Why would a one piece overheat and a 2 piece not? how do you interpret that? I'd assume the one piece has more thermal mass, so should heat up less. Neither has fins? Both are solid steel?
Afaik, the argument for 2 piece is the heat isnt transferred into the hub. i havent heard claims that it dissipates heat better.
Aluminum dissipates heat around 15 times faster than steel. 2-piece rotors, "CAN" have better heat management.
I write "CAN" because I really don't know how good of a thermal engineering job the Chiner rotor manufactures do in their products. (In the automotive world, 2-piece rotors are by far superior at rotor heat management/dissipation than solid steel)
I am a retired mechanical engineer and haven't worked on thermal heat transfer designs in many years, I'm not sure if I will properly boil an explanation down to a few concise statements
- Heat flows from hot to cold at a measurable rate: (Thermal conductivity of Steel to air, steel to Aluminum, Aluminum to air.)
- When brake pads are pressed onto the rotor, heat is generated. This heat is primarily dissipated through air. But, on long continued intense heavy braking, heat builds up in the rotor and starts transferring more of the heat through the rest of the rotor towards the hub. If you add in an aluminum center section, the aluminum will transfer heat at a faster rate than a one-piece solid steel rotor. The thermal conduction path and heat dissipation properties of aluminum can make the difference between a glowing hot rotor and brake fade.
- The complication: There are types of 2-piece steel/aluminum rotors in the market. 1. True floating 2-piece rotors. 2. 2-piece pinned rotors. Unless the manufacturer does a good job of designing in a good thermal transfer path between the interfaces, one could loose a lot of the heat management benefits of a true 2-piece rotor.
There are a lot of variables involved that will affect the overall performance of a rotor on a bicycle. The caliper, the brake pad material, and how long and how hard the brakes are applied between cooling periods. The rider's weight/overall mass you are trying to stop impacts the amount of heat generated.
All of these variable may never come into play for the majority of riders to ever notice a difference between a sold steel rotor and a 2-piece rotor. I often weigh in the 210 pound range. On long steep descents, all this weight adds to the heat generated during heavy braking. Riding on 2-piece rotors, really helped in my downhill braking situations.
