7 ms·

Most rail profiles are like: |\ /| _| |_ _| |_ But BARTD's are more like: _ _ | | | | _| |_ _| |_ Correctio

by mc32 4y ago

Most rail profiles are like: |\ /| _| |_ _| |_ But BARTD's are more like: _ _ | | | | _| |_ _| |_ Correction: the rails are flat, it's the wheels that are also flat. So pretend the above are the upper half of the wheel profiles. End result is more surface contact = more noise/squealing, also less self-centering of the wheels on the tracks, so the lips also brush against the rails. I think this also exacerbates the washboarding of the rails.

xnx 4y ago
Are you sure about that? I thought it was most common to have flat tracks and conical wheels. That's what I see on Google Image Search too.
- phs2501 4y ago
  BART had regular rail profile and non-tapered wheels, so flange squeal (and wear) was apparently a real issue. I'm not sure why they decided on this, since it runs counter to normal railway practice. This article has a story but I wonder if it wasn't just more NIH. https://sf.streetsblog.org/2018/06/08/bart-makes-headway-against-headache-inducing-howl/ https://sf.streetsblog.org/2018/06/08/bart-makes-headway-aga...
- mc32 4y ago
  Ah, yes. Correction. BARTD wheels are flat like the tracks, unlike other rail profiles which are tapered on flat. BARTD wheels are flat on flat.
DougN7 4y ago
Are you sure? I was just watching a video that explained the wheel edges are sloped, not the rails, to help fixed axel wheels to around curves. Wish I could find it again - quite ingenius!
- texaslonghorn5 4y ago
  https://youtu.be/WAwDvbIfkos https://youtu.be/WAwDvbIfkos (video of Feynman explaining how trains stay on the track) Is this it? I found it in my YouTube history. Or this one, which superimposes free body diagram (of forces) on the wheels to show how it is self stabilizing? https://youtu.be/XzgryPhtc1Y https://youtu.be/XzgryPhtc1Y
  - lazide 4y ago
    That is for normal trains no?
    - texaslonghorn5 4y ago
      Thought they were asking about trains in general. Oops
- hadlock 4y ago
  Have you ridden BART? Worst acoustical echo chamber I've ever ridden in (twice, once to the fox theater, once home from SFO, never again)
  - jrowley 4y ago
    The new trains aren’t as loud due to their conical wheels.
    - hadlock 4y ago
      ....sometimes. You can still hear them from a mile away.
- inferiorhuman 4y ago
  Yes. The wheels were flat, apparently Bombardier saw room for improvement and suggested a tapered profile that was retrofitted to the older cars. Unfortunately the track pitch (I'm forgetting what the proper term is) is also wrong, the spacing in the curves is too tight. Normally you'd increase the spacing a bit in curves to encourages the wheels to rotate at different speeds instead of destroying the track. Unfortunately even with the new profile BART is still incredibly loud, and it's not helped at all by low speed curves like the Oakland Wye.
  - iggldiggl 4y ago
    > the track pitch I think the term you're looking for is track gauge. And as for a need to widen it in curves, the answer is basically it depends. On mainline railways gauge widening in curves basically stems from the days of steam locomotives, which mostly didn't have bogies (at least for the main driving wheels) and comparatively enormous wheelbases. On straight(ish) track, some minor gauge widening as compared to the nominal standard gauge might nowadays also be beneficial, especially on high speed lines. Within the context of streetcars/trams, which often need to negotiate very tight curves on the other hand, current state of the art seems to indicate that a slight gauge tightening might actually be more beneficial in order to avoid too large a skew angle between the bogies and the rails. (Compare http://interfacejournal.com/archives/472 http://interfacejournal.com/archives/472) I'm no wheel/rail-interface guru, so quite what the conclusions from that would be for BART I can't say, either, but just wanted to point out that the topic is somewhat more complex, and nowadays gauge widening in curves might not actually always be the right answer.
    - inferiorhuman 4y ago
      > I think the term you're looking for is track gauge. No. Track gauge is the nominal width, no? I'm talking about how the width changes in the curves. A while back a former BART engineer went on the record claiming the spacing in the curves was contributing to the noise. Top to bottom BART fully embraced NIH. Fifty years ago there was good reason but now the rest of the rail industry has moved on and BART is stuck with a lot of tech and engineering debt. For e.g. BART can't grease or sand their tracks either, and you can feel the older trains slip and slide (and damage the track) as they exit the station. Edit: Here's an interesting piece on noise. Note how nearly every other rail system lubricates the tracks (BART can't) and how no other rail system uses (noisy) solid aluminum wheels. https://www.bart.gov/sites/default/files/docs/noisereport.pdf https://www.bart.gov/sites/default/files/docs/noisereport.pd...
- iggldiggl 4y ago
  Normally you do both: The running surface of the wheels is sloped, but the rails are also mounted onto the sleepers with a corresponding inclination. (Some grooved rail profiles for streetcars/trams have the inclination built directly into the profile of the railhead, so the rail itself is mounted upright, but you get the same effect at the railhead level.)
dredmorbius 4y ago
It's rail wheels which are beveled, with the radius decreasing to the outside of the wheel. Your top image has the wheels reversed. There's also a flange which extends below the rail depth on the inner edge of the wheel Should be, showing from the axel to the rail: _ __________________________ _ _ __________________________ _ | | | | | | | | | | | | \ | | / \_| |_/ | | As the car rounds turns, the inner wheel rides down and the outer wheel rides up, increasing the relative rotational radius of the outer wheel. This compensates for the fact that rail wheelsets rotate on a single fixed axis, not independently, and without a differential. For a non-ASCII image see: https://steemitimages.com/DQmUjS7PbhzAYpkTi5A3c2QQQa7jn1c52sfFu9RDK3hnDzX/32r34t5y67543.PNG https://steemitimages.com/DQmUjS7PbhzAYpkTi5A3c2QQQa7jn1c52s... Longer explanation: https://www.popularmechanics.com/science/a25581/science-behind-train-tracks-wheels/ https://www.popularmechanics.com/science/a25581/science-behi... Wikipedia also discusses geometry: https://en.wikipedia.org/wiki/Train_wheel https://en.wikipedia.org/wiki/Train_wheel
- geokon 4y ago
  is there a reason why they don't have the wheels spin separately? Does it make it more complicated? Or there is a disadvantage?
  - nikanj 4y ago
    It makes it more complicated. The wheels need to be fixed to the axel for breaks to function, so you can’t just stick a bearing in the wheel and call it a day.
    - dredmorbius 4y ago
      This seems unlikely.
    - geokon 4y ago
      I'm imagining two half length axels. I guess this creates some torque but the breaks and bearing would be identical

by mc32 4y ago

xnx 4y ago

Are you sure about that? I thought it was most common to have flat tracks and conical wheels. That's what I see on Google Image Search too.

phs2501 4y ago
BART had regular rail profile and non-tapered wheels, so flange squeal (and wear) was apparently a real issue. I'm not sure why they decided on this, since it runs counter to normal railway practice. This article has a story but I wonder if it wasn't just more NIH. https://sf.streetsblog.org/2018/06/08/bart-makes-headway-against-headache-inducing-howl/ https://sf.streetsblog.org/2018/06/08/bart-makes-headway-aga...
mc32 4y ago
Ah, yes. Correction. BARTD wheels are flat like the tracks, unlike other rail profiles which are tapered on flat. BARTD wheels are flat on flat.

DougN7 4y ago

Are you sure? I was just watching a video that explained the wheel edges are sloped, not the rails, to help fixed axel wheels to around curves. Wish I could find it again - quite ingenius!

texaslonghorn5 4y ago
https://youtu.be/WAwDvbIfkos https://youtu.be/WAwDvbIfkos (video of Feynman explaining how trains stay on the track) Is this it? I found it in my YouTube history. Or this one, which superimposes free body diagram (of forces) on the wheels to show how it is self stabilizing? https://youtu.be/XzgryPhtc1Y https://youtu.be/XzgryPhtc1Y
- lazide 4y ago
  That is for normal trains no?
  - texaslonghorn5 4y ago
    Thought they were asking about trains in general. Oops
hadlock 4y ago
Have you ridden BART? Worst acoustical echo chamber I've ever ridden in (twice, once to the fox theater, once home from SFO, never again)
- jrowley 4y ago
  The new trains aren’t as loud due to their conical wheels.
  - hadlock 4y ago
    ....sometimes. You can still hear them from a mile away.
inferiorhuman 4y ago
Yes. The wheels were flat, apparently Bombardier saw room for improvement and suggested a tapered profile that was retrofitted to the older cars. Unfortunately the track pitch (I'm forgetting what the proper term is) is also wrong, the spacing in the curves is too tight. Normally you'd increase the spacing a bit in curves to encourages the wheels to rotate at different speeds instead of destroying the track. Unfortunately even with the new profile BART is still incredibly loud, and it's not helped at all by low speed curves like the Oakland Wye.
- iggldiggl 4y ago
  > the track pitch I think the term you're looking for is track gauge. And as for a need to widen it in curves, the answer is basically it depends. On mainline railways gauge widening in curves basically stems from the days of steam locomotives, which mostly didn't have bogies (at least for the main driving wheels) and comparatively enormous wheelbases. On straight(ish) track, some minor gauge widening as compared to the nominal standard gauge might nowadays also be beneficial, especially on high speed lines. Within the context of streetcars/trams, which often need to negotiate very tight curves on the other hand, current state of the art seems to indicate that a slight gauge tightening might actually be more beneficial in order to avoid too large a skew angle between the bogies and the rails. (Compare http://interfacejournal.com/archives/472 http://interfacejournal.com/archives/472) I'm no wheel/rail-interface guru, so quite what the conclusions from that would be for BART I can't say, either, but just wanted to point out that the topic is somewhat more complex, and nowadays gauge widening in curves might not actually always be the right answer.
  - inferiorhuman 4y ago
    > I think the term you're looking for is track gauge. No. Track gauge is the nominal width, no? I'm talking about how the width changes in the curves. A while back a former BART engineer went on the record claiming the spacing in the curves was contributing to the noise. Top to bottom BART fully embraced NIH. Fifty years ago there was good reason but now the rest of the rail industry has moved on and BART is stuck with a lot of tech and engineering debt. For e.g. BART can't grease or sand their tracks either, and you can feel the older trains slip and slide (and damage the track) as they exit the station. Edit: Here's an interesting piece on noise. Note how nearly every other rail system lubricates the tracks (BART can't) and how no other rail system uses (noisy) solid aluminum wheels. https://www.bart.gov/sites/default/files/docs/noisereport.pdf https://www.bart.gov/sites/default/files/docs/noisereport.pd...
iggldiggl 4y ago
Normally you do both: The running surface of the wheels is sloped, but the rails are also mounted onto the sleepers with a corresponding inclination. (Some grooved rail profiles for streetcars/trams have the inclination built directly into the profile of the railhead, so the rail itself is mounted upright, but you get the same effect at the railhead level.)

dredmorbius 4y ago

It's rail wheels which are beveled, with the radius decreasing to the outside of the wheel. Your top image has the wheels reversed. There's also a flange which extends below the rail depth on the inner edge of the wheel Should be, showing from the axel to the rail: _ __________________________ _ _ __________________________ _ | | | | | | | | | | | | \ | | / \_| |_/ | | As the car rounds turns, the inner wheel rides down and the outer wheel rides up, increasing the relative rotational radius of the outer wheel. This compensates for the fact that rail wheelsets rotate on a single fixed axis, not independently, and without a differential. For a non-ASCII image see: https://steemitimages.com/DQmUjS7PbhzAYpkTi5A3c2QQQa7jn1c52sfFu9RDK3hnDzX/32r34t5y67543.PNG https://steemitimages.com/DQmUjS7PbhzAYpkTi5A3c2QQQa7jn1c52s... Longer explanation: https://www.popularmechanics.com/science/a25581/science-behind-train-tracks-wheels/ https://www.popularmechanics.com/science/a25581/science-behi... Wikipedia also discusses geometry: https://en.wikipedia.org/wiki/Train_wheel https://en.wikipedia.org/wiki/Train_wheel

geokon 4y ago
is there a reason why they don't have the wheels spin separately? Does it make it more complicated? Or there is a disadvantage?
- nikanj 4y ago
  It makes it more complicated. The wheels need to be fixed to the axel for breaks to function, so you can’t just stick a bearing in the wheel and call it a day.
  - dredmorbius 4y ago
    This seems unlikely.
  - geokon 4y ago
    I'm imagining two half length axels. I guess this creates some torque but the breaks and bearing would be identical