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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How long does it take a train to stop?
The whole question is flawed.
Steel on steel brakes WORK just fine. The automotive mechanic forums are full of people showing up with completely worn out brake pads and only complaining about a slight noise (if anything).
In fact, steel on steel often has a higher coefficient of friction than brake pads.
Steel on steel just wears much, much faster than brake pads do.
Examples:
Yeah, go ahead and turn the rotors for me!
byu/gaphoogy inJustrolledintotheshop
Just another paper thin rotor
byu/collegefurtrader inJustrolledintotheshop
Another difference: your brake pads slide across the discs. When a train slows down, the wheels don’t slide over the tracks, so the friction is greater. If the wheels slide deceleration is reduced, just like on your car or bike. When a train slows down, the wheels are charging a battery.
Railroader here… steel on steel has very little traction. Locomotives have “sanders” that can throw sand onto the rail to help get the traction to start if necessary. And to piggy back off what others said, when a train needs to stop asap, they are basically sliding steel on steel for 1 to 2 miles, so not what you want your car brakes to do. Great question!
Trains have very little traction. With them it’s more a case of reducing friction and making them more efficient. They don’t really need traction so much as they don’t have many turns and their behaviour is very predictable.
Look up some videos of trains going up hills. They start slipping on the tiniest of slopes.
Steel on steel brakes do actually work, just not as well. But even trains don’t use metal to slow down , they have brake pads like cars (plus electric braking).
There is always traction between any two materials. It’s just a compromise between wanting more friction for accelerating but less friction for rolling along. Also the locomotives can drop sand on the tracks to help get more traction, and the locomotives are extremely heavy. like hundred thousand pounds heavy. That extra weight increases the friction on the drive wheels. As a final note, train tracks are very very flat. so they don’t have to pull the train uphill usually. So all the locomotive has to do is overcome the rolling resistance of all the cars behind it. There is not much resistance with steel wheels on steel tracks.
Heating is one reason. Hot brakes don’t brake very well. Your brake pads help dissipate heat by wearing out, while steel just gets hot. The brake pads also don’t retain heat very well, so not much heat gets back to the steel backing plate. Friction on the backing plate itself will make it extremely hot, and that heat will be transmitted to the caliper, and thus the brake fluid, which really doesn’t do well under high heat.
Worn brakes on your car do work, they just work very poorly when the pads are worn down to steel.
There is still friction with a steel to steel contact, but much less than with brake pad to steel.
Trains take a mile to stop. Your steel on steel brakes will stop in a mile. Same thing except the train doesn’t have anything in front of it. You have several cars, trucks and a bus full of nuns in front of you.
[Practical Engineering on YouTube just released a video talking about this](https://www.youtube.com/watch?v=Nteyw40i9So).
Summary:
* steel rails last longer
* steel wheels have less friction and can carry more weight
Train wheels and rails both have to be replaced periodically, just like the brakes in your car.
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