What Are Wheel Flats?

[Doomotron]

Self-explanatory. I've heard about them but have no clue what they actually are.

 

[aleggatta]

Self-explanatory. I've heard about them but have no clue what they actually are.

A train brakes and skids for a fraction of a second, resulting in metal being rubbed away from the wheel and creating literally a flat spot

 

[hexagon789]

Self-explanatory. I've heard about them but have no clue what they actually are.

When a train brakes and the level of grip (adhesion) is too low to maintain the rate of deceleration then the wheels will tend to lock-up either ceasing rotating fully or turning at a slower rate than the train is moving, effectively the train starts sliding.

When this happens the wheels, being softer than the rails will tend to wear away at the contact point with the rail causing flat-spots and additionally the contact point will heat up through friction, causing the metal to alter in propery and spall-out (I forget the precise details of what this means), which I believe can cause cracks in the wheel surface.

Flat wheels require the turning on a lathe to make them smooth and properly round again, though this can only be done a number of times before the tyres must be replaced.

The likelihood of wheels locking-up or sliding can be mitigated by provision of Wheel Slide Protection (WSP), this is a system which will partially release and re-apply the brakes to seek to allow the wheels to rotate while maximising the possible deceleration rate that can be sustained with the available adhesion. Some WSP systems are better than others.

 

[Cowley]

A train brakes and skids for a fraction of a second, resulting in metal being rubbed away from the wheel and creating literally a flat spot

For the OP: You can quite often hear it - a thump thump thump sound from the wheels.
Particularly noticeable nearer the end of a vehicle, or anywhere on a Pacer in autumn...

Edit - And what Hexagon says.

 

[hexagon789]

For the OP: You can quite often hear it - a thump thump thump sound from the wheels.
Particularly noticeable nearer the end of a vehicle, or anywhere on a Pacer in autumn...

Edit - And what Hexagon says.

Our 156s are pretty bad for it in leaf-season, though never so bad I could 'feel' the flats. Though the sound is rather annoying!

 

[aleggatta]

When this happens the wheels, being softer than the rails will tend to wear away at the contact point with the rail causing flat-spots and additionally the contact point will heat up through friction, causing the metal to alter in propery and spall-out (I forget the precise details of what this means), which I believe can cause cracks in the wheel surface.

the friction of the skid causes metal fatigue, hardening the metal, then the repeated impacts cause cavities to form as the hardened metal breaks up and falls out.

 

[Cowley]

Our 156s are pretty bad for it in leaf-season, though never so bad I could 'feel' the flats. Though the sound is rather annoying!

With the 143s around here you can hear the wheelflats when they go past sometimes.

 

[hexagon789]

the friction of the skid causes metal fatigue, hardening the metal, then the repeated impacts cause cavities to form as the hardened metal breaks up and falls out.

Precisely the explanation I was looking for, thank you .

 

[hexagon789]

With the 143s around here you can hear the wheelflats when they go past sometimes.

Among many other sounds I would imagine!

I must say I've never noticed the noise of wheelflats other than at close quarters or on the unit itself; though I can hear trains on my two local lines from my house quite clearly most of the time.

 

[Taunton]

Not just a recent problem. E L Ahrons, a pioneer railway book author a century ago https://en.wikipedia.org/wiki/Ernest_L._Ahrons had a brilliantly dry sense of humour. He wrote that the Great Northern local 4-wheeled stock used out of Kings Cross was fitted with octagonal wheels ...

 

[Cowley]

I've heard some good wheelflats on various Mk1s on preserved railways over past few years.
It must be difficult and expensive for these lines to rectify such problems I assume?

 

[hexagon789]

I've heard some good wheelflats on various Mk1s on preserved railways over past few years.
It must be difficult and expensive for these lines to rectify such problems I assume?

I wouldn't have thought they would've needed much heavy braking running at 25mph or less, though I suppose with a single-pipe vacuum brake it's quite a bit more difficult to modulate the application quickly to try to prevent sliding as compared with modern EP-controlled air brakes.

I would guess it must be quite expensive to fully replace tyres and possibly some lines don't have their own lathes I would guess?

 

[anamyd]

I've heard some good wheelflats on various Mk1s on preserved railways over past few years.
It must be difficult and expensive for these lines to rectify such problems I assume?

i think the lathes are really expensive. TfW's at Cardiff Canton is over 30 years old, assuming they haven't replaced it yet

 

[big all]

as an aside
because off the curved rail head and conical shape off a wheel it often happens that smaller flats will only be audible when that part off the wheel makes contact with the rail
also worth noting with friction brake blocks whilst they would clean the wheel and rail tread they can often grip a fraction harder where the flats are and cause the flat to catch again on the rail head if due diligence and skill was not used

 

[TheEdge]

Among many other sounds I would imagine!

I must say I've never noticed the noise of wheelflats other than at close quarters or on the unit itself; though I can hear trains on my two local lines from my house quite clearly most of the time.

I remember as a kid standing near the line at Cullompton hearing huge bangs as GW HSTs went past. Clearly no one was that bothered in the late 90s early 00s about getting stuff reprofiled!

 

[big all]

I remember as a kid standing near the line at Cullompton hearing huge bangs as GW HSTs went past. Clearly no one was that bothered in the late 90s early 00s about getting stuff reprofiled!

the problem is
you get a a bad friction situation this will often not cause damage to vehicle 3/4 or even 5 because the level off contamination is so extreme that damage will only happen where the first few vehicles will be well lubricated by bedris on the rail head and clean as they skid till cleaning causes a coach mid train to be badly effected
this requires train crew to report as the driver will be less aware off the problem as more remote to the noises

 

[edwin_m]

Network Rail has detectors fitted around the network (Wheelchex and more recently Gotcha) which report the uneven track forces associated with wheel flats (among other problems) and hopefully allow the train in question to be stopped if they are approaching dangerous levels. However it does still happen, for example:
https://www.gov.uk/raib-reports/rep...rack-damage-between-ferryside-and-llangennech

The investigation found that one of the wheelsets on the damaged wagon had locked up and slid, causing severe wheel flats, before starting to rotate again. Impacts from the wheel flats subsequently damaged the rails as well as equipment mounted on the bogie, some of which partially detached and was dragged under the train, causing damage to track-mounted equipment. The wheelset had locked up and then started rotating again because an object became caught between one of the wheels and the adjacent brake block holder.

 

[furnessvale]

When a train brakes and the level of grip (adhesion) is too low to maintain the rate of deceleration then the wheels will tend to lock-up either ceasing rotating fully or turning at a slower rate than the train is moving, effectively the train starts sliding.

When this happens the wheels, being softer than the rails will tend to wear away at the contact point with the rail causing flat-spots and additionally the contact point will heat up through friction, causing the metal to alter in propery and spall-out (I forget the precise details of what this means), which I believe can cause cracks in the wheel surface.

I doubt that wheel steel is any softer than rail steel (but I stand to be corrected).

The flat is caused by a single point on the wheel being in contact with a moving length of rail therefore the wheel takes the damage. Conversely, with a rail burn, a moving wheel is in contact with a single point of the rail and it is the rail that takes the damage.

 

[The Lad]

Wheelflats will also occur if handbrakes are left hard on.
New set of 8 tyres about £20k, turning £3k?

 

[cjp]

So in the old videos one sometimes sees of a steam loco with all the driving wheels whizzing around as it tries to get going it will be the track rather than the driving wheels which take a beating?

 

[hexagon789]

I doubt that wheel steel is any softer than rail steel (but I stand to be corrected).

The flat is caused by a single point on the wheel being in contact with a moving length of rail therefore the wheel takes the damage. Conversely, with a rail burn, a moving wheel is in contact with a single point of the rail and it is the rail that takes the damage.

I may be wrong, but I'm sure I heard in a training video that the wheels wore in preference to the rail.

 

[furnessvale]

So in the old videos one sometimes sees of a steam loco with all the driving wheels whizzing around as it tries to get going it will be the track rather than the driving wheels which take a beating?

Basically, yes, although it doesn't do rails or wheels any good.

 

[DarloRich]

Self-explanatory. I've heard about them but have no clue what they actually are.

you will have heard them before you heard of them

I've heard some good wheelflats on various Mk1s on preserved railways over past few years.
It must be difficult and expensive for these lines to rectify such problems I assume?

i think the lathes are really expensive. TfW's at Cardiff Canton is over 30 years old, assuming they haven't replaced it yet

Wheel lathes are REALLY expensive bits of kit which is why they don't get replaced very often.

I doubt that wheel steel is any softer than rail steel (but I stand to be corrected).

I understood there was a difference in hardness with the wheel being the softer metal. Might all be wibble mind!

So in the old videos one sometimes sees of a steam loco with all the driving wheels whizzing around as it tries to get going it will be the track rather than the driving wheels which take a beating?

no: Both will take a hammering and i doubt it does the locomotive motion much good!

 

[aleggatta]

So in the old videos one sometimes sees of a steam loco with all the driving wheels whizzing around as it tries to get going it will be the track rather than the driving wheels which take a beating?

Generally, in this case the rail would take the brunt of the damage, as the wheel slips the friction will cause heat, the wheel will have time to cool down, where as the rail will be in constant contact with something causing a build up of heat in that part of the rail only. Think of it as if you were running a rope through your hands, the rope will not get damaged from the heat, but your hands will as they are the stationary part? I realise we are comparing steel and rope/skin but the same principal applies.

 

[43096]

I remember as a kid standing near the line at Cullompton hearing huge bangs as GW HSTs went past. Clearly no one was that bothered in the late 90s early 00s about getting stuff reprofiled!

Great Western have always been reactive in their approach to wheelflats and maintenance in general. The likes of East Coast, MML and Grand Central all fitted new Westinghouse/Knorr-Bremse wheelslide protection systems to their coaches and power cars (power cars got them with the upgraded Brush electronics package, as well as improved Wheelslip control systems). LNER and EMT (and GC when they had HSTs) also operate a policy of doing preventative tyre turning which skims the wheels to remove tread damage before bigger problems develop and a much bigger chunk of wheel needs to be removed on the lathe. As a result passengers get a better ride - it is rare to find a wheelflat on an EMT HST even during the leaf fall season, for example - and the operator can get extended wheelset life. As a result those operators have extended time between power car bogie overhauls out to at least 588,000 miles and in some cases 630,000 miles. Meanwhile GWR - who have never done the power car upgrades and have been haphazard with the trailers - are still at 480,000 miles between power car bogie changes and routinely run vehicles with severe flats.

 

[Dougal2345]

I remember reading in a railway magazine, a traveller writing that he 'did his duty and reported the flat to the guard'. Is this what one should do?

 

[Dougal2345]

So in the old videos one sometimes sees of a steam loco with all the driving wheels whizzing around as it tries to get going it will be the track rather than the driving wheels which take a beating?

Take a look at this video, about 5:20 in, for some track damage:

 

[Cowley]

Great Western have always been reactive in their approach to wheelflats and maintenance in general. The likes of East Coast, MML and Grand Central all fitted new Westinghouse/Knorr-Bremse wheelslide protection systems to their coaches and power cars (power cars got them with the upgraded Brush electronics package, as well as improved Wheelslip control systems). LNER and EMT (and GC when they had HSTs) also operate a policy of doing preventative tyre turning which skims the wheels to remove tread damage before bigger problems develop and a much bigger chunk of wheel needs to be removed on the lathe. As a result passengers get a better ride - it is rare to find a wheelflat on an EMT HST even during the leaf fall season, for example - and the operator can get extended wheelset life. As a result those operators have extended time between power car bogie overhauls out to at least 588,000 miles and in some cases 630,000 miles. Meanwhile GWR - who have never done the power car upgrades and have been haphazard with the trailers - are still at 480,000 miles between power car bogie changes and routinely run vehicles with severe flats.

Very interesting 43096.

 

[The Lad]

That is quite bad. If you look at the track in stations on a gradient you will often see a mark, perhaps the size of a 2p coin at the same point on both rails caused by slips. Sometimes repeated at bogie wheelbase centres.
Slides are not so obvious on the rails

 

[DarloRich]

I remember reading in a railway magazine, a traveller writing that he 'did his duty and reported the flat to the guard'. Is this what one should do?

I have twittered the TOC to tell them about bad wheel flats. I have told the guard when i got off about flats in the front carriage of a unit. Cant hurt to speak up.