Bike porn

It'd be an interesting comparison if someone actually worked out the kinetic energy being turned into heat for an MTB going down (say) a ski-slope in the summer, and a road bike going down the same mountain via the roads.

As a percentage of the total system weight a 15lb road bike isn't so different from a 35lb MTB.

I'm 171 pounds, so with roadbike and kit probably 190? Me on an MTB would be (say) 215.

The MTB would (assumption!) see fairly frequent braking whereas the road bike would see repeated, heavy but relatively infrequent decelerations from ~50mph down to 15mph or lower (hairpins).

I cannot help but think that there is a reason they fit rally cars with much bigger brakes for asphalt courses.

If we assume that neither rider is pedalling much, the amount of energy in play is just the gravitational potential energy, i.e.
mass of bike & rider x hight descended x strength of gravity

That energy goes three ways as they descend: into rolling resistance, air resistance, and brake heat. The MTB will have slightly higher rolling resistance, but by moving faster and further the road bike will have much higher air resistance, leaving less for the brakes. So on average, the slower bike's brakes work harder.