[MOC] Dublin Luas - Alstom Citadis tram

[dw1812]

On 10/5/2023 at 12:41 PM, Hod Carrier said:

The main connection and pivot is at floor level and is actually quite a simple arrangement. It does have a certain amount of "float" to deal with pitch and roll as well as yaw so that the suspended sections can move relative to the wheeled sections. 

Just scrolling about, don't mind me...

Ah, this part isn't strictly true. Most trams, like Combinos and 30m Citadises, actually feature a two-part articulation model (40m models use 3-part articulation) to ensure rigidity and stability throughout the tram. Only one joint (two in 40m models) in the vehicle is capable of both pitch and yaw change, with the rest only being capable of yaw. Not sure if the 502s in Dublin are capable of pitching across three joints...

 

Edited December 31, 2023 by dw1812

[Duq]

On 11/15/2023 at 7:49 AM, Man with a hat said:

Oh, wow. That's really well done. Again clever work.

Thank you!

8 hours ago, dw1812 said:

Just scrolling about, don't mind me...

Ah, this part isn't strictly true. Most trams, like Combinos and 30m Citadises, actually feature a two-part articulation model (40m models use 3-part articulation) to ensure rigidity and stability throughout the tram. Only one joint (two in 40m models) in the vehicle is capable of both pitch and yaw change, with the rest only being capable of yaw. Not sure if the 502s in Dublin are capable of pitching across three joints...

 

With some hump-back bridges and dips like this one the Dublin trams must be capable of handling pitch in all their joints:

 

[dw1812]

1 hour ago, Duq said:

With some hump-back bridges and dips like this one the Dublin trams must be capable of handling pitch in all their joints[...]

Again, not true. Take a look at this Siemens Combino cresting quite a sharp bridge:

There has to be an element of rigidity, so only every other joint* is capable of bending up and down.

*Segments are usually in pairs, however there will almost always be a group of three segments that are only able to bend left and right. In a 7-segment tram, this will be in the middle.

Edited December 31, 2023 by dw1812
Removed unnecessary image

[UltraViolet]

1 hour ago, dw1812 said:

Again, not true. Take a look at this Siemens Combino cresting quite a sharp bridge:

There has to be an element of rigidity, so only every other joint* is capable of bending up and down.

*Segments are usually in pairs, however there will almost always be a group of three segments that are only able to bend left and right. In a 7-segment tram, this will be in the middle.

  It is a delicious irony that the cited example, the 1st-gen Siemens Combino, infamously tore its articulation frames and body structure apart:

https://en.wikipedia.org/wiki/Siemens_Combino

"On March 12, 2004, Siemens admitted to problems concerning the stability of the car bodies and, as a precautionary measure, instructed all public transportation services to take all Combinos with a service distance of more than 120,000 kilometres (74,565 mi) out of service. Torsion forces generated in S-curves were much higher than anticipated, leading to cracks around the articulations between the car modules. Subsequently, hairline cracks were found in the joints of the aluminium bodies, which could cause the roof to collapse in the case of an accident."

  Of course I say this in good-natured jest, as I'm not disputing your technical assessment - I just find it funny that the illustrative example chosen had such horrible design flaws.   I've had the opportunity to visit Amsterdam and to ride these cars myself.  The elastic oscillating whiplash they exhibit when navigating turns at speed has to been experienced to be believed.  Much of this is was a result of the stiff rubber-cone-bushing mounting system for the bogies, though this dynamic behavior is a common issue to many trams of this segmented design.

  Multi-segmented articulation is quite the interesting and complex engineering topic.  On the Bombardier Flexity 5-segment vehicle I have been building in LEGO, I had to face significant challenges in making it both flexible and rigid enough to function.  I decided it was easier to rigidly mount the motor bogies than to have movement in both the bogie mounts and the articulations.  This required full flexibility at every articulation joint while still maintaining balance and level body segment alignment.  Such is the nature of the game when 'approximating' complex engineered systems in coarse, simplified LEGO at reasonable scale.  My next project is thankfully a two-segment, single articulation light rail model - so much easier!  So far, all my headaches on that build have been limited to pure 'cosmetics'.

Edited December 31, 2023 by UltraViolet

[Duq]

I was looking for a photo of the Luas going over Mayor Street Bridge but couldn't find one... I'll take a closer look next time and try to take a picture.

I guess it depends on wheel arrangements. You can have section with and one without wheels connected with a pitch only hinge, as long as the bogie allows for yaw.

My next tram project (later next year) will be 5 sections so I'll have to figure it all out at that point ;-)

[dw1812]

23 minutes ago, Duq said:

My next tram project (later next year) will be 5 sections so I'll have to figure it all out at that point ;-)

This photo should help! And yeah, so long as each floating section is connected to at least one bogied section and vice versa, the tram SHOULD be rigid. Here's a good example of a Citadis 402 (similar to the Luas vehicles):

Note how the tram is split into a 2-3-2 formation.

Here's how Citadis X02 vehicles are segmented, just as a reference:

20m vehicles:

◢█-▒-█◣ (3 segments, no pitch joints)

30m vehicles:

◢█-▒•█-▒-█◣ (5 segments, one pitch joint)

40m vehicles:

◢█-▒•█-▒-█•▒-█◣ (7 segments, two pitch joints)

50m vehicles:

◢█-▒•█-▒-█•▒-█•▒-█◣ (9 segments, three pitch joints)

 

[UltraViolet]

Great illustration! 

  I'm definitely not a fan of this format of tram, as there are many compromises that have to be made just for the sake of achieving 100% low-floor,.  I do, however, have a great appreciation for the engineering that goes into making it work, and it's all very interesting.