Rail & wheel design

[edwin_m]

I've found that it's actually quite easy to make a tram lose it's feet (M5000), especially on street running sections. I spent an entire evening last week fighting a tram all the way to Ashton and back (twice) every time I went for a bit more than the first notch of power the thing was spinning its wheels.
Braking can also be quite interesting at this time of year.
We're always slipping and sliding around.

That's interesting as the M5000 is a close cousin of the Croydon design. Presumably there weren't any faults such as empty sandboxes?

 

[HSTEd]

Makes me wonder why they got rid of tread brakes in the first place? Even with disc brakes you can still have tread brakes quite happily working alongside them! Near enough all modern Japanese EMU's use both on the same bogie and I think I spotted both on the new bogie for the Desiro City.

The Japanese demand ridiculously short stopping distances from all their trains for reasons of increasing capacity and protection against earthquakes.

IIRC they demand 750m stopping distance from all narrow gauge EMUs and they had a project to develop a 360kph train that could stop in only 4000m, which requires something like 0.25g decelleration. (It had controlled skid friction brakes and air brakes).

Tread brakes tend to cause more wheelflats and have inferior performance to disc brakes, which is why they have largely been eliminated.

 

[Chz]

That's interesting as the M5000 is a close cousin of the Croydon design. Presumably there weren't any faults such as empty sandboxes?

I'd thought that tram adhesion on acceleration was still problematic (even with sand), but track brakes make stopping a doddle. The downside is extreme track wear at regular stopping areas, but when you're sharing the street with traffic it beats ploughing into people/cars.

 

[DJL]

The shortest EMUs are 3 cars (such as the 313).

Sorry to pick up a minor point but really?

466 is 2 car, as is 456 and I'm sure many more.

 

[jopsuk]

But then does a 2car EMU (DM+DM) already deliver power to every wheel?

Class 456 have a single powered bogie on just one of the two cars
Class 466 are power car and trailer car- not sure if both bogies of the power car are motored.

In fact, looking through the many southern region 2-car classes, it appears they've all bean Driving Motor-Driving Trailer formations.

Then (not a two-car) as far as I understand it, some of the powered bogies on class 378 have a powered axle and and an unpowered axle.

 

[181]

Readers of this thread may be interested in this brief account of an attempt to improve adhesion by using unusual wheels. (The rest of the site at http://www.douglas-self.com/MUSEUM/museum.htm is also worth a look). I suppose a rack railway is an extreme case of using non-smooth wheels.

As well as (presumably) having all axles powered, the Snaefell trams mentioned earlier in the thread have the advantage that there aren't many trees on Snaefell and so not many leaves. What's more puzzling is how the Bernina line in Switzerland copes with gradients of about 1 in 14 with quite a small percentage of powered axles (in the train shown here only the two vehicles at the front are powered). Much of it runs through forest too, but it's mostly coniferous.

 

[DJL]

Perhaps they don't get much of the light rain that causes so many problems.

Today's rain was heavier and caused no issues that I could see.

 

[pablo]

The light rain or drizzle is a product of a warm sector, a triangle between the warm front and the following cold front. It is tropical maritime and probably Caribean in origin, cooling and becoming saturated as it comes north-eastwards. Whenever a low pressure passes to the north of Switz, they get it too. They also have frequent fog or low cloud in the mountainous half, which dumps a light precipitation. So.....

 

[bronzeonion]

The Japanese demand ridiculously short stopping distances from all their trains for reasons of increasing capacity and protection against earthquakes.

IIRC they demand 750m stopping distance from all narrow gauge EMUs and they had a project to develop a 360kph train that could stop in only 4000m, which requires something like 0.25g decelleration. (It had controlled skid friction brakes and air brakes).

Tread brakes tend to cause more wheelflats and have inferior performance to disc brakes, which is why they have largely been eliminated.

Don't see why you couldn't have that here though to be honest, it wouldnt be a bad thing to have better braking performance from our EMUs, particularly on routes such as the London Overground lines or the South West Trains stopping commuter services.

I don't see how tread brakes cause more flats than disc brakes, they'd cause less surely because they scrub the crap off the wheel treads whilst the disc brake do not.

 

[HSTEd]

Scrubbing the wheels with tread brakes damages them.

 

[bronzeonion]

Scrubbing the wheels with tread brakes damages them.

The benefits of having the crap scrubbed off the treads and having better braking performance at 'Metro' speeds definitely outweighs the negligible 'extra' wear on the tyres that they always used to get!

Theoretically the amount of tyre worn away from tread brakes would be less than the amount taken off by the lathe when getting rid of flats caused by disc brakes.

 

[HSTEd]

The benefits of having the crap scrubbed off the treads and having better braking performance at 'Metro' speeds definitely outweighs the negligible 'extra' wear on the tyres that they always used to get!

Theoretically the amount of tyre worn away from tread brakes would be less than the amount taken off by the lathe when getting rid of flats caused by disc brakes.

The wear would not be problematic if it was even and smooth, but it's not. It damages the tread and causes ride issues. The brakes can also potentially overheat the tyre with other problems becoming evident as a result.

There is a reason tread brakes are dissapearing outside minor niches.

 

[bronzeonion]

The wear would not be problematic if it was even and smooth, but it's not. It damages the tread and causes ride issues. The brakes can also potentially overheat the tyre with other problems becoming evident as a result.

There is a reason tread brakes are dissapearing outside minor niches.

Fair enough about the overheating issue.

I don't understand how is it not smooth though?

 

[JGR]

Given that tread brakes have inferior performance to disc brakes, but have the advantage of having some level of scrubbing effect, would it be possible to add some sort of tread-contact device for the purpose of removing mulch, rather than braking per se?

Also, eddy-current brakes would seem to be a convenient way to get around adhesion issues (for braking at least), though whether devices like track-circuits would approve of this is probably an issue.

 

[HSTEd]

Fair enough about the overheating issue.

I don't understand how is it not smooth though?

The brakes take a short period to apply compared to the rotation of the wheel and will tend to vibrate on and off, potentially setting up 'waves' in the tyre surface.

 

[edwin_m]

Tread brakes produce heat at the wheel tread and are limited in performance otherwise the wheels get too hot, so can't be used for high rates of deceleration or from high maximum speeds. Disc brakes produce heat in the discs, which are larger and easier to cool. For high performance brakes the discs are on the axle but separate from the wheels with a space between them to allow air circulation.

Eddy current brakes are used in Germany but people are nervous of them because they produce their heat in the rail, and if consecutive trains are all braking at the same place the rail could get too hot. I should imagine it is also difficult to make them failsafe.

Modern units use electric braking where possible (but retain friction brakes which are specified to stop the train on their own in an emergency). Even if the energy can't be returned to the supply, getting rid of it as heat in resistor grids is easier than having it produced in the brake gear (that's why you often get heat haze off the top of Voyagers). There are also no wearing components so less maintenance is needed.

 

[sevenhills]

Hmm. Again I am not claiming to be an expert but my understanding of why car types have a tread, and why F1 change to treaded types in wet weather is bevause it allows the water to move put of the way resulting in more of the (remaining) tyre to actually contact the road rather than the water.

Has someone has said, the contact area is very small on solid wheel to solid track contact. On cycle tyres because the area of contact is very small, tread is generally not needed on solid roads.

 

[DJL]

Has someone has said, the contact area is very small on solid wheel to solid track contact. On cycle tyres because the area of contact is very small, tread is generally not needed on solid roads.

My bike (and expensive carbon fibre Boardman had narrow tires and a very shallow tread.

I can tell you that it is quite scary in the wet.
Even in slightly damp conditions I reduce my speed as I do not want to slide in the event of having to stop in a hurry!

My older bike with its fatter tyres and and deeper tread was not so scary in this regard.
Although it was obviously harder work - hence why I changed it.

So is it fair to say the wider wheels are better for grip but worse for speed and likewise a deeper tread is better for grip in the wet but may be prone to other issues in the dry.

 

[edwin_m]

As mentioned, the contact area between the wheel and the rail is only the size of a 5p piece - it is not infinitely small because the steel deforms elastically to a very small extent. A contact area of this size wouldn't be noticeably better at squeezing out water if there was a tread in the metal surface. And adding a tread to the wheel would reduce the size of the contact area further, probably forcing it into plastic (non-reversable) deformation unless the axle load was reduced.

Because rubber is much more flexible the contact area on a rubber tyre is much larger even at the lesser axleload of a road vehicle. So there is more need to provide somewhere (the grooves in a tread) for the water to go to. Incidentally this also means that the rubber tyre flexes much more than the steel wheel during rotation, which is the main reason rail vehicles roll more easily and use less power than road vehicles.

 

[DJL]

As mentioned, the contact area between the wheel and the rail is only the size of a 5p piece - it is not infinitely small because the steel deforms elastically to a very small extent. A contact area of this size wouldn't be noticeably better at squeezing out water if there was a tread in the metal surface. And adding a tread to the wheel would reduce the size of the contact area further, probably forcing it into plastic (non-reversable) deformation unless the axle load was reduced.

Because rubber is much more flexible the contact area on a rubber tyre is much larger even at the lesser axleload of a road vehicle. So there is more need to provide somewhere (the grooves in a tread) for the water to go to. Incidentally this also means that the rubber tyre flexes much more than the steel wheel during rotation, which is the main reason rail vehicles roll more easily and use less power than road vehicles.

That's interesting - so you are effectively saying that the reduced contact area of the solid train wheel actually reduces the effect of water under the wheel (as compared to the water not being there). If true that would presumably mean that increasing this contact area might actually make things worse!


I had some other ideas this morning. They are probably stupid for other reasons but here goes anyway.

What about a different shaped rail where the wheel kind of fits inside the rail?
i.e. it is sheltered a bit from the rain and other contaminants.

This would probably require smaller wheels which might have undesirable consequences.
I'm also not sure what effect this would have on the way points work - probably they wouldn't!

As a silly side note - is there anywhere in the world where the maximum line speed is set because a train going too fast could take off when it reaches a sharp hill crest?
The flange on wheels is not huge but I guess it would take a lot to lift a heavy train even 0.5 inch off the rails!

 

[edwin_m]

That's interesting - so you are effectively saying that the reduced contact area of the solid train wheel actually reduces the effect of water under the wheel (as compared to the water not being there). If true that would presumably mean that increasing this contact area might actually make things worse!


I had some other ideas this morning. They are probably stupid for other reasons but here goes anyway.

What about a different shaped rail where the wheel kind of fits inside the rail?
i.e. it is sheltered a bit from the rain and other contaminants.

This would probably require smaller wheels which might have undesirable consequences.
I'm also not sure what effect this would have on the way points work - probably they wouldn't!

As a silly side note - is there anywhere in the world where the maximum line speed is set because a train going too fast could take off when it reaches a sharp hill crest?
The flange on wheels is not huge but I guess it would take a lot to lift a heavy train even 0.5 inch off the rails!

I'm not sure how you'd shelter a rail from the weather without having something that encloses the wheel, resulting in some sort of monorail and as you say making points much more difficult. Incidentally the rubber tyre wheels used on Paris Metro are pretty slippy when wet, so best for underground routes.

The angle of the flange and the rail when they touch is critical to preventing derailment on curves and points - if it is too shallow or worn in other ways it can result in a "flange climbing derailment".

"Wheel unloading" is another cause of derailments, where due to some combination of twisted track and uneven weight on the wheels, one wheel carries much less than its share of the weight and can jump off the rail. It's pretty unlikely an entire train would do this. Although there are limits on vertical curves these are mainly to do with preventing passengers getting that lift-going-down feeling when going over a summit.

 

[Bald Rick]

As a silly side note - is there anywhere in the world where the maximum line speed is set because a train going too fast could take off when it reaches a sharp hill crest?
The flange on wheels is not huge but I guess it would take a lot to lift a heavy train even 0.5 inch off the rails!

Plenty, although the limits are set for passenger comfort. There's a good example at Tipton.

 

[snowball]

There's a big difference between the reduction of vertical force needed to cause slight passenger discomfort, and the reduction of vertical force needed to risk taking off.

By my calculation a 100mph train would need a vertical radius of 203.7m (668 ft) or less to take off. (Assuming g=9.81 m/s/s)

 

[jopsuk]

As far as water goes, there's two issues:

Lubrication, where a thin film of water remains between the tyre and road. This can be mitigated by larger surface area (thus in the wet a big bike tyre with tread has more grip than a skinny road slick)

Aqua planing, where the speed is so high that larger amounts of water cannot get out the way of the tyre, causing it to skip across the service like a well thrown stone. This is more of a problem the wider the tyre is, but can be mitigated using clever tread patterns, grooves etc. As far as I know a road bike's skinny tyre would aquaplane at somewhere around 150-200mph. An F1 slick meanwhile aquaplanes at much, much lower speeds! Due to the contact area being tiny and mass being huge, plus the rail head usually being above water level(!), trains are unlikely to aquaplane.

 

[DJL]

As far as water goes, there's two issues:

Lubrication, where a thin film of water remains between the tyre and road. This can be mitigated by larger surface area (thus in the wet a big bike tyre with tread has more grip than a skinny road slick)

Aqua planing, where the speed is so high that larger amounts of water cannot get out the way of the tyre, causing it to skip across the service like a well thrown stone. This is more of a problem the wider the tyre is, but can be mitigated using clever tread patterns, grooves etc. As far as I know a road bike's skinny tyre would aquaplane at somewhere around 150-200mph. An F1 slick meanwhile aquaplanes at much, much lower speeds! Due to the contact area being tiny and mass being huge, plus the rail head usually being above water level(!), trains are unlikely to aquaplane.

Thanks for that explanation - it all makes much more sense now!

 

[HSTEd]

According to tests performed by the FASTECH 360 programme in Japan, the tractive effort from the fifth vehicle rearwards is the same in dry and wet conditions. It appears that any water and ice on the rail has been boiled away or forced off the rails at that point.

This is one of the reasons that 400m train sets would have shorter stopping distances in poor weather conditions than 200m ones with modern technology.

 

[DJL]

According to tests performed by the FASTECH 360 programme in Japan, the tractive effort from the fifth vehicle rearwards is the same in dry and wet conditions. It appears that any water and ice on the rail has been boiled away or forced off the rails at that point.

This is one of the reasons that 400m train sets would have shorter stopping distances in poor weather conditions than 200m ones with modern technology.

Also interesting.

Does that mean that buying/finding some cheap unpowered carriages that don't carry any passengers or weigh very much and shoving them on the front of any train that is not already at it its maximum operational length could help in poor weather?

Obviously then you would simply swap the complaints for "why can't we use all these empty carriages?" instead.

 

[krus_aragon]

As far as water goes, there's two issues:

Lubrication, where a thin film of water remains between the tyre and road. This can be mitigated by larger surface area (thus in the wet a big bike tyre with tread has more grip than a skinny road slick)

Aqua planing, where the speed is so high that larger amounts of water cannot get out the way of the tyre, causing it to skip across the service like a well thrown stone. This is more of a problem the wider the tyre is, but can be mitigated using clever tread patterns, grooves etc. As far as I know a road bike's skinny tyre would aquaplane at somewhere around 150-200mph. An F1 slick meanwhile aquaplanes at much, much lower speeds! Due to the contact area being tiny and mass being huge, plus the rail head usually being above water level(!), trains are unlikely to aquaplane.

The clever tread patterns on car tyres are designed to shove the water sideways and out from underneath the tyres as they turn. Aquaplaning is such an issue for car/F1 tyres because they have a flat base where they contact the road. Bicycle and motorbike tyres have a "tubular" shape, with a curved cross-section. As the vehicles tilt from side to side in use, it's a sensible shape, and it acts like a knife through butter, shoving all the water to the sides as you ride, without needing any tread. It's like pushing a bowl down into a sink-full of dishwater, as opposed to pushing a plate: the bowl will sink through the water easier.


Bicycles are in greater danger from wet, slippery manhole covers, as opposed to semi-porous tarmac. If a car tyre loses traction on a manhole cover, that's only a quarter of it's contact interface with the road. For a (motor)bike, it's a half, and the other wheel invariably follows it onto the hazard. Off-road bikes that go traipsing through the mud do need a treaded tyre to get all that grimy muck out of the way, but or a road bike, there isn't really a need for a tread.

You can read the words of a more authoritative cyclist (Sheldon Brown) on the matter, here.

 

[edwin_m]

Bicycles are in greater danger from wet, slippery manhole covers, as opposed to semi-porous tarmac. If a car tyre loses traction on a manhole cover, that's only a quarter of it's contact interface with the road. For a (motor)bike, it's a half, and the other wheel invariably follows it onto the hazard.

Besides which if a car loses traction on one wheel it's still supported at each corner so it might slide a bit but won't tip over unless it happens to slide into a ditch. A (motor or push) bike losing grip, especially when turning, has the potential to slip sideways and topple.