Chris M
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Post by Chris M on Nov 5, 2010 0:53:20 GMT
My understanding is that the DLR trains use a unique (in Britain at least) wheel profile that is closer to that of low-speed trams than high speed rail. This wheel profile is better suited to tight corners than the one used for mainline trains, but the trade-off is significant 'hunting' on faster stretches of track (especially notable through the tunnel to Woolwich Arsenal).
The DLR system has grown a lot since it's early days, with longer distances between stations, and designs incorporating greater curve radii and some of the really tight corners on the 1980s and early 1990s vintage sections now (partly) disused. Has the time now come to adopt a wheel profile better suited to the current gentler geometry and higher speed sections?
This would give a smoother ride on the fast sections (maybe even allowing small speed increases?) but possibly at the expense of slightly slower travel on the remaining tight corners (I can only really think of Delta Junction and between Bow Church and Pudding Mill Lane on passenger lines)?
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Post by astock5000 on Nov 5, 2010 1:50:03 GMT
the remaining tight corners (I can only really think of Delta Junction and between Bow Church and Pudding Mill Lane on passenger lines)? And also Heron Quays - Crossharbour, and Poplar - All Saints. I've read somewhere on the internet about the wheel profile on Manchester Metrolink's M5000 trams being changed due to this problem. |
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Post by Deleted on Nov 5, 2010 6:52:15 GMT
The DLR system has grown a lot since it's early days, with longer distances between stations, and designs incorporating greater curve radii and some of the really tight corners on the 1980s and early 1990s vintage sections now (partly) disused. Has the time now come to adopt a wheel profile better suited to the current gentler geometry and higher speed sections? This would give a smoother ride on the fast sections (maybe even allowing small speed increases?) but possibly at the expense of slightly slower travel on the remaining tight corners (I can only really think of Delta Junction and between Bow Church and Pudding Mill Lane on passenger lines)? Just curious which bits you're thinking of as being abandoned - the only parts of the DLR to my memory that have been closed are the lines to the original Mudchute and Island Gardens stations and to Stratford platform 4, and neither of those lines had curves as sharp as those remaining on the system today. Tower Gateway approach has been remodelled, but again that was pretty straight and still is. Delta Junction has been expanded but the original tight curves remain, and the same at Canning Town. Other than that I think all of the alignments that DLR runs on today are pretty much as built. The maximum speed on DLR has always been 80kph, and on much of the original 1987 railway that was permissable. Sharp curves are and have always been a relatively small proportion of DLR track, put in only where necessary. Sharp curves remain at Bow, Poplar, Delta Junction, Heron Quays, South Quay reverse curves (put in to avoid a building that was demolished before DLR opened!), Millharbour, Greenwich, Canning Town and Gallions Reach. These are crucial to operation of the entire system so any thought of reducing speed on these sections would have implications on journey times and headways. I'm not sure that it's as simple as a different wheel profile allowing higher speeds and a smoother ride on the straight sections while reducing speed on sharp curves. From what I studied some years ago if the wheel profile doesn't suit the sharp curves you would run the risk of losing traction at low speed, and the wear on the rails and the wheelsets would increase exponentially, possibly increasing the risk of fatigue failure. A slightly different problem was found with BR's Pacer DMUs in 1986-7 where the long wheelbase fixed two-axle layout per car couldn't handle some curves (not as sharp as on DLR) on Cornish branches and bogie stock had to return. The hunting is not at all comfortable I agree - the first manifestation of this was on the 'high-speed' section between Devons Road and All Saints with the original P86 stock around 6-9 months after the railway opened. However it is not dangerous (and certainly not as alarming as some early British Railways freight wagons ability to throw themselves off the track at a similar speed in the 60s) and given the gearing of the traction equipment any proposed increase in speed above 80kph would be at the expense of acceleration, again with an adverse impact on journey times and frequencies. Effectively the sharp curves are the lowest common denominator around which the equipment has to fit. Before thinking about changing the profile of the wheels some careful study would have to be carried out about whether doing so would actually just move the problem elsewhere and make it worse. |
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Post by Deleted on Nov 5, 2010 13:41:36 GMT
What is the exact "hunting" of the bogies? And why weren't steerable axles used?  |
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Post by phillw48 on Nov 5, 2010 17:27:52 GMT
What is the exact "hunting" of the bogies? And why weren't steerable axles used? Hunting is when a bogie is trying to turn on straight track. There are many causes of this problem but it is usually caused by wear in the bogie or wheel bearings or sometimes by dragging brakes. It can cause derailments especially on pointwork. |
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North End
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Post by North End on Nov 5, 2010 22:38:16 GMT
My understanding is that the DLR trains use a unique (in Britain at least) wheel profile that is closer to that of low-speed trams than high speed rail. This wheel profile is better suited to tight corners than the one used for mainline trains, but the trade-off is significant 'hunting' on faster stretches of track (especially notable through the tunnel to Woolwich Arsenal). The DLR system has grown a lot since it's early days, with longer distances between stations, and designs incorporating greater curve radii and some of the really tight corners on the 1980s and early 1990s vintage sections now (partly) disused. Has the time now come to adopt a wheel profile better suited to the current gentler geometry and higher speed sections? This would give a smoother ride on the fast sections (maybe even allowing small speed increases?) but possibly at the expense of slightly slower travel on the remaining tight corners (I can only really think of Delta Junction and between Bow Church and Pudding Mill Lane on passenger lines)? Unfortunately it's not that simple. If you change the wheel profile, you are likely to introduce a whole new range of problems - in particular changes to wheel & track wear. The Woolwich Arsenal tunnel has introduced another type of scenario - a fast section of track with what is fairly tight curvature for 50mph. I recall reading there have been significant issues with track wear in the Woolwich Arsenal tunnel. The final design is very much a compromise, getting the balance right between wheel and rail is immensely difficult, on a railway like the DLR which continues to grow, the best compromise may still be very much less than ideal. |
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Post by Deleted on Nov 6, 2010 5:52:36 GMT
What is the exact "hunting" of the bogies? And why weren't steerable axles used? Hunting is when a bogie is trying to turn on straight track. There are many causes of this problem but it is usually caused by wear in the bogie or wheel bearings or sometimes by dragging brakes. It can cause derailments especially on pointwork. Ah, now I get it. Thanks. |
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Post by Deleted on Nov 6, 2010 6:45:47 GMT
Articulated three-bogie vehicles also seem more prone to hunting, depending on the how the weight is distributed between the axles. It seems to be a common problem on American light rail lines, which tend to use this kind of vehicle, and it seems to get worse when the train is going down a gradient at speed.
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Deleted
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Post by Deleted on Nov 6, 2010 7:30:36 GMT
Would that be an argument to favour proper bogie placements? ;D
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Post by phillw48 on Nov 6, 2010 8:54:27 GMT
Would that be an argument to favour proper bogie placements? ;D In a word yes! A cure for the problem is effective damping which is easy to achieve on a rigid vehicle but a lot harder on an articulated vehicle that has to maneuver round sharp curves. |
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Post by Deleted on Nov 6, 2010 12:19:56 GMT
Would that be an argument to favour proper bogie placements? ;D In a word yes! A cure for the problem is effective damping which is easy to achieve on a rigid vehicle but a lot harder on an articulated vehicle that has to maneuver round sharp curves. TBH I've always favoured proper bogies, no artic wishwash... |
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rincew1nd
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Post by rincew1nd on Nov 8, 2010 20:09:42 GMT
What is the exact "hunting" of the bogies? And why weren't steerable axles used? Hunting is when a bogie is trying to turn on straight track. There are many causes of this problem but it is usually caused by wear in the bogie or wheel bearings or sometimes by dragging brakes. It can cause derailments especially on pointwork. The result is that the train 'waddles' along straight track, as if hunting for the track (is it over here; no, how about over here; no, is it over there.... et cetera) |
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Post by Deleted on Nov 9, 2010 4:18:09 GMT
Hunting is when a bogie is trying to turn on straight track. There are many causes of this problem but it is usually caused by wear in the bogie or wheel bearings or sometimes by dragging brakes. It can cause derailments especially on pointwork. The result is that the train 'waddles' along straight track, as if hunting for the track (is it over here; no, how about over here; no, is it over there.... et cetera) ...which meakes for a less-than-smooth ride... |
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Post by plasmid on Dec 3, 2010 23:07:07 GMT
hunting - this is surely not limited to the small size of the wheels but the strength of the suspension (shocks and springs) too.
when lots of people board a DLR train, you can notice the train bouncing up and down.
the suspension is really soft to allow for tight cornering at low speeds but suffers with hunting at high speed.
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