Unstable when non-bogie rolling stock be running, but why?

[Gostav]

I have interested to know why the non-bogie (usually means four wheeler) rolling stock is not safe for high speed. Many old movies shows the old four wheeler was running and amazing shaking. My friend who is a SNCF staff, he told me the four wheeler is really unsafe because they are no hunting, the wheel always would be sliding to side and hunting for non-bogie and bogie stock is different. Then he left when l want ask more detail to him.
I cannot fully understand, it is similar to the six wheeler? Any different between French and British old four wheeler?

 

[edwin_m]

A couple of suggestions which may or may not be part of the answer.

A bogie vehicle is likely to run more smoothly because it has two sets of suspension, one between the wheels and the bogie and another between the bogie and the body. This may not make a great deal of difference to safety but certainly affects comfort.

A bogie vehicle can have horizontal dampers (shock absorbers) between the bogie and the body to make the bogie less easy to rotate, which reduces the tendency for hunting. With a four-wheel vehicle the only restraint is at the buffers and couplings to the next vehicle, which may be very little depending on the type of coupling and how they are adjusted.

 

[dubscottie]

As posted above its the suspension, especially when running empty that is the problem.

Empty 4 wheel wagons will bounce themselves of the track at speed and usually have a lower max speed when running empty for that reason.

 

[Spartacus]

It all depends on what the stock is designed for, there's bogie vehicles around only rated for 60mph while there's 4 wheelers rated for 75mph, though I wouldn't be keen on taking any of the latter above that I know at least some bogied wagons rated for 60mph empty that were regularly taken up to 75mph when used on a circuit with other sets that were rated 75mph.

 

[pdeaves]

Also, compared to a bogie wheelbase, a four wheel vehicle has a long wheelbase less suited to curves (at least not at speed). Bogie rotation makes it possible to match more closely the axle alignment with the track alignment.

 

[edwin_m]

Also, compared to a bogie wheelbase, a four wheel vehicle has a long wheelbase less suited to curves (at least not at speed). Bogie rotation makes it possible to match more closely the axle alignment with the track alignment.

I was thinking that too, but also that the longer wheelbase makes it more resistant to hunting on straight track.

There were many derailments of short-wheelbase four-wheel wagons in the UK in the 1960s at speeds typically around 60mph, despite the fact that similar trains had run at similar speeds for many years without such problems. This was one reason for a research programme into wheel-rail interaction, which led to much better understanding of hunting and similar events and contributed significantly to more modern bogie designs that emerged in the 1970s. I'm not sure if they ever got to the bottom of the derailment issue however, but the original short wheelbase wagons were restricted to lower speeds and a new generation of longer wheelbase four-wheelers with better suspension appeared that were mostly suitable for 75mph. These were also the basis for the "Pacer" four-wheel passenger trains that are complained about so much on this forum. However since privatisation nearly all new wagons in the UK have been on bogies.

 

[rebmcr]

There were many derailments of short-wheelbase four-wheel wagons in the UK in the 1960s at speeds typically around 60mph, despite the fact that similar trains had run at similar speeds for many years without such problems. I'm not sure if they ever got to the bottom of the derailment issue

I wonder whether the incidents coincided with improvements in track geometry standards? PWay crews beginning to put more precision into their work practices bears strking resemblance to the stories of improved punctuality also causing unforseen problems.

 

[coppercapped]

The studies carried out by BR Research in the 1960s into the wheel-rail interaction mentioned by edwin_m in post #6 led directly to modern four wheel and bogie suspension designs. Wickens and others showed that if the creep forces between the wheel and the rail could be properly controlled then vehicles could be made safe at any speed.

The first hardware was the High Speed Freight Vehicle suspensions. Four wheel vehicles built with these designs were shown to be stable at 90mph when hauled on normal track and at 120mph on the rolling test rig at Derby; a photo of one of the variants may be found here. These concepts formed the basis of the suspension of the Advanced Passenger Train as well as that of the Pacers and fed into the design of the High Speed Diesel Train, aka the HST, and all subsequent rolling stock

The derailments of short wheelbase (10ft or so) freight vehicles mentioned by edmin_m occurred at much lower speeds than 60mph; Palvans and similar were found to have critical speeds, that speed at which 'hunting' of the wheelsets commenced, of around 35mph. The reason that few modern high capacity four wheel wagons have been built has nothing to do with stability but all to do with the ratio of tare weight to full load. In comparison to one bogie wagon two four wheel wagons have two extra headstocks, sets of buffers and couplings - by avoiding their use the empty, tare, weight can be reduced while keeping the same maximum weight even though the bogie frames add some mass compared to single axle suspensions.

The knowledge of how to build stable 4-wheel vehicles has been available since the late 1960s. If built at any time since then no four wheel vehicle should be unstable. If they have been built it smacks of criminal negligence as unstable vehicles are a danger to life and limb.

 

[Asterix2012]

The studies carried out by BR Research in the 1960s into the wheel-rail interaction mentioned by edwin_m in post #6 led directly to modern four wheel and bogie suspension designs. Wickens and others showed that if the creep forces between the wheel and the rail could be properly controlled then vehicles could be made safe at any speed.

The first hardware was the High Speed Freight Vehicle suspensions. Four wheel vehicles built with these designs were shown to be stable at 90mph when hauled on normal track and at 120mph on the rolling test rig at Derby; a photo of one of the variants may be found here. These concepts formed the basis of the suspension of the Advanced Passenger Train as well as that of the Pacers and fed into the design of the High Speed Diesel Train, aka the HST, and all subsequent rolling stock

The derailments of short wheelbase (10ft or so) freight vehicles mentioned by edmin_m occurred at much lower speeds than 60mph; Palvans and similar were found to have critical speeds, that speed at which 'hunting' of the wheelsets commenced, of around 35mph. The reason that few modern high capacity four wheel wagons have been built has nothing to do with stability but all to do with the ratio of tare weight to full load. In comparison to one bogie wagon two four wheel wagons have two extra headstocks, sets of buffers and couplings - by avoiding their use the empty, tare, weight can be reduced while keeping the same maximum weight even though the bogie frames add some mass compared to single axle suspensions.

The knowledge of how to build stable 4-wheel vehicles has been available since the late 1960s. If built at any time since then no four wheel vehicle should be unstable. If they have been built it smacks of criminal negligence as unstable vehicles are a danger to life and limb.

I believe part of the problem was the introduction of continuous welded rail, jointed track helped prevent the hunting motion building up.

 

[Spartacus]

I think the first which was attributed to it may have been Rugby in 1961, found here; http://www.railwaysarchive.co.uk/eventsummary.php?eventID=148 although it seems unlikely that there weren't others beforehand that were probably dismissed due to there being less serious consequences. There's a lot of good info here too on the developments to prevent; it http://www.traintesting.com/HSFV1-4.htm It's easy to see how the chassis of a Pacer is related to those early developments. Thirsk seems to have relaunched investigations.

I'm sure I've heard that about welded rail too, jointed track disrupted the waddling effect by introducing a slight fore and aft movement over joints which broke up any hunting.

 

[Gostav]

I was thinking that too, but also that the longer wheelbase makes it more resistant to hunting on straight track.

There were many derailments of short-wheelbase four-wheel wagons in the UK in the 1960s at speeds typically around 60mph, despite the fact that similar trains had run at similar speeds for many years without such problems. This was one reason for a research programme into wheel-rail interaction, which led to much better understanding of hunting and similar events and contributed significantly to more modern bogie designs that emerged in the 1970s. I'm not sure if they ever got to the bottom of the derailment issue however, but the original short wheelbase wagons were restricted to lower speeds and a new generation of longer wheelbase four-wheelers with better suspension appeared that were mostly suitable for 75mph. These were also the basis for the "Pacer" four-wheel passenger trains that are complained about so much on this forum. However since privatisation nearly all new wagons in the UK have been on bogies.

So that today all four wheel wagons are operation on mainline are nearly close to retirement? But very clearly that the six wheel wagon really is dead on mainline.

 

[edwin_m]

So that today all four wheel wagons are operation on mainline are nearly close to retirement?

That's probably true for the UK at least, although wagons can stay around for quite a while and I wouldn't like to suggest a date when they are all gone.

But very clearly that the six wheel wagon really is dead on mainline.

I've been observing the railway since the 1970s and I don't think I've ever seen a six-wheel wagon in service. The main use was for milk traffic, which is quite heavy and needs to travel fast, but that went onto the roads in the early 70s. There were historically six-wheel vans for things like sausages (no idea why) and parcels, but the last ones probably disappeared about the same time. Round about 1900 there were quite a few six-wheel passenger coaches

 

[theageofthetra]

Albeit articulated aren't some Talgo units essentially four wheelers?- how fast has one of them gone?

 

[Spartacus]

I've been observing the railway since the 1970s and I don't think I've ever seen a six-wheel wagon in service. The main use was for milk traffic, which is quite heavy and needs to travel fast, but that went onto the roads in the early 70s. There were historically six-wheel vans for things like sausages (no idea why) and parcels, but the last ones probably disappeared about the same time. Round about 1900 there were quite a few six-wheel passenger coaches

The only ones I can think of are the independent snowploughs which are 6 wheelers, some of which started life as 8 wheeled Class 40 bogies, the rest I think use steam loco tender frames.

 

[jopsuk]

Albeit articulated aren't some Talgo units essentially four wheelers?- how fast has one of them gone?

Talgo is a whole different ball game- they don't have the live axles that are key component of how trains work. Between each carriage they have a pair of independent wheels which are effectively steered by the "flex" of articulated train. They have a post extending up from the wheel axle, and the main suspension and weight bearing connection to the carriage is then up near roof level, with the gangway between. They used this to enable low centre of gravity natural tilt.

remember that the essential principle with a normal train is that the two coned wheels act on the track through a curve such that the rotation speed is matched as the outer wheel effectively has a larger diameter to the inner one. This keeps the axle orthogonal to the rails. With multiple axles in a rigid body, the further the distance between the axles the more strain you're putting on the system, especially as you go in or out of a curve where one axle is trying to turn whilst the other is still naturally going straight