M3/M4 to Get Steel Driveshafts Beginning Nov. 2017. Sales top 68,000 to date.

[CanAutM3]

Originally Posted by Absurdium

In which way did I simplify the physics? You could argue that the CF could possibly have a different weight distribution but a CF driveshaft is hollow just like a steel one. Also, how has it been demonstrated that the CF driveshaft specific to the F8x has a lower moment of inertia than the new steel one if there hasn't been any solid specs release on it? : I agree that for two shaft of similar diameters, a CF shaft's weight reduction would have a greater effect on reducing the moment than a thinner steel shaft as I have demonstrated in my post should the new shaft be 75mm. But since the thickness of the new shaft has not been confirmed, isn't my point of the new shaft being potentially better in terms of minimizing drivetrain loss valid?

As for the benefit of a stiffer material, I totally agree that the CF would have benefits that the steel one cannot reproduce. However, I've mentioned that in my post and that wasn't what I was questioning; I am just talking about minimizing drivetrain loss due to unsprung weight.

Oversimplifying in the sense that a driveshaft is way more complex than a simple annular tube. See picture below, you need to factor the end connecting points, CV joint, bearings, etc... The dimensions and weights are not fully clear either, but the numbers you mention don't seem quite right. The AM&S article mentions 84mm diameter for CF shaft and a varying 56 to 75mm diameter for the steel one. Also, assuming that the CF driveshaft is 40% lighter as per the original F8X press release and reading the weight of the CF shaft off RealOEM.com, means the steel shaft is about 5kg (11lb) heavier than the CF one.

I agree that, in haste, I went bit ahead of myself saying that the CF shaft has a lesser polar moment of inertia; we do not have sufficient data to assess that. What I meant is that it has been demonstrated, using reasonable assumptions, that the effective mass of the CF driveshaft is noticeably lower than the effective mass of a steel driveshaft. To put all this in perspective though, it is worth about 5~6kg of effective mass on the total vehicle. Every bit counts, but it is not that significant.

Further, you might be confusing un-sprung weight, rotating mass and drivetrain loss. Un-sprung weight affects the ability of the tires to stay in contact with the road when hitting bumps. Rotating mass has an effect on the forces/power needed to accelerate and decelerate the car. Un-sprung weight has no direct correlation with the effects of rotating mass and the driveshaft is most definitely sprung weight. Also, the effect of rotating mass is not considered a loss per se since it can be recuperated; it is simply mass that needs to be accelerated, just like the entire vehicle mass.

[Absurdium]

Originally Posted by CanAutM3

Oversimplifying in the sense that a driveshaft is way more complex than a simple annular tube. See picture below, you need to factor the end connecting points, CV joint, bearings, etc... The dimensions and weights are not fully clear either, but the numbers you mention don't seem quite right. The AM&S article mentions 84mm diameter for CF shaft and a varying 56 to 75mm diameter for the steel one. Also, assuming that the CF driveshaft is 40% lighter as per the original F8X press release and reading the weight of the CF shaft off RealOEM.com, means the steel shaft is about 5kg (11lb) heavier than the CF one.

I agree that, in haste, I went bit ahead of myself saying that the CF shaft has a lesser polar moment of inertia; we do not have sufficient data to assess that. What I meant is that it has been demonstrated, using reasonable assumptions, that the effective mass of the CF driveshaft is noticeably lower than the effective mass of a steel driveshaft. To put all this in perspective though, it is worth about 5~6kg of effective mass on the total vehicle. Every bit counts, but it is not that significant.

Further, you might be confusing un-sprung weight, rotating mass and drivetrain loss. Un-sprung weight affects the ability of the tires to stay in contact with the road when hitting bumps. Rotating mass has an effect on the forces/power needed to accelerate and decelerate the car. Un-sprung weight has no direct correlation with the effects of rotating mass and the driveshaft is most definitely sprung weight. Also, the effect of rotating mass is not considered a loss per se since it can be recuperated; it is simply mass that needs to be accelerated, just like the entire vehicle mass.

Ah okay, I had not taken the joints of the steel shaft into consideration. However from the picture you attached, it appears that even the widest part of the joint is still thinner than the CF shaft. I understand that the exact shafts we are discussing may have different dimensions, but it doesn't seem that the joints themselves would affect the effective rotating mass's diameter by much, if at all. All of the numbers I used are actually from the article. The article says that the CF shaft compared to the new steel one is 2.5kg lighter. I assumed 20lbs as a reasonable guess for the weight of a generic driveshaft. However if we assume higher weights, that would just skew the numbers towards the steel shaft actually.

No worries, I was simply just raising the question and to be honest, I also assumed that the CF shaft would have a lesser moment of inertia when I first heard the news. It wasn't until after I read through some of the given data that I started to wonder.
The purpose of my post as to question that exact assessment. Given the data from the AM&S article, it actually seems that the new steel shaft may not be inferior to the CF shaft due to the potentially much lower diameter, and less weight difference than what BMW initially said during the F8x press release (2.5kg vs. the 5kg like you mentioned). My question really stemmed from the fact that after using reasonable assumptions to perform a rough calculation, I actually arrived at a conclusion that the steel shaft could be better. What I'm questioning now is just the validity of the AM&S article and whether or not those are reliable numbers.

I believe you are correct that I have mixed up unsprung weight with rotating mass. What I meant was minimizing the rotational mass so that more power can be translated to the wheels, instead of being lost by propelling a shaft with a higher polar inertia.

All in all, I really just got curious from reading through the posts from this thread. I actually have an October production date so having the CF shaft being the superior component does me no harm, rather the opposite. I originally assumed BMW's statement of "keeping the superlative performance" with the new shaft as marketing BS as well . However, after reading the AM&S's article, it seems like maybe they weren't just making it up. If the article is valid, the new steel driveshaft may not be as bad as people think.

[georgere]

There is no way that steel shaft will be better for the car vs the CF version.
The reason BMW switches to steel is cost. They want to sell car at the same price while saving on some materials.
They will keep CF in GTS and CS models where they keep higher margins and where you can pitch the use of CF materials vs base models.
Of course steel shaft won't reduce the performance much but if you had a choice between the two versions which would you take?

[dinonz]

Going by the M4 CS, I would expect the M3 CS to have a carbon drive shaft.

So - all those who threw their arms up in anger and ordered early to get the carbon shaft, so you really feel like you've been shafted now? Who would have preferred to wait and order a CS with the carbon shaft?

[Absurdium]

Originally Posted by georgere

There is no way that steel shaft will be better for the car vs the CF version.
The reason BMW switches to steel is cost. They want to sell car at the same price while saving on some materials.
They will keep CF in GTS and CS models where they keep higher margins and where you can pitch the use of CF materials vs base models.
Of course steel shaft won't reduce the performance much but if you had a choice between the two versions which would you take?

This was my original thought as well. Of course CF should be better.. right? But I did some quick but effective math based on the recent article that was released in regards to the specs of the new steel driveshaft and from a physics stand point, the steel could be better. What I am questioning now is not that CF driveshafts are worse; I just want some confirmation on the specs as I'm unsure if that article is a reliable source.

At the end of the day, of course there are other tangible benefit to CF driveshafts other than weight such as stiffness and its one-piece construction, so I would still lean towards a CF shaft. I was simply trying to verify the info that's been posted and maybe see if anyone had an update to the situation.

[dinonz]

Originally Posted by Absurdium

This was my original thought as well. Of course CF should be better.. right? But I did some quick but effective math based on the recent article that was released in regards to the specs of the new steel driveshaft and from a physics stand point, the steel could be better. What I am questioning now is not that CF driveshafts are worse; I just want some confirmation on the specs as I'm unsure if that article is a reliable source.

At the end of the day, of course there are other tangible benefit to CF driveshafts other than weight such as stiffness and its one-piece construction, so I would still lean towards a CF shaft. I was simply trying to verify the info that's been posted and maybe see if anyone had an update to the situation.

I think unless you're fully involved in the Traffic Light Le Mans, the difference will be negligible...

[georgere]

Originally Posted by dinonz

I think unless you're fully involved in the Traffic Light Le Mans, the difference will be negligible...

Haha, pple pay 5k for zcp to get 10 extra hp at 4500+ RPM and claim they feel the extra 10 there
Trust me we feel everything

[dinonz]

Originally Posted by georgere

Haha, pple pay 5k for zcp to get 10 extra hp at 4500+ RPM and claim they feel the extra 10 there
Trust me we feel everything

I know. I got ZCP and I CAN FEEL IT!

No - I got the ZCP for the stripes in the seatbelts.

[Demxsr]

How hard is it to swap a driveshaft swap on these cars? Once you get to driveshaft is it just a few bolts?

[thatjeffsmith]

Originally Posted by dinonz

No - I got the ZCP for the stripes in the seatbelts.

You joke, but I seriously considered it for that specifically, and the seats.

[CanAutM3]

Originally Posted by Absurdium

This was my original thought as well. Of course CF should be better.. right? But I did some quick but effective math based on the recent article that was released in regards to the specs of the new steel driveshaft and from a physics stand point, the steel could be better. What I am questioning now is not that CF driveshafts are worse; I just want some confirmation on the specs as I'm unsure if that article is a reliable source.

At the end of the day, of course there are other tangible benefit to CF driveshafts other than weight such as stiffness and its one-piece construction, so I would still lean towards a CF shaft. I was simply trying to verify the info that's been posted and maybe see if anyone had an update to the situation.

The steel driveshaft IS WORSE from a effective mass point of view. Period. We discussed that already, your math and physics are incomplete.

Albeit, as we have already discussed, the difference in effective mass between the two is rather marginal. The main benefit of the CF driveshaft stems from less drivetrain elasticity due to greater stiffness.

[JoeFromPA]

Originally Posted by CanAutM3

The steel driveshaft IS WORSE from a effective mass point of view. Period. We discussed that already, your math and physics are incomplete.

Albeit, as we have already discussed, the difference in effective mass between the two is rather marginal. The main benefit of the CF driveshaft stems from less drivetrain elasticity due to greater stiffness.

Can you put this in context for me?

For example, I know that going from a dual-mass flywheel to a single mass flywheel of the same total weight will decrease drivetrain elasticity due to greater stiffness. And I have a good feel for the relative effect on throttle response as a result.

How comparable is the CF stiffness vs steel stiffness when it comes to reducing drivetrain flex during transition/load phases? Are we talking the equivalent of switching to stiffer motor mounts?

[Absurdium]

Originally Posted by CanAutM3

The steel driveshaft IS WORSE from a effective mass point of view. Period. We discussed that already, your math and physics are incomplete.

Albeit, as we have already discussed, the difference in effective mass between the two is rather marginal. The main benefit of the CF driveshaft stems from less drivetrain elasticity due to greater stiffness.

We have not discussed it to agree that it's worse. You even acknowledge the fact that since the specification of the new driveshaft is not released, it would be hasty to say that the new one is worse/better. As for my computations, you keep telling me they're incomplete or too simplified but have yet to provide any calculations of your own... By no means am I a engineer, so if any one who knows the subject more wants to shed some light with proper proof, I'm all ears. But the only number you've stated about the driveshaft being 5kg lighter is not even an official number. Simply capitalizing and bolding words does not make it true.

Once again, I agree with the drivetrain elasticity, as I've said in my own post; I even said I'd probably still lean towards a CF shaft...

Look, all I wanted to do was really to just confirm the AM&S's info as with the specs they've released, the shaft may indeed better ("may" being the keyword here). I haven't once claimed which shaft is better, only speculating based on the little information that we have been given. However I feel like our discussion has become too heated and we're not really creating any new insight as much as just mentioning what we have already said. My apologies for any confusion or misunderstanding but I just really want to confirm the spec of the new shaft, not get into a debate about the inherent benefits of a CF shaft.

[CanAutM3]

Originally Posted by Absurdium

We have not discussed it to agree that it's worse. You even acknowledge the fact that since the specification of the new driveshaft is not released, it would be hasty to say that the new one is worse/better. As for my computations, you keep telling me they're incomplete or too simplified but have yet to provide any calculations of your own... By no means am I a engineer, so if any one who knows the subject more wants to shed some light with proper proof, I'm all ears. But the only number you've stated about the driveshaft being 5kg lighter is not even an official number. Simply capitalizing and bolding words does not make it true.

Once again, I agree with the drivetrain elasticity, as I've said in my own post; I even said I'd probably still lean towards a CF shaft...

Look, all I wanted to do was really to just confirm the AM&S's info as with the specs they've released, the shaft may indeed better ("may" being the keyword here). I haven't once claimed which shaft is better, only speculating based on the little information that we have been given. However I feel like our discussion has become too heated and we're not really creating any new insight as much as just mentioning what we have already said. My apologies for any confusion or misunderstanding but I just really want to confirm the spec of the new shaft, not get into a debate about the inherent benefits of a CF shaft.

OK, I'll elaborate a little more.

As was previously stated, a driveshaft is a much more complex geometry that a simple hollow tube. For the CF driveshaft, most of the mass is located in the steel connecting endpoint flanges, not in the tube itself. So it is only a rather small portion of the total driveshaft mass that is moved farther away from the rotating axis with the increase in diameter. So using the simplified hollow tube formula I=mr^2 for the total shaft grossly overestimates the impact on inertia. Based on the geometry, even without having all the specifications of the driveshafts, one can make a reasonable assumption that it takes a rather large increase in the CF tube diameter compared to the steel one to significantly increase the inertia of the total driveshaft.

Further, to be able to asses the impact of a rotating mass on vehicle acceleration, one needs to assess the effective mass of that component (if you want to find out more on the topic do some research on effective mass, mass factors and equivalent mass. "Fundamentals of Vehicle Dynamics" by Gillespie is a good starting point). Effective mass takes into consideration the combined effect of translational acceleration and angular acceleration of a given component. Simply put, when the car accelerates, the driveshaft not only spins faster and faster on itself but also moves forward with the car. To calculate effective mass, one needs to determine the mathematical relationship between the translational and rotational speed of the component. For the driveshaft example, one needs to consider the rear wheel rolling radius of .337m and the final drive ratio of 3.462 in the calculation. If you start doodling with the formulas for a driveshaft, you'll find out that it takes a rather significant increase in inertia for the effect of the angular acceleration to outweigh the impact of the mass of the component on translational acceleration.

As a tidbit, I also did a little more research looking at driveshaft weight in RealOEM.com, and it seems that the 2.5kg quoted in the AM&S article is more representative of the total weight difference between the CF and steel driveshafts. I am not sure where BMW took the 40% they quoted in the original F8X press release.

And BTW, no offense taken

[Absurdium]

Originally Posted by CanAutM3

OK, I'll elaborate a little more.

As was previously stated, a driveshaft is a much more complex geometry that a simple hollow tube. For the CF driveshaft, most of the mass is located in the steel connecting endpoint flanges, not in the tube itself. So it is only a rather small portion of the total driveshaft mass that is moved farther away from the rotating axis with the increase in diameter. So using the simplified hollow tube formula I=mr^2 for the total shaft grossly overestimates the impact on inertia. Based on the geometry, even without having all the specifications of the driveshafts, one can make a reasonable assumption that it takes a rather large increase in the CF tube diameter compared to the steel one to significantly increase the inertia of the total driveshaft.

Further, to be able to asses the impact of a rotating mass on vehicle acceleration, one needs to assess the effective mass of that component (if you want to find out more on the topic do some research on effective mass, mass factors and equivalent mass. "Fundamentals of Vehicle Dynamics" by Gillespie is a good starting point). Effective mass takes into consideration the combined effect of translational acceleration and angular acceleration of a given component. Simply put, when the car accelerates, the driveshaft not only spins faster and faster on itself but also moves forward with the car. To calculate effective mass, one needs to determine the mathematical relationship between the translational and rotational speed of the component. For the driveshaft example, one needs to consider the rear wheel rotating radius of .337m and the final drive ratio of 3.462 in the calculation. If you start doodling with the formulas for a driveshaft, you'll find out that it takes a rather significant increase in inertia for the effect of the angular acceleration to outweigh the impact of the mass of the component on translational acceleration.

As a tidbit, I also did a little more research looking at driveshaft weight in RealOEM.com, and it seems that the 2.5kg quoted in the AM&S article is more representative of the total weight difference between the CF and steel driveshafts. I am not sure where BMW took the 40% they quoted in the original F8X press release.

And BTW, no offense taken

Ah. Okay this is making more sense now. So I guess assuming the endpoint flanges are relatively similar between a CF and steel shaft, the middle tube section of the shaft is what the CF component is replacing. Therefore, any weight lost compared to a steel shaft would be from the middle section only, thus increasing the actual % mass reduced. (say 2.5kg / 5kg for the middle section, vs. 2.5kg / 10kg of the whole shaft) Thinking about it like this, I do agree with you now; In regards to the relationship between CF tube and steel tube diameter, BMW would need a significantly thinner steel tube to maintain the same inertia as the CF, which seems unlikely without compromising on structural integrity.

Ah okay. I think I understand the gist of the concept you've mentioned in your second paragraph. I'll definitely look into this to gain a deeper understanding. Like I said, I'm definitely not a physicist haha; maybe a biophysicist at best. What is your educational background if you don't mind me asking?

Yeah that's what I did too. I wonder how they came to that number as well... Now seems like more of a marketing exercise if they could've produced a steel driveshaft didn't have such a big weight difference to the CF. However of course, the stiffness benefit of the CF is still present, which to me could be worth it alone.

Although we don't have the exact specs yet, I think your assumption is quite reasonable now. I understand that the effect of any of this is quite minuscule when looking at a high powered and high torque vehicle such as the F8x, but it is rather disappointing that the CF shaft is now going away. Although the new stringent regulations that forced the changed is not their doing, they could've found a better solution perhaps.

Thanks for the detailed explanation, appreciate it immensely! Good to have an intellectual conversation without either party being offended haha. Cheers

[Absurdium]

Originally Posted by JoeFromPA

Can you put this in context for me?

For example, I know that going from a dual-mass flywheel to a single mass flywheel of the same total weight will decrease drivetrain elasticity due to greater stiffness. And I have a good feel for the relative effect on throttle response as a result.

How comparable is the CF stiffness vs steel stiffness when it comes to reducing drivetrain flex during transition/load phases? Are we talking the equivalent of switching to stiffer motor mounts?

CF in general is just a much stronger and stiffer material. Being also that it's a one piece unit versus a two piece unit like in a conventional steel driveshaft, the stiffness of the CF driveshaft should have a significant effect on the way the throttle behaves. Significant as in statistically significant in that it's tangible, but maybe not a drastic difference. I would imagine this would translate into a more direct throttle response as the CF shaft would connect the power "faster" from the engine to the differential, as there is less elasticity.

[JoeFromPA]

Originally Posted by Absurdium

CF in general is just a much stronger and stiffer material. Being also that it's a one piece unit versus a two piece unit like in a conventional steel driveshaft, the stiffness of the CF driveshaft should have a significant effect on the way the throttle behaves. Significant as in statistically significant in that it's tangible, but maybe not a drastic difference. I would imagine this would translate into a more direct throttle response as the CF shaft would connect the power "faster" from the engine to the differential, as there is less elasticity.

I understand the concepts you are stating, what I'm trying to extract is compared to what?

We agree this would be a result (not necessarily a benefit) in high load transitional states such as going WOT.

But there are lots of things that flex when going WOT. The engine itself, engine mounts, dual mass flywheels, transmission mounts, the driveshaft, the differential, the axles, and the rubber on the road. Etc.

So what does the difference in elasticity reduction in this chain compare to? Is it the equivalent reduction in elasticity as putting stiffer engine mounts in? More? Less?

This is a long thread about the ramification of this driveshaft change yet there's no context or benchmark - just subjective statements about things like CF stiffness vs. steel.

If I put a more restrictive air filter on and lose 5hp, I say "Well I lost 5 out of 425". That's my context.

If a car becomes torsional stiffer from one generation to the next, we get a context statement like "The chassis is now 34% stiffer than previous"

...

Let's put some stakes in the ground here

[dinonz]

Originally Posted by thatjeffsmith

You joke, but I seriously considered it for that specifically, and the seats.

I was happy with the whole package - love the seats too!

[CanAutM3]

Originally Posted by JoeFromPA

Can you put this in context for me?

For example, I know that going from a dual-mass flywheel to a single mass flywheel of the same total weight will decrease drivetrain elasticity due to greater stiffness. And I have a good feel for the relative effect on throttle response as a result.

How comparable is the CF stiffness vs steel stiffness when it comes to reducing drivetrain flex during transition/load phases? Are we talking the equivalent of switching to stiffer motor mounts?

For me drivetrain elasticity is mostly about power transitions, particularly in on-power to compression transitions when adjusting the line in a corner where the drivetrain needs to quickly go from loading in one direction to the opposite direction.

It's gonna be difficult to put hard numbers on this since we have very limited access to the entire drivetrain specifications. It is also difficult to assess the impact of the added flex couplings in the steel driveshaft in terms of added slack and lessened rigidity. As you stated, the driveshaft is part of a complete system and incremental improvements to each component improves the entire system. The rigidly mounted subframe is a good example of an element increasing the drivetrain system.

Let me put it this way though, driver's only using the Efficient and Sport throttle settings will not be able to tell the difference .

[FC]

Originally Posted by CanAutM3

As a tidbit, I also did a little more research looking at driveshaft weight in RealOEM.com, and it seems that the 2.5kg quoted in the AM&S article is more representative of the total weight difference between the CF and steel driveshafts. I am not sure where BMW took the 40% they quoted in the original F8X press release.

Perhaps it's possible they were considering the reduction in mass over the equivalent length of CF, meaning, excluding the steel end couplings.

Or maybe they just flat-out compared the density of the steel they used against the density of the final CF composite as used (including the resin, etc). Leave it to MARCOM to twist engineering specifications to suit their needs during a press release.

[Absurdium]

Originally Posted by JoeFromPA

I understand the concepts you are stating, what I'm trying to extract is compared to what?

We agree this would be a result (not necessarily a benefit) in high load transitional states such as going WOT.

But there are lots of things that flex when going WOT. The engine itself, engine mounts, dual mass flywheels, transmission mounts, the driveshaft, the differential, the axles, and the rubber on the road. Etc.

So what does the difference in elasticity reduction in this chain compare to? Is it the equivalent reduction in elasticity as putting stiffer engine mounts in? More? Less?

This is a long thread about the ramification of this driveshaft change yet there's no context or benchmark - just subjective statements about things like CF stiffness vs. steel.

If I put a more restrictive air filter on and lose 5hp, I say "Well I lost 5 out of 425". That's my context.

If a car becomes torsional stiffer from one generation to the next, we get a context statement like "The chassis is now 34% stiffer than previous"

...

Let's put some stakes in the ground here

http://www.dexcraft.com/articles/car...-of-materials/

I did a bit of research and it seems that the specific modulus of CF vs. steel at the same weight is about 3 times higher, depending on the condition. Since the CF shaft is built to be a bit lighter, a reasonable assumption would be that it's has 200% of the stiffness of the steel shaft. This doesn't take into consideration that the CF is one piece versus the steel which is two piece so the actual % increase in stiffness could be higher. Keep in mind that this is just for the driveshaft itself so it's hard to measure the quantitative impact on the overall car as a whole..but this gives us a rough estimate for our question.

[CanAutM3]

Originally Posted by Absurdium

http://www.dexcraft.com/articles/car...-of-materials/

I did a bit of research and it seems that the specific modulus of CF vs. steel at the same weight is about 3 times higher, depending on the condition. Since the CF shaft is built to be a bit lighter, a reasonable assumption would be that it's has 200% of the stiffness of the steel shaft. This doesn't take into consideration that the CF is one piece versus the steel which is two piece so the actual % increase in stiffness could be higher. Keep in mind that this is just for the driveshaft itself so it's hard to measure the quantitative impact on the overall car as a whole..but this gives us a rough estimate for our question.

According to the AMS article, a non-negligible reasons mentioned for the M4 CS to retain the CF driveshaft is the higher torque rating, implying that the CF driveshaft has a higher load bearing capability.