Originally Posted by wilbar00c
When I get a chance some morning, I'll run the engine, with with a fan blowing rearward, in front of the bike, in the driveway, to see if the rear drive retains it's "cool". I would not have guessed that the ring and pinion would generate all that heat to the rear wheel with the 180F block and hotter exhaust system just ahead, while the front wheel feels cold.
Yep -- hard to believe at first blush, but if you do back-of-the-envelope heat transfer calcs, you'll see there's enough waste heat deposited in the final drive to get it up to a steady-state temp in the 140-160F (or more) range, depending on load.
As far as the engine/cat not being the source, think of it this way: if they were the primary source, *everything* in the lower back half of the bike would be getting very,very hot, not just the final drive and tire -- and the things between the engine/cat and the final drive (i.e., closer to the engine) would be even hotter than the drive.
The fact is they're not, because the engine and cat heat is not being deposited there in any great quantity -- by far the greatest amount of heat rejection from the engine is out the sides of the bike from the radiators, and out the end of the exhaust. And while the cat gets hot, it's not rejecting enough heat, at a high enough rate, to significantly raise the temp of all the mass in the tire/wheel, at least not before the wheel itself can reject the heat over its large surface area. I'll also note that the final drive is on the wrong side of the tire relative to the cat, which is toward the left side of the bike; all this adds up to it being very unlikely to even contribute significant heat to the drive, let alone be a primary source.