These are classic things to screw up when modifying locomotive models. A lot of third party locomotives have these broken to some extent. You don't need to screw them up, because it's not too difficult to get them right if you remember a few basic principles.
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You can forget tricks like quartering the wheels, or like properly angling the crankpins for multi-cylinder locomotives. It would be nice if we could do this, but unfortunately the game is simply not coded to handle that sort of geometry. It will only handle one basic layout, and that is what we are stuck with.
The layout we are stuck with is easy to remember and fairly easy to implement. The only tricky bits about it are setting up the piston and connecting rod .3dp files to do the graphics correctly, because this involves a bit of basic trigonometry. It just does, so don't grumble about it because that wont change anything.
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The first thing you need to know is that the points in the .3dp files where the connecting rod and coupling bar attach to the drivewheel must be vertically below the drivewheel's axle when you are looking at it from the side. In other words, the attachment points for connecting rod and coupling bar must have the same y axis value as the drivewheel axle, and must have a lower z axis value.
There are no exceptions to this rule. If you try to mess around with this geometry you will end up chasing your tail. If you use this positioning and things don't look right, then you need to work on your connecting rod and/or coupling bar gfx, because it will be those that are wrong.
The next thing you need to know is that regardless of how your graphics are set up, when looking at the locomotive from the side the piston will always travel on a straight line directly between the point defined in the piston file and the axle of the drivewheel. This means that your gfx for your cylinder and piston need to be set up so the central axis of the piston and cylinder bore are along this line. If they aren't, the piston will slice vertically across the cylinder to some extent during its stroke. You can see this happening on quite a few third party locomotives if you watch the piston in slow motion. How bad it will be depends on the difference in angle between this line and the axis of the cylinder and piston graphics.
Also, the initial position of the piston graphics needs to be such that it wll look right at top dead centre and bottom dead centre in the cylinder, bearing in mind that this initial positioning you are stuck with will usually put the piston roughly halfway through its stroke. I say usually and roughly because there can be exceptions.
The Beuth has cylinders that are steeply angled, but the drivewheel attachment point still has to be vertically below the drivewheel axle for things to work properly. This means the intial position of the Beuth's piston is not roughly halfway through its stroke, and the same will apply to any locomotive with similar cylinder positioning. To get it right you will need to use trigonometry.
Even if the cylinders are exactly in a horizontal line with the drivewheel axle, the piston will still not be exactly halfway through its stroke because of the angle of the connecting rod. It will actually be slightly more than halfway to bottom dead centre. Yup, trigonometry may be required if things are tight.
Again, there are no exceptions to this rule. If it looks wrong, your graphics are wrong. Work on those.
The third thing you need to know is that the point defined in the connecting rod file determines where the connecting rod attaches to the drivewheel. As already mentioned, this must have the same y axis value as the drivewheel axle, and must have a lower z axis value than the drivewheel axle.
The graphics for the connecting rod have to be set up so that it runs between this point and the point defined in the piston file. If they aren't, it will look wrong. Again, don't try and mess around with this geometry or you will end up chasing your tail. Fix the connecting rod graphics, because they will be the problem.
The last thing you need to know is that the coupling bar must attach to the drivewheel at the same y axis and z axis values as the connecting rod. The x axis value is usually offset (should be*) so that the coupling bar will be inside the connecting rod, but don't try and mess with the y and z values. Yes, you guessed it, you will end up chasing your tail again. If it looks wrong, fix the coupling bar graphics, because those will be the problem.
![Mr. Green :mrgreen:](./images/smilies/icon_mrgreen.gif)
Don't worry about anything else the coupling bar might do. It always stays parallel to the track. As long as its attachment point to the drivewheel is set properly to start with it will be fine.
So there you go. RT3 drivetrain basics 101.
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*ETA: Just found out something else. Actually the x value for the coupling bar and connecting rod shouldn't be offset. It turns out the game engine is a bit trickier than that.
What I've discovered since writing this post is that the defining points for connecting rod attachment to the drivewheel, and coupling bar attachment to the drivewheel, and the offset of the rotation centre for all wheels (drivewheel, coupled, bogies whatever) should all have the same X axis value. This sorts the drivetrain geometry perfectly. To get the connecting rod and coupling bar visually outside the wheels, the points for their graphics are offset relative to the attachment points.
If the attachment point of the coupling bar is set further out than the rotation centre of the drivewheel this will throw things out of whack. No matter how much you try to offset the coupling bar it will still look like it's stuffed through the drivewheels from some angles, even though the points that define its graphics are well outside the wheels. As soon as you put the attachment point at the same X axis value as the rotation centre of the drivewheel, suddenly the coupling bar graphics pop outside the wheels to the spot where you want them.
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