Collision Boxes & Multi-part locomotives

[savv_nz]

Hello All,

I have run into an issue. I am currently modelling a New Zealand Railways Class 'Eo' electric locomotive, these locomotives were Bo+Bo with the bogies directly coupled to one another with the body sitting atop them. As this was to reduce forces on the locomotive frames (they were used only on the 1:33 gradient Otira Tunnel) the couplers were attached to the bogie.

So far so good. But this means that the couplers move with the bogie so I have split the bogies into two separate dummy locomotives with the body section its own item.

In order to make it all work, the three parts need to come together, I have defined the couplings and collision boxes as per this image:

Untitled-1.jpg

Parts 1 & 3 are mirror images of each other their blueprint settings as per:

Part1Main.jpg

This causes me no problems, the two bogies can be placed together in the sim and work fine.

Part 2, the middle body section has the following blueprint settings:

Part2Main.jpg

Placed by itself, this portion also has no problems. Placed at the center of the bogies the sim makes all the appropriate coupling noises and the locomotive sits "intact" on the track.

The problem appears when I try to drive the loco, or couple anything else to it outside of the editor (drive up a different loco to couple on for example). This causes the whole lot to launch itself into the air and derail.
I have done some experimenting, and if I extend Part 2's collision box definition to be the entirety of the body portion, so that the bogies sit ahead and behind the locomotive (so the body floating in the air) there are no issues - though if the bogie is even slightly beneath the body the launching into the air re-occurs), and so I come down to the problem being the collision boxes are somehow interfering with each other.

I'd appreciate any help than can be thrown my way on this matter,

[dick8299]

Its been a long time since I have modeled a locomotive or wagon, but I have an idea for you to check.

Take a look at your collision height entries and maybe reduce those (to zero?).

[savv_nz]

The mystery deepens.

I can reduce the collision centre Y and collision height to no less than 0.5, below that and the wheel sets when placed individually begin flying all around once more.

I have set up a test series of locomotives using the 3 parts:

1. Part 2 (the body) works fine by itself
2. Part 1 and Part 3 work fine when placed by themselves
3. Part 1 and Part 3 work fine when coupled directly together
4. Part 1 and Part 2 work fine when coupled directly together
5. Part 2 and Part 3 work fine when coupled directly together

* All these combinations do have a bit of "wobble" when clicked on, twisting side to side around the pivot point.

The problem combination is when Part 1, Part 2 and Part 3 are all coupled together in the proper set-up. This causes the bogie parts (1 and 2) to leap into the air or spin madly around causing a derailment due to consist tilt.
It doesn't matter which way around Part 2 is orientated in relation to the other parts (ie it can be reversed compared to the bogies) and the problem persists.

[savv_nz]

Further to the above, if trialled with just the one bogie this is what happens:

20171015234508_1.jpg

This is Part 1 and Part 2 coupled together (also happens when its part 2 and part 3). Nothing I do seems to right the bogie so that it sits level (including moving the end pivot points, wheel positions as defined in the bogie blueprint)
I believe this happens when both the bogie parts are under the loco causing the thing to go haywire.

At first I thought it was possibly a mis-match in the defined coupler heights, but they all match up at 0.762 so I'm at a loss.

[Bananarama]

The issue I see is that the bounding box between the two bogies can't touch. Since those bogies never leave the loco, there's no reason you can't imply that the bogies touch - visually instead of physically. Try making the collision gap between the bogies a meter or two and things should smooth out.

[jpetersjr]

Wow, that engine looks very similar to some of the ones also used here in the US. Would love to later see something like the Virginian or the New Haven boxcab for RW.

Virginian.jpg

[savv_nz]

collision gap between the bogies a meter or two and things should smooth out.

Thanks Marc, thats got her sitting on her "shoes" like she should be.

20171016145540_1.jpg

I now, however, have an altogether different problem.
When coupled to wagons as a single locomotive "unit" there is no issues, the locomotive pulls everything along just as expected.

When coupled to another of the locomotive "units" it attempts to the bogie (part1/part3) out from under the body portion, this then stretches out for a bit, before the body portion and the trailing bogie catch up - it doesn't move the second "unit" all together smoothly, it kind of moves caterpillar (or slinky?) style until it suddenly snaps together (or stretches too far apart) launching everything into the air and giving the 'train derailed due to consist tilt' error.

It almost looks to me like coupler slack causing what is termed here in New Zealand as 'run in' when the faster moving tail end of a train catches up to the slower moving front end.
Below is my coupler blueprint For going from part 1 to part 2 and part 2 to part 3) I am wondering if a value other than 0 is required in the bottom two fields, though when I set both to 10,000 earlier it made no difference.

Untitled-1.jpg

I am using the same coupler blueprint as I do for the wagons on the ends that connect to them, so there should be no issue there though I note that the gap between two different loco "units" also stretches out further than it should.

Could this be to do with the shortened collision box?

[Bananarama]

It shouldn't, as the box was only shortened in the middle of the locomotive, no?

[savv_nz]

The middle part box couldn't be shortened much more than it was, so I had to shorten the two bogie piece collision boxes, the below image details how it now all fits together - blue areas are collision boxes, green & pink areas are the limits as defined by the front and rear X pivot locations. I've also marked the coupler receiving points, the pivot points for the couplers are inside the areas defined by the front/rear X pivot box (which is how I've always done it before with no issues).

Untitled-1.jpg

I am wondering if I maybe need to reduce Part 2's Front/Rear Pivot Point so that it doesnt touch the ends of Part 1/3? Still unsure as to how this produces a slinky like effect...

[Bananarama]

Why do you have a middle collision box?

[savv_nz]

Thought it needed one, have now set it to zero - however this hasn't stopped the stretching out between couplers. When coupled to another loco "unit" the thing still moves slinky fashion until the point of derailment.

[savv_nz]

An update should anyone else encounter this problem in the future, the solution to the slinky effect was to increase the bogie mass up to 5 ton from 0.5. Problems solved... who knows why.