F8X M3/M4 CF Drive shaft mod

[bigjae46]

No, these have been in since 2012…I just didn't drop the subframe fully, I should have, would have made my life easier in a number of ways replacing everything back there, but I like taking the long way disguised as a shortcut…


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lol...been there! I remember looking at BW's fuel starvation kit. $300? WTF...I can do it for less!!! So fast forward, 3 failures on track, lots of climbing through the cage to remove the pump, $450, and a constant fear of "is that fuel I smell?"...realized I should have just bought the BW kit for $300.

[9kracing]

I know you guys hate him, but has anyone read the entry Matt did about the driveshafts in these cars?

Seems like this is an issue he already ran into.









The E46 M3's driveshaft challenge.


The E46 M3 presents unique challenges to develop a carbon driveshaft. The engine and transmission sit at a slight angle with the rear subframe and differential. The driveshaft and chassis do not sit at perfectly parallel.

BMW M's original 2 piece steel driveshaft solves this problem by using a tapered tube design, a center supporting bearing and a U-joint as seen below.

The downside to this design is reduced performance & feel from increased weight and reduced stiffness. These components will eventually wear, clunk and vibrate.

Unfortunately, BMW driveshafts are not rebuildable. It's partly why we upgrade to a carbon driveshaft.

The V2's carbon tube limitations.


With carbon tubes, we have neither options available to us.

Due to their winded carbon construction, the carbon tubes cannot be tapered nor conform to transmission tunnel shapes.

As previously discussed, carbon tubes stiffness generally increases with diameter. With the V2, the original diameter developed for the E9x M3 and E6x M5/6 was retained due to its extensive development time.

Its strength and heat resistance has been demonstrated on over 1,000 + M cars with extremely low failure rates. The V2's selected diameter is stronger than its original steel counterpart with a torque rating of 4,000 ft/lbs.



The E46 M3's V2 development focused on new flanges specific to the E46 M3. It featured a smaller flexdisc and PCD compared to the E9x/E6x M cars.



The SMG2 also presented clearance constraints with the shifter linkage - an elongated flange was designed as a result. The V2's flange also required longer bolts.



Additionally, the E46 M3's original shaft uses a constant velocity joint without a cap due to tighter flange clearances. The bearings are thus exposed - and will require greasing. Below is an example with the carbon driveshaft.



Many will confidently state the E46 cannot be fitted with a 1 piece driveshaft due to the driveline's angle. The challenge is not the angle per say, but rather the tunnel clearances.

The constant velocity joints are specifically designed to absorb driveline shock and movement in any angle within its mechanical limitations. I've uploaded a short to my YouTube channel explaining this here.

My limited V2 experience.


In 2022, I fitted the V2 carbon driveshaft to my E46 M3 6spd. Back then, my E46 M3 was close to stock with original suspension, bushing and mounts.

I was able to drive the car in the Fall prior to its complete tear down for restoration and modernization of the body. I reported no issues at the time.

The product's manufacturer had been selling this variant prior to my installation and hadn't raised any concerns. Under my scope, the V2 carbon driveshaft was sold to dozens of E46 M3 owners worldwide starting in 2022.

This is where I failed: too much trust and not enough verification.

The V2's problems.


In late 2023, issues started arising in an E46 M3 race car with vibrations above 200 kph.Vibrations are not normal on this carbon driveshaft design considering the use of the original flexdisc and the constant velocity joint.

Upon further inspection, the carbon tube had come into contact with the chassis, leading to structurally damaged tubes.

In early 2024, 3x new cases were reported to me with all being on SMG2 equipped M3s. Upon further inspection, the same tunnel contact problem was identified. This was an identifiable pattern that would be later confirmed with new cases in late 2024 including the first 6spd equipped cars with a stiffened chassis. Local testing.


As my car was not available for testing, I provided a V2 driveshaft to a local E46 M3 for hands-on testing. It was a 6spd car with stiffer bushings and lowered, stiffer suspension. The owner tracks the car at local HPDEs.



Vibrations arose at 190 kph. Upon inspection, tunnel contact was also identified specifically where the original center supporting bearing is installed.

Below are pictures of the expected damage if it was caught early on. Continued driving would eventually lead to tube failure; it would split completely.



Additionally, I added pictures of what will likely happen if you fit the constant velocity joint with the cap. There are signs of contact with the differential pinion flange. This likely played a role in further vibrations.

In 2024, I pressed my concerns with the manufacturer.

They agreed to engineer a new variant with a smaller diameter carbon tube, along with upgrades to flanges along the way.

Up next: I document the development process, failures and successes that led to the current V3.

[fattycharged]

Are we still putting poly subframe bushings in cars? WHY???

No, these have been in since 2012…I just didn't drop the subframe fully, I should have, would have made my life easier in a number of ways replacing everything back there, but I like taking the long way disguised as a shortcut…


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[bigjae46]

I would recommend not using urethane or rubber subframe bushings with this driveshaft, including AKG, yeah. Not saying it will definitely cause problems, but the likelihood of problems is way too high to get a recommendation from me. Solid bushings is the way here.

As an aside, I went from AKG urethane to solid aluminum on my car and noticed almost no NVH change at all. And for comparison, I’ve removed things like solid RTAB mounts and urethane strut tower mounts because I was unhappy with the NVH they introduced. I cannot think of a reason to recommend urethane subframe bushings.

Are we still putting poly subframe bushings in cars? WHY???

[Bry5on]

So the consensus is if you run AKG subframe bushing, this is a no-no?

I would recommend not using urethane or rubber subframe bushings with this driveshaft, including AKG, yeah. Not saying it will definitely cause problems, but the likelihood of problems is way too high to get a recommendation from me. Solid bushings is the way here.

As an aside, I went from AKG urethane to solid aluminum on my car and noticed almost no NVH change at all. And for comparison, I’ve removed things like solid RTAB mounts and urethane strut tower mounts because I was unhappy with the NVH they introduced. I cannot think of a reason to recommend urethane subframe bushings.

[bigjae46]

So the consensus is if you run AKG subframe bushing, this is a no-no?

Should be OK...I don't think they raise the subframe closer to the body.

[fattycharged]

So the consensus is if you run AKG subframe bushing, this is a no-no?

[0-60motorsports]

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…

I didnt think of this. Thanks for pointing that out. Let us know when you do the conversion about the clearance please.

[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.