What are the 'unwanted' gases that exist between the pad and the brake disc comprised of ?

The so called gases that get wiped away by grooves in the rotor.

Is it comprised of resin that previously held the brake lining together but escapes off the surface of the pad?

Or is it a hot pocket of air? Maybe a mixture of both?


My understanding is at higher temperatures adherent friction is dominant where a layer of the brake lining transfers to the disc. Would this transfer layer be smaller on a grooved disc than a non grooved, since some of it (I assume) is being wiped away?


Thanks in advance.

I think it is vaporised adhesive. It acts as a lubricant in vapour form. Having grooves or holes 'wipes' this gas away at the cost of increased pad wear. I believe cross drilled rotors wear faster than grooved.

Hmm, ChrisLane is right, they are/were mostly vaporised adhesive from the manufacturing process.

You still learn about those gases at university or somewhere else but the fact is that they disappeared nearly completely within the last decade after the sintermetallic technique was introduced.
Even with modern organic pads (which are not recommended for FSAE) you shouldn't have those problems as long as you stay below 400˚C. And I never heard of a team which broke those numbers.

Really, at those temperatures you're probably boiling your brake fluid anyways, especially if you're using good old DOT-3. That generally contributes more to brake fade than pad off-gassing nowadays.

Don't forget that another result of drilling/slotting is to decrease rotor mass, allowing it to heat up faster.

I agree that pad gassing is nearly gone in today's pads and that it isn't really a concern.

The grooves in the rotor are now mostly used to increase the number of "leading edges" on the rotor. Some theorize that during braking, these edges grab more on the friction material and increase braking force. I've even heard of people saying that they are there to squeeze pad material into so that you obtain higher braking force at the expense of higher pad wear. Reality is that the pads don't squeeze into the rotor grooves/holes and it doesn't cause significantly more braking force or contribute to pad wear tremendously. I was told by engineers at Performance Friction and AP Racing that increased numbers of leading edges do help to increase initial bite on the pads but don't do much past that.