Fiesta ST Rotational Inertia of BKK Kits

[viki888056]

Was doing some fun research about rotational mass of BBK's and wanted to show what I got. I have a felling that it might be incorrect but it can give a perspective more or less each BBK. (this is brake discs only)

Stock Brakes
6.8kg 278mm = 0.263 kg-m^2

WillWood
4.5kg 310mm = 0.216 kg-m^2

EBC
6.8kg 300mm = 0.306 kg-m^2
7.0kg 330mm = 0.381 kg-m^2

REVO by Alcon
7.66kg 332mm = 0.422 kg-m^2

Tarox
6.2kg 300mm = 0.279 kg-m^2

Tried to find the weight of AP BBK but cant find the info for them.

 

[sonicmn]

Was doing some fun research about rotational mass of BBK's and wanted to show what I got. I have a felling that it might be incorrect but it can give a perspective more or less each BBK. (this is brake discs only)

Stock Brakes
6.8kg 278mm = 0.263 kg-m^2

WillWood
4.5kg 310mm = 0.216 kg-m^2

EBC
6.8kg 300mm = 0.306 kg-m^2
7.0kg 330mm = 0.381 kg-m^2

REVO by Alcon
7.66kg 332mm = 0.422 kg-m^2

Tarox
6.2kg 300mm = 0.279 kg-m^2

Tried to find the weight of AP BBK but cant find the info for them.

Cool idea as a comparison! Is this just quick math using the stated weight of the rotor or does this break down the distribution of weight in the rotor?

I only ask as rotational inertia is also dependent on the distance of the weight from the center of rotation, e.g. the Willwood rotor may be lighter but most of that weight loss is in the aluminum center hat right near the hub/axle which has less of a savings compared to lightening the entirety of the rotor, and even with the lighter rotor weight overall, the larger diameter may end up with a similar moment of inertia in the end.

The other interesting comparison is the heat capacity of the rotor itself, I believe that's the approach that the "big boy" cars take, if you make the braking surface of the rotor thicker (heavier) it has higher heat capacity and does a better job of maintaining braking performance throughout a track session as the braking surface doesn't get as hot as fast. With big power in a corvette, 911, etc. the added weight of huge rotors is less of an issue, but on the fun little go-carts we have, its probably much more noticeable!

Anyhoo, cool calculations! its an interesting comparison between the different manufacturers of BBK's

 

[Dialcaliper]

Interesting. Curious how you came up with these numbers - can you elaborate?

Short of full on drag cars, which tend to if anything go for smaller brakes to save both weight and reduce inertia (not bigger), rotational inertia is going to have a very small effect, compared to lower unsprung weight.

Larger rotors have both more thermal mass and also more surface area inside the vanes to reject heat.

The Wilwood kit rotors despite the larger diameter are also quite narrow compared with the stock rotors, which with the aluminum hats means reduced weight.

 

[Dpro]

Cool idea as a comparison! Is this just quick math using the stated weight of the rotor or does this break down the distribution of weight in the rotor?

I only ask as rotational inertia is also dependent on the distance of the weight from the center of rotation, e.g. the Willwood rotor may be lighter but most of that weight loss is in the aluminum center hat right near the hub/axle which has less of a savings compared to lightening the entirety of the rotor, and even with the lighter rotor weight overall, the larger diameter may end up with a similar moment of inertia in the end.

The other interesting comparison is the heat capacity of the rotor itself, I believe that's the approach that the "big boy" cars take, if you make the braking surface of the rotor thicker (heavier) it has higher heat capacity and does a better job of maintaining braking performance throughout a track session as the braking surface doesn't get as hot as fast. With big power in a corvette, 911, etc. the added weight of huge rotors is less of an issue, but on the fun little go-carts we have, its probably much more noticeable!

Anyhoo, cool calculations! its an interesting comparison between the different manufacturers of BBK's

It’s an excercise for fun but really just calculating how much unsprung weight each kit manages to drop is much more useful than the slight differences in rotational mass. As it is my Wilwood kit drops quite a bit of unsprung weight . The Whoosh kit drops 7lbs per corner over stock and the Wilwood dropped another 3-4 pounds per corner over that . . All in all between my brakes and Dekagrams I have dropped at least 18lbs per corner in unsprung weight .

 

[viki888056]

Cool idea as a comparison! Is this just quick math using the stated weight of the rotor or does this break down the distribution of weight in the rotor?

I only ask as rotational inertia is also dependent on the distance of the weight from the center of rotation, e.g. the Willwood rotor may be lighter but most of that weight loss is in the aluminum center hat right near the hub/axle which has less of a savings compared to lightening the entirety of the rotor, and even with the lighter rotor weight overall, the larger diameter may end up with a similar moment of inertia in the end.

The other interesting comparison is the heat capacity of the rotor itself, I believe that's the approach that the "big boy" cars take, if you make the braking surface of the rotor thicker (heavier) it has higher heat capacity and does a better job of maintaining braking performance throughout a track session as the braking surface doesn't get as hot as fast. With big power in a corvette, 911, etc. the added weight of huge rotors is less of an issue, but on the fun little go-carts we have, its probably much more noticeable!

Anyhoo, cool calculations! its an interesting comparison between the different manufacturers of BBK's

You are right about this that the weight is much different as most of the two-piece rotors have mass outside but one-piece rotors also have most weight outside not the same it would be hard to calculate and find the weight of all of the hub caps for each BBK. let's say that two-piece rotors have 90% of the mass in the outer ring and one-piece rotors have 70%. but this is just a rough draft of it gives more or less weight to each system I think that this can be used when choosing a BBK.

We can say that the difference with BBK is even bigger.

 

[viki888056]

Interesting. Curious how you came up with these numbers - can you elaborate?

Short of full on drag cars, which tend to if anything go for smaller brakes to save both weight and reduce inertia (not bigger), rotational inertia is going to have a very small effect, compared to lower unsprung weight.

Larger rotors have both more thermal mass and also more surface area inside the vanes to reject heat.

The Wilwood kit rotors despite the larger diameter are also quite narrow compared with the stock rotors, which with the aluminum hats means reduced weight.

I agree but in Europe drag racing is not that popular so this is more for time attack or people that like to do track days. like as you said that the willdoods are lighter but larger i would say its the same as stock when it come the rotational inetsia.

 

[viki888056]

It’s an excercise for fun but really just calculating how much unsprung weight each kit manages to drop is much more useful than the slight differences in rotational mass. As it is my Wilwood kit drops quite a bit of unsprung weight . The Whoosh kit drops 7lbs per corner over stock and the Wilwood dropped another 3-4 pounds per corner over that . . All in all between my brakes and Dekagrams I have dropped at least 18lbs per corner in unsprung weight .

I agree this is just a fun research i did wile being sick and obsessed with my fist. but isn't there like a rule that each 1kg you lose in the hub is 3kg in reality? Im not sure maybe you guys will know better.

 

[viki888056]

Interesting. Curious how you came up with these numbers - can you elaborate?

Short of full on drag cars, which tend to if anything go for smaller brakes to save both weight and reduce inertia (not bigger), rotational inertia is going to have a very small effect, compared to lower unsprung weight.

Larger rotors have both more thermal mass and also more surface area inside the vanes to reject heat.

The Wilwood kit rotors despite the larger diameter are also quite narrow compared with the stock rotors, which with the aluminum hats means reduced weight.

I used a basic inersia calculator online that calculates this for me. i can send the link to it if you what to dobble check.

 

[sonicmn]

You are right about this that the weight is much different as most of the two-piece rotors have mass outside but one-piece rotors also have most weight outside not the same it would be hard to calculate and find the weight of all of the hub caps for each BBK. let's say that two-piece rotors have 90% of the mass in the outer ring and one-piece rotors have 70%. but this is just a rough draft of it gives more or less weight to each system I think that this can be used when choosing a BBK.

We can say that the difference with BBK is even bigger.

That makes sense to me! Just figured I'd ask for clarification, I'm an engineer and your post brought me back to statics and dynamics engineering classes. Fun topic to think about when considering upgrades as it will change the ride/steering of our vehicles.