Hi!

I built a Lego train and it has a 2-wheel bogie. If the train enters into a straight section from a curve the bogie doesn't turn back. Also, another problem is that the in a straight section I can move the bogie freely.

I can't use 4-wheeled bogies, because the train that I built doesn't have that bogie in real life.

Hi there, welcome to Eurobricks!

that is the same problem people have with long freight or passenger cars when using only two axles. Without pivot point for each axle friction becomes readily too large. With pivot point as in your case you need some "restoring force" once the axle is not perpendicular to the tracks anymore (i.e. coming from a curved segment into a straight).

This may be done with a rubber band - if you have any chance you should get hold of a copy of Holger Matthes' "The LEGO Trains Book", where he addresses exactly this problem.

However, this is exactly true for cars only - since then the 1 axle front bogie is also forced (against the restoring force) into the curved position by the coupling to the train or preceding car/engine.

I bet other people here have much better answers!

Best wishes,
Thorsten      

Hi

Thanks for helping me! 

 

I think rubber bands could work but: If they are too weak they won't fix the problem, if they are too strong they keep the wheels straight all the time. I highly doubt you will find rubber bands in the right size and strength for this application. The real train most likely has fixed axles too and so should your model. 

3 hours ago, McWaffel said:

I think rubber bands could work but: If they are too weak they won't fix the problem, if they are too strong they keep the wheels straight all the time. I highly doubt you will find rubber bands in the right size and strength for this application. The real train most likely has fixed axles too and so should your model. 

Entirely true!

The options are quite subtle though: There are 4 different types of original LEGO rubber bands, red, yellow, blue, and white. And there are 1/2 stud lengths to adjust the "strength" of the forces exerted by the rubber band. And then there are the €3 boxes with myriads of rubber bands ...

I don't know anything about the fixed axles of the real train - but I really believe that this train does not have to negotiate the real world extrapolation of LEGO curved track

Regards
Thorsten    

You could try LEGO Technic flexible axles to "spring" them back.

How many studs are between your wheels?  If they are less than 16, you might be able to get away using fixed bogie frame.

Marklin use a spring between the two axles ...you can do the same, or if you prefer you can use two springs: each one from the axle to the underbody structure of the wagon

 

you can use a rubber band from set 40153

It is more than 16 studs. I tried it before and it didn't work.

I can try to build a spring mechanism like in the track switches, but I don't know if they can work.

If you have a train of multiple cars, you could rigidly attach the outermost axles to the bodies of the outermost cars, then connect the inner axles in pairs to form bogies:

Sort of like Jacobs bogies in the middle. The long distance to the outermost axles might still cause problems, though.

Forgive my ignorance, but is there really any need for having centring springs at all? Wouldn't the natural castoring effect of having the pivot point for the axle mounted inboard of the axle itself not produce the desired result, the same as for the wheels on a supermarket shopping trolley? Naturally you may need to limit the amount of articulation to prevent the wheels falling between the rails, especially on longer cars, but the steering impetus can be imparted to the leading axle of each car through the coupling to the car in front while the trailing axle will simply follow on behind obeying the laws of physics.

Has anyone tried this?

3 hours ago, Hod Carrier said:

Forgive my ignorance, but is there really any need for having centring springs at all? Wouldn't the natural castoring effect of having the pivot point for the axle mounted inboard of the axle itself not produce the desired result, the same as for the wheels on a supermarket shopping trolley? Naturally you may need to limit the amount of articulation to prevent the wheels falling between the rails, especially on longer cars, but the steering impetus can be imparted to the leading axle of each car through the coupling to the car in front while the trailing axle will simply follow on behind obeying the laws of physics.

Has anyone tried this?

I haven't - could maybe work, but there are some doubts ...

I believe that the difference with regard to the shopping trolley is: 4 individually pivoting wheels instead of a "two fixed-axle arrangement" in addition to "no rails in the supermarket". I believe that the two wheels on one axle may "jam" or better lock-up in the angled position when the friction forces (caused by the still pushing driving motor) overcome any realignment forces. SwengX said that this often happens when the train comes out of a curve, which then easily locks-up the axle position (due to friction).

Man, all wrong nomenclature, and all just believes - where are the physicists or mechanical engineers when you need them? 

All the best
Thorsten     

    

This train is a Hungarian Bzmot, and normally it is only one cart. It can't have jacobs. 

@SwengX Oh blimey!! And I thought it was only us Brits who were crazy enough to come up with a self-propelled 4-wheel railcar.

If the idea is to run that as a single car and not as part of a larger consist then even castoring is unlikely to help, because at least the leading axle will need to be lead through a curve in order to impart steering impetus.

@Toastie SwengX doesn't appear to have set up his MOC to take advantage of castoring because it appears that his axle pivot is directly on top of the axle itself. For that you need to move the pivot point away from the axle, in the manner of the supermarket trolley wheel. Apart from that there are few similarities between the dynamics of trains and supermarket trolleys, I agree.

I'm beginning to think that I might have to build a proof-of-concept prototype to test this out.

You will probably need to compromise to get this train to run reliably. The simplest solution would be to use a double bogie at each end, but hide the innermost wheelset on each bogie behind a shroud that mimics the under frame detail. This approach would allow you to easily motorize the train, and will give you a reliable runner.

@scruffulous Funny you should mention that, but I was just in the middle of knocking up something along those very lines on LDD.

GIRO_VAGONES by MTorralbo, on Flickr

10 detalle ejes by MTorralbo, on Flickr

I don't know if this will work, but I found it an interesting solution. It might be worth a try.

Hi

I tried every method and sadly I didn't find a solution. I tried the 4 wheel bogie and it worked but the I don't know how I can make a drivable 4 wheel bogie. There is one 1x6 brick with technic holes and the first axle is in the first hole and the second is in the last hole. It is not a big distance between the two axles, so I don't know how can I make that drivable.

What do you mean by "driveable"?

Do you mean powered? You would need some sort of motor. But connecting an external, fixed-in-position-motor to a moving axle/bogie is not going to be easy, if possible at all... since you aren't going to use single-axle bogies anymore, why not use a normal PF train motor?

A few years ago, I made a 2 axle Pacer railcar.  I modified it to use Power Functions driving one axle.  It ran terrible like the real prototype. 

Brickshelf pics

 

 

Hi

I made this Robel maintenance Train a few years ago. 2 axles, both Powered by 1 9V technic motor, 9V battery snotted in the driving cabin and non Lego (sorry) cables back to the RC unit that sits in the bottom of the wagon, thus enabling it to be Remote controlled. Did not run very well, very slow driving, but that is also the case with most Robels. The biggest problem is the distance between axles and curve radius.

Good Luck!