Question about chassis construction of leading/trailing axles

[Amoreternum]

Hello everyone and a happy new year!

At the moment, I am planning and building a new locomotive and as usual, starting with the frame and wheelbase. I came across several designs on the internet and came across different ways to construct leading or trailing axles, one solution with regular turntables or a pivoting point, with another one being the one that can be seen in this one: click here! Credits go to the builder michaelgale on pinterest, of course. What are the benefits of using the second technique? I can think of force distribution on pushing and pulling cars or to look more realistic, but I'd like to get your opinion or even better, facts on that.

Thanks ahead!

[zephyr1934]

The obvious problem is that the standard Lego curves are so unrealistically tight. A car or engine with two bogies is fine, the trucks just turn a little bit more, but a steam locomotive has problems, the wheels have to follow the track but the boiler is rigid and has to stay in a straight line. There are many ways to allow this to happen. If you use hinges or turntables, you typically have to articulate with an arm of some sort (e.g., a 2x plate or a technic beam), which then requires rotation on both ends of the arm. The problem with using an arm like this for the pilot truck is that it has a large range of motion, but a steam engine has cylinders that have to occupy the same space. I believe the solution from your link limits the range that the pilot wheels conflict with, making more room for the cylinders.

[Amoreternum]

Making room for the cylinders makes perfect sense, I did not think of that. Won't be a problem with my prototype but I can see that leading to a lot of issues on steam locomotives. Thanks alot for your in depth answer!

[zephyr1934]

Good to hear and no problem. There's one more challenge with a steam locomotive with pilot and trailing trucks- uneven track. A steam engine typically has three points of contract with the rails. If you plan to run over a bumpy surface (e.g., transitioning from carpet to hard floor, or running on uneven tables) that adds more challenges. When going over a dip, the middle wheels can be lifted off the track or going over a hill the front or rear wheels can leave the track. If the flanges leave the track it can cause a derailment. However, if you are only running on a pretty flat surface you should be fine. Always best to build the mechanicals first and test to make sure you do not have any unanticipated problems before you build the rest of your locomotive. For all of these reasons, it is easiest to build an 0-6-0 as ones' first steam MOC, but as long as you are up for a bit of trial and error there's no reason one has to start with a simple engine like that.

[Amoreternum]

Actually it's not a steam engine I'm building but an electric one, but all of your points would apply as well. Wheels slipping off the rails due to uneven foundation is another point I'd have to think about but I like to test everything after basically every change that I make. Layout is supposed to be flat but I'd love to have a bridge somewhere, would have to use spirals up and down and there I might have trouble with derailing pilot or trailing trucks. Thanks for your input! 

[Phil-B259]

  On 1/2/2022 at 4:04 PM, zephyr1934 said:

The obvious problem is that the standard Lego curves are so unrealistically tight. A car or engine with two bogies is fine, the trucks just turn a little bit more, but a steam locomotive has problems, the wheels have to follow the track but the boiler is rigid and has to stay in a straight line. There are many ways to allow this to happen. If you use hinges or turntables, you typically have to articulate with an arm of some sort (e.g., a 2x plate or a technic beam), which then requires rotation on both ends of the arm. The problem with using an arm like this for the pilot truck is that it has a large range of motion, but a steam engine has cylinders that have to occupy the same space. I believe the solution from your link limits the range that the pilot wheels conflict with, making more room for the cylinders.

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Please remember there is a big difference as to where those cylinders can be put!

Many smaller locomotives designed in the UK / Europe up until the first world war used inside cylinders!

As the name suggests these were mounted inside the locos frames and not stuck on the outside where they could get in the way of the leading bogie. Some of the many examples from the UK include the flowing:-

https://sremg.org.uk/steam/b4-b4x.shtml
https://sremg.org.uk/steam/t9class.shtml
https://sremg.org.uk/steam/lclass.shtml
https://en.wikipedia.org/wiki/LMS_Class_2P_4-4-0
https://en.wikipedia.org/wiki/GER_Classes_S46,_D56_and_H88
https://en.wikipedia.org/wiki/GCR_Class_11F
https://en.wikipedia.org/wiki/LNWR_Whale_Precursor_Class
https://en.wikipedia.org/wiki/Caledonian_Railway_72_Class

Granted the curvy slashers might be an issue to replicate in Lego but if you are no slavishly following a specific prototype then I'm sure something pleasing could be created

In Lego terms this greatly simplifies the loco building as you don't need to worry about the front bogie hitting the cylinder.

Though if you want a change there were also a few inside cylindered 4-6-0s..... ( e.g. https://en.wikipedia.org/wiki/GER_Class_S69 )

 

 

Edited January 3, 2022 by Phil-B259

[zephyr1934]

  On 1/3/2022 at 5:39 PM, Phil-B259 said:

Please remember there is a big difference as to where those cylinders can be put!

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Most definitely, I was just offering one of the reasons why someone would not want to let a leading truck swing too far.

 

  On 1/3/2022 at 5:30 PM, Amoreternum said:

Layout is supposed to be flat but I'd love to have a bridge somewhere, would have to use spirals up and down and there I might have trouble with derailing pilot or trailing trucks.

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Layouts are always in flux, if it is a distant idea, don't worry too much about it now. When the spirals eventually come, you can design them around this engine or redesign this engine to meet the needs of the specific spirals. If the spirals are more than 6 mo off you will likely be a better builder and have a better selection of available parts to tackle that hill (grin, pun intended). Otherwise, if the spiral is concurrent with the locomotive, develop them together so that they are harmonious from the start.

You learn a lot by building tricky locomotives and necessity really is the mother of invention. Hence my suggestion to not worry much about future spirals. It was not until my third steam engine that I learned to add vertical clearance, but I was running on very uneven tables at my club's shows. Members would stand on the tables to assemble tall skyscrapers, so the surface warped. The tables held 18 baseplartes (3x6) so my trains were hitting peaks and valleys all the time. So I went overboard in response,

(the important bit you do not see is that there is room for both trucks to move up to handle a sag, and that the trucks should include something like the 1x4 double curved slope to prevent from catching when moving left-right)