Some thoughts on heat, thickness, and drilling/slotting:

Consider a 2ft piece of flat steel. You can hold one end in your hand comfortably while you torch the other end. So we can see that heat propagates in metal by conduction.
The heat on a brake rotor is generated on the pad surfaces, and the rotor is cooled in the middle. Seems like that would increase the temperature difference--that's bad, right? But it's the only way to cool it, so we have to do it.

Trey mentioned that thicker rotors last longer and have less tendency to crack under severe conditions.

Suppose that the pad sides of a rotor are at temperature T1, the middle (cooling "side") is at T2, and that T1 and T2 are constant regardless of rotor thickness. The more distance (thickness) between T1 and T2, the more gradual the temperature gradient will be in temperature per unit distance. Then the heat stress (which we are considering to be an expansion gradient) will be distributed over more material and therefore will not be as severe. So thicker rotors are better in theory and in practice.

Machining operations (such as drilling or slotting) cause mechanical stresses, which are exacerbated by heat stress. In addition to that, holes (even chamfered ones) and slots have sharp edges which are stress risers. Now consider what happens when you put a torch to something with a sharp edge or corner on it: the edge glows red before the rest of the object. So the edge is hotter, right? That's another temperature gradient. So now we have
heat stress,
mechanical stress,
stress risers,
and additional heat stress around the holes or slots.
Theoretically, this would make cracks more likely than just a plain rotor, and the cracks would probably be in the area of the most stress--right by the holes/slots. Lo and behold, this is exactly what happens in real life.

So in extreme braking applications, assuming that the pad does not generate gases, a flat rotor is better than cross-drilled or slotted.

Just a chemist's 2 cents.

 

Don't forget the massive amounts of air passing across the outside surfaces of the rotor. It may be as effective as the air thru the vents, especially in non-directional or straight vented rotors. In solid rotors that's all there is.

 

Boy, this thread turned into a mess

Trey, the thicker the rotor, the greater the temp difference between the inside and the outside of the "'cheek", generally speaking. The greater the temp differential, the greater the chance for mechanical stress and therefore cracking, especially when you've provided a path (your drilled hole) for the crack to propagate. I think NovaMan already said this much (and more).

Non-vented rotors reign supreme on motorcycles. Of course, the GP bikes run carbon-carbon brakes.

I, too, am experiencing hair loss, causing a fast loss of heat in the winter, and sunburn in the summer. Fortunately, I don't often use my head for braking purposes, and when I do, I try to wear my bicycle helmet (which is ventilated with holes and occasionally cracks, helping to bring this conversational thread back on-topic ).

 

Alright, so we have established that a plain smooth rotor is the best. Steel is the most practical material to make brake rotors out of also.

Now, as for carbon ceramic brakes. They can handle heat much better right? And they last longer because of this. Is this their only benefit? I know I am leaving out something but I just cant put my finger on it. Help me out.

The carbon brakes used on the Enzo are amazing, but also cost a buttload. Anyone know where some carbon or ceramic rotors can be found for under 20 grand?

Hunter

 

Yeah, you can get the carbon rotors for a Top Fuel Dragster or Funny car from Strange for about $1100 each, but I think that is probably still out of everybody's price range!

One thing about carbon is that they don't work to well cold-the hotter they get, the better they work. Normal street driving probably doesn't generate enough heat to stop well enough to warrant the couple extra bucks spent on them!

 

GET SOME CUSTOM MADE ALUMINUM ONES

save 10lbs a wheel

thats prob a good 1/3 sec or close to in a stock car

-40lbs of rototing mass = faster braking and accel

 

A picture is worth 1,000 words:

Keiths' WS6.com Baer Brake Rotors

Cryo treating extended the rotor life from 4K miles to 19K miles..... but they still cracked. Makes me inspect my rear Baer drilled/slotted rotors every time I have a wheel off.

 

Ya that pic, just makes me stray farther and farther away from buying drilled rotors, and makes me giggle inside when I see guys blow 100 grand on a Porsche with drilled rotors.

I know that carbon rotors take a while to work well, but what about ceramic rotors?

Also, any benefit in cooling to aluminum rotors? Wouldn't they warp due to quick heating and cooling periods?

Hunter

 

Im not sure about warping- but they are VERY light and stay VERY cool.


Also- Eric,
I was again talking to my manager. He showed me some heat analasys on on some test (blank) rotors.

On a single stop, the temp can varry on a rotor by over 500*! The outside edges of a rotor were the hottest and coolest in the front/hat sections.

After a few stops, the outer diameter of the rotor was the hottest and got progressivly cooler as we got to the inside.
If anyone is intrested, i can try to post some pictures. Thermoexpansion on these things is NUTS too. that thing moves ALOT.

Also, I talked to him about your theory of how thicker cross drilled rotors crack easier. he said that although he never designed or tested one, he was VERY confident thicker was better.
Looking through his "rotor thickness vs cracking" test, the results are clear as glass. Thicker is MUCH MUCH stronger!


Let me know if there is intrest in seeing these pictures before i go off scanning them for fun

 

I would love to see those pics.

Hunter

 

The holes in Porsche (i.e. Brembo) rotors are actually cast in the rotor when its made rather than being drilled afterwards. This supposedly helps eliminate the stress risers which causes normal drilled rotors to crack.

If I had an extra $2k laying around I'd love to have a set of Porsche Big Reds on my car. Holes or no holes those things would stop a 747. As good as they are on F-Bodies I can only dream as to how well they work on a lighter car...

 

Unless of course you have a fat head

 

This seems to be an awfully fluff filled thread for advanced tech… forgive me if I repeat something that someone already said, it was painful enough reading that I only skimmed most of it.

In a nutshell:
- cross drilled rotors are a hold back to the 60’s. Back then pads would outgas as you used them and rotors were cross drilled to allow a path for these gasses to escape so the pads would stay in contact with the rotor surface. Pad compounds have gotten much better and out gassing is no longer an issue. There is NO reason why a car with modern pad compounds and a steel or iron rotor should be cross drilled. They do nothing for cooling. They do little for weight reduction, and the hard/drilled edges are stress risers which create a location for cracks to start.
- Slotted rotors are really designed for rally and similar applications. The slots do not cool, they do not lighten or anything else. They clean contaminants from the pad surface so when you apply the brakes right after driving through a mud puddle you actually have brakes. They’re sort of a combination scraper and groove to flow crap away from the pad surface (think aquatread tires). BTW, if they are not properly designed they become a great source of stress risers and cracks. For 99% of us, slotted rotors will do nothing but cause our pads to wear faster and decrease the life of the rotor.
- Larger rotors are not used for more stopping power. Any rotor that can lock the brakes can exert more torque then the tires can apply to the ground (in other words, all the stopping power that the chassis could use), in most cases, this would be a very small rotor. What bigger rotors do is act as a bigger heat sink. They have more mass, more thermal inertia so that if you apply the same amount of heat to them they do not get as hot (stopping converts the kinetic energy of the car to heat). The idea is that if you can control the range of temperatures that the rotors can possibly see to a smaller range you will have an easier time designing pads that will give consistent braking performance. If it wasn’t for heat then the best design would be the smallest rotor that you can generate enough torque to lock the wheels, these things are heavy, and remember that rotating mass is like adding 4x the stationary mass to the car WRT to the necessary force you must generate to accelerate (decelerate) the car. For street use, almost no one needs larger brakes, they just need pads that better suit their driving style.
- Most drilled and slotted rotors are sold for their appearance… it’s a ricer mod. People associate it with high performance brakes. The super high end Porsche stuff is the only exception that I know of. Their “crossdrilled” are actually cast (with a taper and chamfer) in ceramic composite rotors that are meant to work with a special pad. They’ve discontinued and redesigned them at lest 2x that I know of because they did not work correctly or wouldn’t live.
- Another effort that a lot of experimenting in this field is going in is better heat dissipation. If you can dissipate more heat you can run a smaller rotor with the same results. One of the ‘cooler’ (sorry) things that I’ve seen are rotors with lots of little cones cast into the finned vented area to increase surface area and better dissipate heat.

And then my real pet peeve… if you can run smaller rotors you can run smaller diameter, lighter wheels and tires

 

Well, thanks for that great brake rotor lesson. I think I am gonna look in the Porsche rotors. Ceramic brakes are waht I really wanna do, and these guys know their stuff.

What do u think about aluminum rotors?

Hunter

 

No kidding, those Porch brakes rock, wish I could have some too. My old azz 240Z brakes make the Z28 feel like a friggin bus.