Why are Metrolink trams so slow?

[craigybagel]

Phase 2 (Clifton / Toton) of NET was only ever designed for 70kmh.

The former 80kmh stretches between Wilkinson Street - Basford and up the Hucknall branch have all been reduced to 70kmh, and have been as such for well over a year now. Not much point in having 80kmh sections when most of your fleet is limited to 70.

Which does beg the question - why did they order 70kmh trams on an 80kmh system?

 

[Ianno87]

Which does beg the question - why did they order 70kmh trams on an 80kmh system?

Guess that there are so few, and short 80kph sections that it makes virtually no journey time or capacoty difference.

 

[edwin_m]

Most continental tramways have maximum speeds of 70km/h or less, especially if they use vehicles with relatively short body sections like the Citadis. The 80km/h Metrolink and Croydon vehicles are based on designs that are closer to being light metros. If the vehicle Alstom proposed to use is only designed for 70km/h then they aren't going to change it for what in Citadis terms is a small order (they've sold over 2000 of them) and to save only a few seconds.

 

[td97]

The 5 mph crawl into/out of Altrincham is an absolute joke. Can walk it faster than the tram down the platforms.
Same for at Queens Rd, and at Victoria heading out of town. Ridiculously slow speed limits over the points there.

 

[td97]

But they accelerate fast - was at Navigation Road where the NR and Metrolink platforms are side by side. The Pacer set off at exactly the same moment as the tram I was on (Altrincham-bound), and the tram absolutely left it for dead from a standstill!

 

[AndyB28]

Morning All! Just joined and my first post so seems appropriate to kick off with something close to home.
I use the MML system quite often so it was interesting to hear some of the comments - especially about the speeds achieved. My question is why the current vehicles suffer from horrendous juddering at speed? This is certainly my experience on the stretch I know best - Derker to Shaw. My best guess was that it was somehow due to design compromises to allow the vehicles to be both city-centre trams and suburban rail services? Be nice to get an expert opinion though.

 

[Mojo]

My best guess was that it was somehow due to design compromises to allow the vehicles to be both city-centre trams and suburban rail services?

Sort of. It depends how conical the wheels are. The greater the conicity the tighter the corners the vehicles can go round but on straight track at higher speeds you get ‘hunting’ which is when the vehicle sways from side to side.

 

[AndyB28]

Sort of. It depends how conical the wheels are. The greater the conicity the tighter the corners the vehicles can go round but on straight track at higher speeds you get ‘hunting’ which is when the vehicle sways from side to side.

Many thanks for the reply Mojo. I understand what you're saying but, as a passenger, it's hard to detect any 'swaying' as such (under the conditions described). The 'judder' is quite high frequency. I don't really know the rest of the network well so it would be interesting to know if anyone else has noticed anything similar on other sections.

 

[edwin_m]

Hunting is actually when the bogie goes along the track in a zigzag instead of a straight line, so it is much higher frequency than the body sway. Trains typically have dampers (shock absorbers) mounted longitudinally outside the bogie frame, attached via brackets to the bogie at one end and the body at the other. These make it more difficult for the bogie to rotate quickly and therefore rduce the tendency for hunting. But this isn't possible on a tram, where the bogies have to rotate a lot further to get round a 25m radius curve. A damper in this position wouldn't have enough travel, and possibly the extra resistance to bogie rotation might make them keep going straight when they should be turning.

 

[AndyB28]

Hunting is actually when the bogie goes along the track in a zigzag instead of a straight line, so it is much higher frequency than the body sway. Trains typically have dampers (shock absorbers) mounted longitudinally outside the bogie frame, attached via brackets to the bogie at one end and the body at the other. These make it more difficult for the bogie to rotate quickly and therefore rduce the tendency for hunting. But this isn't possible on a tram, where the bogies have to rotate a lot further to get round a 25m radius curve. A damper in this position wouldn't have enough travel, and possibly the extra resistance to bogie rotation might make them keep going straight when they should be turning.

Many thanks for the clarification. Certainly sounds like what's happening but, surely, that can't be doing the rails and vehicle itself much good!

 

[Greybeard33]

Many thanks for the clarification. Certainly sounds like what's happening but, surely, that can't be doing the rails and vehicle itself much good!

This is a long running issue. See the following detailed explanation from the Metrolink Ride Quality thread in May 2017. From my experience as a passenger, I do not think there has been much overall improvement since then.

Bombardier have been trying to improve the ride quality of the M5000s over the last three years. It has improved greatly to be fair, but is still an ongoing process.
They have wheel profile issues as mentioned further up thread. This initially manifested itself as a low frequency hunting motion, you could actually see the nose of a vehicle coming towards you deviating from the centre line by about 6 inches in each direction. A change in wheel profile, and a step change in the pace of the rail grinding programme largely eliminated this issue. Unfortunately the new wheel profile has resulted in a high frequency hunting motion (the vehicle feels like it is shaking and bouncing) in certain areas. This is currently being looked at, and has resulted in a temporary speed restriction between Dane Road and Old Trafford on the Altrincham line where it was worst.

Some vehicles (3024 and 3105 onwards) have a different Damper set up, this gives a much firmer ride largely eliminating most of the 'bouncyness' at speed at the expense of low speed ride comfort. It was originally intended to roll this new set up across the fleet as and when vehicles came up for overhaul (first ones will be due in a year or two) however there have been issues with the new set up and I'm not sure if this will still go ahead.

It is worth noting that when the K5000s were ordered for Köln, they were intended for 100kph running. It was quickly found that there were issues with high speed ride quality and they were demoted to lower speed routes and replaced with Refurbished Stadtbahn B sets which they were originally intended to replace. It should therefore be no surprise that the M5000 ride quality is.... lively.

As for the ride quality in Croydon being better, that could be down to the CR4000s being low floor (lower centre of gravity etc) or it could have something to do with their track being in better condition.

The T68s had air suspension, which largely eliminated many of the track defects, so it wasn't as important to keep the track in 100% tip top condition, the M5000s feel every single dip and kink in the track however.

So yes, it is an issue, and yes, it is being worked on. However it's a bit like unravelling a ball of string.

(For those that don't know, I'm a Metrolink Driver)

The susceptibility of individual trams to hunting depends on the condition of the wheels and suspension. At worst, the shaking can be bad enough to make it impossible to use a touchscreen. If a single unit, or the front unit of a double, is affected, the driver will usually slow down, but the driver may be unaware of conditions in the rear unit of a double.

From time to time, temporary speed restrictions have been introduced on sections of track that are particularly badly affected. These have eventually been lifted after rail grinding work.

 

[_toommm_]

Morning All! Just joined and my first post so seems appropriate to kick off with something close to home.
I use the MML system quite often so it was interesting to hear some of the comments - especially about the speeds achieved. My question is why the current vehicles suffer from horrendous juddering at speed? This is certainly my experience on the stretch I know best - Derker to Shaw. My best guess was that it was somehow due to design compromises to allow the vehicles to be both city-centre trams and suburban rail services? Be nice to get an expert opinion though.

The scientific term is 'hunters oscillation'. As the wheels try to find (or 'hunt') equilibrium in the middle of the track, it can't maintain that, especially at higher speeds, so it judders back and forth between the two rails. Dampers, well, dampen this, but it will be more noticeable on trams vs. trains as trams are lighter and slower, so have less effective dampers. Less wheels obviously means less dampers too, hence why Pacers are so unbearably uncomfortable.

 

[rebmcr]

Less wheels obviously means less dampers too, hence why Pacers are so unbearably uncomfortable.

No, Pacers are uncomfortable because there are no bogies whatsoever. There is nothing to damp.

 

[_toommm_]

No, Pacers are uncomfortable because there are no bogies whatsoever. There is nothing to damp.

I never said Pacers have dampers, I was staying that they're uncomfortable as they have less. 0 dampers is less than any other number of dampers, therefore my statement is still correct.

 

[Bletchleyite]

No, Pacers are uncomfortable because there are no bogies whatsoever. There is nothing to damp.

Pacers do have suspension and springs and dampers associated with it. Essentially the whole coach is one long bogie - so there is only primary suspension and not secondary. (Primary is wheel to bogie, secondary is bogie to body).

They indeed don't have rotational dampers because there's nothing to rotate, which will as you say affect the ride quality because the whole vehicle will hunt/judder in the way a bogie will on its own.

 

[507 001]

The scientific term is 'hunters oscillation'. As the wheels try to find (or 'hunt') equilibrium in the middle of the track, it can't maintain that, especially at higher speeds, so it judders back and forth between the two rails. Dampers, well, dampen this, but it will be more noticeable on trams vs. trains as trams are lighter and slower, so have less effective dampers. Less wheels obviously means less dampers too, hence why Pacers are so unbearably uncomfortable.

Fewer wheels does not necessarily equal less comfort.

Why do you think a lot of high speed stock is Articulated?

 

[_toommm_]

Fewer wheels does not necessarily equal less comfort.

Why do you think a lot of high speed stock is Articulated?

Fewer wheels in this case does though as the Pacers have no dampers. I'm not imposing a blanket statement saying less wheels directly equates to less comfort, but in this case, it does

 

[507 001]

Fewer wheels in this case does though as the Pacers have no dampers. I'm not imposing a blanket statement saying less wheels directly equates to less comfort, but in this case, it does

No, pacers ride badly due to the poor design of suspension. They do have Dampers. You can see them quite clearly in this picture;
https://simple.m.wikipedia.org/wiki/File:Roof_142022.jpg They are the vertical grey pieces.
Per wheel, the Pacers probably have more dampers than similar era conventional bogied sprinter stock, which IIRC only has a single one on each side per wheel set.

 

[_toommm_]

No, pacers ride badly due to the poor design of suspension. The number of dampers may be part of that but it isn’t the sole cause.

Again, I never said that was the sole, and primary cause.

 

[507 001]

Again, I never said that was the sole, and primary cause.

You pretty much did.

Please see my edited post.

 

[_toommm_]

You pretty much did.

Please see my edited post.

You've changed your mind now, going from me definitively saying it to pretty much. I didn't say it categorically though.

 

[507 001]

Good lord, it was a turn of phrase!

“Fewer wheels in this case does though as the Pacers have no dampers”

I’d say that was pretty definitive, wouldn’t you?

 

[_toommm_]

Good lord, it was a turn of phrase!

“Fewer wheels in this case does though as the Pacers have no dampers”

I’d say that was pretty definitive, wouldn’t you?

Good Lord!

But I didn't say it was the primary cause. To break down what I said, I said, to alternate the phrasing, that Pacers are uncomfortable because of the lack of dampers. That means that the lack of dampers is a cause of the discomfort. Nowhere did i mention that that was the primary cause, the ultimate cause etc.

But we're reading too far into it now, aren't we.

 

[507 001]

Good Lord!

But I didn't say it was the primary cause. To break down what I said, I said, to alternate the phrasing, that Pacers are uncomfortable because of the lack of dampers. That means that the lack of dampers is a cause of the discomfort. Nowhere did i mention that that was the primary cause, the ultimate cause etc.

But we're reading too far into it now, aren't we.

But as we’ve pointed out, They do have Dampers, so you were wrong anyway!

 

[edwin_m]

The dampers on Pacers are to do with vertical ride and nothing to do with hunting. As well as being off-topic.

 

[Jozhua]

I know this thread is quite old but speaking of Metrolink line speeds, does anyone have an idea what the speeds for the Trafford Park extension are going to look like?

I've had a look at the flyover for the route and it seems that there aren't too many curves, so hopefully it'll have a chance of being faster than a bus!

 

[WatcherZero]

Probably 30 mph for the most part, the stops aren't particularly far apart.