Originally Posted by SpazOnaZX
The big question for me is: why in a hydraulic system like this is the amount of pad changing where the lever starts to bite at race track temperatures? The new pads bite within a 1/4" of the lever being pulled (at track temp), whereas the old pads will not bite until the lever has been pulled almost halfway back. No amount of 'pumping' the lever would bring the bite closer either (rules out the rotors pushing the pads out at speed... I did think of that too)If it's air somewhere, why is it only effecting the system when the pistons are further out? And why only when it's hot.
Pretty strange... but thanks for the feedback... I'd like to get to the bottom of this someday ;)
Air expands when it gets hot , the bubble gets bigger, sometimes the bubble will divide into smaller bubbles or even ends up as froth.
As for the bite feeling being different this would depend on the pad and rotor combination, different pads have different cooefficients of friction and these too vary with operating temperatures so greater pressure is required for a pad with a lower coeeficient.
Another thing to consider is more heat transfer through thinner pads , the thinner they get the hotter they get under the same conditions, the hotter they get the the more they wear ,then they get hotter !!
We spend a considerable amount of time calculating brake wear for long races i.e 24 hours of Daytona , 50% of wear thickness is not 50% of the time left to go before they have worn out, it becomes exponential, and calculating the time to change the pads during a race is a nightmare.
But for a track day I would use new pads for better consistancy anyway.
As for the spongy feeling when the pistons are further out , it could be vacuum related.