We're studying thermodynamics in Physics right now and the transfer and storage of heat. A couple of facts got me thinking.
1) Specific Heat is defined as "heat capacity per unit mass"
2) the equation defining this is Q=cm ∆T where Q is the amount of heat an object gains or loses, c is the specific heat of the material (a constant) and ∆T is the change in temperature of the material from its initial temp to its final temp (Tf - Ti)

Different materials can be considered when making brake rotors. Several important things must be considered, including but not limited to: strength, cost, workability/machinability (how hard it is to work with), weight, and Specific Heat.

Some specific heats and weights of commonly used metals:
Iron: 0.447 calories/grams x Kelvins , weighs 55.7 grams per mole
Aluminum: 0.215 cal/gxK , weighs 27 grams per mole.
Titanium: 0.523 cal/gxK , weighs 47.9 grams per mole

Iron and Aluminum are most common as they are cheap, easy to work with, and strong. Titanium is more expensive and harder to work with. Now, the above information taken into account, it stands to reason that the best metal for brake rotors would be Titanium, as it is the strongest, has the highest specific heat, and weighs less than Iron. Having the highest specific heat is key because it takes more heat to heat up the metal as it can store more of it.

Now, here is my theory. The specific heat of water is 1.00 cal/gxK which is nearly twice that of Titanium, more than twice that of iron, and more than 4 times that of aluminum. Now obviously we can't make a brake rotor out of water, as it is a liquid. Nor can we run water over the rotors as it is a lubricant and would seriously diminish brake performance. Now, here is my idea: A rotor that is hollow, but the hollow pocket is filled with water. The water will absorb a lot of the heat generated by braking, as the metal and water will want to reach thermal equilibrium. To do so, the metal must keep transferring heat to the water, so the metal looses heat, improving brake performance in endurance type situations.

Now, to further enhance the set up, you would use vented brakes, though the vents would go only as far as the wall of the water pocket. This would increase surface area to expedite cooling of the metal and the water core. The brakes could also be slotted, as that is done to the surface of the rotor. Partially cross drilling would also be possible in the vented outer area of the disc, but obviously not towards the center where the water core is.

The ideal set up would be one of the following:
1) Titanium rotors with a water core, vented, slotted, and cross drilled. Titanium is very strong, lighter than iron, and has the highest specific heat. The rotors would be hard to heat up to begin with and any heat they absorb would be transferred to the water, cooling the metal as the materials attempt to reach thermal equilibrium. Venting, slotting, and cross drilling them would further expedite rotor cooling.

2) Aluminum rotors with water core, vented, slotted, and cross drilled. Same as the titanium rotors but with the properties inherent to aluminum, those being its lighter weight, reduced strength, and much lower specific heat. The possible advantages there are the weigt savings, as water is heavy and will weigh down either rotor design. The other possible advantage is that aluminum, with its lower specific heat, would transfer generated heat to the water quickly and thus cool down more quickly. On the other hand, it would also heat up more quickly. The trade off would have to be tested to determine whether its a better set up than titanium.

This design, regardless of the metal chosen, has and inherent advantage and disadvantage. The disadvantage is the additional weight of the water in the rotor, as well as an additional cost to produce them. The advantage, other than those already covered, is that the heavy water is located at the center of the rotating assembly (the wheel and rotor). Keeping the weight centered stabilizes and balances the rotating object and allows it to rotate faster.

Of course, initially the real world application of this design would be for high end racing teams, as it would be expensive. Its primary advantages would be in endurance and Auto Cross racing where there is heavy brake use. But eventually it could be brought to consumer autos, as many racing innovations have been.

So what do you guys think of my idea/theory? Would the potential advantages described here be enough to make a significant difference in stopping distance, heat build up, brake fade, etc over coventional brakes? Is my reasoning sound or am I totally off my rocker?

Thanks for reading my dissertation of sorts there

A friend of mine and I are talking about it and he made some good points.

The rotor would take longer to heat up, but at the same time it would take longer to cool down as water retains heat very well. Not good in this application. All the venting, slotting and drilling in the world won't make that huge of a difference, though it will help.

The water would expand inside the rotor. Now since its sealed, it could only expand to a point, but the pressure would increase exponentially. If the rotor is not strong enough in its materials and its design, the rotor would crack or explode as the water pressure gets extremely high. But there's no guarentee the rotor and water would ever get that hot.

He also pointed out that the almost constant expansion and contraction of the water may have and adverse effect on the integrity of the rotor and could wear out the metal over time causing it to warp or become weak.

Just a few more things to think about. Still seems like an idea to pursue to me.

No one has mentioned that with all that added mass that now is rotating the effect it would have on the suspension.

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I think that after about 8 seconds of braking the water would turn to steam and explode your rotor resulting in 0 braking power from then on

True, another draw back. The additional weight of each wheel and rotor assebly would definalty effect the suspension geometry.
Also, the added weight would hurt the rotational inertia of the wheel, which would hurt your speed, so the same amount of power from the engine wouldn't move the car as fast.

Ok, some basics on brakes. The whole intent of braking is to convert the kinetic energy of the car's momentum into heat. You need to dump that heat as fast as possible because if the pads get too hot then superheated gases (plasma?) will form between the pads and the rotor surface, lubricating the pads and seriously reducing braking friction. That's why racing brakes are crossdrilled and slotted, to allow a place for these gases to vent, and that's why on a street car those same slots and holes are purely decorative because it's near impossible for a street car to get its brakes hot enough for this to happen.

Now, anything that causes heat to be retained in the rotors is a bad thing. It's not about absorbing heat, if it was then rotors would be giant chunks of iron. It's all about dumping heat, the faster the better.

The ultimate rotor would be able to dump all of the heat generated by braking right to the atmosphere as it is being generated at maximum braking force, but in reality that can't really happen in a car braking system. Rotors gain heat during hard braking and then dump heat during the rest of the drive, acting as a thermodynamic accumulator as it were.

If you really wanted to deal with the heating issue, and this would probably only be worth it on a race car, you could blow water mist down the inside of the rotor vent slots while braking. The heat of braking would vaporize the water to steam, absorbing a huge amount of heat during the phase change. Downsides of this would be thermal stresses that could crack the rotor, so the rotor would have to be steel or titanium instead of iron, and having a limited ability to carry water.

JazzMan

Throw heat transfer and fluid flow and heat sink equations into the mix and it gets more confusing. Consider the fin area of a typical radiator with the fin area of the typical vented rotor. Consider the coolant volume of an engine and the coolant volumn of the rotor. You're talking orders of magnitude difference.

But remember the cardinal rule of engineering: K.I.S.S. (Keep It Simple, Stupid)

You don't always want a more complex solution to a problem. Consider the Ferrari Enzo with Carbon brake rotors similar to F1 cars. Far superior thermal and strength qualities (for a brake rotor) than the typical metals used, but a simple design that doesn't rely on unusual cooling, internal water jackets, etc.

But it's a neat idea. You're thinking outside the box, and that's the important thing. You'll throw out 1000 bad ideas to get to one good one. But that one good one will be worth it.

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[This message has been edited by Formula88 (edited 02-14-2004).]

Ahh I see, thank you Jazzman. I thought that, since the rotors were going to heat up anyway, it would be better to transfer that heat inward to the water pocket and away from the actual braking surface and allowing the heat to disipate via the vents from there, thus keeping the surface cooler. It would work in addition to the outside atmoshpere absorbing the heat. The idea behind using a metal with a higher specific heat is that it would take more heat to get the rotors hot, as they would absorb a lot. The pit fall is that it would take them longer to lose the heat they accumulate.

Thinking more about what Jazzman said, maybe the rotors should be wider as well as bigger in diameter. Making them wider allows for A) more surface area on the vented part as the vents are wider and B) more air flow through the vented area. Also, making them wider pulls heat that the rotor does absorb away from the surface that is being heated. Due to thermal equilibrium, heat would move away from the source of the heat in order to heat everything it is in thermal contact with. So with the pads clamping on the outer sides, the heat would move inward, into the widened vented section where it could better be absorbed by the atmosphere. The additional metal would expedite the heat transfer away from the braking surfaces and towards the vented area.

Now I have another idea, one quite difficult to engineer.
It would be similar to the cooling system of a car, where in the water is circulated. Of course, the immediate problem its the fact that the rotors are spinning. That gives two possibilities.
1) a water pipe that is in frictional and thermal contact with the rotor via ball bearings made of a strong metal that transfers heat well.
2) Or, running very small water lines through the calipers and against the back side of the brake pads.
Don't know how well either of those would work though. Either way there would have to be a water pump and radiator just for that system.

Try this on for size: The brakes don't have to be at the wheels.

With the braking function handled inboard you open up all sorts of additional possibilities because you eliminate the size/geometry restrictions of being inside the wheel.

JazzMan

More brainstorming.

The drum brake was the earlier design and worked well, though it had a serious problem with heat build up. The big heavy solid drum get hot quick and stayed hot. However, it had an advantage over the disc brake in that it provided much more surface area and thus more friction and more stopping power. The disc gives up surface area for better cooling and reduced brake fade.

Since the disc came in, the drum is relegated to the rear brakes or is not used at all.

Would not a better idea would be to design a better drum? Instead of a solid piece of metal, make it like a car rim with spokes. Right there you get better ventilation. Add vents to the outside edge all the way around for cooling. Some braking surface could be sacrificed to allow the vents to go all the way down and through to let gases and heat escape directly into the atmosphere.

Any way you do it, you could still get a lot more contact area with the brake pads, thereby increasing friction and stopping power. With all the venting and proper air ducting in and around the wheel wells, I'd bet you could get better braking performance out of a well designed drum than a disc.

Are we talking racing brakes or everyday brakes..

I like the idea of a Ti ring brake (think open drum)

For racing apps.. what kinds of exotic materials could you come up with that have a high heat shed rate?.. Ceramic faced/Ti backed/slotted rotors?

Give me 2 more hours and some more Dt coke and I'll have it all worked out..

It'll work great till you ingineer types start mucking it up with your fancy theories and why things are physically impossible.

Remember, there's a fine line between practical and possible.

An idea I saw once that looked promising: The rotor disk's outside diameter matched that of the inside of the wheel, such that the disk was attached to the wheel directly, and had no hub. The caliper was mounted to the knuckle with the "C" facing outwards to grip the rotor from its inside diameter. This arrangement allows for easy placement of two or more calipers, and provides huge amounts of surface area to shed heat.

Another approach for real emergency braking would be an anchor fired straight downward into the pavement, attached to a steel cable that was tied to the car with a big spring. Sixty to zero in 5 feet.

JazzMan

Dont forget the experimental electomagnetic braking systems. They will make disc and drum brakes look like Flinstone brakes.

You would need to use the metal of the cars around you to push against with your magnetic fields in order to stop yourself. You decellerate, they accellerate. Hmm... now that has some possibilities...

JazzMan

quote Originally posted by Fastback 86: Would not a better idea would be to design a better drum?

Welcome to 1965. Pontiac already did that.


The finned 8 lug rim is actually the brake drum. They were far superior to the regular drum brakes, but disc brakes were better still.

Oh, and they looked really cool, too.

[This message has been edited by Formula88 (edited 02-15-2004).]

FYI, Titanium would not make a good brake rotor

It may have a somewhat higher specific heat than iron, but it's got MUCH less mass. You do the math.
Just go down to your local Porsche dealership and order a set of ceramic rotors and pads for a 911 turbo or GT2.

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'87 Fiero GT: Low, Sleek, Fast, and Loud
'90 Pontiac 6000 SE AWD: None of the Above

Luck is for those who lack the courage to define their own fate

Well looks like some ppl thought of this ceramic deal before I did....

http://www.tirerack.com/brakes/tech/why_ceramic_brake_pads.html

http://waw.wardsauto.com/ar/auto_brembo_busting_ceramic/

Guess I'm back to the open drum idea..

Some are even going to carbon fiber rotors. If your going to disapate heat in the rotor itself, why not make them sodium filled like valves for cooling?

Got around to reading this whole thing...
Yes, inboard mounted brakes offer several advantages over outboard mounted brakes. One of which is less unsprung weight. Older Jaguars had inboard mounted rear brakes. When in generally good condition, those cars actually ride better than new Jaguars.

I'm amazed that more manufacturers don't mount the brakes inboard, at least on driving axles. I don't think ducting would be difficult, as the rotor is basically its own centrifugal air pump.

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'87 Fiero GT: Low, Sleek, Fast, and Loud
'90 Pontiac 6000 SE AWD: None of the Above

Luck is for those who lack the courage to define their own fate

Don't most NASCAR racers use forced air with ducts to cool each rotor, intake at the front?
Not much good in school zones, though.

TG

[This message has been edited by TennT (edited 02-15-2004).]

Wow, I was just playing around with the idea of indboard brakes, and here it is. The rear would just take some design and fabrication. But...

If one were to have CV-jointed driveshafts on the front leading to inboard discs, what effect would it have on steering, handling, etc. I don't mean AWD, just inboard brakes.

Would the loss in unsprung weight would be sufficent to compensate for the added sprung weight of the axles, etc. The other benefit I saw, that has been mentioned here is space since it wouldn't have to fit inside the rim.

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perk - todd's hot rods
Shotgun - the ultimate toy toter
Blade - the street-legal Fiero-based race car
Rock-it - the Fiero that will soon be for sale

Hummers (the real ones) have inboard mounted discs. The geared hubs on those vehicles act to double brake torque at the wheels.

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'87 Fiero GT: Low, Sleek, Fast, and Loud
'90 Pontiac 6000 SE AWD: None of the Above

Luck is for those who lack the courage to define their own fate

Some general thougths on this... (If someone didn't get to this already...)

When you think about brakes, think heat pump. The rotor literally pumps heat from the friction faces to the air.

You can cool brakes too fast and/or too much. Most friction materials need a certain amount of heat to brake properly. This is already a problem with certain aftermarket pads that won't stop the car hardly at all when cold. Rotor designs account for the max heat they think the car will generate but also control the rate of heat loss so the pads work properly.

Moving the brake rotor inboard does solve some issues but creates others. For one thing you may need a stronger axle to take the torque. Braking can generate more torque than stomping the go pedal. Brakes can also generate huge load surges that the engine usually can't. This would mean all parts of the axle have to be up for the job, not just the shaft. A CV joint not up to this could snap off or literally explode. Normal axles never see those kind of load spikes.

Iron is used not only because it is cheap, it lasts a heck of a long time. The iron used is usually a ductile formula that takes quite allot of abuse when it comes down to it. Putting a nick in an Iron rotor isn't usually a fatal flaw. A nick in some other materials will cause rapid failure. I've see iron rotors so ground up you have to wonder how the hell they didn't fail. The stuff can be rather amazing.

quote The major drawback here is the other half of the coin. With the high specific heat, the rotors and water would take much longer to cool down than another set up.

Just like with heatsinks, thermal conductivity (rather than heat capacity) and surface area would be the key elements of brake cooling. Like mentioned above, the idea isn't for the rotor to store as much heat possible, but rather for it to transfer the heat as quickly as possible to the surrounding air.

But, as the Ogre mentioned, there is a caveat. The brake rotors and pads do need to heat up a certain amount to achieve optimum performance. So rather than cooling the rotors as much as possible, you're now looking at a scenario where you need to keep the rotors within a certain temperature range... similar to your engine's cooling system.

This may sound low-tech and mundane, but a great way to improve the performance of a heatsink is to have a fan blowing cool air on it. Look inside your computer and you'll see several examples of this cheap yet effective cooling technique. The great thing about vented brake rotors is that they're basically a heatsink and a fan rolled into one. But the rotors aren't always spinning at the optimum speed. As a matter of fact, when your rotors get the hottest, they're probably not spinning very fast (they just finished stopping your vehicle).

So I'd like to propose a novel alternative to liquid-filled rotors and exotic materials: fan-cooled brake rotors. You could build a shroud into the splash guard (or to replace it) with an inlet for an air duct, to pump fresh air directly into the center of the rotor. At the other end of the duct, you'd have a fan or blower, with a pyrometer-controlled circuit that varies the fan speed to keep the rotor within a designated temperature range. It's probably already been done before, but I think it would be neat.

[This message has been edited by Blacktree (edited 02-15-2004).]

Have the brake rotor attached to the outside of the rim, and the caliper mounted to the suspension

You'd have a huge brakeing area, you could have fins on the mag to direct airflow onto the rotor.

however, you are loosing your mechanical advantage the closer to the centerline the caliper is placed..

James Z

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The Buel (sp?) bikes I saw at the Harley shop have a brake system like the one pictured above. Pretty cool looking setup.

A fan cooling system is somewhat achieved by using vented rotors that have curved vanes rather than the straight vanes most commonly found on cars

quote Originally posted by SplineZ: however, you are loosing your mechanical advantage the closer to the centerline the caliper is placed..

What are you talking about?

The same reason that buses have larger steering wheels than most cars.. I guess "centerline" isnt the best word... being closer to the "center" of the wheel i meant

James Z

quote Originally posted by Will: What are you talking about?

Not necessarily. Take something cone shaped, set it on its base on a bar stool or something else that spins, then spin it really fast. First, place your hands on it near the pointy end, where you're closer to the axis of rotation. Then spin it again and place your hands down at the bottow where its farther from the axis and it will require less force and or less time to stop.

The big disadvantage with internally mounted calipers like shown above, They have a Huge disk that adds LOTS of rotating mass and unsprung weight, Our enemies. Not to mension the complications of mounting them. But it would certainly look cool. Monster trucks have 2 disk brakes for all four wheels, they are mounted on the pinion of the differentials. They have 2 Final drive reductions AFTER the brakes, imagine the stopping power there! but there is still the rotating mass issue (bye bye horsepower!) But if your into thinking outside the box. Try to come up with a better form of energy other then heat, that can be produced from brakes.

But like stated before, the electromagnetic brakes can be used to turn deceleration into something more useful then heat, Electricity. But I feel it is alittle beyond gasoline cars to have that kind of electric generation, hybrid's or electric cars, they could use them.

quote Originally posted by SplineZ: The same reason that buses have larger steering wheels than most cars.. I guess "centerline" isnt the best word... being closer to the "center" of the wheel i meant James Z

The determining factor is where the centroid of the pad area is located, not where the caliper itself is. The internally mounted caliper pictured would have a heck of a lot more leverage than a convnetional caliper.

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'87 Fiero GT: Low, Sleek, Fast, and Loud
'90 Pontiac 6000 SE AWD: None of the Above

Luck is for those who lack the courage to define their own fate

Weight does not effect geometry. Geometry is geometric.

More importantly the additional weight would increase rotating mass which would hurt brake performance. Ironicly, that is what you were trying to improve.

Carrol Smith in Engineer to Win has plenty of braking thoughts if you want to read what happens real world and not in a physics textbook.

quote Originally posted by Fastback 86: True, another draw back. The additional weight of each wheel and rotor assebly would definalty effect the suspension geometry. Also, the added weight would hurt the rotational inertia of the wheel, which would hurt your speed, so the same amount of power from the engine wouldn't move the car as fast.