[HN Gopher] Train Wheels Are Cones
___________________________________________________________________
Train Wheels Are Cones
Author : trekhleb
Score : 190 points
Date : 2021-08-29 18:06 UTC (4 hours ago)
(HTM) web link (awesci.com)
(TXT) w3m dump (awesci.com)
| jdblair wrote:
| 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.
| johnwalkr wrote:
| 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.
| Lammy wrote:
| > 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/List_of_railroad_truck_parts#A...
| aaaaaaaaaaab wrote:
| Feynman https://www.youtube.com/watch?v=WAwDvbIfkos
| waynesonfire wrote:
| This whole series is amazing.
| modeless wrote:
| Here it is in better quality, the whole thing, and with
| subtitles: 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 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
| fouronnes3 wrote:
| 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]
| erk__ wrote:
| There is also an excellent Numberphile video on the subject
| https://www.youtube.com/watch?v=Ku8BOBwD4hc
| hprotagonist wrote:
| 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 wrote:
| Different mechanisms of action but similar implementation:
|
| Wing Dihedral -
| https://en.wikipedia.org/wiki/Dihedral_(aeronautics)
|
| Crowned Pulleys -
| https://woodgears.ca/bandsaw/crowned_pulleys.html
| garbagetime wrote:
| 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.
| syncsynchalt wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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...
| dharmab wrote:
| Yes, I'm talking about motorcycles. Most touring bikes
| have cruise control these days. Ducati's newest
| Multistrada even has radar cruise control and auto-
| braking.
| gpsx wrote:
| Just to add this explicitly, to expand on some specifics,
| motorcycles have the added complexity of angular momentum and
| its effects. As you mention, turning the wheel effects your
| lean angle through angular momentum conservation. The cone then
| is the dominant factor causing the bike to turn.
|
| The speed matters because as the speed is faster, a smaller
| angle change in the handlebars corresponds to a bigger sideways
| tilt motion of the bike.
| noir_lord wrote:
| Counter-steering.
|
| It comes intuitively if you've ever ridden a bike at more than
| a sedate speed.
| bolangi wrote:
| Uh, no. Motorcycle racers learned about countersteering in
| the 70s. Before they had no idea.
| noir_lord wrote:
| You realise that motorcycles existed for literally decades
| before it had a name, how where they turning corners at
| speed, hell how was I doing it in the 90s on scramblers
| when I was a kid, no one taught me, it's intuitive.
| ksaj wrote:
| Totally correct. At speed, you simply cannot turn without
| counter steering. If you tried to turn by turning your
| bars in the direction you wanted to go, you simply won't
| go there.
|
| At some point, someone noticed racing bikes counter steer
| really severely so you can easily see the wheel is
| actually pointing the opposite way. But the reality is
| that even at "won't fall over" speed on a bicycle, you're
| already doing the exact same thing. When you lean, you
| counter steer. Otherwise you'll high-side like a missile.
| syncsynchalt wrote:
| One of the Wright brothers has a quote about this (before
| they were airplane makers they were bicycle makers, the
| bicycle being the latest mechanical marvel of their
| time). The summary of the quote is that the action is
| intuitive but nobody realizes (or even admits) that
| they're doing it:
|
| > I have asked dozens of bicycle riders how they turn to
| the left. I have never found a single person who stated
| all the facts correctly when first asked. They almost
| invariably said that to turn to the left, they turned the
| handlebar to the left and as a result made a turn to the
| left. [...] I have never found a non-scientific rider who
| had particularly noticed it and spoke of it from his own
| conscious observation and initiative.
|
| The existence of counter-steering is still controversial
| to some riders, to the point where machines like the "No
| B.S. Bike" were created to demonstrate it as a necessary
| effect: https://soundrider.com/archive/safety-
| skills/nobsbike.aspx
| ksaj wrote:
| Another way to demonstrate it on a bicycle, you can do
| something pretty much every child has already done
| numerous times: ride without your hands on the handle
| bars (hands free).
|
| To turn, you lean in the direction you want to go. But
| what way does the handle bar turn when you do that? It
| counter steers! You _will_ fall if it doesn 't (which is
| essentially what the No B.S. Bike demonstrates).
| ksaj wrote:
| The irony is that the majority of people will think you're
| making it up when describing counter steering, yet they are
| already doing it without even thinking about it all the time.
|
| The first time I read about counter steering, I thought "man,
| this'll take forever to practice" until I realized it's
| really the only way one ever does it.
| dharmab wrote:
| The main advantage to knowing is that you can stop wasting
| energy with footpeg weighting or unnecessary leaning and
| use bar pressure. (weighting and body position have their
| uses but for most riders in most situations countersteering
| is the most efficient technique.)
| ksaj wrote:
| That's when they call it push steering. But it's really
| the same thing. When you push, you are causing your bike
| to counter steer. You are literally pushing the bar
| forward, even if you think you are pushing downward,
| which is by definition a counter steer.
|
| I think a lot of people assume those are different
| things. But they aren't. You simply _can 't_ turn at
| speed without counter steering, regardless of how you
| visualize the mechanics.
|
| You are spot on for people who think pushing with their
| foot on the inner foot peg has an effect. I'm sure it
| does, but it's a lot of wasted effort given that the end
| result you are looking for is the bar turning the
| opposite direction, and you'll be doing that whether you
| are conscious of it or not. It takes far less effort to
| simply not think about it, and do what comes natural
| since your arms and hands will inevitably do the right
| thing without any "different" theories interfering.
|
| It's also why 3 wheel bikes are notorious for throwing
| the rider high-side if they corner too quickly. It stops
| you from leaning, so there is no ability to counter
| steer. You end up behaving more like a London double-
| decker bus in the turn.
| Hamuko wrote:
| If it came so intuitively, there should be less videos like
| this: https://www.youtube.com/watch?v=VVE79XT8-Mg
| underwater wrote:
| This is a helmet cam video from a motorbike rider riding
| straight into a truck at high speed. It should probably
| have a NSFW tag or warning.
| 3pt14159 wrote:
| 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 wrote:
| 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 wrote:
| > 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 wrote:
| Another thing I learned: Most engineers can build something
| that works. It is much, much harder to build something that
| optimizes for cost.
| bch wrote:
| "Anybody can build a bridge that stands - it takes an
| engineer to build a bridge that _barely_ stands."
| dougSF70 wrote:
| And it takes an architect to build a bridge that rarely
| stands...cf. the millennium bridge in london. Designed by
| architects, fixed by engineers.
| elzbardico wrote:
| You do know that the architects hand off their work to
| structural engineers on anything more complex than a
| single pavement building before it gets built, do you?
| sbisson wrote:
| To be fair to the architects of the Millennium Bridge,
| the structural engineers used a vehicle bridge model, not
| one for pedestrians.
| 3pt14159 wrote:
| I know right? I also loved the times where I got to the math
| and all that internal intuition lined up completely.
| Torsional deformation for example. I almost blamed myself for
| not inventing the math myself it so obviously matched my
| intuitions. It's kinda fun no matter which way it goes.
|
| Black body radiation never quite sat right with me. One of
| the few subject areas where I just resigned myself to
| memorizing the formulas and moving on with life. Same with
| non-integer dimensional spaces for the most advanced partial
| differential course I took. I can visualize 2 million
| dimensional spaces just fine professor. But one and a half?
| What does this even mean?
| goldenkey wrote:
| It just means that the degrees of freedom aren't used
| fully. This is usually seen in fractals, where there is a
| level of redundancy that is respective of the fractional
| dimension missing.
|
| Think about it like this. If I have a 2d field (x,y) and I
| enforce every point's x value to be 0, I pretty much just
| made the x degree of freedom redundant, and can now call
| the field a 1d field. If instead, I enforce every 3rd
| point's x value to be 0, I've now got a 1+2/3 dimensional
| space. Because there is some redundancy, I no longer get
| the full entropy that 2 dimensions provide.
| tambourine_man wrote:
| That is a great, simple explanation, thanks.
| GuB-42 wrote:
| Isn't black body radiation a problem that drove scientists
| crazy for many years before Einstein found an explanation
| that got him a Nobel prize. That explanation is what
| started the huge mess that is quantum physics.
|
| So I don't think anyone can be blamed for not getting it
| intuitively.
| gmueckl wrote:
| I'm not 100% sure whether or not you're attributing the
| right physicist here ;).
|
| Max Planck wrote about quantized energy emissions from
| black bodies first in 1900. With this assumption, the
| spectrum of black body radiation could be derived
| successfully. That won him the Nobel Price in 1919.
| Albert Einstein postulated that light itself was
| quantized in one of his famous series of papers in 1905.
| This paper won him the Nobel Price in 1922.
|
| [Side note: confusingly, Max Planck was awarded the 1918
| Nobel Price and Albert Einstein was awarded the 1921
| Nobel Price. This happened because the committee decided
| in 1918 and again in 1921 that none of the candidates met
| their standards and withheld the price for later.]
| CamperBob2 wrote:
| Yep. It was literally a catastrophe for the
| traditionalists in physics.
| jacquesm wrote:
| Almost anything when done at the fine edge of engineering for
| optimum price point without sacrificing reliability is going to
| be an amazing thing to do a deep dive in. Windmills are
| another, they seem so simple and obvious until you dig in.
| na85 wrote:
| Hey Jacques, whatever happened to your wind turbine you were
| giving away?
| voz_ wrote:
| 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 wrote:
| 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?
| MathMonkeyMan wrote:
| His stories about college fraternities are quite outside the
| usual.
| lostlogin wrote:
| 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
| amelius wrote:
| Toy trains not having this feature is a wasted opportunity.
| ksaj wrote:
| 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 wrote:
| What's also interesting is how subtle the slope the wheels are. I
| can barely see it in the pictures.
| dmix wrote:
| I'm getting redirected to an ad when the page loads?
| ruined wrote:
| have you tried not doing that
| cpach wrote:
| Try the HTTPS link instead: https://awesci.com/train-wheels-
| are-amazing/
| code4money wrote:
| summary: train wheels use the physical shape of the wheel to
| turn, and the stopper is only for emergencies. very cool!
| foo92691 wrote:
| But not on BART!
| m0llusk wrote:
| Shinkansen use cylindrical wheels, but the tracks and wheels
| are kept maintained to an unusually high standard.
| kentonv wrote:
| As of 2018, more than half the train cars had been updated to
| conical wheels:
| 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.
| matttproud wrote:
| If you're curious about the constraints of BART and the
| history involved in its development that led to why it is the
| way it is, I can certainly recommend Michael Healy's book
| BART: The Dramatic History of the Bay Area Rapid Transit
| System.
|
| https://books.google.ch/books/about/BART.html?id=ubbwDwAAQBA.
| ...
| kentonv wrote:
| Heh... I don't think I have the patience to read a book
| about BART but I'd read the cliff notes on the cylindrical
| wheels decision. There are so many mentions of the
| cylindrical wheels on the internet but none of them explain
| _why_ they were chosen when conical wheels were already
| well-understood at the time... I 'm sure the engineers
| weren't just ignorant.
| jcrawfordor wrote:
| Cylindrical wheels were expected to reduce hunting
| oscillation and rail wear, which were particularly
| significant problems for BART because of the high speeds
| it operated at. The basic problem is that computer
| modeling was not yet available, and so the new design was
| validated experimentally using a set of instrumented test
| carriages on a short rail section built for the purpose.
| This found positive results on improved ride, but failed
| to detect the long-term problematic track wear. BART
| wheels have mostly been re-trued to a new profile which
| is not cylindrical, but also not quite a traditional
| conical section, and was designed with extensive use of
| computer modeling.
|
| The cylindrical wheel decision is closely related to the
| decision to use Indian/broad gauge, which was expected to
| provide a smoother ride as well as allowing more support
| equipment to be mounted under the car where it would
| produce less vibration.
|
| Both are decisions that have not stood the test of time,
| although the choice of Indian gauge cannot practically be
| reversed. But I think the discussion around this often
| pays the original designers far too little credit: BART
| was intentionally a highly innovative design with
| numerous aspects that were somewhat experimental. BART's
| automated control system, for example, was such a debacle
| that BART initially operated with signal towers and the
| control system required nearly complete replacement. But
| it was a completely trailblazing design, and the same
| missteps would have to be made _somewhere._ BART was used
| once again as a test platform for an innovative radio
| control scheme in the 2000s, evidence of which can still
| be seen mounted trackside on the SFO wye.
|
| Many lessons learned from BART's performance have
| contributed to later designs around the world, including
| notably the DC Metro which was built just shortly after
| by some of the same contractors.
| userbinator wrote:
| I wonder what the reason for not making them conical in the
| first place was, given that this knowledge has been around
| for over a century now. Maybe they were worried about
| https://en.wikipedia.org/wiki/Hunting_oscillation ?
| Lammy wrote:
| Some more info and photos can be found here (PDF warning):
|
| https://www.apta.com/wp-
| content/uploads/Resources/mc/rail/pr...
|
| https://www.bart.gov/sites/default/files/docs/New%20wheel%20.
| ..
| [deleted]
| henearkr wrote:
| 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 wrote:
| 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 wrote:
| The Montreal metro also uses rubber-tired trains.
| _jal wrote:
| Some MTA lines (NYC) are awful, too. The J comes to mind.
| nabilhat wrote:
| Another way to think of how this works is to look at crowned
| pulleys:
|
| 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.
| GhostVII wrote:
| 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.
| aaaaaaaaaaab wrote:
| They do both.
| morpheos137 wrote:
| The main reason other than turns is to avoid hunting
| oscillation on straight track.
|
| https://en.m.wikipedia.org/wiki/Hunting_oscillation
| AnimalMuppet wrote:
| 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 wrote:
| 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 wrote:
| > 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?
| jcrawfordor wrote:
| 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.
| ben11kehoe wrote:
| 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 wrote:
| 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 wrote:
| my understanding is the later:
| https://youtu.be/agd8B-31bjE?t=106
| ben11kehoe wrote:
| 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.
| Rume wrote:
| The engineers can of suprise me base on the train wheels that was
| cones
| punnerud wrote:
| 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.
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(page generated 2021-08-29 23:00 UTC)