F8X M3/M4 CF Drive shaft mod

[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…


Sent from my iPhone using Tapatalk

[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.

[0-60motorsports]

Thanks for the detailed great feedback

[x Spades x]

Thank you for taking the time, energy, and patience for installing/testing/write up. This was awesome to read, as this thread journey has been a tumultuous one, 😆 🤣 😂. My deepest appreciation!

[Bry5on]

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!

[x Spades x]

Awesome work, THANK YOU!!

[CrisSilberGrau]

Bry5on Thank you I appreciate it, that clears it up.

[Bry5on]

Bry5on please indicate the exact points you measured the length from.

I did that above, but reiterating for the sake of clarity:
front: guibo flange surface
rear: solid end of the driveshaft

You do not need a rear CV joint installed to build one of these using the dimensions above. So no nonsense about whether the CV joint is fully or partially extended or anything like that.

[CrisSilberGrau]

Bry5on please indicate the exact points you measured the length from.

[Bry5on]

Ok, thanks to EthanolTurbo the driveshaft is in my hands. Some info:

Measurements (~ +/-2mm):
Stock driveshaft: 1365mm
Modified F80-e46 carbon driveshaft: 1360mm

Weights, like-for-like naked with rear CV only (bathroom scale, using me as tare weight, repeated 6x each alternating back and forth between driveshafts, ~ +/- 0.1lb):
Stock driveshaft: 17.5lb
Modified F80-e46 carbon driveshaft: 14.1lb

Weight savings: 3.4lb, 19%

Extra measurements:

Diameter, carbon center shaft: 79.8mm
Diameter, carbon steel ends: 88.5mm

Diameter, e46 front: 75.1mm
Diameter, e46 rear: 60.3mm

Length, e46 front section, from flange as in photos: ~635mm
Length, e46 rear section, to solid end of shaft (not CV): ~730mm

Hope this clears some things up. Will try to measure runout and get to a test fit as soon as I get the chance. I'm betting that being 5mm short doesn't end up being an issue because I'm running solid subframe bushings and there's more than 5mm of play in the subframe bushings alone.

Edit: I don't think this has a moment of inertia benefit, so this *shouldn't* feel like a lightweight flywheel in first gear. So we'll see what the difference is with the stiffer shaft and fewer moving joints.

[EthanolTurbo]

I’m game to do a test if it installs without dropping the rear end! Alternatively we can take some detailed measurements of the stock shaft against the modified one to verify you got the dimensions sorted out with the last go around. I moved down to SD a few months back, it’s been a long time coming!

Awesome yeah, either works. You have to rock the diff backwards to install.

I'll PM you my phone #.

[0-60motorsports]

I think grape mint is what he smokes, he probably copied you. Message me on IG so I can start your driveshaft conversion.

Hahahahhahah YES! I can't find you bud. Can you please add me 060motorsports on ig

[Bry5on]

Hah, when did you move to SD? I thought you were in the Bay. I can go grab you a non-telescoping CF driveshaft and take your donor shaft to the drive shaft shop in Garden Grove for the conversion. Or you can test my already converted CF DS.

I’m game to do a test if it installs without dropping the rear end! Alternatively we can take some detailed measurements of the stock shaft against the modified one to verify you got the dimensions sorted out with the last go around. I moved down to SD a few months back, it’s been a long time coming!