search: >'train-track grinding/profiling'< @DDG : <https://html.duckduckgo.com/html/?q='train-track grinding/pr...>
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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.
Should be, showing from the axel to the rail:
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For a non-ASCII image see: https://steemitimages.com/DQmUjS7PbhzAYpkTi5A3c2QQQa7jn1c52s...
Longer explanation: https://www.popularmechanics.com/science/a25581/science-behi...
Wikipedia also discusses geometry: https://en.wikipedia.org/wiki/Train_wheel
This is what causes a lot of noise and wear and tear. I actually researched this and it seems that it was not as stupid of a decision as a lot of people say it is. There is a problem with beveled wheels -- it is called hunting. This is when the car oscillates from side to side. Hunting is unpleasant, also causes wear and tear and noise and creates a danger of derailing.
Hunting becomes more likely if the car is lighter and if it is going high speed. The bart was designed to be lighter than most existing metro systems and to go much faster than existing metro systems in certain corridors. So they wanted to prevent hunting and thus made a portion of the wheel cylindrical.
So it seems that they just made another engineering trade off. And I am sure they had justification. But I do wish that when engineers make these trade offs they take human comfort into account. I does not seem that they gave sound much of a weight when making their trade offs.
I think the solution is to keep the wheels cylindrical but just decouple them. I.e., not to have the right and left wheel connected by an axle. If they are independent they can move at different speeds on the turns and will not cause grinding and noise. And the wheels that are driven may be driven by different electric motors that drive the wheels by the necessary different speeds on turns. This is called vectoring and is already being done for some electric cars.
BART's since adopted a very-slightly-beveled wheel design pictured here:
https://sf.streetsblog.org/wp-content/uploads/sites/3/2016/0...
Story:
https://sf.streetsblog.org/2016/04/07/new-bart-cars-show-age...
Note that that's as of 2016, six years ago.
The hunting phenomenon is real and has some interesting consequences. For freight trains, empty boxcars have a lower speed limit than fully-laden ones, as the hunting behaviour is greater with less weight.
High-speed rail systems required understanding and managing hunting action. Getting a case of the wobblies at 300 kph is an exciting but short ride.
The DLR has very tight corners, and was originally built on very tight budget. As a result it uses very steeply bevelled wheels to handle the corners.
The wheels do a great job on the corners, but on the straight they suffer from significant hunting, resulting in the trains wobbling side-to-side (and slightly up-and-down), at a rather alarming frequency and amplitude. Not quite enough to make you sea sick, but it’s pretty close.
I suspect the wheels have high conicity. The rolling stock is also likely lighter than a heavy-rail system.
https://en.m.wikipedia.org/wiki/Docklands_Light_Railway_roll...
Here’s good video about the DLR time stamped to point that demonstrates the hunting well.
The video confirms what I'd surmised: that tight turns were the reason for more conical wheels. The sections I watched didn't actually show the wheel geometry, though, and I've not been able to find any photos online (DDG and Google Image search, various combinations of "DLR | Docklands Light Railroad | (conical|cone|conicity) wheel"). The WIkipeidia DLR Rolling Stock article mentions the conicity and hunting issues, but doesn't show the wheels either:
https://en.wikipedia.org/wiki/Docklands_Light_Railway_rollin...
(Large stretches of the original network built in the 1980s ran on elevated track, supported on pylons over ground that's a mixture of chalk and clay and tends to waterlog. Heavy rail can be built over and under that part of the Thames, but it has its own special challenges and the DLR was built as the cheapest way of getting commuters in and out of the Canary Wharf office towers, from Tower Gateway and the actual London Underground network to the west. Then from the 1990s onwards mission creep set in ...)
It's also something of a two-edged sword: the property value boost makes further transit rights of way far more expensive to acquire. I've noticed in particular that much rail development happens either as greenfield projects within countries as they're first industrialising (UK and US in the 19th century, China in the late 20th), or following a devastating war or other economic setback --- Japan's shinkansen and to a somewhat lesser degree France's TGV are both legacies of WWII.
In the US, aside from political opposition from other parts of the transportation sector, it's wealthy landowners who are the most significant obstacle to new development.
Thanks for the, er, bedrock explanation as well.
Links a 2016 article.
Does it make it more complicated? Or there is a disadvantage?
As the wheels move off centre, the bevel makes one wheel have a larger effective diameter and the other wheel a smaller effective diameter. Because the wheels have a solid axel, that forces one wheel to start travelling slightly faster than the other, causing the pair of wheels to turn.
With the bevel making the wheels larger on the inside, it means that as a pair of wheels drifts right relative to the tracks, the right hand wheel rides up, starts behaving as if it’s larger, and then starts to overtake the left wheel (which has ridden down, and behaves as if it’s smaller), creating a turning effect to the left, correcting the rightward drift.
When setup correctly, and travelling at a reasonable speed, the big metal lip on train wheels should never touch the track. The self-centering effect does all the work, the lip is just a backup to deal with overly tight turns, points and extremely slow running.
Or are you saying the whole setup actually tilts the carriage into the turn - helping to prevent it from tipping?
Weight and load have no impact of the effect. The only requirement is that the wheels remain adhered to the track, and don’t slip. Which is expected anyway, because it would be tricky to accelerate and decelerate a train with slipping wheels.
With independent wheels you have no self-centering effect, so only think keeping your wheels point in the right direction would be inner flange on the wheels colliding with the track. Such collisions obvious involve quite a bit of friction, noise and track/wheel wear.
Not to mention independent wheels are more complex to build. Two wheels directly attached to a single fixed axel is about a simple as you can get. You only need two couplings between your single axel and non-rotating part of your train. With independent wheels you need four, or more complex wheel designs to allow bearings at the wheel/axel coupling.
There no need to tip carriages into turns to prevent tipping, gravity, track banking, and speed limits already assure that tipping isn’t an issue. Tilting trains only exist to increase passenger comfort, not safety. Tilting trains are prefect capable of operating a full line speed with their tilting mechanism disabled or jammed, it’ll just be uncomfortable for the passengers.
That isn’t true. No train network is going to allow train to operate at speed, if the safety of those trains depends on non-failure of a complex tilting mechanism. Having a tilting mechanism fail or jam before a corner (which has happened) can not result in a high speed derailment. That risk is simply unacceptable on modern railway.
Tilting trains make it possible to run at higher speeds on older lines by allowing older lines to have the camber on corners increased to accommodate the higher speeds. But the camber can’t increased enough to hit optimal amount of camber without make the rail unusable to older trains.
Tilting trains provide a useful compromise. Increase the camber enough for stability (but not comfort), while keeping the camber acceptable for older trains, then make up the rest of the tilt with carriage tilting mechanism. The total amount of tilt is then comfortable for passengers. There is a minor safety element provided by tilting trains, in that lateral forces in cornering are lower, thus passengers are less likely to fall, and items won’t come of tables. But the tilting mechanism certainly is a safety critical element to running faster on old lines.
It’s also worth mentioning that carriages on passenger trains have a pretty low center of gravity, as all the heavy equipment (motors, bogies, breaks, suspension) sits below the passenger compartment. As a result tilting a passenger train won’t significantly move its center of gravity, so impact on total train stability is going to pretty minor.
Tilting a carriage isn't going to adjust the CoG by any appreciable amount. It will tend to make the net centripetal moment more virtical from the reference frame of passengers, however.
Wikipedia's article corroborates this:
As a train (or other vehicle) rounds a curve at speed, objects inside the train experience centrifugal force. This can cause packages to slide about or seated passengers to feel squashed by the outboard armrest, and standing passengers to lose their balance. Tilting trains are designed to counteract this by tilting the carriages towards the inside of the curve, thus compensating for the g-force.
For a good explainer see: https://yewtu.be/watch?v=SRsm7mv0Oh8
With separted axles, you wouldn't get the climbing behaviour of the outside wheel. With a fixed axle, the idealised shape of the wheels is of a double-ended cone (see the video for an example) which can ride higher on the outside of a turn. Even with conical wheels, with independent axels there's no way to impart a turning force on the carriage. I'm not entirely sure what the end result would be, but I suspect it would be a far harsher ride, and much more track and wheel wear. Probably a much higher probability of derailments as well.
Cone angle varies by type of train and speeds. The video shows both freight and high-speed rail designs and discusses characteristics of both.
A solid axel allows placing an external bearings between the truck and the axel.
Hub bearings are internal and service would require dealing with the nontrivial weight of a loose wheel.
See https://en.m.wikipedia.org/wiki/List_of_railroad_truck_parts
https://connectorsupplier.com/regenerative-braking-systems-r...
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
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.
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)
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.
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.pd...
I'm not intimately familiar with the English terms, but I think it still all boils down to the track gauge. That Boston article I linked to certainly talks about track gauge and gauge-widening and the like.
https://sf.streetsblog.org/2018/06/08/bart-makes-headway-aga...