If you know anything about us around here, we like us some good old experimentation and datalogging. The nerds over at Turn-in Concepts (who, by the way, blew minds in the Redline Time Attack “Street AWD” class this year on the East Coast), donned their lab coats and got to work on the topic of heat transfer from brake rotor to wheel bearing/hub.

With their permission, here is a copy of their write-up, pictures, and graphs:

High Temperatures Suck.

You know it, we know it, your wheel bearings live it.

If you look through our postings over the years, you’ll see that one of the things we’ve been trying to do for you guys is to come up with something that can make your wheel bearings last longer. The big thing to combat is heat. Heat is the bearing killer. Anything you can do to keep heat out of the wheel bearings is going to make them last longer. The problem is that you’ve got the amazing source of heat called “your brakes” with a direct metal-to-metal contact to your hub and then your bearings. Even worse, it’s iron-to-steel, making it a 12-lane heat superhighway. So we thought we’d through up some construction barrels.

Titanium is a metal, sure, but it’s a metal that hates heat. Heat stole Titanium’s woman a few years ago and Titanium hasn’t moved on. Titanium expresses this hate by having a low (for a metal) thermal conductivity. Grade 5 Titanium, the one we’re using, has the lowest thermal conductivity of any Titanium alloy you can buy without a defense contract paying for it. Grade 5 Ti has a thermal conductivity of 6.7 W/m*K. Compare that to an average alloy of steel at nearly 50 W/m*K or 6061 Aluminum, who’s a total heat whore, at 167 W/m*K. Don’t worry about the units, just focus on how big the numbers are compared to Grade 5 Ti. This is the same Titanium we’re using in our brake pad shims (same idea is this, just between the brake pad and the piston, rather than the rotor and hub).

So there’s this big heat superhighway running between your brake rotor, through your hubs, and into your bearings. Since it’s all iron and steel, we’re talking heat Autobahn here. Then TiC comes along and starts putting up construction signs, orange barrels, and speed limits. We toss Titanium in there between the rotor and the hub. Suddenly, heat’s getting pulled over and charged with Furious Driving. Bam, your bearings are having a party.

“This is all good marketing”, you say, “but show us the proof!”. Waaaay ahead of you. The TiC intern happens to have friends (yes, this was surprising to us too). One of his friends is an engineer who deals with the control of systems using heat for a living. Toss one rotor, one complete knuckle, a baggie of Titanium goodies, and a TiC intern in a car and send them off with a mandate to do science. It’ll be like Mr. Wizard, Bill Nye, Jamie, and Adam all decided to have a party on TiC’s dime.

So here’s the test setup. Take one knuckle, complete, and clamp it in the vise. As you can see, we media blasted the face of the hub to maximize the contact between the rotor and the hub. We wanted to make sure that this was a worst-case test. In the real world, rust and other corrosion on the hub and rotor would create slight air gaps. By cleaning everything, we’re giving out parts an even harsher test.

Next, we started hooking thermocouples to everything. When you think thermocouple, think “fancy-pants lab thermometer”. This makes it easier to get accurate, precise, and consistent results. As you can see, we placed a thermocouple on the back of the rotor, the back of the knuckle, and the splines inside the back of the hub where the axle interlocks. We also recorded the temperature of the rotor using a lab-grade non-contact IR thermometer.

The thermocouples read out to electronic displays, which were then carefully logged to a spreadsheet by the TiC-intern’s Fiancee. Thanks TiC-inter’s Fiancee! To begin with, all 4 measurements were recorded at 30-second intervals. At 5 minutes, the recording interval dropped to once every 60 seconds. At 30 minutes, the recording interval dropped to every 300 seconds. Then we made pretty graphs.

To simulate braking, we put the rotor in a oven. The rotor was brought up to 600F (bake, not broil) and then left for 30 minutes to heat soak. The rotor was taken out of the oven, carried across to the knuckle, and at the call of “NOW!” from the TiC-Intern’s Fiancee, the rotor was slammed down against the knuckle and fastened tightly with an impact wrench. Immediately, we began recording temperatures and continued doing so for the next 40 minutes.

Now, you may be asking why this all looks like a workshop rather than the a lab, as promised. The answer to that is simple: Fire. This workshop had the oven and the vise as close as possible. Yet it still set us on fire (yes really) just carrying it 10 feet across the room. Even more interestingly, it took about 15 seconds to get the rotor out of the oven and bolted to the hub. In that time, the temperature of the rotor dropped 100F from 600F to 500F. It’s amazing how fast that rotor is getting rid of heat when it’s at braking temperatures.

Once the 40 minutes was up, we separated the rotor and hub. The rotor went back in the oven and the hub got a fan. Blowy kind, not the kind that wants autographs. Eventually, the rotor was heat soaked and the hub was back down to room temperature. At this point, we installed the HTS (Heat Transfer Shield) onto the hub. As you can see, it’s a flat disc that goes between the rotor and hub. Additionally, there are two washers (not shown) that go between the caliper bracket and the knuckle to space the caliper over exactly the same amount as the rotor (clever, aren’t we?). So the flat HTS fits over the lugs and then gets pressed flat against the face of the hub.

More shouting of numbers, more catching on fire, and more watching parts cool. Once the data was all collected, it was time to make pretty graphs!

So, what did we find? Here’s the most basic, and also least informative, graph:

As you can see, the hub that was protected by the HTS stayed colder than the unprotected hub. It had a lower peak temp and took longer to get there. Now, that “took longer to get there” is important. In the real world, you have braking going on. Braking means moving. Moving means airflow. So the rotor heats up and cools back down rapidly. All the HTS has to do is keep the hub a little bit cooler for a little bit longer and the rotor will cool back down to the point that it’s no longer hammering on the rotor.

Another thing to think about with this graph is repeated braking zones. We simulated one stop here and the hubs started at the same temperature in both tests. In the real world, you stop more than once. So stop one heats the hub up. The unprotected hub is now 30F hotter than the HTS-protected hub. Stop two does the same thing again, except for the fact that the unprotected hub is starting 30F hotter. Boom, the HTS-protected hub is now 60F colder than the unprotected hub. Repeat this corner after corner, lap after lap, and the unprotected hub is going to be seeing significantly higher temps than the HTS-protected hub.

Time for a new graph. What you really care about is how well the HTS helps the hub stay colder than the rotor. In other words, you want to know the temperature difference between the rotor and the hub. Why, look what we have here:

In this graph, higher is better. As you can see, the HTS-protected hub always has a bigger difference between it and the rotor than the unprotected hub. In fact, on average, the HTS-protected hub has about a 2.5x better temperature gap and reaches as much as a 5x better gap! That means that, no matter how hot you get your rotors, you’re consistently keeping the hubs, and therefore the bearings, cooler with the HTS in place.