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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Friction force = normal force x coefficient of friction
The coefficient of friction is how “rough” the two surfaces are. For steel-on-steel this is very low.
The normal force is how much weight is pressing directly on the surface. For a train this value is immense.
Trains make up for very poor friction by weighing hundreds of tons. That minuscule friction between two steel surfaces on a tiny contact patch is magnified by 40,000 tons and becomes enough to stop a train.
Of course the momentum of the train is also 40,000 tons, so they can’t exactly start and stop on a dime either. They do have relatively poor traction compared to a road vehicle and can’t go up or down steep grades.
Trains have very little traction compared to a car, it’s why they take so long to speed up or slow down, and why they have such problems climbing hills.
When your car takes a mile to come to a stop, you call that the brakes not working. When a train takes a mile to come to a stop, that’s normal.
Car brakes are designed in a way that they can still move while braking (to an extent, they can still lock) and so that they wear in a specific way.
What wears are the brake pads and discs. The calipers that hold the pads are designed for mechanical strength. That means different steel composition and treatment than the pads and discs. The pads and discs also are different types of steel.
When the brakes are used up, they do not work because the materials that make contact are no longer materials designed for braking.
Steel is the name of a family of alloy. There are many different types of steel made for different purposes.
Train rails and wheels are designed for a different purpose and use different types of steel than the ones used in brakes.
ETA: The part about trains not having much traction has been covered by others. The wear parts on brakes are designed for the opposite.
The trains weigh a lot ^((citation needed)), so they generate a lot of friction force.
You could use more force on your worn out brakes and they would work, or you could wait a long time. But the brake material means you can use less force and work much faster. The downside is you have to replace them more often. (note also that the grip of the tires is very important for braking too). For a train this would be both cost prohibitive and unnecessary since the inherent weight of the train already provides the necessary force to create friction for movement.
It’s also worth noting that trains take forever to stop and they can’t go up much of an incline, so the steel on steel isn’t giving them the best performance possible, but for the purposes they are designed for it’s a good balance of cost and performance. Even if we could make them stop faster, that would almost certainly result in catastrophic failure including derailment. You can’t just stop 1 million tons of cargo on a dime. We expect different performance characteristics for a car (for example we want it to stop in a matter of feet, not miles) and thus use materials that meet that performance. For example, we use rubber tires which provide better grip but which can’t carry as much weight as a train.
Trains are very big and very heavy. Relative to their size they actually don’t have that much traction which is why they can’t stop on a dime or quickly accelerate. Also their wheels and the rails are fairly smooth and do get worn over time but they’re very thick. Car brakes on the other hand are not built to work like this, so if you had no brake pads left you’d still brake, but the brake rotor would be damaged a lot and the braking would be far worse than normal.
Steel on Steel actually has a decent coefficient of friction – about 0.4 when sliding and 0.8 when stationary.
The issue for trains is that they have a relatively low contact area (a result of hard wheels and tracks to reduce rolling resistance) and are very heavy.
If a train full on slams the brakes and stops the wheels from rolling they’ll start to slide, and the shear force on the wheel (due to the fairly high friction) will be greater than the shear capacity of the small section of wheel material in contact with the tracks, so will shear off and result in a flat portion on the wheel. This is obviously bad, so instead trains brake slowly to avoid this type of uneven wear on the wheels. Note that in this process the wheel is always rolling, not grinding, and the brake pads for the train work in a similar way as those on a car – the energy is still dissipated through the pads, not the wheels on the tracks except in an emergency.
In a car braking system the same typically applies – most braking tries to avoid skidding (both to prevent tire wear and because once skidding you actually lose braking power).
The pads, in both systems, exist to provide a way for the braking system to wear without damaging the brake disc/wheels. Both trains and cars could have steel on steel brake pads, and the coefficient of friction for this isn’t bad, but they’d wear down very very quickly.
Trains do not have good traction. Trains take a long time to slow down and speed up. They cannot go up steep hills because the wheels of the locomotive would slip too much.
They work just fine, other than the unsettling grinding noises. They wear out quickly and will wear through the piston and drain the brake fluid rapidly. Which will then quit working. Losing too much brake fluid for the hydraulic system to continue functioning is the real danger.
Every other content talking about friction coefficients and drag coefficients etc are nonsense. These people must have no experience driving a car with brake pads that are worn through.
Car brakes actually work decently well when the pad is completely worn and its the steel backing against the disk/drum.
Many folks get to this point and have no idea its happening.
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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