16 ms·

Train Wheels Are Cones

401 points by trekhleb 5y ago

code4money 5y ago
summary: train wheels use the physical shape of the wheel to turn, and the stopper is only for emergencies. very cool!
GhostVII 5y ago
Are the cones mainly for keeping the train centered, or for allowing the outer wheel on a turn to be effectively larger? Not sure if the differential effect is an added benefit that isn't really necessary, or if it is the main goal of the design.
- ben11kehoe 5y ago
  They keep the train centered by the differential effect. The angle of the cone is very slight, nowhere near enough for gravity to overcome friction to cause the train to slip laterally into the center. And then keeping the train centered as the track turns results in the train turning with the track.
  - GhostVII 5y ago
    Sure, but I guess I'm wondering if that is just a convenient effect of the centering, or if it is actually necessary to prevent the wheels from skipping.
- lbotos 5y ago
  my understanding is the later: https://youtu.be/agd8B-31bjE?t=106 https://youtu.be/agd8B-31bjE?t=106
- aaaaaaaaaaab 5y ago
  They do both.
- AnimalMuppet 5y ago
  It's the same thing. If the train becomes uncentered, then one wheel is the outer wheel, and will turn the axle back toward the center. Or, looked at the other way, when the track curves, then the axle becomes uncentered.
  - smartscience 5y ago
    To demonstrate this more fully, consider the case of having the wheel flanges on the outside, with the conicity of the wheels pointing the other way. Gravity would still tend to centre this arrangement, but I'm told that if you build such a system in practice, then it won't run nearly as smoothly. (PhD was 'Residual stress in rails', for what that's worth. Judging from the profiles of the rails I saw, direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. But on roughly straight track, I'm satisfied that the argument about conicity applies).
    - lostlogin 5y ago
      > direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. The London Underground has some lines that are horrifically loud. The squealing must surely be at dangerous sound levels. I’d always assumed it was the flange against the rail, and you appear to be confirming that?
      - AnimalMuppet 5y ago
        That's called "flange squeal". Yes, it can be ear-shatteringly loud. But it mostly (totally?) happens on very tight curves. It shouldn't happen much or at all on gentler curves. (Of course, this is circular, because I'm kind of defining "gentler" and "tight" based on whether they cause flange squeal. Still, there's a point - there is something like a threshold of curve tightness where flange squeal becomes much more probable.)
        idroveatrain 5y ago
        It can happen on straightrail on an incline, too. It's hard to assess precisely what's happening in the locomotive, but under traction I believe the running gear will toe out, and align based on the path of least resistance. I believe this leads to the flange pressing, with immense force, against the rail. You also get a lot of wheel slip in this condition. I surmised this running 2 motors up a 3%(?) grade with 20k ton gross at 10mph. It's about the only explanation I could come up with is that the running gear was twisting under the gravity and the energy being put down to work against it. It might also just be a stringline sort of effect dragging the motors to one side of the track and pressing the flange. Maybe one of the rail engineers will come holler at me for my poor trainhandling skills.
- morpheos137 5y ago
  The main reason other than turns is to avoid hunting oscillation on straight track. https://en.m.wikipedia.org/wiki/Hunting_oscillation https://en.m.wikipedia.org/wiki/Hunting_oscillation
- jcrawfordor 5y ago
  Both are factors in good centering, but mostly the change in diameter. In turns, there is a natural tendency for the train to shift towards the outside of the curve due to inertia. The wheel diameters become asymmetric which helps to re-center the train. It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. The relationship between these two effects is a bit complex (depends on weights and speeds of trains) so it's usually all a bit approximate. The conical section of the wheels is mostly intended to prevent hunting on straight track, and the shape can't be made too aggressive without increasing the wear on wheels on rails. So on curves the superelevation is added to provide the extra force required. Because conical wheels do increase wear and can contribute to oscillation in their own way, there have been experiments with cylindrical wheels especially on higher-speed trains---BART is a well known example. It ultimately didn't work very well and so they have been re-trueing the wheels to a non-cylindrical profile, although still not quite a traditional conical one. Basically in higher-speed operation the re-centering effect is too significant and causes one wheel to "chatter," which over time creates a significant vibration in the rail. Trouble is cylindrical wheels tend to cause the same thing to happen on the other side. It was a very hard problem before computer modeling became available.
  - iggldiggl 5y ago
    > It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. [...] So on curves the superelevation is added to provide the extra force required. I've never heard about that theory as for why superelevation/cant is supposedly being used until now. Given that most of the time you'll end up with a remaining net force to the outside of the curve even after application of cant, it doesn't seem to make that much sense, either.
    - jcrawfordor 5y ago
      That's the conventional explanation of superelevation, although I worded it in sort of an odd way. But I'm describing the same thing that e.g. Wikipedia does. Superelevation directs the force of the car more "straight down" in relation to the rails which improves centering and balance of the load by the same token. The thing I said about "shifting gravity" is unnecessarily confusing because it depends on reference frame. I think for low-speed freight the balance needs to be pretty close on to ideal to meet regulations, e.g. FRA regulations give calculations for acceptable ranges. But since it's dependent on running speed it's hard to get correct for freight and passenger mixed operation which is the subject of this FRA report that has a lot of detail on the calculations: https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19085/Superelevation.pdf https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19...
- tus89 5y ago
  The goal is for the train to NOT be centered when cornering, the cones allow for this. If the train was centered during corning, or simply with non-coned wheels, the outer wheel would necessarily slip.
ben11kehoe 5y ago
I think this may be the primary reason why narrow-gauge railways are better at tighter curves: the shorter axle means the same wheel radius difference (caused by lateral displacement) causes a smaller turn radius versus a standard-gauge axle.
aaaaaaaaaaab 5y ago
Feynman https://www.youtube.com/watch?v=WAwDvbIfkos https://www.youtube.com/watch?v=WAwDvbIfkos
- waynesonfire 5y ago
  This whole series is amazing.
  - modeless 5y ago
    Here it is in better quality, the whole thing, and with subtitles: https://www.youtube.com/watch?v=nYg6jzotiAc https://www.youtube.com/watch?v=nYg6jzotiAc Particularly good parts are the explanation of fire and trees ("trees come out of the air"): https://youtu.be/nYg6jzotiAc?t=440 https://youtu.be/nYg6jzotiAc?t=440 and the explanation of the mirror problem, i.e. how does a mirror know to reverse left and right but not up and down: https://youtu.be/nYg6jzotiAc?t=1976 https://youtu.be/nYg6jzotiAc?t=1976
    - fouronnes3 5y ago
      The mirror thing is the one that every time I think: yes! this time I understand it! Then I think about it a bit more and nope. Black magic.
      - deleted 5y ago
        [deleted]
      - deleted 5y ago
        [deleted]
- erk__ 5y ago
  There is also an excellent Numberphile video on the subject https://www.youtube.com/watch?v=Ku8BOBwD4hc https://www.youtube.com/watch?v=Ku8BOBwD4hc
- Nition 5y ago
  I realise this isn't a very helpful comment in itself, but I hope Hacker News comments don't go the way Reddit comments have, where they routinely post and upvote content that's already in the linked article (usually because few people read beyond the title). In this case - just reposting the Feynman video - it's fine, but in other cases it leads to a lot of uninformed or unnecessary discussion, sometimes speculating on some hypothetical that the article already answers.
hprotagonist 5y ago
a similar thing becomes true for motorcycle and bicycle tires in a curve, without the differential effect of two wheels on one axle: when leaned over, the contact patch of the tire deforms conically and the effect is like rolling a solo cup on the ground: it “wants” to keep turning. Of course, pneumatic tires have cones that adjust their shape on the fly...
jcims 5y ago
Different mechanisms of action but similar implementation: Wing Dihedral - https://en.wikipedia.org/wiki/Dihedral_(aeronautics) https://en.wikipedia.org/wiki/Dihedral_(aeronautics) Crowned Pulleys - https://woodgears.ca/bandsaw/crowned_pulleys.html https://woodgears.ca/bandsaw/crowned_pulleys.html
syncsynchalt 5y ago
Motorcycles turn at speed by a similar principle, though the cones arrangement is a bit flipped around. I usually demonstrate it with two solo cups put mouth-to-mouth, to make a pair of facing cones that represents the motorcycle tire. The starting condition is that you're above parking lot speeds, and the bike is stable and is dynamically inclined to stay perfectly upright. To go left, you turn the bars right to upset the stable bike onto the left cone, and it goes left. To go right you turn the bars left and it upsets the bike onto the right cone, and goes right.
- foepys 5y ago
  This can also be easily experienced with a bicycle. Just push the handlebar forward on one side and watch/feel it tip over to that side instead of the other side where the wheel is pointing to. Just be careful when doing this and don't fall.
  - AnotherGoodName 5y ago
    In fact even for a bicycle at the lowest speeds counter-steer is required. Now that people have read this there will be a whole new group of people who on their next bike ride will think "According to theory i must be subconsciously turning the handlebars left in order to perform a right hand turn?" And then will you notice yourself doing it. It's quite remarkable. All those years you thought you turned the handlebars into the turn. You've actually been turning them the other way subconsciously in order to lean into the turn.
    - dharmab 5y ago
      One of the ways I keep myself occupied on long empty highway rides is to turn my cruise control on and keep myself in the lane by "punching" my handlebar. Punch the left side to turn left and the right side to turn right.
      - lostlogin 5y ago
        I’m completely lost now. You’re talking about a motorcycle? I didn’t know any of them had cruise control. I think the parent post is talking about a push bike. But on the off chance there is a push bike with cruise control…
dmix 5y ago
I’m getting redirected to an ad when the page loads?
- ruined 5y ago
  have you tried not doing that
- cpach 5y ago
  Try the HTTPS link instead: https://awesci.com/train-wheels-are-amazing/ https://awesci.com/train-wheels-are-amazing/
foo92691 5y ago
But not on BART!
- kentonv 5y ago
  As of 2018, more than half the train cars had been updated to conical wheels: https://www.bart.gov/news/articles/2018/news20180606 https://www.bart.gov/news/articles/2018/news20180606 But yes, historically the awful screeching around corners was because BART used cylindrical wheels. It's also, apparently, why they can't run all night -- the tracks need nightly maintenance due to the grinding.
  - Lammy 5y ago
    Some more info and photos can be found here (PDF warning): https://www.apta.com/wp-content/uploads/Resources/mc/rail/previous/2017rail/presentations/Presentations/Holland,%20Ben.pdf https://www.apta.com/wp-content/uploads/Resources/mc/rail/pr... https://www.bart.gov/sites/default/files/docs/New%20wheel%20profile%20Nov%202018%20R4.pdf https://www.bart.gov/sites/default/files/docs/New%20wheel%20...
  - henearkr 5y ago
    I know other horribly screeching trains, e.g. the Paris metro. May it be for the same reason? If so, then the fix seems straightforward.
    - Jyaif 5y ago
      The Paris metro's noise does not compare with the incredible screeching of the BART. Also, I think the Paris metro has special requirements, including quite sharp turns which I'm not sure trains can handle. Some of the lines in Paris actually use tires with side rails to channel the train to handle this.
      - frosted-flakes 5y ago
        The Montreal metro also uses rubber-tired trains.
nabilhat 5y ago
Another way to think of how this works is to look at crowned pulleys: https://woodgears.ca/bandsaw/crowned_pulleys.html https://woodgears.ca/bandsaw/crowned_pulleys.html In the train's case (if we ignore the gap between the wheels), the pair of wheels work like a crowned pulley, and the track finds center like a belt.
3pt14159 5y ago
Very few things surprised me as much as rail design when I was studying structural engineering. Surface tension and fluid dynamics were both trippy too, but I expected those things to be complicated and while surface tension blew my mind due to the relative simplicity of the proof, and fluid dynamics blew my mind because it was somehow 100x more complex than I estimated. Walking into rail design was hilarious. I worked on motorcycles and did some car stuff. I figured it was obvious, and sorta dismissed this assignment as a joke. Nope. My dismissive intuitions were just flat out wrong. It kinda leaves an impression on you to sorta avoid saying you know for sure before putting in some amount of work.
- dharmab 5y ago
  I worked on motorcycles for years using DIY guides and YouTube tutorials. Opening up real engineering books was an eye-opening and humbling experience that made me a better mechanic, driver/rider, homeowner and software engineer. (For the curious motorcyclist, I recommend "Honda Common Service Manual" as a starting point.)
  - Swizec 5y ago
    > opening real engineering books was eye-opening Turns out designing a new system to fit requirements is orders of magnitudes harder than fixing a system somebody else designed. You see this in software all the time. Anyone can follow a tutorial. But can you start from scratch and build something novel? Can you build it such that others can maintain long after you’re gone? That’s hard. Same with cooking. Anyone can follow a recipe. But can you design a recipe?
    - dharmab 5y ago
      Another thing I learned: Most engineers can build something that works. It is much, much harder to build something that optimizes for cost.
      - bch 5y ago
        “Anybody can build a bridge that stands - it takes an engineer to build a bridge that barely stands.”
amelius 5y ago
Toy trains not having this feature is a wasted opportunity.
- ksaj 5y ago
  It exists. They are called fast angle wheels in the modeling world. http://cs.trains.com/ctt/f/95/t/79912.aspx To quote: Fast angle wheels first came out when MPC took over Lionel. The wheels are not squared off where they ride on the rail. They are angled to the flange. "Fast angle" is a toolmaker's term for adding an angle to a surface so the part can be quickly removed from the tool without marring the surface during manufacture. Hence the term "fast angle wheel" was coined by Lionel employees. The fast angle did more than benefit manufacture. Because the wheels are fixed to the axel, it benefits them on curved track. The wheelsets can drift to a point where one wheel diameter point touching the rail is slightly larger than the opposite wheel diameter point touching the rail. This reduces friction because the outside rail is longer in circumference than the inside rail. Especially sharp 031 or 027 curves. If you look closely, you can see the cars lean into the curves as the outside wheels drift to a larger diameter.
etaioinshrdlu 5y ago
What’s also interesting is how subtle the slope the wheels are. I can barely see it in the pictures.
punnerud 5y ago
The wheels also have to have the right size to not get resonance. This have been a problem in Norway when the train reach 200km/h, because they forgot (?) to factor this in. This feels like a really bumpy road at high speed, and stop if the train driver reduce the speed just a little bit.
MathMonkeyMan 5y ago
His stories about college fraternities are quite outside the usual.
- lostlogin 5y ago
  The author? Or Feynman? I can’t find anything from either - though did read about the institutional racism dated by Feynman. Imagine being the person who questions his suitability for a Phd. https://en.m.wikipedia.org/wiki/Richard_Feynman https://en.m.wikipedia.org/wiki/Richard_Feynman
  - joveian 5y ago
    If Fynman, there is this story: https://sloth.hell.pl/~szymon/archiwalia/humor/feynman.html https://sloth.hell.pl/~szymon/archiwalia/humor/feynman.html
  - MathMonkeyMan 5y ago
    Yeah I was referring to Feynman. At the end of the video embedded in the article, he ends a story by saying something like "these were the sorts of things you had to know in the fraternity." He didn't mention shotgunning beers.
garbagetime 5y ago
Extremely common knowledge. I'm not against it being posted I just find it funny the writer seems to think this isn't something that many random primary school students know.
voz_ 5y ago
Meta observation: The top two comments are indicative of quality drift in HN. The first one, from 3pt14159 is inquisitive, interested, and humble. The second one, from the aptly named garbagetime, is dismissive and rude. Let's all please try to be more like the former, and less like the latter.
- c0nducktr 5y ago
  meta meta observation: You commented rather early in the posts history. Currently garbagetime's posts is near the bottom, while 3pt14159's is at the top. How many posts were there at the time of writing? Did your comment influence the subsequent voting? Would garbagetime have been naturally downvoted if given enough time? Is hacker news actually declining in quality, or is it just tendency to favor good things when remembering the past? Does any of this matter at all?
Rume 5y ago
The engineers can of suprise me base on the train wheels that was cones
jdblair 5y ago
Why are train wheels connected with an axle? Is it structural? If the wheel pairs were independent then it wouldn't matter how fast each wheel in a pair rotates.
- Lammy 5y ago
  > Is it structural? Yes, the cars' weight rests on the end of each axle via a "bogie" that holds the suspension and brakes and such, and then the multi-axle bogie itself rotates on a center pin: https://en.wikipedia.org/wiki/Bogie#Components https://en.wikipedia.org/wiki/Bogie#Components https://en.wikipedia.org/wiki/List_of_railroad_truck_parts#Axlebox https://en.wikipedia.org/wiki/List_of_railroad_truck_parts#A...
  - CRConrad 5y ago
    But that doesn't explain why the wheels are rotationally fixed to each other by an axle.
- johnwalkr 5y ago
  They are, and for non-driven wheels, the bogey and train cars basically sit on top. There's only a few minor things like brake hardware that need to be removed to remove a wheel. When there is a derailment, many of the wheel sets fall off. The axles are tough. Each axle weighs about 1 ton if I remember correctly. Each wheel can be reworked on a lathe several times (either with the wheel set removed or in situ on a drive-through floor-mounted lathe). After a few years, the diameter of the wheel is out of spec, and new ones are pressed on the axle. Axles can last about 75 years.
- twobitshifter 5y ago
  On some low floor trams they are independently rotating such as the Siemens avenio. https://en.m.wikipedia.org/wiki/Siemens_Avenio https://en.m.wikipedia.org/wiki/Siemens_Avenio Technical study https://assets.new.siemens.com/siemens/assets/api/uuid:2dbbe04aeb88674e9b6c332c43d452456b59019e/wheel-sets-independent-en.pdf https://assets.new.siemens.com/siemens/assets/api/uuid:2dbbe...
- 5y ago
jet_32951 5y ago
Conic frusta, to be accurate.
dehrmann 5y ago
At least on the old cars, wheels on BART are not cones. I keep wondering how much money running custom trains on a custom gauge costs them. https://www.bart.gov/news/articles/2018/news20180606 https://www.bart.gov/news/articles/2018/news20180606
- xsmasher 5y ago
  Anyone know why BART had cylindrical wheels, if the advantages of tapered wheels were known for a hundred years?
  - divbzero 5y ago
    Elsewhere in this thread, jcrawfordor explains that hunting oscillation from conical wheels can cause “chatter” at higher speeds which led to BART’s experimentation with cylindrical wheels [1][2]. Others also note that the wheels of high-speed trains are closer to cylinders than cones. [3][4] [1]: https://news.ycombinator.com/item?id=28350423 https://news.ycombinator.com/item?id=28350423 [2]: https://news.ycombinator.com/item?id=28350667 https://news.ycombinator.com/item?id=28350667 [3]: https://news.ycombinator.com/item?id=28350755 https://news.ycombinator.com/item?id=28350755 [4]: https://news.ycombinator.com/item?id=28352896 https://news.ycombinator.com/item?id=28352896
  - bobthepanda 5y ago
    BART in general was designed by aerospace engineers, because people in mid-century America thought that rail was an old outdated thing to replaced with monorails, personal rapid transit and maglev. The people who designed BART from the ground up on first principals (and ignoring everything that rail had done previously.) The problem with this is that newer technologies were not competing with rail purely on the technological merits, but with rail's massive amount of competition in suppliers and economies of scale. It turns out that with not a lot of effort, you can apply much the same savings and technological improvements to traditional rail, except without the huge added expense of converting to a standard you are currently incompatible with. Newer technologies have the reverse problem, in that they have very few, sometimes even a single supplier, and are basically custom projects with all the expense that entails. So the monorails, PRT, maglev, and other weird system are mostly unique specimens, or very few in number.
baybal2 5y ago
High speed rail wheels are not cones, or much less slanted cones, but they are turning in a much larger radius, and sometimes having independent wheels.
TedShiller 5y ago
It’s fairly widely known actually
thspimpolds 5y ago
They are also pressure fitted onto the axles. They are not bolted or otherwise fixed. The only thing which keeps them on the train is friction and the rail itself. I was on a commuter train which derailed because it LOST a wheel. I don't understand why they aren't at least locked in mechanically. That wheel went rolling at 80MPH and blasted straight through trees near the tracks.
anticensor 5y ago
It is for the same reason wheels grab the rail from the inside, not from above.

401 points by trekhleb 5y ago

summary: train wheels use the physical shape of the wheel to turn, and the stopper is only for emergencies. very cool!

Are the cones mainly for keeping the train centered, or for allowing the outer wheel on a turn to be effectively larger? Not sure if the differential effect is an added benefit that isn't really necessary, or if it is the main goal of the design.

ben11kehoe 5y ago
They keep the train centered by the differential effect. The angle of the cone is very slight, nowhere near enough for gravity to overcome friction to cause the train to slip laterally into the center. And then keeping the train centered as the track turns results in the train turning with the track.
- GhostVII 5y ago
  Sure, but I guess I'm wondering if that is just a convenient effect of the centering, or if it is actually necessary to prevent the wheels from skipping.
lbotos 5y ago
my understanding is the later: https://youtu.be/agd8B-31bjE?t=106 https://youtu.be/agd8B-31bjE?t=106
aaaaaaaaaaab 5y ago
They do both.
AnimalMuppet 5y ago
It's the same thing. If the train becomes uncentered, then one wheel is the outer wheel, and will turn the axle back toward the center. Or, looked at the other way, when the track curves, then the axle becomes uncentered.
- smartscience 5y ago
  To demonstrate this more fully, consider the case of having the wheel flanges on the outside, with the conicity of the wheels pointing the other way. Gravity would still tend to centre this arrangement, but I'm told that if you build such a system in practice, then it won't run nearly as smoothly. (PhD was 'Residual stress in rails', for what that's worth. Judging from the profiles of the rails I saw, direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. But on roughly straight track, I'm satisfied that the argument about conicity applies).
  - lostlogin 5y ago
    > direct contact with the wheel flange plays a substantial role in keeping the train in place on curved track. The London Underground has some lines that are horrifically loud. The squealing must surely be at dangerous sound levels. I’d always assumed it was the flange against the rail, and you appear to be confirming that?
    - AnimalMuppet 5y ago
      That's called "flange squeal". Yes, it can be ear-shatteringly loud. But it mostly (totally?) happens on very tight curves. It shouldn't happen much or at all on gentler curves. (Of course, this is circular, because I'm kind of defining "gentler" and "tight" based on whether they cause flange squeal. Still, there's a point - there is something like a threshold of curve tightness where flange squeal becomes much more probable.)
      - idroveatrain 5y ago
        It can happen on straightrail on an incline, too. It's hard to assess precisely what's happening in the locomotive, but under traction I believe the running gear will toe out, and align based on the path of least resistance. I believe this leads to the flange pressing, with immense force, against the rail. You also get a lot of wheel slip in this condition. I surmised this running 2 motors up a 3%(?) grade with 20k ton gross at 10mph. It's about the only explanation I could come up with is that the running gear was twisting under the gravity and the energy being put down to work against it. It might also just be a stringline sort of effect dragging the motors to one side of the track and pressing the flange. Maybe one of the rail engineers will come holler at me for my poor trainhandling skills.
morpheos137 5y ago
The main reason other than turns is to avoid hunting oscillation on straight track. https://en.m.wikipedia.org/wiki/Hunting_oscillation https://en.m.wikipedia.org/wiki/Hunting_oscillation
jcrawfordor 5y ago
Both are factors in good centering, but mostly the change in diameter. In turns, there is a natural tendency for the train to shift towards the outside of the curve due to inertia. The wheel diameters become asymmetric which helps to re-center the train. It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. The relationship between these two effects is a bit complex (depends on weights and speeds of trains) so it's usually all a bit approximate. The conical section of the wheels is mostly intended to prevent hunting on straight track, and the shape can't be made too aggressive without increasing the wear on wheels on rails. So on curves the superelevation is added to provide the extra force required. Because conical wheels do increase wear and can contribute to oscillation in their own way, there have been experiments with cylindrical wheels especially on higher-speed trains---BART is a well known example. It ultimately didn't work very well and so they have been re-trueing the wheels to a non-cylindrical profile, although still not quite a traditional conical one. Basically in higher-speed operation the re-centering effect is too significant and causes one wheel to "chatter," which over time creates a significant vibration in the rail. Trouble is cylindrical wheels tend to cause the same thing to happen on the other side. It was a very hard problem before computer modeling became available.
- iggldiggl 5y ago
  > It's usually not sufficient on its own, which is why superelevation is used as well - the outside rail is somewhat higher than the inside rail which shifts relative gravity to pull the train back towards the inside as well. [...] So on curves the superelevation is added to provide the extra force required. I've never heard about that theory as for why superelevation/cant is supposedly being used until now. Given that most of the time you'll end up with a remaining net force to the outside of the curve even after application of cant, it doesn't seem to make that much sense, either.
  - jcrawfordor 5y ago
    That's the conventional explanation of superelevation, although I worded it in sort of an odd way. But I'm describing the same thing that e.g. Wikipedia does. Superelevation directs the force of the car more "straight down" in relation to the rails which improves centering and balance of the load by the same token. The thing I said about "shifting gravity" is unnecessarily confusing because it depends on reference frame. I think for low-speed freight the balance needs to be pretty close on to ideal to meet regulations, e.g. FRA regulations give calculations for acceptable ranges. But since it's dependent on running speed it's hard to get correct for freight and passenger mixed operation which is the subject of this FRA report that has a lot of detail on the calculations: https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19085/Superelevation.pdf https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/19...
tus89 5y ago
The goal is for the train to NOT be centered when cornering, the cones allow for this. If the train was centered during corning, or simply with non-coned wheels, the outer wheel would necessarily slip.

ben11kehoe 5y ago

I think this may be the primary reason why narrow-gauge railways are better at tighter curves: the shorter axle means the same wheel radius difference (caused by lateral displacement) causes a smaller turn radius versus a standard-gauge axle.

aaaaaaaaaaab 5y ago

Feynman https://www.youtube.com/watch?v=WAwDvbIfkos https://www.youtube.com/watch?v=WAwDvbIfkos

waynesonfire 5y ago
This whole series is amazing.
- modeless 5y ago
  Here it is in better quality, the whole thing, and with subtitles: https://www.youtube.com/watch?v=nYg6jzotiAc https://www.youtube.com/watch?v=nYg6jzotiAc Particularly good parts are the explanation of fire and trees ("trees come out of the air"): https://youtu.be/nYg6jzotiAc?t=440 https://youtu.be/nYg6jzotiAc?t=440 and the explanation of the mirror problem, i.e. how does a mirror know to reverse left and right but not up and down: https://youtu.be/nYg6jzotiAc?t=1976 https://youtu.be/nYg6jzotiAc?t=1976
  - fouronnes3 5y ago
    The mirror thing is the one that every time I think: yes! this time I understand it! Then I think about it a bit more and nope. Black magic.
    - deleted 5y ago
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erk__ 5y ago
There is also an excellent Numberphile video on the subject https://www.youtube.com/watch?v=Ku8BOBwD4hc https://www.youtube.com/watch?v=Ku8BOBwD4hc
Nition 5y ago
I realise this isn't a very helpful comment in itself, but I hope Hacker News comments don't go the way Reddit comments have, where they routinely post and upvote content that's already in the linked article (usually because few people read beyond the title). In this case - just reposting the Feynman video - it's fine, but in other cases it leads to a lot of uninformed or unnecessary discussion, sometimes speculating on some hypothetical that the article already answers.

hprotagonist 5y ago

a similar thing becomes true for motorcycle and bicycle tires in a curve, without the differential effect of two wheels on one axle: when leaned over, the contact patch of the tire deforms conically and the effect is like rolling a solo cup on the ground: it “wants” to keep turning. Of course, pneumatic tires have cones that adjust their shape on the fly...

jcims 5y ago

Different mechanisms of action but similar implementation: Wing Dihedral - https://en.wikipedia.org/wiki/Dihedral_(aeronautics) https://en.wikipedia.org/wiki/Dihedral_(aeronautics) Crowned Pulleys - https://woodgears.ca/bandsaw/crowned_pulleys.html https://woodgears.ca/bandsaw/crowned_pulleys.html

syncsynchalt 5y ago

Motorcycles turn at speed by a similar principle, though the cones arrangement is a bit flipped around. I usually demonstrate it with two solo cups put mouth-to-mouth, to make a pair of facing cones that represents the motorcycle tire. The starting condition is that you're above parking lot speeds, and the bike is stable and is dynamically inclined to stay perfectly upright. To go left, you turn the bars right to upset the stable bike onto the left cone, and it goes left. To go right you turn the bars left and it upsets the bike onto the right cone, and goes right.

foepys 5y ago
This can also be easily experienced with a bicycle. Just push the handlebar forward on one side and watch/feel it tip over to that side instead of the other side where the wheel is pointing to. Just be careful when doing this and don't fall.
- AnotherGoodName 5y ago
  In fact even for a bicycle at the lowest speeds counter-steer is required. Now that people have read this there will be a whole new group of people who on their next bike ride will think "According to theory i must be subconsciously turning the handlebars left in order to perform a right hand turn?" And then will you notice yourself doing it. It's quite remarkable. All those years you thought you turned the handlebars into the turn. You've actually been turning them the other way subconsciously in order to lean into the turn.
  - dharmab 5y ago
    One of the ways I keep myself occupied on long empty highway rides is to turn my cruise control on and keep myself in the lane by "punching" my handlebar. Punch the left side to turn left and the right side to turn right.
    - lostlogin 5y ago
      I’m completely lost now. You’re talking about a motorcycle? I didn’t know any of them had cruise control. I think the parent post is talking about a push bike. But on the off chance there is a push bike with cruise control…

dmix 5y ago

I’m getting redirected to an ad when the page loads?

ruined 5y ago
have you tried not doing that
cpach 5y ago
Try the HTTPS link instead: https://awesci.com/train-wheels-are-amazing/ https://awesci.com/train-wheels-are-amazing/

foo92691 5y ago

But not on BART!

kentonv 5y ago
As of 2018, more than half the train cars had been updated to conical wheels: https://www.bart.gov/news/articles/2018/news20180606 https://www.bart.gov/news/articles/2018/news20180606 But yes, historically the awful screeching around corners was because BART used cylindrical wheels. It's also, apparently, why they can't run all night -- the tracks need nightly maintenance due to the grinding.
- Lammy 5y ago
  Some more info and photos can be found here (PDF warning): https://www.apta.com/wp-content/uploads/Resources/mc/rail/previous/2017rail/presentations/Presentations/Holland,%20Ben.pdf https://www.apta.com/wp-content/uploads/Resources/mc/rail/pr... https://www.bart.gov/sites/default/files/docs/New%20wheel%20profile%20Nov%202018%20R4.pdf https://www.bart.gov/sites/default/files/docs/New%20wheel%20...
- henearkr 5y ago
  I know other horribly screeching trains, e.g. the Paris metro. May it be for the same reason? If so, then the fix seems straightforward.
  - Jyaif 5y ago
    The Paris metro's noise does not compare with the incredible screeching of the BART. Also, I think the Paris metro has special requirements, including quite sharp turns which I'm not sure trains can handle. Some of the lines in Paris actually use tires with side rails to channel the train to handle this.
    - frosted-flakes 5y ago
      The Montreal metro also uses rubber-tired trains.

nabilhat 5y ago

Another way to think of how this works is to look at crowned pulleys: https://woodgears.ca/bandsaw/crowned_pulleys.html https://woodgears.ca/bandsaw/crowned_pulleys.html In the train's case (if we ignore the gap between the wheels), the pair of wheels work like a crowned pulley, and the track finds center like a belt.

3pt14159 5y ago

Very few things surprised me as much as rail design when I was studying structural engineering. Surface tension and fluid dynamics were both trippy too, but I expected those things to be complicated and while surface tension blew my mind due to the relative simplicity of the proof, and fluid dynamics blew my mind because it was somehow 100x more complex than I estimated. Walking into rail design was hilarious. I worked on motorcycles and did some car stuff. I figured it was obvious, and sorta dismissed this assignment as a joke. Nope. My dismissive intuitions were just flat out wrong. It kinda leaves an impression on you to sorta avoid saying you know for sure before putting in some amount of work.

dharmab 5y ago
I worked on motorcycles for years using DIY guides and YouTube tutorials. Opening up real engineering books was an eye-opening and humbling experience that made me a better mechanic, driver/rider, homeowner and software engineer. (For the curious motorcyclist, I recommend "Honda Common Service Manual" as a starting point.)
- Swizec 5y ago
  > opening real engineering books was eye-opening Turns out designing a new system to fit requirements is orders of magnitudes harder than fixing a system somebody else designed. You see this in software all the time. Anyone can follow a tutorial. But can you start from scratch and build something novel? Can you build it such that others can maintain long after you’re gone? That’s hard. Same with cooking. Anyone can follow a recipe. But can you design a recipe?
  - dharmab 5y ago
    Another thing I learned: Most engineers can build something that works. It is much, much harder to build something that optimizes for cost.
    - bch 5y ago
      “Anybody can build a bridge that stands - it takes an engineer to build a bridge that barely stands.”

amelius 5y ago

Toy trains not having this feature is a wasted opportunity.

ksaj 5y ago
It exists. They are called fast angle wheels in the modeling world. http://cs.trains.com/ctt/f/95/t/79912.aspx To quote: Fast angle wheels first came out when MPC took over Lionel. The wheels are not squared off where they ride on the rail. They are angled to the flange. "Fast angle" is a toolmaker's term for adding an angle to a surface so the part can be quickly removed from the tool without marring the surface during manufacture. Hence the term "fast angle wheel" was coined by Lionel employees. The fast angle did more than benefit manufacture. Because the wheels are fixed to the axel, it benefits them on curved track. The wheelsets can drift to a point where one wheel diameter point touching the rail is slightly larger than the opposite wheel diameter point touching the rail. This reduces friction because the outside rail is longer in circumference than the inside rail. Especially sharp 031 or 027 curves. If you look closely, you can see the cars lean into the curves as the outside wheels drift to a larger diameter.

etaioinshrdlu 5y ago

What’s also interesting is how subtle the slope the wheels are. I can barely see it in the pictures.

punnerud 5y ago

The wheels also have to have the right size to not get resonance. This have been a problem in Norway when the train reach 200km/h, because they forgot (?) to factor this in. This feels like a really bumpy road at high speed, and stop if the train driver reduce the speed just a little bit.

MathMonkeyMan 5y ago

His stories about college fraternities are quite outside the usual.

lostlogin 5y ago
The author? Or Feynman? I can’t find anything from either - though did read about the institutional racism dated by Feynman. Imagine being the person who questions his suitability for a Phd. https://en.m.wikipedia.org/wiki/Richard_Feynman https://en.m.wikipedia.org/wiki/Richard_Feynman
- joveian 5y ago
  If Fynman, there is this story: https://sloth.hell.pl/~szymon/archiwalia/humor/feynman.html https://sloth.hell.pl/~szymon/archiwalia/humor/feynman.html
- MathMonkeyMan 5y ago
  Yeah I was referring to Feynman. At the end of the video embedded in the article, he ends a story by saying something like "these were the sorts of things you had to know in the fraternity." He didn't mention shotgunning beers.

garbagetime 5y ago

Extremely common knowledge. I'm not against it being posted I just find it funny the writer seems to think this isn't something that many random primary school students know.

voz_ 5y ago

Meta observation: The top two comments are indicative of quality drift in HN. The first one, from 3pt14159 is inquisitive, interested, and humble. The second one, from the aptly named garbagetime, is dismissive and rude. Let's all please try to be more like the former, and less like the latter.

c0nducktr 5y ago
meta meta observation: You commented rather early in the posts history. Currently garbagetime's posts is near the bottom, while 3pt14159's is at the top. How many posts were there at the time of writing? Did your comment influence the subsequent voting? Would garbagetime have been naturally downvoted if given enough time? Is hacker news actually declining in quality, or is it just tendency to favor good things when remembering the past? Does any of this matter at all?

Rume 5y ago

The engineers can of suprise me base on the train wheels that was cones

jdblair 5y ago

Why are train wheels connected with an axle? Is it structural? If the wheel pairs were independent then it wouldn't matter how fast each wheel in a pair rotates.

Lammy 5y ago
> Is it structural? Yes, the cars' weight rests on the end of each axle via a "bogie" that holds the suspension and brakes and such, and then the multi-axle bogie itself rotates on a center pin: https://en.wikipedia.org/wiki/Bogie#Components https://en.wikipedia.org/wiki/Bogie#Components https://en.wikipedia.org/wiki/List_of_railroad_truck_parts#Axlebox https://en.wikipedia.org/wiki/List_of_railroad_truck_parts#A...
- CRConrad 5y ago
  But that doesn't explain why the wheels are rotationally fixed to each other by an axle.
johnwalkr 5y ago
They are, and for non-driven wheels, the bogey and train cars basically sit on top. There's only a few minor things like brake hardware that need to be removed to remove a wheel. When there is a derailment, many of the wheel sets fall off. The axles are tough. Each axle weighs about 1 ton if I remember correctly. Each wheel can be reworked on a lathe several times (either with the wheel set removed or in situ on a drive-through floor-mounted lathe). After a few years, the diameter of the wheel is out of spec, and new ones are pressed on the axle. Axles can last about 75 years.
twobitshifter 5y ago
On some low floor trams they are independently rotating such as the Siemens avenio. https://en.m.wikipedia.org/wiki/Siemens_Avenio https://en.m.wikipedia.org/wiki/Siemens_Avenio Technical study https://assets.new.siemens.com/siemens/assets/api/uuid:2dbbe04aeb88674e9b6c332c43d452456b59019e/wheel-sets-independent-en.pdf https://assets.new.siemens.com/siemens/assets/api/uuid:2dbbe...
5y ago

jet_32951 5y ago

Conic frusta, to be accurate.

dehrmann 5y ago

At least on the old cars, wheels on BART are not cones. I keep wondering how much money running custom trains on a custom gauge costs them. https://www.bart.gov/news/articles/2018/news20180606 https://www.bart.gov/news/articles/2018/news20180606

xsmasher 5y ago
Anyone know why BART had cylindrical wheels, if the advantages of tapered wheels were known for a hundred years?
- divbzero 5y ago
  Elsewhere in this thread, jcrawfordor explains that hunting oscillation from conical wheels can cause “chatter” at higher speeds which led to BART’s experimentation with cylindrical wheels [1][2]. Others also note that the wheels of high-speed trains are closer to cylinders than cones. [3][4] [1]: https://news.ycombinator.com/item?id=28350423 https://news.ycombinator.com/item?id=28350423 [2]: https://news.ycombinator.com/item?id=28350667 https://news.ycombinator.com/item?id=28350667 [3]: https://news.ycombinator.com/item?id=28350755 https://news.ycombinator.com/item?id=28350755 [4]: https://news.ycombinator.com/item?id=28352896 https://news.ycombinator.com/item?id=28352896
- bobthepanda 5y ago
  BART in general was designed by aerospace engineers, because people in mid-century America thought that rail was an old outdated thing to replaced with monorails, personal rapid transit and maglev. The people who designed BART from the ground up on first principals (and ignoring everything that rail had done previously.) The problem with this is that newer technologies were not competing with rail purely on the technological merits, but with rail's massive amount of competition in suppliers and economies of scale. It turns out that with not a lot of effort, you can apply much the same savings and technological improvements to traditional rail, except without the huge added expense of converting to a standard you are currently incompatible with. Newer technologies have the reverse problem, in that they have very few, sometimes even a single supplier, and are basically custom projects with all the expense that entails. So the monorails, PRT, maglev, and other weird system are mostly unique specimens, or very few in number.

baybal2 5y ago

High speed rail wheels are not cones, or much less slanted cones, but they are turning in a much larger radius, and sometimes having independent wheels.

TedShiller 5y ago

It’s fairly widely known actually

thspimpolds 5y ago

They are also pressure fitted onto the axles. They are not bolted or otherwise fixed. The only thing which keeps them on the train is friction and the rail itself. I was on a commuter train which derailed because it LOST a wheel. I don't understand why they aren't at least locked in mechanically. That wheel went rolling at 80MPH and blasted straight through trees near the tracks.

anticensor 5y ago

It is for the same reason wheels grab the rail from the inside, not from above.