Rotor Temp Testing

[argonaut]

Came across some good information regarding rotor temps seen on the track. This was posted by a long time member on corvette forum. The car is a C5, prepped for racing. It should be fairly transferable to a Mustang - rear driver, same ballpark HP, 3200+ lbs. He used high temp paint to mark the rotor in several spots and show the max temp reached. This was for 2-piece DBA5000 rotors, running A6 tires and Wilwood Calipers. I don't know what track:

- Edge of the rotor (where air exits from the vanes) 1400 degrees
- inner surface of the rotor face 800-1000 degrees
- rotor hat 670 degrees

More info:
- he was using brake ducts and doesn't think they do much. He based this on comparing the temp of the inside face to the outer face...the inside is much hotter but he didn't indicate how much. Needs more testing IMO before writting off their value.
- He added 2 "shims" made out of old backing plates and saw 300 degree drop on the plate nearest the pistons compared to no shims. (note thats a heck of a lot of "shim", indicating he was running some pretty worn pads and thus temps would be higher than brand-new pads)
- He said next time he'd try to test 1 piece rotors.

Not real scientific but some interesting data none the less. Rotors get hot...like real HOT. Thats a Big drop between the edge of the rotor and the hats. It would be very interesting to see 1-piece vs 2-piece - I suspect 2-piece have a big advantage here.

I've always wanted to get that paint and try this for myself.

Anybody else have some data like this?
The consensus opinion is brake ducts are a must (I've followed this advice for years) but does anyone have real data? I've heard more than one counter opinion on this - ducts don't do squat. But Pro teams run them as standard equipment and they would know. So until shown real data otherwise I'm going to continue believing in them.

 

[Philostang]

I've also wanted to try out that paint. Mostly I think it's academic, since I'm happy enough with my current combo that I'm not likely to change much, but I think it would be cool just to know.

On the ducts, we need more info on how he set up his system. Some of the home spun set ups don't seem optimal in how they direct (or in some cases trap) the outgoing air. The inside face being hotter could be a normal condition for his brake set up, or there may be something going bad with his calipers. Hard to say without really seeing what he did, the conditions under which he was testing, etc.

I'm with you though, I'll run ducts until seeing back-to-back tests that refute their value. Besides, the relevant test isn't differential temps, it's temps and heat transfer with vs. without them.

Best,
-j

 

[SoundGuyDave]

Inner face vs. outer face:

Inner: Relatively static air, trapped in a wheelwell. You do NOT want underbody air moving into this area, as that would start eliminating underbody aero, and thus downforce. This trapped air layer acts as an insulator, preventing heat dissipation.

Outer: There is actually quite a bit of air movement around and through the wheel, at least until it gets blocked by the brake rotor. Also, please note that the hot exhaust air from the rotor circumference will exhaust partially into the inner wheelwell, where it's trapped, and partly outward, where it's exhausted through the wheel. Thats why you see a LOT more brake dust on the outer half of your wheels with the inner, assuming you run ducts.

In short, ducts work, and work well on a variety of chassis types. At one track, I absolutely destroyed a set of brakes in a day, because I failed to remove my duct covers before tracking the car. Mental error on my part. The next day, with another FRESH set of pads, I had no problems. The lap times were very similar (+/- 0.75 seconds for best lap per day), and the only difference was functional ducts. At Road America, my ducts work TOO well, and I have to block off part of the inlets, or I go undertemp on the pads...

That all sounds pretty functional to me.

 

[DusterRT]

- Edge of the rotor (where air exits from the vanes) 1400 degrees
- inner surface of the rotor face 800-1000 degrees
- rotor hat 670 degrees

Thats a Big drop between the edge of the rotor and the hats. It would be very interesting to see 1-piece vs 2-piece - I suspect 2-piece have a big advantage here.

This all makes sense. The 'edge' of the rotor is closest to the pads which is where the heat is generated, and lack any direct cooling. The vanes are in the center of the sandwich, and have a limited amount of contact area for heat transfer, and at least on my Stoptech rotors, are considerably thinner than the outer surfaces and also have the most contact with cool air (they are the air pumps, after all). The inner surface is between the two, and connected to the vanes, so it makes 100% sense that their temperatures are pretty much right in between the edge and vanes.

The rotor hat is farthest from the friction, and is sandwiched between two large heat sinks (wheels and hub). 2-piece rotors also have less contact with the friction ring, with something like 8-10 points of contact where they bolt together, so between that and aluminum's heat transfer advantage over iron, a significant temperature delta there is no surprise.

More info:
- he was using brake ducts and doesn't think they do much. He based this on comparing the temp of the inside face to the outer face...the inside is much hotter but he didn't indicate how much. Needs more testing IMO before writing off their value.

The consensus opinion is brake ducts are a must (I've followed this advice for years) but does anyone have real data? I've heard more than one counter opinion on this - ducts don't do squat. But Pro teams run them as standard equipment and they would know. So until shown real data otherwise I'm going to continue believing in them.

Brake ducts are proven to help, and be downright necessary in racing applications. But not all ducting is created equal...maybe his setup isn't effective. I don't have numbers, but I can say they were a big help in my application..but I also have a pretty robust setup for a street car (~5" openings in the bumper feeding 3" hose to some pretty nice ducts).

- He added 2 "shims" made out of old backing plates and saw 300 degree drop on the plate nearest the pistons compared to no shims. (note thats a heck of a lot of "shim", indicating he was running some pretty worn pads and thus temps would be higher than brand-new pads)

This is nothing new, people have used titanium shims to block heat transfer to the calipers for decades. Pros being it keeps the calipers cooler, cons being it makes the calipers a little less effective at being a heat sink for the pads.

- He said next time he'd try to test 1 piece rotors.

I honestly doubt he'll see much, if any, difference, but it would also be hard to get an apples-to-apples comparison since 2-piece rotors are probably going to have a more effective vane design than inexpensive 1-piece rotors.

Rotors get hot...like real HOT.

Ya think?

 

[Chris B.]

Anybody else have some data like this?

I've had my rotor temps shot in the pits at Watkins Glen last fall. It was a 55 degree F day. The outer face of my front rotors near the edge were 920 degrees F and the outer face of my rear rotors near the edge were around 760-780 degrees F. There was some time between my last hard braking and when the car was stopped in the pits when the rotor temps were shot. I was told that under braking the rotor temps would be at least 200 degrees F higher than they were in the pits.

The consensus opinion is brake ducts are a must (I've followed this advice for years) but does anyone have real data? I've heard more than one counter opinion on this - ducts don't do squat. But Pro teams run them as standard equipment and they would know. So until shown real data otherwise I'm going to continue believing in them.

I don't have any data on the effectiveness of brake cooling ducts on the track, but I noticed after adding them that my Motul 600 RBF isn't nearly as dark after a track day as it was before adding them.

With the openings to m ducts covered, I can get my rotor temps up to 570 degrees F on the street when driving on some of my favorite twisty mountain roads. With them open, the rotor temps rarely hit 300 degrees F on the same roads.