[medphysdave]

Originally Posted by CSBM5

Notice they don't explain the details on rotational mass in the BMW link. For an engineer, it leaps right out at you when you see all the discussions on line -- a very important variable in rotational mass discussions is the radius of gyration. The radius of gyration is the radius where the rotational mass can be considered located in the basic moment of inertia calculation (i.e. I = m k^2, where m is mass and k is the radius of gyration).

Note that the moment of inertia is a function of the radius of gyration to the 2nd power; hence k has a large effect on rotational inertia as it changes.

In the case here a quick estimate of k for the front brake rotor is about 150mm or 5.9". If you wanted to compare mass located at that k versus say the same mass located at roughly the wheel k, it's k1^2/k2^2. If we consider a 19" wheel to have a radius of gyration located at about 8.5", then popping in these approximate numbers yields 0.48; hence the rotor weight is only 48% as important as the wheel weight in this approximate example.

Put another way, wheel weight contributes 2.1 times more to rotational inertia than disc weight.

This is all back-of-the-envelope quick estimates. However, the effect of radius of gyration is very important in moment of inertia calculations and shouldn't be left out of information any source is providing on "rotating mass" changes.

It's somewhat analogous to the way aero drag coefficients are bandied about by car makers, etc. Comparing two different cars, say one with a Cd of 0.30 and the other a Cd of 0.33 is perhaps "interesting" from an aero design perspective, but without the specified frontal area of the two cars, it tells you absolutely nothing about the aerodynamic drag comparison of the two as the car with the lower Cd could actually have a similar or larger aerodynamic drag...you just don't know without specification of the frontal area.

Sorry...pet peeves of two commonly misunderstood (marketing exploited too) things in the car world.

Word, and that's why reducing the weight of the rotor hat to reduce rotational mass likely has minimal affect. You'd have a larger impact on suspension response. I love this stuff. Lighter wheel and tire. And the wheel needs to be "lighter" in the right location. Reduce that effective radius baby.

[MomoM3]

Originally Posted by CSBM5

Notice they don't explain the details on rotational mass in the BMW link. For an engineer, it leaps right out at you when you see all the discussions on line -- a very important variable in rotational mass discussions is the radius of gyration. The radius of gyration is the radius where the rotational mass can be considered located in the basic moment of inertia calculation (i.e. I = m k^2, where m is mass and k is the radius of gyration).

Note that the moment of inertia is a function of the radius of gyration to the 2nd power; hence k has a large effect on rotational inertia as it changes.

In the case here a quick estimate of k for the front brake rotor is about 150mm or 5.9". If you wanted to compare mass located at that k versus say the same mass located at roughly the wheel k, it's k1^2/k2^2. If we consider a 19" wheel to have a radius of gyration located at about 8.5", then popping in these approximate numbers yields 0.48; hence the rotor weight is only 48% as important as the wheel weight in this approximate example.

Put another way, wheel weight contributes 2.1 times more to rotational inertia than disc weight.

This is all back-of-the-envelope quick estimates. However, the effect of radius of gyration is very important in moment of inertia calculations and shouldn't be left out of information any source is providing on "rotating mass" changes.

It's somewhat analogous to the way aero drag coefficients are bandied about by car makers, etc. Comparing two different cars, say one with a Cd of 0.30 and the other a Cd of 0.33 is perhaps "interesting" from an aero design perspective, but without the specified frontal area of the two cars, it tells you absolutely nothing about the aerodynamic drag comparison of the two as the car with the lower Cd could actually have a similar or larger aerodynamic drag...you just don't know without specification of the frontal area.

Sorry...pet peeves of two commonly misunderstood (marketing exploited too) things in the car world.

I have no idea what you're talking about but this is the reason I love this community. Thanks!

[Higgs Boson]

Originally Posted by CSBM5

Notice they don't explain the details on rotational mass in the BMW link. For an engineer, it leaps right out at you when you see all the discussions on line -- a very important variable in rotational mass discussions is the radius of gyration. The radius of gyration is the radius where the rotational mass can be considered located in the basic moment of inertia calculation (i.e. I = m k^2, where m is mass and k is the radius of gyration).

Note that the moment of inertia is a function of the radius of gyration to the 2nd power; hence k has a large effect on rotational inertia as it changes.

In the case here a quick estimate of k for the front brake rotor is about 150mm or 5.9". If you wanted to compare mass located at that k versus say the same mass located at roughly the wheel k, it's k1^2/k2^2. If we consider a 19" wheel to have a radius of gyration located at about 8.5", then popping in these approximate numbers yields 0.48; hence the rotor weight is only 48% as important as the wheel weight in this approximate example.

Put another way, wheel weight contributes 2.1 times more to rotational inertia than disc weight.

This is all back-of-the-envelope quick estimates. However, the effect of radius of gyration is very important in moment of inertia calculations and shouldn't be left out of information any source is providing on "rotating mass" changes.

It's somewhat analogous to the way aero drag coefficients are bandied about by car makers, etc. Comparing two different cars, say one with a Cd of 0.30 and the other a Cd of 0.33 is perhaps "interesting" from an aero design perspective, but without the specified frontal area of the two cars, it tells you absolutely nothing about the aerodynamic drag comparison of the two as the car with the lower Cd could actually have a similar or larger aerodynamic drag...you just don't know without specification of the frontal area.

Sorry...pet peeves of two commonly misunderstood (marketing exploited too) things in the car world.

From a practical standpoint, if a regular M2 wheel is 24 lbs and the CS wheel is 21lbs then we have saved 3 lbs per wheel or 12 lbs.

The CCBs are advertised as 42.8 lbs lighter according to the link posted.

In this case the contribution of the CCBs are more effective than the lighter wheels.

All I am saying is the gyroscopic effect can't be measured in a vacuum like that. You have to compare the change in weight of the brakes vs the change in weight of the wheels.

Granted, in a vehicle with much heavier 35lb wheels going to a super light 15-18lb race style wheel, the effects of the wheel will be more pronounced, but since this is the M2 forum, I used M2 data.

[CSBM5]

I think you totally missed the entire point of my (correct) post....

[medphysdave]

Originally Posted by CSBM5

I think you totally missed the entire point of my (correct) post....

Your post was good. The physics behind it is straightforward.

[Acetech]

The weight savings on the CS CCBs is 55lbs, now considering that the absolute majority of that weight savings is spinning in the 14.5 inch range or 7 inch from center of rotation, it's pretty substantial.
Now where is the weight reduction happening on a set of lighter wheels? Is it primarily in the drum at 18 inches 9" from center or in the spokes at 9 inches 4.5" from center or hub at 3 inches.
Worth considering

[medphysdave]

Originally Posted by Acetech

The weight savings on the CS CCBs is 55lbs, now considering that the absolute majority of that weight savings is spinning in the 14.5 inch range or 7 inch from center of rotation, it's pretty substantial.
Now where is the weight reduction happening on a set of lighter wheels? Is it primarily in the drum at 18 inches 9" from center or in the spokes at 9 inches 4.5" from center or hub at 3 inches.
Worth considering

That's my question about the wheels. How much design magic is in the 763M. I don't want to see this thread dive into a CCB debate. I think they are a great option and superior to the iron brakes in most cases. I did a lot of research on CCBs prior to my purchase. It's that 14.5in range where the gains matter if looking at CS with and without the option. If comparing a C and CS, you can't just compare the weight difference between the stock wheels because the distribution of the weight matters a lot. The disc weight difference between ccb and iron is substantial. Would an iron brakes car be more stable at high speed than a ccb equipped car due to the iron cars resistance to changing direction. Is it the rotating mass that the driver would notice, or would it be the increased response of less unsprung mass? I would think the latter as the rotational mass would be more noticeable when starting from a stop. CCB seem to be at home on track. Repeated low speed to high speed acceleration and many dramatic directional changes at speed. Both situations where CCB should be superior. Maybe I'll get the opportunity to drive a 6mt with ccb to feel the difference first hand.

[Higgs Boson]

Originally Posted by CSBM5

I think you totally missed the entire point of my (correct) post....

I didn't say anything you said was incorrect.

[MadBimmeRad]

Originally Posted by medphysdave

That's my question about the wheels. How much design magic is in the 763M. I don't want to see this thread dive into a CCB debate. I think they are a great option and superior to the iron brakes in most cases. I did a lot of research on CCBs prior to my purchase. It's that 14.5in range where the gains matter if looking at CS with and without the option. If comparing a C and CS, you can't just compare the weight difference between the stock wheels because the distribution of the weight matters a lot. The disc weight difference between ccb and iron is substantial. Would an iron brakes car be more stable at high speed than a ccb equipped car due to the iron cars resistance to changing direction. Is it the rotating mass that the driver would notice, or would it be the increased response of less unsprung mass? I would think the latter as the rotational mass would be more noticeable when starting from a stop. CCB seem to be at home on track. Repeated low speed to high speed acceleration and many dramatic directional changes at speed. Both situations where CCB should be superior. Maybe I'll get the opportunity to drive a 6mt with ccb to feel the difference first hand.

You can drive mine the next time you are going by Adelaide South Australia

[Artemis]

Next controversial topic: inflating tires with helium.

[Davil]

Originally Posted by Artemis

Next controversial topic: inflating tires with helium.

Don't go there. It's a noble gas that rarely ends with a noble thread.

[CSBM5]

Originally Posted by medphysdave

That's my question about the wheels. How much design magic is in the 763M. I don't want to see this thread dive into a CCB debate. I think they are a great option and superior to the iron brakes in most cases. I did a lot of research on CCBs prior to my purchase. It's that 14.5in range where the gains matter if looking at CS with and without the option. If comparing a C and CS, you can't just compare the weight difference between the stock wheels because the distribution of the weight matters a lot. The disc weight difference between ccb and iron is substantial. Would an iron brakes car be more stable at high speed than a ccb equipped car due to the iron cars resistance to changing direction. Is it the rotating mass that the driver would notice, or would it be the increased response of less unsprung mass? I would think the latter as the rotational mass would be more noticeable when starting from a stop. CCB seem to be at home on track. Repeated low speed to high speed acceleration and many dramatic directional changes at speed. Both situations where CCB should be superior. Maybe I'll get the opportunity to drive a 6mt with ccb to feel the difference first hand.

Yeah, if you now look into the effect of rotating mass on car performance, it's 100% a function of acceleration and hence has its largest effect when acceleration is largest and visa-versa. Acceleration falls off significantly and continually as velocity climbs; deceleration from any speed however can be rather constant at a high level (maximum braking).

I would more think that one would enojy the total unsprung weight reduction of the wheel and CCB disc combination...something like on track in a bumpy (high frequency bumps) 70 mph sweeper taken at the limits (M2C with 788M and 2NH, and M2 CS with 763M and CCB; same tires). Something like turn 10 at Summit Point (at least in the old days) would be a great place to see how nice a big reduction in unsprung weight feels.

[CSBM5]

Originally Posted by Higgs Boson

I didn't say anything you said was incorrect.

No you didn't. The point I was making is that in the car industry (especially advertising) simply referring to "rotating mass" changes is commonplace and without the context of radius gyration and actual calculation of moment of inertia changes, there is no basis for valid comparisons (or advertising) although the vast majority of people assume "rotating mass" is all the same. The car makers know (clearly) and exploit that lack of understanding intentionally.

[Higgs Boson]

Originally Posted by CSBM5

No you didn't. The point I was making is that in the car industry (especially advertising) simply referring to "rotating mass" changes is commonplace and without the context of radius gyration and actual calculation of moment of inertia changes, there is no basis for valid comparisons (or advertising) although the vast majority of people assume "rotating mass" is all the same. The car makers know (clearly) and exploit that lack of understanding intentionally.

I agree, was trying to add, not subtract.

[f80_MG]

Bumping this up instead of creating a new thread.
I remember this from the Corvette guys, figured it's an idea worth sharing since I'm sure all CCB owners have thought about how to most carefully remove wheels.

Personally I don't think I would ever make this effort (even though it's cheap and easy), just use lug extenders.

https://www.corvetteforum.com/forums...-s-manual.html

[JoeMatt]

Not sure if this was mentioned, but the consumables warranty add on doesn’t include carbon breaks.

I did opt for the add on for 4 years, since it covers a lot and I plan on tracking and they said NBD if I track it. Though if I damage the wheels at the track (or construction zone) they’re not covered but the finance guy literally told me he has it and just lie no ones will follow up.

Never got the warranty before, but I curb the shit out of wheels, and the price of just 2 rims or 1 set of breaks covers the cost right there. No deductible.

Was going to make another thread actually to see if anyone else is getting the extra coverage.

Maybe it’s a scam, but my friend swears by them on his X5 he upgrades every few years. Especially with NY roads with potholes, nails, glass chips, vandalism etc.