I've found something interesting while shooting temperatures of my rotors.

I posted yesterday thinking my e-brake was stuck on because I was finding that the hub area that the rim bolts to was 100 degrees hotter than the friction surface of the disk. However, after shooting the hub of the front brakes, I'm seeing a 60-70 degree difference there too.

I would've thought the outer friction area would run the hottest and the entire rotor would get cooler as you go toward it's center.

The only thing I can come up with is I'm not shooting temperatures quick enough after stopping and the friction surface is cooling much quicker than the hub as I'm taking off my seatbelt and getting all of my things out of the car.

Since this is the case, I've thought about putting a nice shiny surface on the rim and the hub where they meet so the brakes can transfer more heat to the rims and act like a large heat sink. Of course I would monitor them to make sure the heat never makes it to the tire area.

Still though, I was very surprised to see this kind of a temperature difference in solid metal an inch or two apart. If this is common knowledge, someone please let me know.

 

less Airflow on the hub. i believe heat gases float along the rotor surface and are swept away by ambient air.

whereas the hub has a rougher surface that can act as pockets and hold the air in. this, in tern, makes the hotter air accumulate around the hub area.

 

Interesting. I would not had thought this was the case either. Thanks for sharing the discovery.

My first thought was the same as you. The inner hat cools more slowly.

But upon further thought, I have a paradox. The heat should be a factor of the mass also. Sort of a "thermal flywheel effect". And, the outer disc would have more mass, thus holding more of the heat. With the hat, less so... less mass, less heat. Paradox

One could say that the wheel is adding mass. However, I doubt that the heat would transfer quick enough into the wheel rim to apply into this logic regarding mass.

Was the rotors completely cool before doing this one test? If not, then heat from prior driving would had gradually transferred into the hat and wheel over a period of time. But, still I am confused. The less mass of the hat (including lug nuts and spindle) should still allow the hat to be cooler.

Here is one scary theory. The wheel bearings produce enough heat to make the inner rotor hotter than the outer rotor. Yes, I know this is far fetched. But, it needs to be "put on the table" as a possibility.

I feel my rotors almost once per day. I will start feeling the hats too to compare the temp difference.

 

I chose to ignore this option on purpose. Something about a whole can of worms lol.

 

Easily tested however.

When you take a trip again, come to a coasting stop with no brakes along the side of the road. Then get out, bend over, and insert thermometer..............to compare hat versus outer disc temps.

 

Ok. I got home today after my 1 hr 10 min commute. I pulled into the garage, got out, bent over, and stuck my finger up my rear............. up my rear rotor.

Because I have my splash sheilds removed, I am able to touch my spindle hubs.

It is not the bearings. Both front and rear spindle hubs were almost ambient temps.

The front outer disc was hotter than the rear outer disc. But, the rear hat was warmer than the front hat

The front outer disc was noticeably hotter than the front hat.

The rear outer disc was nearly same temp as the rear hat.

Both front and rear wheels (aka, rims) were very cool. absolutely ambient temp. Even parts of the wheel directly mated against the rotor were extremely cool.

I was very surprised that zero heat transferred from the hat into the spindle hub. How can this be ? Metal against metal should transfer heat

 

I think it's that the rotor surface is exposed to air, so will dissapate heat faster. Also the disk is designed to flow air and dissapate heat; the hub less so.

Even under braking more than 1/2 the disk is exposed to air and cools WHILE braking. The hub takes on heat and holds it, I would assume, at a fairly constant temp during a braking event or a series of closely spaced braking events.

 

Just wanted to bump this since it seemed interesting...

With all this in mind, I assume that two-piece rotors would have cooler hat temps? and does the hat temp really have an effect on braking?

 

The rotor surface is vented, on the fronts at least, so they will cool faster than the hats which are not exposed to as much air flow. The rear rotors I can only guess that even thought the rotor is solid it is still disapating heat faster due to air flow around it. All just a guess though.