Comment by hristov | Hacker News Reader

Comment by hristov | Hacker News Reader

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Yes, it is the wheels that are beveled. However, the BART wheels are not beveled, they are flat (cylindrical) at the part where the cars usually ride. See https://www.reddit.com/r/bayarea/comments/2t7x49/the_bart_wh....

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.

I was explaining the shape of traditional rail wheels.

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.

Of anyone wants to see what hunting looks like, then watch a video of the London DLR.

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.

The DLR is a low-speed network with tight turning radii. I haven't found any footage clearly showing hunting (in part because it's difficult to distinguish other lateral-movement-inducing elements such as turns and switches), but the basic operating parameters are fairly clear.

I suspect the wheels have high conicity. The rolling stock is also likely lighter than a heavy-rail system.

https://yewtu.be/watch?v=i1pAfKsL1bk

Well, there's a reason they named it the Docklands Light Railway!

(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 ...)

its like the lifeline of east london, and it connected large aread of land that used to be borderline slums. Now there are highriae apartments and the fact that they have access to piblic trandport has increased their valuation by billions.

Providing improved transport and transit is amongst the best way of increasing property values. A stellar example is Denver, Colorado, which grew in size tenfold in a decade after a branch line connected it to the Transcontinental Railroad in 1870, becoming the dominant Rocky Mountain region city, a title which might otherwise have gone to Chyenne, Wyoming.

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.

Ha! Absoltuely correct and I'm honoured to be Strossed.

Thanks for the, er, bedrock explanation as well.

Oh trust me, the hunting on the DLR is pretty bad. You’ll notice the second the trains get up any speed on a straight. The problem is bad enough for it appear in its own section of the DLR Rolling stock wiki page.

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.

https://youtu.be/_WosYRVYMEM?t=907

Thanks.

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...

The Shinkansen also use cylindrical wheels on flat rails. The solution is to spend a large amount of effort and money into maintaining both the wheels and the rails. The amount of maintenance that BART train wheels and tracks get is much lower than what is done to keep Shinkansen running.

Hunting in modern rail vehicles is largely a solved problem using axle yaw dampers and other systems. I struggle to see why BART doesnt use a standard wheel profile and buy from a rolling stock supplier with experience of designing for good ride quality.

They do now, if you search for it there is a presentation on the transition to conic wheels somewhere out there.

https://news.ycombinator.com/item?id=32033094

Links a 2016 article.

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