Are brake hats part of the thermal mass?

[j-c-c]

I really mean, are they intentionally part of the thermal mass of the rotor? Seems alum gives a lot of strength approaching 400F, and a well used rotor should be able to get the hat that temp, so why does it seem no effort is made to insulate the two, like SS washers/bolts, etc?

[astroracer]

SS washers or bolts will do nothing to "insulate" the hat from transmitted heat. They are just as good a heat conductor as any other steel. The hat is part of the thermal mass, no way around it. Any effort to try and isolate it from the rotor, say with gaskets or non-conductive materials, will just result in loose or sloppy connections after a few hot/cold cycles. Besides that, the braking forces would immediately rip the rotor off the hat if there were no "hard" connections to distribute the forces into the hat.
Mark

[TheJDMan]

So how does a floating rotor work without a hard connection?

[astroracer]

So how does a floating rotor work without a hard connection?

The hard connection is through the carrier bolted to the hub. The floater disc still has to react against something that is hard mounted to the hub and the floater is generally bolted to the carrier. The difference is they use small springs around the floater bolts to let the disc move separate from the carrier.

[j-c-c]

SS washers or bolts will do nothing to "insulate" the hat from transmitted heat. They are just as good a heat conductor as any other steel. The hat is part of the thermal mass, no way around it. Any effort to try and isolate it from the rotor, say with gaskets or non-conductive materials, will just result in loose or sloppy connections after a few hot/cold cycles.
Mark

Not according to this source.

http://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html

[mmosley]

Part of the thermal mass... suppose so, but the rotor is much more effective at dissipating heat due to surface area and convection. In a two piece rotor the surface area shared between the hat and the rotor proper is relatively small, any heat transfer has to be conducted across that mating surface, and to a smaller extent through the bolts and washers. If you have a aluminum hat that may actually allow more heat to flow through the hat since Aluminum conducts heat more readily than steels, but the cross section area of the hat is rather thin (not much mass or path). The heat then continues from the hat into your wheel also. Overall I would not consider the hat an effective way to shed heat if that is what you are after, and it is relatively low mass so it would not hold much heat either.

[astroracer]

Not according to this source.

http://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html

As far as conductivity goes stainless is less of a conductor, yes, in it's pure form. BUT, do you really want to use pure stainless in this situation? Stainless needs to be alloyed to increase it's strength and, as you increase the carbon content you increase the conductivity. Using a stainless bolt for this connection is not going to be as strong as a grade 8 piece and I wouldn't take the risk for a minimal decrease in conductivity. Think about it. Race teams do this all day long with brake temps reaching upwards of 1600 degrees. I have never heard of a brake hat melting and causing an accident. Like Mike Mosley stated, the hat is low mass and heat dissipation with go thru the rotor and the wheel. The hat will transmit the heat with no adverse affect to itself. This is nothing to worry about. Build the brake assembly with the correct fasteners and you will be good.

[sccacuda]

This is what Wilwood told me when I was questioning the amount of play in my hat/rotor. If you have an assembled hat/rotor using T-nuts, you'll notice play between the rotor and hat connection. You can actually move the rotor slightly against the hat as the T-nuts move in the grooves. At temperature, the slop disappears as the aluminum expands. The hat absorbs and conducts heat, but lacks enough mass and surface area to be an effective heat sink alone. It is part of the thermal mass, but more as a conductor than sink.