Christopher Sykes
1 HN recommender · recommended in 4 months, 2013–2022
2:27 · 820K views · Sep 5, 2008
A short clip from the 1983 BBC series Fun to Imagine in which Richard Feynman explains how a train stays on the track. He covers the geometry of tapered wheels that steer the train back toward the center of the rails.
HN people link it as Feynman's charming and clear explanation of why train wheels are tapered, whenever trains, bicycles or steering come up.
4 comments from Hacker News
That doesn’t seem to be a charitable reading and in any case it has to do with wheel taper, more like cone than cylinder. More specifically the nature of the solution was getting two parts of the org, track and train, to work together. While that may seem elementary stuff, anyone at a big org can attest to how often it is tremendously difficult to do sometimes. I write this not as an apologist for commuter rail, which I find excessively expensive and inflexible outside of very circumstances, but as an excuse to link to the lovely Feynman description of why train wheels are tapered: this video
Indeed. Feynman talked in ‘surely you’re joking..’ about his affection for MIT students in his fraternity setting one another puzzles, and he talked about the question of how trains go round corners being one he found particularly interesting. In fact he repeats it in this interview: this video He doesn’t answer the question ‘how does a train stay on the track’ with ‘some trains don’t’; he doesn’t switch to talking about rollercoaster trains and insist that the way they stay on the track is with horizontal wheels. He finds the logical, geometrically satisfying answer quite delightful. And in fact, in many cases the flanges which he says are just for safety and aren’t supposed to impact the track because otherwise they make a terrible sound… are expected to hit the track to help the train make it round particularly tight turns. Coming from New York and having ridden the subways you’d think Feynman would know there are some corners subway trains take where the flanges rub all the way round, and the lovely conical wheels aren’t what keeps the train on the track at all. So I definitely find this a weird characterization of a Feynman-ish approach to this kind of problem. I think he’d probably have delighted in the geometric neatness of why circular or triangular manhole covers can’t fall down their holes, and been happy to ignore the practical fact of square ones for the sake of a good puzzle story.
If Feynman were still around, he'd set things straight. I don't think a bicycle staying upright is much different from a train keeping course [1], but I can't describe why. Instead of the rail veering off and the wheel adapting, it's like the bike does what it wants and the ground shifts beneath it. If you've read about Einstein's pail-of-milk-on-a-lazy-susan, it's a similarly unintuitive frame of reference. Also, if the experts haven't figured it out: I don't know what I'm talking about. I find it fascinating that we as a species can observe, define, and exploit Maxwell's law, neutrino physics, etc, but we can't clearly explain bicycles. Or why wings provide lift. EDIT: forgot the Feynman video [1] this video
> doesn't a speed change imply a force on the bearing .... imply friction Not necessarily. A speed change could also be because the track is longer. That's how trains stay on the tracks, the wheels are slanted, and if they turn off the track the length of the wheel changes, which changes the speed (relative to the wheel on the other side) and steers it back onto the track. Feynman explains it: this video