# A quantity to measure whether brakes should be repaired

Suppose you can record some suitable parameters each time a driver uses the brakes of a vehicle (assume it's a heavy truck if necessary). Is there a reasonable formula for a dimensionless quantity $$Q$$ such that, say if for example $$Q\in [5,10]$$ in most tests, then the brakes seem to be working properly, but otherwise should be considered to be repaired?

Any help appreciated!

(I just found http://www.engineeringinspiration.co.uk/brakecalcs.html#bf2 and will have a look at it as well.)

• eh... Can't speak for a heavy truck, but in your typical personal automobile, the front brakes do nearly all the work, yet you still need to have the rear ones inspected. Also if they remain aligned, they may work quite well until one of the pads is completely used up, so it is not certain that you would get a useful early indication, unless the pads are designed for this (which they sometimes are, make an annoying noise). You'd probably want to come up with measures to address various specific issues that need service. – Pete W Feb 24 at 0:13
• Here is a thought: how will you deal with engine braking? As not all change in speed is due to braking... – Solar Mike Feb 24 at 9:58
• other issues to think thru besides pad / rotor wear: air in brake lines (soft response), problems with action of caliper guides, like friction or failure of spring, if present (uneven distribution of force and wear -- between inside/outside pad on same wheel, between wheels on same hydraulic circuit), misalignment of rotors (vibration, accelerated/uneven wear), worn out rotors, rips in of the rubber boot protecting the cylinders and guides (loss of lubrication, accelerated rust, eventually jammed up guide or even catastrophic failure in cylinder, loss of brake fluid, brief moment of panic) – Pete W Feb 24 at 10:49