Lighter Brake Rotors Not Better?

[polysynth]

From what I understand, brakes work by converting the car's kinetic energy into heat in the rotors. And we all know that I takes mass/weight in order to hold heat. For example it takes much less energy to heat up a pot-full worth of water than to heat up a pool. If that's the case then is it a good idea to buy lighter rotors for the purposes of saving unsprung mass? Lighter rotors means it'll heat up faster and if it heats up faster it'll brake fade more.

But I am sure that there's more to it though. Big companies like endless, brembo, stoptech, etc...all have lighter components than OEM including the rotors in their big brake kits, yet they perform well. How is that justified? Are heat being dissipated better in these BBK? Are the pads design to operate at a higher temperature?

 

[8.5MS3]

that is why they are cross drilled, slotted, and vented.....to dissipate heat. also a thinner material will cool quicker than a thicker one

 

[drledford93]

Actually, the cross-drilling, etc, is for the reduction and cleaning of the rotors. To keep the brake dust off the disc which can reduce the coefficient of friction. It has been shown that cross-drilling can cause potential cracking and explosion of the disc itself. Read up on it and be careful.

For 1lb of unsprung weight reduction, the car sees an effective reduction of 7lbs of sprung weight. IE/ if you had a collective 1lb weight loss from all 4 wheels, then it would be as if you suddenly lost 7lbs off your belly. (Just a funny example.) So, you can see that any weight you can shed in this way will be multiplied greatly in performance. This is why racing teams spend a huge portion of their budget getting the lightest wheels they can find.

I can attest to this, having used 16" 3rd gen RX7 rims, which are 16lbs. The gain over stock rims was HUGE. With only a CAI, I was getting unbelievable wheelspin in 1st-3rd gears! Amazing. The same would apply to any weight loss in the brake rotors or even calipers, if you wanted to go that far.

Daniel

 

[Darth Vader]

There's a big difference between thermal mass and regular mass. The amount of heat a rotor can take in without destruction is it's thermal mass. This is affected by, yes, the amount of material involved but also the type of material and design factors.

 

[8.5MS3]

There's a big difference between thermal mass and regular mass. The amount of heat a rotor can take in without destruction is it's thermal mass. This is affected by, yes, the amount of material involved but also the type of material and design factors.

well yea. im not gonna shave half the material off my stock rotors for lightness. although i wonder what the outcome would be lol crushed disks?

 

[SRTie4k]

well yea. im not gonna shave half the material off my stock rotors for lightness. although i wonder what the outcome would be lol crushed disks?

They'd just keep slipping and slipping until they shattered.

 

[polysynth]

There's a big difference between thermal mass and regular mass. The amount of heat a rotor can take in without destruction is it's thermal mass. This is affected by, yes, the amount of material involved but also the type of material and design factors.

Hmm...good point. Do you know type of materials these are specifically?

 

[Darth Vader]

No but, my guess for stock would be ductile iron, aka, nodular iron.

Sintered iron, carbon steel, carbon fibre, are other materials is use. These terms cover a wide range of proprietary factors, i.e. one company's choice of a sintered iron may not be the same as another company's.

 

[cymx5]

For 1lb of unsprung weight reduction, the car sees an effective reduction of 7lbs of sprung weight.

Depends on the geometry of the car.

The amount of heat a rotor can take in without destruction is it's thermal mass. This is affected by, yes, the amount of material involved but also the type of material and design factors.

I assume you merely mean the surface area by 'design factors?'

Wow. Being a real engineer, I really need to stay away from this thread and the many misconceptions / incorrect definitions that may come from it.

 

[drledford93]

Depends on the geometry of the car.

Being a real mathematician with an interest in physics, I posted this. Why would the "geometry" of the car affect anything to do with unsprung weight?

 

[8.5MS3]

ive herd 6 but the key thing is your reducing rotational inertia, angular momentum and unspring weight..all of which translate to better acceleration, braking and steering response

 

[cymx5]

The best way to learn is to answer the question yourself...i.e. who/what/where did the 6lb or 7lb answers come from?

 

[8.5MS3]

well its different with each car. its dependent on a number of factors:

wheel diameter
rotor diameter
wheel/rotor weight
where the weight has been removed (closer to the edge or the center)
which corner its removed from (front vs rear)
how much is removed

blah blah blah

dont make me pull out my machine design textbook from back in the day....i am a "real engineer" get off your horse. people do not know everything hence the reason for this thread. dont instigate a pissing match

 

[cymx5]

neh.

 

[drledford93]

neh.

Nice "real engineer" response there, bud.

Others have done the calculations and come up with (I believe) 6.8 lbs. So you got me there.

If you are going to be a negative naybob, please refrain from posting. If you have something to add (like actually disproving the statements here), please do so.

Daniel

PS--I noticed you still didn't answer my question of you about why geometry matters in unsprung weight? Hmm...

 

[Darth Vader]

By "design factors" I mean is it internally vented, is it cross-drilled, are there slots in it, is there ductwork directing cooling air to the rotors, etc. All of these are, after the materials choice that started the discussion, going to affect how much kinetic energy can be converted to heat by the brakes, without them destroying themselves.

Not being a "real engineer", just a guy who puts together the fastest stuff I can with the money I've got, I apologise if I haven't got all the words the way you want them. I ask you to show me where I, fundamentally, don't understand how brakes work.

 

[Mid_Life_Crisis]

PS--I noticed you still didn't answer my question of you about why geometry matters in unsprung weight? Hmm...

I suspect that by "geometry", he means the suspension design specifics. For example, the length of the swingarm (or whatever the technical term is) should affect how much difference weight changes to the rim/tire/brake assembly make. Others could be more specific, I`m sure.

 

[drledford93]

I suspect that by "geometry", he means the suspension design specifics. For example, the length of the swingarm (or whatever the technical term is) should affect how much difference weight changes to the rim/tire/brake assembly make. Others could be more specific, I`m sure.

But it's still unsprung weight, is it not? I'm not terribly familiar with this aspect. I guess it's the length of the torque arm? If this is so, I would imagine it's not going to change things much off of the "average" figure I pointed out earlier of ~6.8lbs. Maybe a few tenths, but not much more. Sure there are probably extremes (supercars with their odd suspensions), but these datapoints can be tossed as insignificant and anomalies.

Daniel

 

[cymx5]

Totally irrelevant comment here, but you would like "Fooled by Randomness" by Nassim Nicholas Taleb. He discusses, as one Amazon.com reviewer explained, "At risk of great oversimplification, Taleb argues quite articulately that extreme occurrences in a distribution happen a lot more frequently than humans are prone to believe." Basically, some of the most significant events in history are due to us generalizing the past, then getting burned by the statistically improbable. He mostly discusses trading in the stock markets, though, so like I said...totally irrelevant here, just a decent book to read.

 

[Mid_Life_Crisis]

But it's still unsprung weight, is it not? I'm not terribly familiar with this aspect. I guess it's the length of the torque arm? If this is so, I would imagine it's not going to change things much off of the "average" figure I pointed out earlier of ~6.8lbs. Maybe a few tenths, but not much more. Sure there are probably extremes (supercars with their odd suspensions), but these datapoints can be tossed as insignificant and anomalies.

Daniel

I didn`t say it made a lot of difference. I don`t know enough about the subject to make an educated statement one way or the other. I was simply suggesting what he might have been referring to.