[darwins]

Just some thoughts about 6 wheel vehicles.

How do we model them? How do we set up the physics?

Consider a 3 axle car, 9m in length with 6m wheelbase. In real life it could certainly negotiate curves of less than 100m radius, so I have made some illustrations based on a 60m radius curve.

Firstly the situation in real life, the ability to negotiate curves depends (mostly) on having some sideplay of the axles:

https://i.imgur.com/5hC8gqP.jpg

The rigid wheelbase doesn't quite work in MSTS or OpenRails as there is no sideways movement of wheels and axles.

https://i.imgur.com/ywssEt6.jpg

So the conventional MSTS and OR modelling solution was to create two false bogies. Each bogie is pivoted 1.5m from the centre axle. One bogie has two pairs of wheels, whilst the other bogie has only one pair of wheels. The visual effect is better:

https://i.imgur.com/XlitJ1x.jpg

Another option is now available in OR. We can have more than two bogies. What would happen if we used three bogies with the bogie pivot at the centre of each axle? Would the result be better or worse than the previous suggestion? Would it look something like this?

https://i.imgur.com/IGJW819.jpg

The model and the physics do not have to be the same. We might use any of the following options

	ORTSNumberBogies ( 0 )

	ORTSNumberBogies ( 1 )

	ORTSNumberBogies ( 2 )
	ORTSLengthBogieCentre ( 3m )

	ORTSNumberBogies ( 3 )
	ORTSLengthBogieCentre ( 3m )

	ORTSNumberBogies ( 3 )
	ORTSLengthBogieCentre ( 6m )

In all of these cases I am assuming that

	ORTSNumberAxles ( 4 )
	ORTSRigidWheelbase ( 6m )

is appropriate. What do others think? Is there a "correct" solution for rolling stock with a long rigid or semi-rigid wheelbase?

[jonas]

Thank you for your elaborately illustrated post darwins! I just have a question about your 2nd picture.

This is my 3-axle bogie with a centered pivot:


As a model builder, I previously assumed that a three-axle bogie with a centered pivot and a symmetrical center axis would also be rotated around this pivot in a curve.
In MSTS this also seems to be confirmed:


With the same bogie I get the following picture in OR:


A closer look at the fundamentally different rotation behavior in the two simulators is irritating. In OR, the pivot seems to be shifted between the 2nd and 3rd wheel axle.

In OR, this also seems to lead to this somewhat premature rotation when the wagon enters the curve, although the bogie is still on the straight track section:


Apparently in OR it is not (only?) the position of the bogie pivot that is decisive, but the position of the wagon pivot itself.

Unfortunately, I can't say anything more about your proposed solution with the false bogies for the time being.

Greetings
Jonas



EDIT 29. Nov. 2023: The same applies to the 3 axle bogies of the original MSTS Dash 9.

[ErickC]

Are your truck centers, number of axles per truck, and truck wheelbase set correctly in your physics files? I suspect this could cause issues.

It's been a hot minute since I built an SD-type locomotive, but I don't seem to recall having any issues with my Hilevel diner, and that has 6 axles. Mind, I don't run it on extreme curves, so maybe there's an effect I don't see.

[Traindude]

In the case of steam locomotives and cars/wagons with rigid-frame wheelsets (aka No trucks/bogies) I usually omit ORTSNumberBogies and ORTSLengthBogieCentre, as the current derail coefficient parameters assume that the vehicle meets two requirements:

1. The vehicle has exactly two trucks/bogies (no more, no less)
2. The pivot points for the trucks/bogies are exactly in the center of the truck/bogie frame. (This may not always be the case on account of the trucks' connections to the rigid main frame and/or devices such as swing-hangers or rocking devices that assist in guiding the vehicle around a curve.)

So, if your vehicle either is a steam locomotive, a rigid-frame short-wheelbase car/wagon/coach/etc, or a vehicle whose trucks have eccentric pivot points, then ORTSNumberBogies and ORTSLengthBogieCentre are useless.

[Laci1959]

Traindude, on 27 November 2023 - 07:52 PM, said:

In the case of steam locomotives and cars/wagons with rigid-frame wheelsets (aka No trucks/bogies) I usually omit ORTSNumberBogies and ORTSLengthBogieCentre, as the current derail coefficient parameters assume that the vehicle meets two requirements:

1. The vehicle has exactly two trucks/bogies (no more, no less)
2. The pivot points for the trucks/bogies are exactly in the center of the truck/bogie frame. (This may not always be the case on account of the trucks' connections to the rigid main frame and/or devices such as swing-hangers or rocking devices that assist in guiding the vehicle around a curve.)

So, if your vehicle either is a steam locomotive, a rigid-frame short-wheelbase car/wagon/coach/etc, or a vehicle whose trucks have eccentric pivot points, then ORTSNumberBogies and ORTSLengthBogieCentre are useless.

Hello.

Once upon a time, the developers of OR started from the assumption that a vehicle has two bogies, and each bogie has four wheels, if the vehicle is not a steam locomotive or a two-axle freight car. I think this principle still exists today, this is well illustrated by #Jonas' pictures. I think this is independent of the ORTSNumberBogies and ORTSLengthBogieCentre posts, but it's also possible that these two posts are just making the situation worse.
For #Darwins's suggestion, all that is said is that the loose bearing guidance really makes it possible for the wheels to fit into the curve, but I think he drew an idealized state. Under a certain arc radius, the wheels no longer line up so nicely. They wear out the rails badly. I have seen this in one or two places during my work. Unfortunately, I cannot give an exact value.

Sincerely, Laci1959

[darwins]

ErickC, on 27 November 2023 - 06:33 PM, said:

Are your truck centers, number of axles per truck, and truck wheelbase set correctly in your physics files?

Can we put a different number of axles on different trucks?
I am thinking here of how we might model the Metropolitan Vickers Co-Bo locomotive or the Lancashire and Yorkshire Railway 10-wheeled dining car. (I think I have also seen an LNER dining car with different bogies under the kitchen end and passenger end.)

Laci1959, on 27 November 2023 - 11:44 PM, said:

Under a certain arc radius, the wheels no longer line up so nicely. They wear out the rails badly. I have seen this in one or two places during my work. Unfortunately, I cannot give an exact value.

Of course there are limits! They will depend on length of rigid wheelbase, the amount of sideplay and or the degree of rotation that a bogie or truck has. Some British steam locos had flangeless centre driving wheels to allow them to negotiate tighter curves.

[Weter]

Quote

flangeless centre driving wheels

Very often solution.

Quote

a different number of axles on different trucks

Also Ganz-Mavag/Arad DMU

[Traindude]

One more thing that should be noted is, regardless if the wheelsets are mounted in a ridgid frame or mounted in a truck/bogie, all rail wheelsets are given some degree of lateral "wiggle room" by having some lateral clearance between the journal box flanges and the frame. By the late steam era, many larger locomotives were equipped with lateral-motion devices that allowed for even more lateral than can be given by mere clearance between the journal box and the frame.




However, MSTS never accounted for this. If there was some way to make the axles shift their X-axis position away from zero when negotiating curves (with relevant parameters in *.wag or *.eng files) that would be great!

On the subject of curve resistance in general, many rail vehicles were equipped with wheel-flange lubricators to minimize the friction between the flange and the rail. Is the effect of flange lubrication something worth modelling?

[Weter]

We have stationary flange lubricators at tight curves sites.
And I always wonder, how such lubrication coexists with friction braking, based on shoe-to wheel's surface pair?

[darwins]

Weter, on 28 November 2023 - 06:33 PM, said:

We have stationary flange lubricators at tight curves sites.
And I always wonder, how such lubrication coexists with friction braking, based on shoe-to wheel's surface pair?

Flange lubricators should put grease only on the flange and not on the wheel tread!

Traindude, on 28 November 2023 - 02:42 PM, said:

One more thing that should be noted is, regardless if the wheelsets are mounted in a ridgid frame or mounted in a truck/bogie, all rail wheelsets are given some degree of lateral "wiggle room" by having some lateral clearance between the journal box flanges and the frame.

However, MSTS never accounted for this. If there was some way to make the axles shift their X-axis position away from zero when negotiating curves (with relevant parameters in *.wag or *.eng files) that would be great!

Yes. Please!

That was the start of this post. That would be a really good solution.

The first 8 wheeled carriages used in UK were "radial" carriages, fitted with radial axles that had a high degree of lateral movement.
https://i.imgur.com/7NfCzCx.jpg