Ever since I ducted my front rotors, I've noticed a tendency to have a pulsing in the front brakes when they get hot. I pulled my rotors a couple days ago, and found something very interesting.

I could see a pattern of discoloration on the outside surface of the rotor which was not present on the inside surface. I finally noticed that it exactly matched up with the spacing of the internal vanes. I think the vanes are being well-cooled by the duct air, so the surface of the rotor gets hotter between the vanes than at the vanes.

I only see this on the outside, presumably because the inside face is seeing some impingement cooling from the duct air.

Do other people notice this? Does it cause you problems?

 

I noticed the exact same discolored pattern after a track day with stock rotors and stock heat shields/no ducting. Didn't feel any pulsing however. I doubt it's due to your ducts.

 

Had exactly the same markings and was puzzled, came to the conclusion that the new rotors I had fitted had overheated, even though I have ducting, so I decided to "mic" both faces up and found they had warped quite a lot!
Might be worth measuring yours, I wish I had measured the rotors before I had put them on and in the future I will be doing as I think that they may have possibly had some high spots which could have contributed to the overheating.

 

does this mean ducting = no good?

 

I get the pulsation, too, but I've chalked it up to pad material building up on the rotor and/or warpage.

 

Like I said, I looked at the rotors. There was a clear pattern of discoloration exactly corresponding to the internal vanes, and it was only on the outside surface of the rotors (the side that is not being actively cooled).

 

I didn't make it clear that I may have been overlooking what you've noticed and have been mistakenly attributing the pulsation to the wrong cause. All my rotors have been turned at this point, but next track day I'll examine them a little more closely.

What I did just realize, though, was that when checking my rotor temps last weekend I noticed a significant difference on my digital temp gauge when comparing the inside and outside part of the rotors (smaller radius would be, say, 290 and the larger radius outer part of the rotor would be 340). The outer part of the rotor is spinning faster than the inside, which means, of course, that the outside part of the rotor should be hotter than the inside. So maybe the discoloration has more to do with that than the ducting?

 

I'm not sure what I noticed is due to the ducting. But I don't seem to recall it from before the ducts went in. On the other hand, I've also picked up a lot of speed since I put the ducts in, so correlation does not imply causation.

I do think the pulsing I'm feeling is due to this phenomenon. But I don't know how much (if at all) the ducting is influencing it. My only reason for suspecting any influence is the difference between the inside and outside surfaces.

That's why I was asking if anybody else has experienced this.

 

If the pattern is with the vanes, they are so close together that it doesn't seem logical that you would pick up a pulsation (the pad spans several vanes), especially when rotating and braking at speed. Is the pulsation a very high frequency that decreases as you decelerate?

 

Quick calculation shows that at 60 mph, the vanes are sweeping past the pads at a rate of 1 every 2 or 3 milliseconds -- way too fast for you to feel anything.

There may be some pad material buildup; what pads are you using?