hunting oscillation and wheel/rail profiles

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Jozhua

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Hi guys,

Got some questions about hunting oscillation.

My understanding is that it is affected by carriage lengths and the profile of wheels/rails? Steeper wheel profiles enable tighter curves, at the expense of hunting at a lower speed?

Does this relate to light rail vehicles, giving them a pretty hard limit on top speed?

Thanks for the help!
 
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Mcr Warrior

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Certainly historically has been an issue with Metrolink trams (both M5000s and before that the original T68s) on certain "faster" sections of track, in particular that on the Altrincham line between Dane Road and Stretford.
 

Dr Hoo

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A lot of light rail vehicles with low floors throughout (not including Manchester, obviously) have 'independent' wheels with no through axles and this can be part of completely different dynamics.
 

Taunton

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Certainly an issue on the DLR, they may have eventually changed the profile. The full speed S-curve descent from Royal Albert to Prince Regent, coming down with the power off (which always seems to make hunting worse) was very noticeable in the leading vehicle.

The Edinburgh vehicles, low floor with independent wheels, seem incapable of handling curves at anything more than about 10mph. A particular tedious nuisance on the long final stretch to the airport, which for some reason was designed with a series of right-angle curves as it negotiates the potato fields.
 

edwin_m

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One critical issue is damping of bogie rotation. The dampers (shock absorbers) you see on the side of many bogies are bracketed to the bogie at one end and the body at the other, so will tend to reduce the rapid rotations that take place during hunting as the wheels oscillate. The flipside is that they also resist the slower rotations needed to go round curves, which tends to result in more flange contact. Tram bogies have to rotate a very long way on minimum radius curves, so damping down hunting is particularly challenging.
 

eoff

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The Edinburgh vehicles, low floor with independent wheels, seem incapable of handling curves at anything more than about 10mph. A particular tedious nuisance on the long final stretch to the airport, which for some reason was designed with a series of right-angle curves as it negotiates the potato fields.
I never cease to be surprised by this at the times when I'm in no hurry and take the tram to the airport rather than the bus.

One critical issue is damping of bogie rotation. The dampers (shock absorbers) you see on the side of many bogies are bracketed to the bogie at one end and the body at the other, so will tend to reduce the rapid rotations that take place during hunting as the wheels oscillate. The flipside is that they also resist the slower rotations needed to go round curves, which tends to result in more flange contact. Tram bogies have to rotate a very long way on minimum radius curves, so damping down hunting is particularly challenging.
Do any carriages/wagons exist that have independent wheels? I'm assuming that driving wheels will be connected.
 
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Jozhua

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Certainly historically has been an issue with Metrolink trams (both M5000s and before that the original T68s) on certain "faster" sections of track, in particular that on the Altrincham line between Dane Road and Stretford.
Oh trust me, I know! Going up towards Oldham can get pretty lively too, although to be honest it's nothing too insane I haven't experienced on other systems. I guess it does make the 50mph limit a pretty hard one for the system.
Certainly an issue on the DLR, they may have eventually changed the profile. The full speed S-curve descent from Royal Albert to Prince Regent, coming down with the power off (which always seems to make hunting worse) was very noticeable in the leading vehicle.

The Edinburgh vehicles, low floor with independent wheels, seem incapable of handling curves at anything more than about 10mph. A particular tedious nuisance on the long final stretch to the airport, which for some reason was designed with a series of right-angle curves as it negotiates the potato fields.
Incapable of handling curves at anything more than about 10mph sounds like the Manchester Metrolink as well lol. Although some of the curves have just incredibly small radi.

Basically, I wanted to know about this because I'm writing a blog post about why they shouldn't replace existing heavy rail with Metrolink in Manchester. Especially to places like Wigan and Glossop, which aren't exactly inner suburbs. I feel like light rail has it's limits and can't really be expected to cope with incredibly tight curves and also running at higher speeds (50mph+).
 

edwin_m

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Do any carriages/wagons exist that have independent wheels? I'm assuming that driving wheels will be connected.
None in the UK as far as I'm aware. The Talgo articulated sets used in Spain and elsewhere have independent wheels, which are kept parallel by linkages to the vehicle bodies.

One idea for trams is to have a separate motor for each wheel, and a control system that will turn the outer wheel faster on a curve. This ought to reduce noise and wheel wear when the curve is too tight for the conicity to take up the difference in radius between the two rails, which currently causes noise and wear. They may be able to do something about hunting too. But I don't think it's got beyond the odd prototype.
 

billio

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The trams on the Bury line had such atrocious hunting I wondered how the drivers could stand it, It was particularly bad on the descent from Whitefield to Radcliffe making me think the tram was about to leave the track. A lengthy journey on Metrolink is a significantly lower quality journey than a heavy rail journey. Something to be endured rather than enjoyed.
 

millemille

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It has been my experience, as the fleet engineer for class 365 back in the 2000's when they suffered from terrible hunting/rough ride problems between KX and Hitchin (particularly through tunnels) that the the factors that initiate and perpetuate hunting are primary suspension lateral stiffness -and to a lesser extent bogie rotational and yaw stiffness, wheel/rail interface relative conicity at the point of contact on straight track and the position of the contact point relative to the flange root. Rail spacing/gauge can also have an impact on propensity for hunting/

Low conicity between wheel and rail reduces the self centring characteristics of the wheel relative to the rail, low primary suspension (and to a lesser extent secondary suspension) lateral stiffness reduces the self centring characteristics of wheelset within the bogie frame and the bogie frame within the vehicle. So a lateral force exerted on the wheelset displaces it laterally more than is ideal across the rail head. As soon as that displacement is sufficient to bring the root of the flange - and the abrupt increase in wheel rolling diameter - into contact with the rail head the lateral movement is abruptly arrested, but there is now a significant amount of potential energy stored in the system because the wheel running on the flange root is raised higher than the other wheel. That potential energy is released and the wheelset displaces laterally in the opposite direction, back towards its starting position. But the lack of stiffness in the suspension and the lack of conicity does little or nothing to resist the lateral movement and the wheelset displaces laterally past the starting point and ends up with the opposite wheel's flange root running on the rail head. And the whole cycle repeats and that's hunting.

On the 365's it was found that a lot of track between KX and Hitchin had been replaced post Hatfield and had been laid tight to gauge to allow for significant profile grinding - to address gauge corner cracking - without the need to replace the rail (laid at 1428 or less). A combination of this and the 365's low primary suspension stiffness needed to allow it to have wide ranging route clearance and the P8 wheel profiles causes the hunting.

The hunting was addressed by reprofiling the wheels to RD9 profile - P8 with a slight increase in conicity in the contact area (AFAICR) but more importantly a much narrow flange - and a systematic program of rail grinding to widen the gauge of the track. In combination this prevented the flange root from coming into contact with the rail head and broke the closed loop described above so hunting did not perpetuate. it took about 5 years for all of the track - in the exception of the sections in the tunnels to be ground - and once that was done the profile reverted to P8 (narrower flanges shorten wheel life) and the only place that hunting occurred after this was in the tunnels and as soon as the train came out of the tunnel the hunting stopped.
 

507 001

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The hunting issue with M5000s originally had a low frequency, then they changed the wheel profile to try and combat it and it changed to a high frequency which *mostly* affected heavily worn rails.

You can still see where the problems with the original low frequency hunt were most prolific as there is severe wear to the gauge face of the rail in 2-3 metre long patches.

I was driving an Altrincham set a few days ago through one of the problem areas for the high speed hunt (Dane Rd-Stratford, still on ex-BR rail) and noticed that it was pretty smooth riding. I realised I hadn’t had one that hunts for quite a long time, so it would appear that the problem has been fixed. Theres been a long standing 40mph TSR through that section, hopefully it can be lifted...
 

supervc-10

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It certainly seems better in that stretch recently- went down to Altrincham before Christmas, and it was pretty smooth, but the first time I took the tram down that way I was genuinely worried!

I wonder if adaptive dampers might be worthwhile for tram stock? Ones that can be firmed up on straighter sections of track, and loosened on the tighter, slower, inner city areas. They're a common enough feature in cars, often using valves to open up secondary sections of damper, so the technology isn't particularly complex. Continuously adaptive ones use magnetic fluid, but that seems beyond the need of a tram!
 

Jozhua

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The trams on the Bury line had such atrocious hunting I wondered how the drivers could stand it, It was particularly bad on the descent from Whitefield to Radcliffe making me think the tram was about to leave the track. A lengthy journey on Metrolink is a significantly lower quality journey than a heavy rail journey. Something to be endured rather than enjoyed.
I do love Metrolink, but I'm inclined to agree that it's not appropriate for longer distances than what it's currently deployed for.
It has been my experience, as the fleet engineer for class 365 back in the 2000's when they suffered from terrible hunting/rough ride problems between KX and Hitchin (particularly through tunnels) that the the factors that initiate and perpetuate hunting are primary suspension lateral stiffness -and to a lesser extent bogie rotational and yaw stiffness, wheel/rail interface relative conicity at the point of contact on straight track and the position of the contact point relative to the flange root. Rail spacing/gauge can also have an impact on propensity for hunting/

Low conicity between wheel and rail reduces the self centring characteristics of the wheel relative to the rail, low primary suspension (and to a lesser extent secondary suspension) lateral stiffness reduces the self centring characteristics of wheelset within the bogie frame and the bogie frame within the vehicle. So a lateral force exerted on the wheelset displaces it laterally more than is ideal across the rail head. As soon as that displacement is sufficient to bring the root of the flange - and the abrupt increase in wheel rolling diameter - into contact with the rail head the lateral movement is abruptly arrested, but there is now a significant amount of potential energy stored in the system because the wheel running on the flange root is raised higher than the other wheel. That potential energy is released and the wheelset displaces laterally in the opposite direction, back towards its starting position. But the lack of stiffness in the suspension and the lack of conicity does little or nothing to resist the lateral movement and the wheelset displaces laterally past the starting point and ends up with the opposite wheel's flange root running on the rail head. And the whole cycle repeats and that's hunting.

On the 365's it was found that a lot of track between KX and Hitchin had been replaced post Hatfield and had been laid tight to gauge to allow for significant profile grinding - to address gauge corner cracking - without the need to replace the rail (laid at 1428 or less). A combination of this and the 365's low primary suspension stiffness needed to allow it to have wide ranging route clearance and the P8 wheel profiles causes the hunting.

The hunting was addressed by reprofiling the wheels to RD9 profile - P8 with a slight increase in conicity in the contact area (AFAICR) but more importantly a much narrow flange - and a systematic program of rail grinding to widen the gauge of the track. In combination this prevented the flange root from coming into contact with the rail head and broke the closed loop described above so hunting did not perpetuate. it took about 5 years for all of the track - in the exception of the sections in the tunnels to be ground - and once that was done the profile reverted to P8 (narrower flanges shorten wheel life) and the only place that hunting occurred after this was in the tunnels and as soon as the train came out of the tunnel the hunting stopped.
That's a really interesting read, thanks!

You forget about some of the nuances of railway engineering, how you're kind of balancing the trains so they centre themselves in the rail. Flanges are generally a "last resort" to stop derailing, except for perhaps some tighter corners and the like.
The hunting issue with M5000s originally had a low frequency, then they changed the wheel profile to try and combat it and it changed to a high frequency which *mostly* affected heavily worn rails.

You can still see where the problems with the original low frequency hunt were most prolific as there is severe wear to the gauge face of the rail in 2-3 metre long patches.

I was driving an Altrincham set a few days ago through one of the problem areas for the high speed hunt (Dane Rd-Stratford, still on ex-BR rail) and noticed that it was pretty smooth riding. I realised I hadn’t had one that hunts for quite a long time, so it would appear that the problem has been fixed. Theres been a long standing 40mph TSR through that section, hopefully it can be lifted...
That's interesting!

As a Metrolink driver, would you say that you think the system is suitable for converting heavy rail too on longer distance trips?

Personally, I think the system would be better expanding onto new alignments (Trafford Park, Airport Line) to cover more areas within a few miles of the city. I think that having to balance both tight curve requirements and a need to go higher than 50mph running (or sustained 50mph running) might be pushing it a bit far!
 

hwl

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Hi guys,

Got some questions about hunting oscillation.

My understanding is that it is affected by carriage lengths and the profile of wheels/rails? Steeper wheel profiles enable tighter curves, at the expense of hunting at a lower speed?

Does this relate to light rail vehicles, giving them a pretty hard limit on top speed?

Thanks for the help!
It worth looking at the history of BR Research's rail - wheel interface programme that heavily focused on hunting "VAMPIRE" which was both the name of the special test coach and later became the name of the software developed that is used globally for looking at this (Currently on V7.1).

Also worth a look at this PhD thesis:
or this masters one:

Huddersfield do quite a bit in this area so there will be a few more useful theses particularly on wear /dimensional impacts.
 

507 001

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I do love Metrolink, but I'm inclined to agree that it's not appropriate for longer distances than what it's currently deployed for.
That's a really interesting read, thanks!

You forget about some of the nuances of railway engineering, how you're kind of balancing the trains so they centre themselves in the rail. Flanges are generally a "last resort" to stop derailing, except for perhaps some tighter corners and the like.

That's interesting!

As a Metrolink driver, would you say that you think the system is suitable for converting heavy rail too on longer distance trips?

Personally, I think the system would be better expanding onto new alignments (Trafford Park, Airport Line) to cover more areas within a few miles of the city. I think that having to balance both tight curve requirements and a need to go higher than 50mph running (or sustained 50mph running) might be pushing it a bit far!

I think that's a discussion for a different thread but briefly, No, I wouldn't agree with you on that. I think Metrolink has traditionally been strongest when it has been able to maintain journey times (or even improve on them) but increase usability and accessibility. The Perfect Metrolink line for me is the Oldham Rochdale Line, good access to the Town and City Centres and good journey times.

It's very telling that the segregated ex-heavy rail lines are far more successful in terms of ridership than the entirely new lines, its all down to journey times.

Now don't get me wrong, there are limits. I think Wigan is pushing it and Chester is definitely inappropriate. Tram Trains will help with the speed issue.
 

Jozhua

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I think that's a discussion for a different thread but briefly, No, I wouldn't agree with you on that. I think Metrolink has traditionally been strongest when it has been able to maintain journey times (or even improve on them) but increase usability and accessibility. The Perfect Metrolink line for me is the Oldham Rochdale Line, good access to the Town and City Centres and good journey times.

It's very telling that the segregated ex-heavy rail lines are far more successful in terms of ridership than the entirely new lines, its all down to journey times.

Now don't get me wrong, there are limits. I think Wigan is pushing it and Chester is definitely inappropriate. Tram Trains will help with the speed issue.
Yeah, I'll look at starting a new thread on it, thanks for the insight!
 

edwin_m

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Was one cure for Hunting the use of slightly tighter sleepers?
With a gauge of 1432mm rather than 1435mm.
I believe that actually made it worse - Modern Railways said at the time that the Selby Diversion was laid to 1432 and nullified the benefits of a new wheel profile.

Too much conicity can also cause hunting, because the wheel may overshoot the optimum running position and carry on until the opposite flange makes contact.
 

Wilts Wanderer

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One of the things that has most surprised me about the Class 80x fleets is the severity of the hunting that some of the vehicles seem to suffer. Strangely it seems to affect both powered outside-bogied vehicles as well as the lightweight trailer cars with inside frame bogies. Either way, it’s not a pleasant experience at 125mph and tends to cause me to contemplate whether we’re about to visit the local farmland (to quote Ian Walmsley.)

Edit - previously the Wessex Trains 143 fleet were afflicted by severe hunting, around the time the fleet maintenance was transferring from Canton to St Philips Marsh and there were issues with tyre turning etc. I recall a run through Sapperton Tunnel that dislodged luggage from the overhead racks and the flanging was excruciatingly loud. In that case the driver would drop off the power and it quickly stopped, so presumably a bit of coasting reduced speed below the critical resonance point, but would immediately recommence once power was reapplied and speed regained.
 

Jozhua

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One of the things that has most surprised me about the Class 80x fleets is the severity of the hunting that some of the vehicles seem to suffer. Strangely it seems to affect both powered outside-bogied vehicles as well as the lightweight trailer cars with inside frame bogies. Either way, it’s not a pleasant experience at 125mph and tends to cause me to contemplate whether we’re about to visit the local farmland (to quote Ian Walmsley.)

Edit - previously the Wessex Trains 143 fleet were afflicted by severe hunting, around the time the fleet maintenance was transferring from Canton to St Philips Marsh and there were issues with tyre turning etc. I recall a run through Sapperton Tunnel that dislodged luggage from the overhead racks and the flanging was excruciatingly loud. In that case the driver would drop off the power and it quickly stopped, so presumably a bit of coasting reduced speed below the critical resonance point, but would immediately recommence once power was reapplied and speed regained.
That's not great on a brand new train! Pacers don't suprise me tho lol.

I haven't experienced oscillation to any noticeable degree on class 801's and 802's I've ridden on.

My guess would be maintainence crews are still getting used to them & how best to optimise performance. Especially if they've had a few months in service for wheels to wear down and the like.
 

TRAX

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Certainly an issue on the DLR, they may have eventually changed the profile. The full speed S-curve descent from Royal Albert to Prince Regent, coming down with the power off (which always seems to make hunting worse) was very noticeable in the leading vehicle.

The Edinburgh vehicles, low floor with independent wheels, seem incapable of handling curves at anything more than about 10mph. A particular tedious nuisance on the long final stretch to the airport, which for some reason was designed with a series of right-angle curves as it negotiates the potato fields.

What makes the trams incapable of handling curves quicker is the fact that the bogies don’t rotate in relation to the body under which they are mounted.
 

edwin_m

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What makes the trams incapable of handling curves quicker is the fact that the bogies don’t rotate in relation to the body under which they are mounted.
However the body sections are much shorter and rotate with the bogies.

Do you have any evidence that this type of tram is intrinsically less able to handle tight curves? The Nottingham ones use the same concept and operate down to 18m radius.

Or are you saying that the transition curves in Edinburgh are insufficient for this type of vehicle? The body section must rotate into the curve over the time taken for the bogie wheelbase to pass over the change in curvature, unlike more traditional trams where it's the distance between bogie centres, so if transitions are too short the lateral acceleration may be uncomfortable.
 

TRAX

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However the body sections are much shorter and rotate with the bogies.

Do you have any evidence that this type of tram is intrinsically less able to handle tight curves? The Nottingham ones use the same concept and operate down to 18m radius.

Or are you saying that the transition curves in Edinburgh are insufficient for this type of vehicle? The body section must rotate into the curve over the time taken for the bogie wheelbase to pass over the change in curvature, unlike more traditional trams where it's the distance between bogie centres, so if transitions are too short the lateral acceleration may be uncomfortable.

Trams with "fake" bogies have to take curves slowly to avoid imposing too much force on the body, because, as you said, the body takes the lateral accelerations that a normal bogie would take on a train with pivoting bogies.
The movements can be quite abrupt and this can be quite noticeable on Citadis trams.
 

AndrewE

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For hunting you should have been on the NE SW trains in the mid-1970s. The crews took full advantage of the speed that the diesels brought, especially between Bristol and Brum, but in the Mk 1 coaches it got quite hairy at times. It took me quite a while to get used to the idea that the train wasn't going to jump off the track! I suspect some nervous passengers would have been put off and gone by bus instead...
 

edwin_m

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Trams with "fake" bogies have to take curves slowly to avoid imposing too much force on the body, because, as you said, the body takes the lateral accelerations that a normal bogie would take on a train with pivoting bogies.
The movements can be quite abrupt and this can be quite noticeable on Citadis trams.
This should not be so if they have managed to include proper transition curves, as the lateral change in acceleration due to change in curvature will be spread over the length of the transition not the length of the wheelbase. The absolute value of acceleration on the curve itself, where the effective radius is constant, will not depend on the bogie arrangement.
 

TRAX

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This should not be so if they have managed to include proper transition curves, as the lateral change in acceleration due to change in curvature will be spread over the length of the transition not the length of the wheelbase. The absolute value of acceleration on the curve itself, where the effective radius is constant, will not depend on the bogie arrangement.

Yet it still is an issue in most low-floor tram systems.
Škoda managed to make a 100 % low-floor design with pivoting bogies on their ForCity design - the bogies are under the cab and between the cars.
 

Jozhua

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For hunting you should have been on the NE SW trains in the mid-1970s. The crews took full advantage of the speed that the diesels brought, especially between Bristol and Brum, but in the Mk 1 coaches it got quite hairy at times. It took me quite a while to get used to the idea that the train wasn't going to jump off the track! I suspect some nervous passengers would have been put off and gone by bus instead...
Sounds kind of exciting!
 

AndrewE

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It always amazes me how trains cope with terrible track, or sometimes even stay on the rails... this video
doesn't show hunting as such, but still is a testament to how resilient trains are...
I think the drivers were showing off for the cameras. How much were they tipped?
A
 

Jozhua

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It always amazes me how trains cope with terrible track, or sometimes even stay on the rails... this video
doesn't show hunting as such, but still is a testament to how resilient trains are...
I think the drivers were showing off for the cameras. How much were they tipped?
A
Amazing, are those working locos, or is it a heritage style thing?

There have been occasions of locos driving down paved roads in emergency situations - one diesel electric was used in I think the US or Canada during a blackout to supply emergency power.

There are crossovers that literally involve vehicles ploughing straight over uncut rails on flanges - again I think this is a US thing - when one line is used a lot more than the other.

I think the key issue with hunting is that it impacts passenger comfort and also efficiency - too much and the amount of energy lost basically limits your top speed.
 
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