Originally Posted by bustert

be careful what you buy as not all so called rotors are floating. heat issue on a street machine is not an issue as mr. max pointed out.
there are several types of retainment anchor points being circular, flat and triangular.
rotors with circular retainment do not float. if it could, large amounts of space is needed and then the rotor would rattle. second , heat expansion has a tendency to wedge against retainer.
the other types allow for movement with out the need for excessive clearance.
axial floating rotor is doomed for failure and can cause braking issues with feedback(knock-back of the pads) into the controls and shortened component life, zero axial float is best.
the only advantage of a floating rotor is that it allows for movement to more fully conform to the pads, more conformity, better braking. really needed for a street machine, well, hummm, your dollar. all other things equal, the heat load would be nearly the same with a slight edge going to the floating rotor generating more heat, remember better conformity.

Originally Posted by Agoober

I know no one asked but the oil in the fork leg that has the rotor and brake on a single rotor system always degrades faster than the other side.
So some heat must go through the brake and mount .

Originally Posted by Max Headflow

The oil turning dark in the caliper leg over the other is a neat observation but I doubt it's due to heat from the caliper. Most oils don't start to oxidize until something above or around 250F.. Ever seen a set of forks get that hot? I would bet that the reason the oil turns darker on the caliper side due to the additional forces added to the fork leg when braking. The bushings see more load / wear.

Originally Posted by Agoober

It's just been an observation over decades of bikes , I've had more fork seals go on the side with disc brakes too , it might act on the bronze slider bush or the rubber seals .
Just a comment no real right or wrong thing , an observation . Spose someone could test the oil ? See what gives.

Originally Posted by Max Headflow

Not questioning your observation. I question you theory as to cause. Have you ever felt the fork lowers after a good hard ride?

Originally Posted by Agoober

No I have not , I will if I remember, but as I said SOME heat must go through to the fork leg with the single disc .Actually my mate has one of those heat reader guns, I'll try to get him to take it for a ride when we go. We always stop at a look out just after a tight twisty section where hard braking is the key to good lap times lol.Might check them there if he's up for it
As you said something is at play an oil test might show what. It could very well be torsional stresses like you said .
I go check my tyre pressures every ride and I have noticed my rear brake disc is always hotter than my front disc. It' less than a mile to the servo where I check , I hardly use my back brake and the ABS doesn't kick in till over the speed I go to check tyre pressures . Might just be lack of clean air to the rear disc.