Hydraulic Disk Brakes

[SuiteSuccess]

Recently replaced a rotor and brake pads on one wheel of my daughter’s car due to a broken  pad that came loose and gouged a groove in the rotor. Took it into my trusted tire shop to check the other wheels since suspected a warped rotor on front (steering wheel vibrations on braking).   Turns out the other three rotors were undersized (car has 100k miles so needed rotors) so had them replaced.  My question, however, pertains to the fact that I was “informed” that rotors/ pads must be replaced in pairs such as both front or both back at the same time to “balance” the braking.   Her pads had been replaced 8 months prior and still had lots of material left on the other three.  Now my question.  From a physics standpoint not sure that this is a valid argument.  The cylinders, fluid, etc has not changed and the pads ride in contact with the rotor so any hydraulic force applied will still be distributed equally to all wheels.  The older pads are not going to have pulled away from a good rotor.   So to my mind the amount of material on each individual pad (as long as safe and adequate)is immaterial and should make absolutely no difference in performance? They should be “balanced” regardless?  Where is my analysis wrong?

[spindrift]

I believe that's the recommendation because any defect (say a lip or gouge in the rotor) will cause some deformity in the pad.  If the rotors are that new, and they're free of any imperfection, I don't believe it would be necessary to change the pair.

[lenp]

I believe it is more of a liability issue.  A shyster lawyer could have field day in court with this.  Juries are gullible!

Similar to Les Schwab's policy of not replacing pads only - they will only replace the entire caliper.  Again, I believe it is a legal issue. 

Lenp

[Deezl Smoke]

 Typically, there is a "manufacturing tolerance", in pad compound chemistry and even in rotor metallurgy. So it's not always the amount of wear, it can be the performance value of pad compound or rotor metallurgy that may cause an amount of braking performance difference. In other words, pull to one side or the other.

[Lloyd]

Speaking as a professional mechanic with 25 years experience, it is highly recommended to replace pads at the same time on each axle, along with machining or replacing the rotors. Every set of pads is going to have a slightly different compound and every rotor is going to have a different metallurgy and surface finish. These will effect the friction coefficient at each corner as well as the brake's ability to shed heat, both of which will determine braking performance.

[SuiteSuccess]

Thanks folks.  Amazing some of the places my mind goes when I’m bored. 😜.  No disrespect to others but Lloyd your explanation makes a lot of sense.