3 ms·
Good to know it was for steadiness against the wind given its light construction and the original intent to have if go on (under?) the GGB deck. I’d like to kn
by mc32 4y ago
Good to know it was for steadiness against the wind given its light construction and the original intent to have if go on (under?) the GGB deck.
I’d like to know why the tracks are flat and not angled and require regular grinding down to even out the washboard effect this causes… which results in that high pitched sound whenever it enters a curve due to slippage.
- Trouble_007 4y agosearch: >'train-track grinding/profiling'< @DDG : <https://html.duckduckgo.com/html/?q='train-track grinding/profiling' https://html.duckduckgo.com/html/?q='train-track grinding/pr...>
- mc32 4y agoMost 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 agoAre 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 agoBART 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 agoAh, 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 agoAre 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 agohttps://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 agoThat is for normal trains no?
- texaslonghorn5 4y agoThought they were asking about trains in general. Oops
- hadlock 4y agoHave you ridden BART? Worst acoustical echo chamber I've ever ridden in (twice, once to the fox theater, once home from SFO, never again)
- 4y ago
- dredmorbius 4y agoIt'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 agois 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 agoIt 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 agoThis seems unlikely.
- geokon 4y agoI'm imagining two half length axels. I guess this creates some torque but the breaks and bearing would be identical
- EamonnMR 4y agoHuh, the MBTA Green Line in Boston has a couple of sharp turns where it makes unbearable high pitched shrieks too, I wonder if it's also using flat tracks...
- IIAOPSW 4y agoProbably the flange scratching the side of the rail.
- iggldiggl 4y agoThe Green Line still is very much a streetcar at heart, and at those kinds of tight radii no amount of rail inclination (that is no reasonable amount of inclination, after all the whole shebang still needs to work on tangent track, too) will be able to compensate for the difference between outer and inner wheels. The best you can try is add some sort of lubrication, but even that won't fully quieten it down, since you can only lubricate the flanges, but not the running surface for hopefully obvious reasons.
- ant6n 4y agoU can individually suspend the wheels, rather than using a fixed axle. Some low floor trams (streetcars) have those, because it allows passenger areas where otherwise the axle would be.
- iggldiggl 4y agoYes, but then you're losing the self-steering, which isn't ideal, either. For low floor vehicles you take that compromise (except possibly at the vehicle ends if you don't need fully 100 % low floor), but otherwise it looks like fixed axles still are the preferred solution.
- jeffbee 4y agoEveryone is trying to explain this but the reason is that BART was designed and built by a company that previously built dams, and in their hubris decided that the rail industry had nothing to teach them. It was the Space Age and Americans were absolutely overflowing with hubris. In short there is no reason. It was a mistake.