eli5 Why do trains with steel wheels have traction on steel rails, but if your brakes on your car are used up and become steel on steel they dont work anymore?
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The big difference is that the train wheels are rolling — there is no sliding between the wheel and the track. Brakes on the other hand are all about sliding — when slowing down, the pads is constantly sliding on the rotors.
The non-sliding friction (static friction) of steel on steel is not bad. It has a coefficient of friction of 0.5 – 0.8 when the surfaces are clean and dry. This drops down to the .4 range if sliding is occurring.
https://www.engineeringtoolbox.com/friction-coefficients-d_778.html
Brake pads have a sliding coefficient of friction around the 0.3 – 0.5 range.
[https://brakepower.com/brake-pad-friction-coefficient.htm](https://brakepower.com/brake-pad-friction-coefficient.htm)
Note that the 0.4 sliding coefficient of friction for steel on steel is about the same as that of brake pads. So why are the brake pads so much better? For a number of reasons. Here are a couple:
Brake pads slide smoothly and don’t grab. With steel sliding on steel (or iron), galling can occur, especially at elevated temperatures. Galling is where the two pieces of metal actually weld themselves together in a small area and are then torn apart by the parts moving. This will cause a highly variable and amount of braking to occur making for jerky and unpredictable braking.
Brake pads don’t wear the metal rotor very fast. If the steel part of the pad stars touching the rotor, then the rotor will ware much faster. The rotor can wear so much that it fails completely, leaving you with no brakes on that wheel.
It’s not that steel on steel won’t work at all for brakes, it just doesn’t work well.
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