75D. The problem is the same for all poly bushings, you can literally push them in without much force. How is that supposed to hold up under load?
If you don't want to go solid, go OEM.

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[Mike RT4]

This pic was when I had the driveshaft in and the diff moved due to shit poly diff bushings, close one...
I don't have a pic now with solid bushings, but it sits dead center.
I run cmp subframe bushings and millway diff bushings. Doesn't matter which brand though. Just stay away from anything poly.

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So you're saying it does clear with the CMP main beam mounts that move it upwards?

Do you think it would be an issue with the standard diff mounts and CMP main beam mounts?

[Hfj]

Bimmerworld I believe, they are non-raising. Stock diff bushings. You want enough room that wearing diff bushings will not cause you to nuke your driveshaft. A totally blown out set of diff bushings will likely end with a failed driveshaft even with solid subframe bushings, it’s pretty tight, too tight for a camera to fit beyond what’s in the video.

Avoid using CMP here as they raise the subframe up. VAC sells both options, that’s probably the safest bet. There are probably a bunch of choices but don’t leave it to guess work.

Thanks for that. I’ll probably end up going AKG or Bimmerworld solid all around. They seem to not lift the subframe to compensate for reinforcement plates.

[Hfj]

This pic was when I had the driveshaft in and the diff moved due to shit poly diff bushings, close one...
I don't have a pic now with solid bushings, but it sits dead center.
I run cmp subframe bushings and millway diff bushings. Doesn't matter which brand though. Just stay away from anything poly.

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Woah. That’s a significant shift. Which poly diff bushings were you running? I have AKG 95A in mine and would like to avoid running solid diff bushings so I can enjoy a freeway drive without a crying diff, but seeing how close that is I’m having second thoughts.

[Bry5on]

Which solid subframe bushings are you running? Are you running solid diff bushings as well? Do you have a picture of the tightest point of the driveshaft to the trans tunnel?

Bimmerworld I believe, they are non-raising. Stock diff bushings. You want enough room that wearing diff bushings will not cause you to nuke your driveshaft. A totally blown out set of diff bushings will likely end with a failed driveshaft even with solid subframe bushings, it’s pretty tight, too tight for a camera to fit beyond what’s in the video.

Avoid using CMP here as they raise the subframe up. VAC sells both options, that’s probably the safest bet. There are probably a bunch of choices but don’t leave it to guess work.

[bigjae46]

Just stay away from anything poly.

Yeah. Everyone once in awhile I need to install poly something somewhere and remind myself. lol

[Altaran]

Which solid subframe bushings are you running? Are you running solid diff bushings as well? Do you have a picture of the tightest point of the driveshaft to the trans tunnel?

This pic was when I had the driveshaft in and the diff moved due to shit poly diff bushings, close one...
I don't have a pic now with solid bushings, but it sits dead center.
I run cmp subframe bushings and millway diff bushings. Doesn't matter which brand though. Just stay away from anything poly.

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[Hfj]

Alright, I had a few free hours this morning and there was an open lift, so here we go!

How it started:


A few minutes later:




And another 30 minutes after that, we're all buttoned up again and ready for a test drive:


This is the F80 driveshaft that EthanolTurbo had modified with e46 M3 flex disc and CV joints. It is exactly 5mm shorter than a factory M3 driveshaft. Because it's shorter, I did not have to unbolt or drop the diff or subframe to install it, as you can see above. This is a tradeoff, as there will be less spline engagement with the driveshaft and CV joint. I think 2-3mm shorter than stock would be the sweet spot if you're not using the telescoping F80 version of the shaft (which is a better solution overall, in my opinion). Install was easy, just like any other driveshaft, only made difficult by the shitty supersprint header joint.

We measured before that this shaft is 80mm OD, versus the 75mm max OD stock shaft, so the math says that this carbon shaft should be much stiffer (bigger diameter, 50% stiffer material, likely also thicker wall) than the original and also bring in some of the inherent damping that carbon weaves have that steel does not. It's also one piece, although I don't think this is likely to have any real noticeable effect from the seat. We measured that it's 3.5lb lighter than the steel part, which I also don't think will be too seat of the pants noticeable, because inertially these math out to be very similar due to the larger OD of the carbon shaft.

Because this shaft is larger OD, it's a very tight fit. This is only suitable for cars with non-raised, solid subframe bushings. My car has *non-raised* & solid subframe bushings with reinforcement plates (4-5mm thick, which helps gain clearance) and I had about 8mm clearance to the transmission tunnel. Any displacement of the subframe/diff under acceleration or due to adding subframe raising solid bushings is likely to put you at risk of hitting the transmission tunnel and disintegrating your driveshaft. Consider this fair warning if you choose to go carbon, and I cannot stress this enough. Carbon does not fail subtly or slowly.

https://youtu.be/umdLZ_PUL_w

Good news, I did not have to make any clearance to my unmodified exhaust heat shield.

So what's the verdict? My initial thoughts say a very minor positive difference versus steel.

First of all, since I'm in San Diego it's easy for me to make a quick trip to Mexico where I tested the car up to 120mph (with stock gearing) and had absolutely no noticeable vibration. I'm sensitive to vibration. Sort of OCD about it. In fact, I'd say that the car is ever so slightly quieter at speed. I was expecting this as it's a common noted difference, so there may be a bit of placebo here.

When I initially took the car for a drive, I was pretty hopeful because I couldn't detect any semblance of the m-clunk. This changed once the car had warmed up. I'd say this was a mild improvement to the clunk (more of a thud now in my car), definitely less of an improvement than the rear end bracing made (my touring cabin X-brace and subframe-v-brace). Any improvement here is welcome, but it's not a magic bullet. Shift & shifter quality is largely the same, just with slightly less of the thud on a hard 1-2 shift.

There is also an improvement during clutch take-up, it just feels smoother and a bit more direct - in the direction of, but not equivalent to, a Porsche. This is maybe the most detectable and biggest difference.

There is no perceptible increase in the car accelerating quicker or revving faster that I can tell. Note that the math also generally agrees with this.

On just one occasion, it did seem that I was able to excite some natural frequency somewhere around 40-50mph under WOT second gear acceleration after a 1-2 shift. It felt similar to a failing CSB, but I wasn't able to reproduce this, so it could have been coming from the road, as I was on a highway onramp in a landfill area.

So overall I'd say this is mostly a smoothness, directness and NVH improvement, in that order. All effects are relatively minor but if you're OCD about NVH and needed a driveshaft balance like me anyway, I'd say go for it. It's an improvement. If you don't need a driveshaft, maybe wait until your next 'while I'm in there' opportunity. If you're debating whether to do this or a chassis stiffening mod first, *definitely* do the chassis stiffening mods first, you'll feel those a lot more.

Hope that's helpful!

Which solid subframe bushings are you running? Are you running solid diff bushings as well? Do you have a picture of the tightest point of the driveshaft to the trans tunnel?

[Tonggi]

Because this shaft is larger OD, it's a very tight fit. This is only suitable for cars with non-raised, solid subframe bushings. My car has *non-raised* & solid subframe bushings with reinforcement plates (4-5mm thick, which helps gain clearance) and I had about 8mm clearance to the transmission tunnel. Any displacement of the subframe/diff under acceleration or due to adding subframe raising solid bushings is likely to put you at risk of hitting the transmission tunnel and disintegrating your driveshaft. Consider this fair warning if you choose to go carbon, and I cannot stress this enough. Carbon does not fail subtly or slowly.

This is what has me worried. I have the raised solid subframe bushings but I also have poly engine and trans. I dont foresee the driveshaft moving much in the transmission tunnel. Only on the actual CF piece would there be issues, the rear flange to the diff should be fine.

Glad I read this, didn't think about this before and thanks for the honest feedback. The marketing behind the CF driveshaft makes it sound like its best invention after sliced bread. I like the idea of it and want one but at least I know to get one when its time to replace rather than FOMO on huge performance

[fattycharged]

great review and thanks for getting the measurements sorted out.
been sitting on a telescoping version for about a year now and need to just bite the bullet on this or pass it along to someone else…


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[bigjae46]

Alright, I had a few free hours this morning and there was an open lift, so here we go!

How it started:


A few minutes later:




And another 30 minutes after that, we're all buttoned up again and ready for a test drive:


This is the F80 driveshaft that EthanolTurbo had modified with e46 M3 flex disc and CV joints. It is exactly 5mm shorter than a factory M3 driveshaft. Because it's shorter, I did not have to unbolt or drop the diff or subframe to install it, as you can see above. This is a tradeoff, as there will be less spline engagement with the driveshaft and CV joint. I think 2-3mm shorter than stock would be the sweet spot if you're not using the telescoping F80 version of the shaft (which is a better solution overall, in my opinion). Install was easy, just like any other driveshaft, only made difficult by the shitty supersprint header joint.

We measured before that this shaft is 80mm OD, versus the 75mm max OD stock shaft, so the math says that this carbon shaft should be much stiffer (bigger diameter, 50% stiffer material, likely also thicker wall) than the original and also bring in some of the inherent damping that carbon weaves have that steel does not. It's also one piece, although I don't think this is likely to have any real noticeable effect from the seat. We measured that it's 3.5lb lighter than the steel part, which I also don't think will be too seat of the pants noticeable, because inertially these math out to be very similar due to the larger OD of the carbon shaft.

Because this shaft is larger OD, it's a very tight fit. This is only suitable for cars with non-raised, solid subframe bushings. My car has *non-raised* & solid subframe bushings with reinforcement plates (4-5mm thick, which helps gain clearance) and I had about 8mm clearance to the transmission tunnel. Any displacement of the subframe/diff under acceleration or due to adding subframe raising solid bushings is likely to put you at risk of hitting the transmission tunnel and disintegrating your driveshaft. Consider this fair warning if you choose to go carbon, and I cannot stress this enough. Carbon does not fail subtly or slowly.

https://youtu.be/umdLZ_PUL_w

Good news, I did not have to make any clearance to my unmodified exhaust heat shield.

So what's the verdict? My initial thoughts say a very minor positive difference versus steel.

First of all, since I'm in San Diego it's easy for me to make a quick trip to Mexico where I tested the car up to 120mph (with stock gearing) and had absolutely no noticeable vibration. I'm sensitive to vibration. Sort of OCD about it. In fact, I'd say that the car is ever so slightly quieter at speed. I was expecting this as it's a common noted difference, so there may be a bit of placebo here.

When I initially took the car for a drive, I was pretty hopeful because I couldn't detect any semblance of the m-clunk. This changed once the car had warmed up. I'd say this was a mild improvement to the clunk (more of a thud now in my car), definitely less of an improvement than the rear end bracing made (my touring cabin X-brace and subframe-v-brace). Any improvement here is welcome, but it's not a magic bullet. Shift & shifter quality is largely the same, just with slightly less of the thud on a hard 1-2 shift.

There is also an improvement during clutch take-up, it just feels smoother and a bit more direct - in the direction of, but not equivalent to, a Porsche. This is maybe the most detectable and biggest difference.

There is no perceptible increase in the car accelerating quicker or revving faster that I can tell. Note that the math also generally agrees with this.

On just one occasion, it did seem that I was able to excite some natural frequency somewhere around 40-50mph under WOT second gear acceleration after a 1-2 shift. It felt similar to a failing CSB, but I wasn't able to reproduce this, so it could have been coming from the road, as I was on a highway onramp in a landfill area.

So overall I'd say this is mostly a smoothness, directness and NVH improvement, in that order. All effects are relatively minor but if you're OCD about NVH and needed a driveshaft balance like me anyway, I'd say go for it. It's an improvement. If you don't need a driveshaft, maybe wait until your next 'while I'm in there' opportunity. If you're debating whether to do this or a chassis stiffening mod first, *definitely* do the chassis stiffening mods first, you'll feel those a lot more.

Hope that's helpful!

Awesome! Thanks for getting most of the information in one spot. But I am almost sure I can think of a different dumb question to ask...lol

[EthanolTurbo]

There you guys go, that's why I handed it to an engineer. 😅 Thank you Bry5on for the super helpful data and analysis.

So it works. If anyone wants one, I can start converting them in batches or you can do it yourself with local shops.