What is minimum curve?

[trv100]

on the side of the carriages where the information panel with ETH index etc on, there's something that says Minimum Curve and then gives data in Chains, when I googled it I understood it even less, what actually is minimum curve in its simplest terms?

 

[zwk500]

It's the smallest radius (distance from centre of circle to edge) of curvature the vehicle can go round whilst staying in gauge/not toppling over etc. The smaller the radius, the tighter the curve that the train can traverse.

 

[30907]

on the side of the carriages where the information panel with ETH index etc on, there's something that says Minimum Curve and then gives data in Chains, when I googled it I understood it even less, what actually is minimum curve in its simplest terms?

To add to zwk500:
a 20m-plus vehicle will overhang at both ends and in the middle on a small-radius curve.
It might therefore come into contact with something at the lineside, or the buffers of adjacent vehicles might lock together instead of touching face-to-face, and that's a possible cause of derailment.

A chain is still a standard railway civil engineer measure, near enough 20m.

 

[randyrippley]

1 chain = 22 yards

 

[Annetts key]

One chain = 22 yards = 66 feet = 20.1168 metres.
80 chains = 1760 yards = 1 mile

The U.K. railway network uses miles, chains and yards extensively.

 

[edwin_m]

To add to zwk500:
a 20m-plus vehicle will overhang at both ends and in the middle on a small-radius curve.
It might therefore come into contact with something at the lineside, or the buffers of adjacent vehicles might lock together instead of touching face-to-face, and that's a possible cause of derailment.

A chain is still a standard railway civil engineer measure, near enough 20m.

Also the bogies might not be able to swivel far enough to go round a small radius curve without suffering some damage.

 

[mr_moo]

To add to zwk500:
a 20m-plus vehicle will overhang at both ends and in the middle on a small-radius curve.
It might therefore come into contact with something at the lineside [..]

Whilst what you state is correct (although it’s true in varying severity for any size of vehicle on any curve), I don't think this would be a factor in the stated 'minimum curve' on a wagon, unless it was a set of wagons with semi-permanent couplings, in which case absolutely.

Track designers will specifically calculate the risk of buffer locking for any given set of curves and traffic, which will feed into design acceptance and published gauging acceptance in the sectional appendix.
Similarly, they will specifically calculate the clearances from any vehicle to any structure that's close to the railway, and this again feeds into the above, it wouldn’t be a restriction on the vehicle itself.
Agreed that buffer compression would be a factor though, although I think you'd have to be on an extremely tight curve for this to be an issue.

Essentially, it's mostly about the ability of the bogie and wheelsets to safely rotate around tight curves without climbing up the rail.
A stiffer bogie will place greater sideways forces onto the wheel, increasing its chance of climbing a rail in a tight curve.
The 'wheelbase', which is the distance between the two wheels on a wheelset, will also be a key factor - the further apart they are, the more stable the vehicle at higher speeds, but also the more difficult it will be for the vehicle to navigate tight curves because the distance apart is fixed.
Some bogies have fixed wheelsets and rely on the rotation of the whole bogie to accommodate curves, which is simpler but means the angle of attack of the wheels gets greater as the curve radius gets tighter, leading to a greater risk of flange climb on tight radii. Some bogies allow each axle to rotate independently, meaning the axles can separately adjust to the curvature, enabling them to go round tighter curves with much lower flange climb risk.
Different types of couplings also have a different ability to flex/rotate etc, so some types will only be able to accommodate a certain radius before being pulled or twisted beyond their normal operating limits. This is especially true for semi-permanent coupling / fixed formation wagons.

Lastly, the wheel profile is also a potential factor. There are several different wheel profiles, some with slightly bigger flanges, some with tighter curves etc. Different profiles have a different risk on different curves and at different speeds, and some are just older designs that have never been updated, but again, this will factor into the minimum safe radius for the vehicle.

 

[zwk500]

Similarly, they will specifically calculate the clearances from any vehicle to any structure that's close to the railway, and this again feeds into the above, it wouldn’t be a restriction on the vehicle itself.

I believe the overhang of the new 80x/Nova units did lead to a number of restrictions on platforms with tight curves, such as at Newcastle, until clearances could be increased or better data was available to determine the restriction had been overly cautious.

 

[edwin_m]

Some bogies have fixed wheelsets and rely on the rotation of the whole bogie to accommodate curves, which is simpler but means the angle of attack of the wheels gets greater as the curve radius gets tighter, leading to a greater risk of flange climb on tight radii. Some bogies allow each axle to rotate independently, meaning the axles can separately adjust to the curvature, enabling them to go round tighter curves with much lower flange climb risk.

Not sure this is correct. Practically all trains have wheelsets where the two wheels on the same axle rotate together but the different axles rotate independently. There are very few exceptions, such as Talgos and some trams that have independent wheels, and some locomotives where one motor drives several axles. On tight curves the wheel flanges will contact the side of the rail to help the wheelset to follow the track. This can indeed lead to flange climbing, but that depends on speed (and on various other factors relating to wheel/rail shape and friction). It doesn't limit the radius of curve a particular vehicle can travel round. It does limit how fast it can do so, but this limitation doesn't depend on the type of train.

I believe the overhang of the new 80x/Nova units did lead to a number of restrictions on platforms with tight curves, such as at Newcastle, until clearances could be increased or better data was available to determine the restriction had been overly cautious.

It did, but that's not about the curve itself, it's about what there is next to the curved track that might be hit by the train.

 

[Lockwood]

Some bogies have fixed wheelsets and rely on the rotation of the whole bogie to accommodate curves, which is simpler but means the angle of attack of the wheels gets greater as the curve radius gets tighter, leading to a greater risk of flange climb on tight radii. Some bogies allow each axle to rotate independently, meaning the axles can separately adjust to the curvature, enabling them to go round tighter curves with much lower flange climb risk.

Do any axles have any form of differential, or is that effect suitable managed by the conical wheel shape?

 

[Dr Hoo]

Do any axles have any form of differential, or is that effect suitable managed by the conical wheel shape?

Essentially “no” to differentials. (Some trams do have independent wheels with stub axles but commonly for low floor gangway reasons rather than tight curvature.)

 

[Ken H]

Isnt there a problem coupling different types of vehicle because of behaviour on curves. I think there were problems with a pacer 142-144 types being coupled to a 23m vehicle like a 158. I think that was covered by an outright ban, not by curve restrictions.

 

[zwk500]

It did, but that's not about the curve itself, it's about what there is next to the curved track that might be hit by the train.

It's related to the curve, because on bigger radii (radiuses?) the train was fine with curved platforms.

 

[edwin_m]

It's related to the curve, because on bigger radii (radiuses?) the train was fine with curved platforms.

On some platforms (Bath springs to mind) the curve radius was increased, mainly to improve stepping distances. But at most of the problem stations the platform was adjusted but the curve was left unchanged.

 

[Efini92]

Isnt there a problem coupling different types of vehicle because of behaviour on curves. I think there were problems with a pacer 142-144 types being coupled to a 23m vehicle like a 158. I think that was covered by an outright ban, not by curve restrictions.

142 & 144 did couple to 158’s.

 

[61653 HTAFC]

Isnt there a problem coupling different types of vehicle because of behaviour on curves. I think there were problems with a pacer 142-144 types being coupled to a 23m vehicle like a 158. I think that was covered by an outright ban, not by curve restrictions.

It absolutely was not. There was no ban on 14x coupling to 23m 15x, it used to happen every day.

 

[ac6000cw]

Not sure this is correct. Practically all trains have wheelsets where the two wheels on the same axle rotate together but the different axles rotate independently. There are very few exceptions, such as Talgos and some trams that have independent wheels, and some locomotives where one motor drives several axles. On tight curves the wheel flanges will contact the side of the rail to help the wheelset to follow the track. This can indeed lead to flange climbing, but that depends on speed (and on various other factors relating to wheel/rail shape and friction). It doesn't limit the radius of curve a particular vehicle can travel round. It does limit how fast it can do so, but this limitation doesn't depend on the type of train.

I think mr_moo was probably talking about steerable-axle bogies versus normal ones e.g. some freight bogies and the class 66 bogies have self-steering axles.

 

[edwin_m]

I think mr_moo was probably talking about steerable-axle bogies versus normal ones e.g. some freight bogies and the class 66 bogies have self-steering axles.

Reading it again you're probably right. It wasn't very clear when I read it the first time.

 

[richieb1971]

I know Bedford used to have one of the sharpest curves at St Johns. I assume these days sharp curves are historic ones and no new ones were built post 1960.

 

[zwk500]

I know Bedford used to have one of the sharpest curves at St Johns. I assume these days sharp curves are historic ones and no new ones were built post 1960.

They're avoided where possible but sometimes it just isn't feasible.

 

[edwin_m]

If I recall correctly the Bacon Factory Curve in Ipswich, built 10 or so years ago, is around 200m radius and that was the largest that could be fitted into the site. Anything less than that requires a check rail.

 

[snowball]

There was a whole thread a few years ago on the tightest curve on the network, with dozens of people submitting their favourite examples but hardly any of them knowing what radius they were.

Most of the tightest ones seem to be in the ballpark of 200m.

 

[Statto]

Not sure the radius is, but Merseyrail on the Liverpool loop line has some of the tightest curves on the rail network, & that's inside a tunnel too.

 

[Taunton]

Not sure this is correct. Practically all trains have wheelsets where the two wheels on the same axle rotate together but the different axles rotate independently. There are very few exceptions, such as Talgos and some trams that have independent wheels, and some locomotives where one motor drives several axles.

Maybe true nowadays, but of course steam locomotives have all driven wheels connected (likewise 08 diesel shunters). I've often wondered how a 9F 2-10-0 for example manages on a normal crossover.

The Soviet Union built a 4-14-4 steam loco in the 1930s, but I believe it never made a trip without derailing. Here's a video of a (semi-articulated) 4-8-8-4 in the USA being worked round a tight curve, along with some very skilled slow driving for such a monster:

There seems to be a real Ker-Twang moment at 5:22 in the film.

 

[edwin_m]

Maybe true nowadays, but of course steam locomotives have all driven wheels connected (likewise 08 diesel shunters). I've often wondered how a 9F 2-10-0 for example manages on a normal crossover.

That just about falls within my mention of locomotives with a motor driving several axles...

Tangential (pun intended) to this topic, the 9Fs and WD 2-10-0s had flangeless middle drivers to assist with traversing switches and crossings. They are now banned from NR because newer designs of S&C aren't compatible. Apparently similar 2-10-0s are still allowed in Germany, despite the crossings there normally having raised check rails which I would expect to be problematic with a flangeless driver.

 

[ac6000cw]

Union Pacific had a fleet of 4-12-2 steam locos, with a design that allowed the 1st and 6th driving axles to move laterally by up to 2 inches on curves (instead of flange-less middle axles). See https://en.wikipedia.org/wiki/Union_Pacific_9000_Class

 

[mjmason1996]

I've often wondered how locos with 3 bogies (such as some of the Japanese electrics) negotiate curves.. I'd imagine the centre one is allowed some degree of lateral movement.

 

[ac6000cw]

I've often wondered how locos with 3 bogies (such as some of the Japanese electrics) negotiate curves.. I'd imagine the centre one is allowed some degree of lateral movement.

AFAIK, that's correct e.g. the Swiss Re620 have that arrangement. An articulated body is also sometimes used e.g. some Italian electric locos like the E.656 - https://en.wikipedia.org/wiki/FS_Class_E.656

 

[MarkyT]

Union Pacific had a fleet of 4-12-2 steam locos, with a design that allowed the 1st and 6th driving axles to move laterally by up to 2 inches on curves (instead of flange-less middle axles). See https://en.wikipedia.org/wiki/Union_Pacific_9000_Class

A Krauss-Helmholtz bogie mechanism is one design that achieves this:

Krauss-Helmholtz bogie - Wikipedia

en.wikipedia.org

A Krauss-Helmholtz bogie (Krauss-Helmholtz-Lenkgestell) is a mechanism used on steam locomotives and some electric locomotives to improve curve running.

GNR and LNER locos and possibly others in UK used a 'Cartazzi' arrangement for the unpowered rear trailling axle under the firebox and cab instead of a pony truck. The Cartazzi design does not pivot, but allows some sideways movement with a centering mechanism:

Cartazzi axle - Wikipedia

en.wikipedia.org

A Cartazzi axle is a design of leading or trailing wheel support used worldwide. The design was used extensively on the former LNER's Pacific steam locomotives and named after its inventor F.J. Cartazzi, formerly of the Great Northern Railway. It should not be confused with a pony truck as it does not pivot at all. The axle does, however, have sideways play built in to accommodate tight curves. Cartazzi's design causes the weight of the locomotive to exert a self-centring action on the trailing wheels.