F8X M3/M4 CF Drive shaft mod

[SliM3]

There you have it. My driveshaft was done improperly. Now I have to figure out why.

I was wondering why your shop welded a new tri-flange to your DS. IMO when going that route they should've at least used the output flange from your transmission to ensure the DS centerline stays aligned with the transmission output shaft centerline.


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[EthanolTurbo]

So, I finally got around to a longer drive with my car.

As for my CF Shaft, there are no downsides I could feel.

The car does feel much more direct, however I am driving a freshly rebuild and more powerful engine, so I am not fully attributing that to the shaft.

No more clunking. I did have quite a bit before and I have not done any changes to the tranny or diff since. So that's a plus.

Due to prior feedback above I also did some runs above 130kph/80mph. No vibrations.

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There you have it. My driveshaft was done improperly. Now I have to figure out why.

[Altaran]

One more thing which may be relevant:
All my drivetrain components are running stiffer mounts that stock so there is a lot less movement.

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[Altaran]

So, I finally got around to a longer drive with my car.

As for my CF Shaft, there are no downsides I could feel.

The car does feel much more direct, however I am driving a freshly rebuild and more powerful engine, so I am not fully attributing that to the shaft.

No more clunking. I did have quite a bit before and I have not done any changes to the tranny or diff since. So that's a plus.

Due to prior feedback above I also did some runs above 130kph/80mph. No vibrations.

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[discoelk]

FWIW, I understand what you are saying. 3rd/8000 is a faster wheel/driveshaft speed than 6th/80mph. 3rd/8000 is closer to 100mph. In my limited experience, discerning drivetrain vibration at high engine speed and hard acceleration is difficult butEthanolTurbo would need to elaborate.

[Radekxpl]

Another twist, if you are in 3rd gear at 7600 rpm at 80mph and then put the car in neutral and coast, does drive shaft stop spinning or even start spinning slower at idle vs 7600 while you are still traveling at 80mph?

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Driveshaft does not stop spinning if you put car in neutral at any speed whether you going 80mph or 20mph it will continue spinning. That I know for a fact.
Which now I think about it 🤔🤦what Discoelk said makes sense is directly related to wheels speed, however it still doesn’t explain no vibrations at 80 in 3rd gear and vibrations 80mph in 6th gears since drive shaft would be spinning at the same rate no matter what gear.

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[mrgizmo04]

Another twist, if you are in 3rd gear at 7600 rpm at 80mph and then put the car in neutral and coast, does drive shaft stop spinning or even start spinning slower at idle vs 7600 while you are still traveling at 80mph?

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[Radekxpl]

Driveshaft speed is directly correlated to wheel speed. Engine speed and gear do not matter. The shaft (heh) sounds out of balance or as someone else mentioned, the flanges are not welded true (misalignment issue).

Even given what your saying is correct that still does not add up since this would mean going 80mph in 3rd gear would result same driveshaft rpm speed as going 80mph in 6th gear, given what our Tester has mentioned is true, that there was no vibration at 8k engine rpm in 1st 2nd, 3rd gears emphasis on the 3rd gear here since m3 in 3rd gear is doing well over 80mph close to redline 8k. This just proves Something it's not adding up here.


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[discoelk]

Driveshaft speed is directly correlated to wheel speed. Engine speed and gear do not matter. The shaft (heh) sounds out of balance or as someone else mentioned, the flanges are not welded true (misalignment issue).

[Radekxpl]

Originally posted by EthanolTurbo View Post
Food for thought: driveshaft is vibration free up to 8k RPM in 1st, 2nd, 3rd gear. It only vibrated at 80 mph in 6th gear and it was around 3k RPM the other night. Will test further. If it was the driveshaft wouldn't it vibrate at high RPM in any gear?

Negative. Driveshaft directly correlates to wheel speed. Engine is geared up through the transmission(really the other way around but that's not important). I.e. 8000rpm/4.227 in first, 8000rpm/2.528 in second.

60mph will always be a set driveshaft rpm, regardless of engine speed. This would only change if you had a different final drive ratio like a 4.10.

That is correct however using this https://www.drive-lines.com/php10/dlrpm.php driveline calculator its only true to certain speed vs gear your in at the time
for example: using 275/35/18 tire size in this example 25.6 tire diameter
M3 traveling in 3rd gear at 80mph which would be somewhere near 7600 engine rpm the driveshaft should be around spinning at 1754 RPM
where
if you traveling at 80mph in 6th gear driveshaft spinning at 871 rpm so roughly half the speed when it was in 3rd gear at the same speed, something doesn't add up here as EthanolTurbo mentioned it wasn't vibrating at 8k in 3rd gear which in that gear it definitely was spinning faster than in 6th going roughly the same speed.
Even if your doing 60mph in 3rd gear it’s still is spinning faster at 1295 then in 6th at 80mph at 871

Given this information either there’s more testing need to be done to confirm thats it’s only vibrating at high speeds in lower gear or maybe person testing can’t detect vibrating in higher gears since the engine is at redline and there’s more engine noise nvh associated at that time as well compared to calm cruising in 6th or there’s something else going on there 🤔

[mrgizmo04]

Food for thought: driveshaft is vibration free up to 8k RPM in 1st, 2nd, 3rd gear. It only vibrated at 80 mph in 6th gear and it was around 3k RPM the other night. Will test further. If it was the driveshaft wouldn't it vibrate at high RPM in any gear?

I was thinking this as well.

Sounds like the shop that did the balancing also thought the same thing, thus there is a problem at higher mph...

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[heinzboehmer]

Negative. Driveshaft directly correlates to wheel speed. Engine is geared up through the transmission(really the other way around but that's not important). I.e. 8000rpm/4.227 in first, 8000rpm/2.528 in second.

60mph will always be a set driveshaft rpm, regardless of engine speed. This would only change if you had a different final drive ratio like a 4.10.

Yes this. Driveshaft rpm is directly proportional to wheel rpm not engine rpm.

[discoelk]

Food for thought: driveshaft is vibration free up to 8k RPM in 1st, 2nd, 3rd gear. It only vibrated at 80 mph in 6th gear and it was around 3k RPM the other night. Will test further. If it was the driveshaft wouldn't it vibrate at high RPM in any gear?

Negative. Driveshaft directly correlates to wheel speed. Engine is geared up through the transmission(really the other way around but that's not important). I.e. 8000rpm/4.227 in first, 8000rpm/2.528 in second.

60mph will always be a set driveshaft rpm, regardless of engine speed. This would only change if you had a different final drive ratio like a 4.10.

[simonnim]

I checked with my local shop, they said no problem. But, I'll have to exchange it for another since the side of mines looks slightly damaged. They said about the same cost as the other invoice placed here a few pages back. I don't want to release shop but apparently according to the shop that I take my car to claimed they're the 'best' around here.

I'll test and see how the results are.

[Projecte46m3]

Food for thought: driveshaft is vibration free up to 8k RPM in 1st, 2nd, 3rd gear. It only vibrated at 80 mph in 6th gear and it was around 3k RPM the other night. Will test further. If it was the driveshaft wouldn't it vibrate at high RPM in any gear?

I was thinking this as well.