Moment of Inertia Weight - Fact or Urban Legend?

[GrussGott]

I'm posting this in the general forum because it's not directly related to brakes. mostly. Also, yes, I've searched and read all kinds of threads, and official BMW content.

So I'm trying to decide on how much marriage capital I want to burn by bringing $8k of brakes to the finance committee.

I would summarize all the CCB threads as:
1.) Not much weight loss (12.5kg/28lbs total or 7lbs a corner)
2.) Not much stopping improvement if any
3.) Are supported on warranty

So on the surface, doesn't seem to be much benefit except they're gold and they're big and they sound cool

Then I read this:

The important fact is, CCB mass is part of the rotating and unsprung mass. Saving weight at these masses is critical, because they are subject to the moment of inertia. Rotating brake discs, that have to be accelerated or braked, or suspension that moves up and down are subject to the law of inertia. In professional Motorsport there is a saying: " 1kg saved at the rotating/unsprung mass is equivalent to about 5kg at the chassis. "

Thus the 3Kg/corner = 15kg/corner = 33lbs/corner or 62.5kg/138lbs total

So here's my question to you experts ... does the moment of inertia equation really hold so that a small amount of weight at the wheel "feels" like a much larger weight?

Also I would think if this is 'true', that handling/dynamics would be greatly (ok somewhat) improved since the wheel has less weight to overcome.

Thoughts?

[CanAutM3]

There are quite a few discussions on this exact topic in the "Brake and Chassis" sub-forum, you should have a look there (you even linked one thread and all the info you seek is in there).

The impact of inertia is real and it is not a myth. However there are multiple factors to consider (gearing, rolling radius, shape of the object, etc...) to assess that impact and you cannot take a blanket number like your 1kg to 5kg correlation.

As I posted here, doing rough calculation for the F8X, the equivalent mass ratio of the brake disc is about 1.20. This means that every lb of disc weight is equivalent to 1.2lb of non-rotating mass on the car. A reduction of 27.5lb on the discs equates to 33lb equivalent mass. Add back the 13lb from the heavier callipers (27.5-14.5) and you end up with a 20lb equivalent mass reduction on the total car (~0.5%). Not significant IMO.

Also don't confuse unsprung mass and rotational mass. Rotational mass increases the equivalent weight of the vehicle and impacts the power needed the accelerate or brake the car. Reducing unsprung mass allows the suspension to better react to changes in the shape of the road and helps keep the tire in contact with the tarmac. IMO, the latter is biggest benefit of the CCB from a performance point of view. For reference, the CCB provide about 6.9lb rotational mass reduction and 3.6lb of unsprung mass reduction per corner. If this is worth $8500, that is entirely up to you .

[bruce.augenstein@comcast.]

Originally Posted by CanAutM3

There are quite a few discussions on exact the topic in the "Brake and Chassis" sub-forum, you should have a look there (you even linked one thread and all the info you seek is in there).

The impact of inertia is real and it is not a myth. However there are multiple factors to consider (gearing, rolling radius, shape of the object, etc...) to assess that impact and you cannot take a blanket number like your 1kg to 5kg correlation.

As I posted here, doing rough calculation for the F8X, the equivalent mass ratio of the brake disc is about 1.20. This means that every lb of disc weight is equivalent to 1.2lb of non-rotating mass on the car. A reduction of 27.5lb on the discs equates to 33lb equivalent mass. Add back the 13lb from the heavier callipers (27.5-14.5) and you end up with a 20lb equivalent mass reduction on the total car (~0.5%). Not significant IMO.

Also don't confuse unsprung mass and rotational mass. Rotational mass increases the equivalent weight of the vehicle and impacts the power needed the accelerate or brake the car. Reducing unsprung mass allows the suspension to better react changes in the shape of the road and helps keep the tire in contact with the tarmac. IMO, the latter is biggest benefit of the CCB from a performance point of view. For reference, the CCB provide about 6.9lb rotational mass reduction and 3.6lb of unsprung mass reduction per corner. If this is worth $8500, that is entirely up to you .

A complete, and completely accurate answer.

Bruce

[RPiM5]

Originally Posted by GrussGott

Then I read this:

Thus the 3Kg/corner = 15kg/corner = 33lbs/corner or 62.5kg/138lbs total

Ask yourself this question, would you personally feel the difference in the weight of the M4 if it has 138lbs less? Maybe not so much on the street to 99% of people driving them, but on the racetrack, HELL YES!

The best way for you to answer this for yourself is to drive a current BMW ///M with CCB's on the street and on the track and decide for yourself. An instructor friend of mine recently did an event where he got to drive an F13 M6 on the racetrack with CCB's. He said after that experience there was no way he could own another ///M car without them.

BUT, if you never intend on tracking your M car, there are other benefits for everyday use on the streets.

Good:
- No Brake Dust (My favorite)
- Look way cooler
- Pads only need to be changed about 50,000 miles and rotors will need to be changed about 100,000 miles (if only using the CCB's on the street if used on the track they will wear out faster, but only if you are tracking like 2 a month or more. If you only go to the track 2 times a year and the rest of the year are just street driving your M Car, you will be fine.)

Bad:
- Brake squeel sometimes. They are racing brakes after all.
- Cost of replacing pads and rotors. On the M5/M6 the cost to replace just a single rotor and nothing else will run you about $3000-$4,000 dollars. I'd imagine a full brake job easily between $10,000-$12,000. For the M3/M4, I'd imagine a little cheaper.
- More easily prone to damage from small rock chips some have said.

If I was buying another M5 or M6, or a new M3/M4 I'd definitely get the CCB's. It wouldn't be an ///M car without them in my opinion.

[jzmundy]

Originally Posted by CanAutM3

There are quite a few discussions on exact the topic in the "Brake and Chassis" sub-forum, you should have a look there (you even linked one thread and all the info you seek is in there).

The impact of inertia is real and it is not a myth. However there are multiple factors to consider (gearing, rolling radius, shape of the object, etc...) to assess that impact and you cannot take a blanket number like your 1kg to 5kg correlation.

As I posted here, doing rough calculation for the F8X, the equivalent mass ratio of the brake disc is about 1.20. This means that every lb of disc weight is equivalent to 1.2lb of non-rotating mass on the car. A reduction of 27.5lb on the discs equates to 33lb equivalent mass. Add back the 13lb from the heavier callipers (27.5-14.5) and you end up with a 20lb equivalent mass reduction on the total car (~0.5%). Not significant IMO.

Also don't confuse unsprung mass and rotational mass. Rotational mass increases the equivalent weight of the vehicle and impacts the power needed the accelerate or brake the car. Reducing unsprung mass allows the suspension to better react changes in the shape of the road and helps keep the tire in contact with the tarmac. IMO, the latter is biggest benefit of the CCB from a performance point of view. For reference, the CCB provide about 6.9lb rotational mass reduction and 3.6lb of unsprung mass reduction per corner. If this is worth $8500, that is entirely up to you .

Posts like this are refreshing to see after some of the unintelligible drivel present elsewhere on this forum.

[BMWJ]

unless you're worried about lap times, I'd save the money.

[noro]

One very common mistake made during these "rotational mass" discussion is the omission of the fact that the distribution of weight and it's distance from the center is absolute key. So for example.

1kg drop in tire > 1kg drop in wheel weight > 1kg drop in rotors.

Dropping a kb in the rotors does not have the same factor of 5 effect. Dropping a kg from the tires does.

I don't think you'll feel forward and backward acceleration improvement with the rotor's mass reduction.

[dhoggm3]

7 lb per corner is a lot. The biggest improvement is over bumps in a turn, where the springs/shocks will recover faster and regain grip faster with a lighter unsprung mass attached.

That said, $8K is an awful lot. (And CCB tend to fry themselves on track.)

[IMrMark]

Originally Posted by CanAutM3

There are quite a few discussions on exact the topic in the "Brake and Chassis" sub-forum, you should have a look there (you even linked one thread and all the info you seek is in there).

The impact of inertia is real and it is not a myth. However there are multiple factors to consider (gearing, rolling radius, shape of the object, etc...) to assess that impact and you cannot take a blanket number like your 1kg to 5kg correlation.

As I posted here, doing rough calculation for the F8X, the equivalent mass ratio of the brake disc is about 1.20. This means that every lb of disc weight is equivalent to 1.2lb of non-rotating mass on the car. A reduction of 27.5lb on the discs equates to 33lb equivalent mass. Add back the 13lb from the heavier callipers (27.5-14.5) and you end up with a 20lb equivalent mass reduction on the total car (~0.5%). Not significant IMO.

Also don't confuse unsprung mass and rotational mass. Rotational mass increases the equivalent weight of the vehicle and impacts the power needed the accelerate or brake the car. Reducing unsprung mass allows the suspension to better react changes in the shape of the road and helps keep the tire in contact with the tarmac. IMO, the latter is biggest benefit of the CCB from a performance point of view. For reference, the CCB provide about 6.9lb rotational mass reduction and 3.6lb of unsprung mass reduction per corner. If this is worth $8500, that is entirely up to you .

awesome details

[jackkk]

Originally Posted by noro

One very common mistake made during these "rotational mass" discussion is the omission of the fact that the distribution of weight and it's distance from the center is absolute key. So for example.

1kg drop in tire > 1kg drop in wheel weight > 1kg drop in rotors.

Dropping a kb in the rotors does not have the same factor of 5 effect. Dropping a kg from the tires does.

I don't think you'll feel forward and backward acceleration improvement with the rotor's mass reduction.

You are right! the best way is to reduce the tire weight than the wheels weight and at last the rotor weight.

So I think the Bremboo BBK kit (not CCB) will be cheaper than the CCB and will be lighter than the stock breaks (as usual they do). And they will operate better on the race track, because the CCB is not a track set up, thay are desined to the public road mostly. So it will be better choice for me.

[Randy M]

CCB's are a waste for the street from a performance perspective. If you're driving on public roads at the level where you're getting the irons to fade you really should take that behavior to the track. The only way I could justify them for the street are aesthetics, low or no dust and bling calipers.

Given that BMW will replace the iron rotors under the service plan and the iron's don't last as long, it appears that those who are buying CCB's are letting BMW off the hook and lining their pockets at the same time.

[BhamDavid]

Another consideration is the car's overall POLAR moment of inertia. A low polar moment of inertia is why mid-engined cars handle so well. Their mass is concentrated near the car's center of gravity, so rotating the car's mass as you enter a turn requires less force. Imagine twisting a dumbbell back and forth in your hand versus a ball of equivalent weight. Interestingly, a dumbbell has a 50-50 weight distribution, it just has a high polar moment of inertia.

To my point: The affect of the force of any mass will increase with the square of the distance from the car's center of gravity. So the affect of weight savings as you move toward the ends of the car are much more significant than near the car's mid-point.

Therefore, the weight savings from CCBs will decrease the force required of the tires turning the car.

[CanAutM3]

Originally Posted by dhoggm3

7 lb per corner is a lot. The biggest improvement is over bumps in a turn, where the springs/shocks will recover faster and regain grip faster with a lighter unsprung mass attached.

That said, $8K is an awful lot. (And CCB tend to fry themselves on track.)

It's ~3.6lb of unsprung mass per corner, not 7lb...

[CanAutM3]

Originally Posted by BhamDavid

Another consideration is the car's overall POLAR moment of inertia. A low polar moment of inertia is why mid-engined cars handle so well. Their mass is concentrated near the car's center of gravity, so rotating the car's mass as you enter a turn requires less force. Imagine twisting a dumbbell back and forth in your hand versus a ball of equivalent weight. Interestingly, a dumbbell has a 50-50 weight distribution, it just has a high polar moment of inertia.

To my point: The affect of the force of any mass will increase with the square of the distance from the car's center of gravity. So the affect of weight savings as you move toward the ends of the car are much more significant than near the car's mid-point.

Therefore, the weight savings from CCBs will decrease the force required of the tires turning the car.

Your statement is true, however I seriously doubt that 3.6lb per corner can be felt from that perspective. The benefit of CCB on the total polar moment of inertia of the car is there, but negligible IMO.

[CanAutM3]

Mods should move this thread into one of the existing CCB threads IMO.

[BhamDavid]

Originally Posted by CanAutM3

Your statement is true, however I seriously doubt that 3.6lb per corner can be felt from that perspective. The benefit of CCB on the total polar moment of inertia of the car is there, but negligible IMO.

IMO, it's like everything else BMW designed for the weight savings and handling capabilities of this car: incremental savings eventually make big differences.

[GrussGott]

Originally Posted by CanAutM3

3.6lb of unsprung mass reduction per corner. If this is worth $8500, that is entirely up to you .

Wonderful info, thanks, solves my dilemma!

[M4TW]

Originally Posted by CanAutM3

Mods should move this thread into one of the existing CCB threads IMO.

I was sad to see the great CCB threads get moved into the cellar. I liked those threads. I saved $8,500 just from reading them.

[BetterLate]

Originally Posted by jzmundy

Posts like this are refreshing to see after some of the unintelligible drivel present elsewhere on this forum.

Agreed! Thanks CanAut!

[FogCityM3]

Not sure of the magnitude of what you'll feel at 3.6 lbs per corner but may be modestly noticeable. I recently did two rounds of weight reduction: lightweight wheels at ~5-6 lbs per corner, which you could definitely feel (directly impacts both unsprung weight and rotating mass), followed by the Essex/AP racing BBK, which reduced weight at 10 lbs/wheel front/ 8 lbs/wheel rear and this did change the handling feel, but not as much it seemed relative to the wheels (about 60% of weight savings was in the caliper vs the rotors). The total of ~60 lbs in unsprung weight savings taken together has been very noticeable and improves ride quality, feeling of agility, reduces understeer, improves grip, steering response and immediacy of throttle inputs. Would say the impact to how the vehicle feels is more than the impact of a 200 lb person in the passenger seat..so in that sense I believe the 4x-5x lb savings impact. I just completed the mods and will be at the track in a few weeks to see how they perform.

While the BBK I chose is not CC, I looked into various CC options and they are quite pricey (even the MOV-IT kit was close to $20k and that is one of the cheaper ones). If I were in your position I would do it. Coupled with some aftermarket wheels later on down the road, could be quite noticeable when taken all together. Every little bit counts!

Edit: realized the OP said it was ~ 7 lbs/per corner of unsprung mass reduction. I think that makes the CCB option even more compelling and yes, based on my experience, think it would be fairly noticeable.

[CanAutM3]

Originally Posted by FogCityM3

Edit: realized the OP said it was ~ 7 lbs/per corner of unsprung mass reduction. I think that makes the CCB option even more compelling and yes, based on my experience, think it would be fairly noticeable.

It is ~3.6lb of unsprung mass reduction per corner, not 7lb

Reducing the weight of the wheels will be much more noticeable since the weight it farther out from the rotation center. I can definitely feel when I put my lighter track wheels on too .

[FogCityM3]

BMW's website says CCB system saves nearly 30 lbs vs the regular braking system. How is the savings not ~7+ lbs per corner, or am I missing something?

Originally Posted by CanAutM3

It is ~3.6lb of unsprung mass reduction per corner, not 7lb

Reducing the weight of the wheels will be much more noticeable since the weight it farther out from the rotation center. I can definitely feel when I put my lighter track wheels on too .