Hod Carrier

Vehicle Dynamics Laboratory investigates the Castering Effect

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On 1/1/2018 at 5:50 AM, Hod Carrier said:

There may be times when you want the axles of a vehicle to steer in opposite directions, such as going through points/switches, which is why my test track includes a couple of S bends.

I was initially concerned about this as well; however, on the Umbauwagen 3yg the front and rear axles always turn together. The three axles are in a straight line in the middle of an S-bend, which works because the track is approximately straight there:s-bend.png

For a wheelbase of this length (20 studs axle-to-axle), the deviation is small enough that the wheel flanges don't rub on the track (Try it and see! Big Ben Bricks wheels are slightly thinner and so are affected less.). The articulation is mostly needed to reduce friction in turns, and to keep the wheels from riding up the point or guard rails of a switch.

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This is where your three axle design has the advantage over the two axle designs.

Because all three of your axles influence and steer each other, the centre axle is centring the two outer axles and reducing the deflection which is the source of the unwanted friction. With a passive steering two axle design the axle at the blue end of the vehicle would still be turning hard left against the direction of the curve because the axle of the red end would be turning into the curve under the action of the coupling to the neighbouring car.

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I agree with @Hod Carrier - the third axle is an important input to the two other axles, since it can suggest "how the track is done in the middle" :sweet: - two axles only cannot have this kind of input and can only rely on themselves, or be helped by the pushing/pulling wagons. :blush:

Great three axle wagon, BTW!!! :thumbup::thumbup::thumbup:  I wanted to create them in 12v style for my BR78 :laugh: - may I try this kind of linking?

 

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I know it's a pain in the ... to get it working, specially on R40 radii, and most of us have more than a Curver box full of them.

I was wondering how the linked cars will behave on larger radii.

Would the problem persist, or should it behave better due to the larger radii?

I expect they would behave better when pushed.

And how would they pass the R104 point from BrickTracks?

see discussion on Eurobricks: 

see: 

Anyone who tried this yet?

 

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On very large radius curves (made with straight parts with custom mountings - so they form a curve) I can keep both axles fixed...or only one turning. 

I could try with Lego flexible track to create a long radius zigzag curve (emulating a switch) and see how the yellow wagon behaves :wink:

 

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You beat me to it!! :laugh:

The bigger the curve radius the happier these vehicles will be. The trick is to tailor the amount of axle articulation to suit the smallest radius that they are likely to encounter; so the bigger the radius the less articulation you will need. Eventually, as @Paperinik77pk says, you will get to a radius big enough that the axles don't need to articulate at all and can be fixed.

Edited by Hod Carrier

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I totally agree with @Hod Carrier, we can manage to create longer radius curve with standard Lego parts, but points are always limited to R40...so we had to work on the "worst case" possible. :classic:

BrickTracks points are way larger than R40, so I think the wagons will be veeery happy to travel over there!!! I just saw the video @Ludo posted yesterday - look at that crocodile locomotive changing track on standard points - it is a pain only to see it - and it has very flexible bogies! :laugh: 

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5 hours ago, Paperinik77pk said:

I totally agree with @Hod Carrier, we can manage to create longer radius curve with standard Lego parts, but points are always limited to R40...so we had to work on the "worst case" possible. :classic:

This is what I was getting at -- a rigid 2-axle car will stay on ordinary track (albeit with increased friction!), but the discontinuous geometry of switches will derail them.

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how do they do this on real life long trains? or do they not have single axle bogie loooong trains, only double axle bogies?

it seems like you are trying to invent a soloution on train, for an issue that is made by the track, where the answer is, modify the track not the train.

the other thing with the buffers steering the single axles off. what happens if you remove the buffer and magnet from the axles, and attach them to the actual carriage, leaving the axles to free steer?

 

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@bradaz11 Real trains work very differently from LEGO ones, and this is the main reason why vehicle dynamics have to be considered differently.

Real train axles naturally self-steer because of the cross-sectional profiles of the train wheels and track. The diameter of the wheel tread is largest next to the flange and reduces slightly as you measure away from it. As the axle enters a bend the wheels move laterally towards the outside of the bend. As a consequence, the diameter of the outer wheel where it is in contact with the rail is larger than the diameter of the inner wheel meaning that it will travel further per revolution which causes the axle to follow a curved rather than a straight path. It is this difference in wheel tread diameter that causes the axle to follow the bend, not the flange itself as is commonly believed. You can push a rail axle down a length of track and it will follow the bends happily without any intervention. Rail vehicles take advantage of this by allowing each axle a certain degree of movement within the suspension to permit each axle to steer itself relative to the position of the vehicle body, allowing the vehicle to take bends without creating unnecessary wear to rails or wheels due to excessive friction.

LEGO rail wheels and track are far cruder meaning that a LEGO axle will not naturally self-steer in the way that a real axle would. Believe me, it would be far, far easier if LEGO axles did behave this way because you could then be sure that they axles will follow the path of the tracks. But because they don't, any steering effect you get can only be achieved by applying some force to the axle assemblies directly. In the case of the vehicles that I and others were testing, this force was applied through the coupling to the neighbouring vehicle. Free steering axles that had no steering force applied would not follow the course of the track, refused to self-centre (even when at the very rear of the train) and were frequently the cause of derailments. I did not experience any problems with the buffers steering the axles off because I made sure there was sufficient articulation to ensure that the buffers never made contact with each other. This might be a problem if they were rigidly mounted to the vehicle itself, but mine were attached to the axle assemblies which prevented any interference.

Certainly one solution to this problem would be simply to use larger radius curves, but not everyone has the luxury of space to use them and many have to make do with LEGO's standard curves. As a result, focusing on the vehicle rather than the track makes the solution accessible to more people. I'm sure that those people or clubs lucky enough to have the space for large radius curves will have looked at this and decided it's not required for them, and that's fine. But for the majority of us it opens the possibility for running scale length vehicles on the standard track without the excessive friction and derailment issues previously experienced by trying to run long-wheelbase vehicles with rigidly mounted axles.

To answer your question about real rail vehicles, there are indeed limits to how large a two-axle vehicle can be. Partly this is down to vehicle dynamics but partly it is due to axle loads. The model VGA wagons that I built on the back of this testing are representations of the largest two-axle vehicle to run on the UK rail network. There may be larger vehicles of this configuration elsewhere in the world, but anything larger than this in the UK would require bogies. The trend in rail freight has moved away from trains formed of large numbers of small wagons towards smaller numbers of large wagons. This gives greater efficiency of operation and permits larger loads to be conveyed in trains of equivalent lengths by reducing wasted space thereby increasing the load density.

I hope this answers your questions, but if you'd like to ask more I will be happy to answer.

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@Hod Carrier

thanks for the reply. When I say buffer, I mean the entire buffer brick, so magnet and buffer. I was refering to your bunching issue when pushing the carriages. If the buffer bricks were mounted to the chassis of the carriage, the connection wouldn't bunch would it? Or would it cause issue because the axle wouldn't align itself to the track, and so although it may not bunch, it may still have the wheels at weird angles or worse, derail?

 

thanks for explaining the real wheel relationship to the rails, that does help clear up what is going on IRL.

the thing I'm trying to ask is, if a real single axle train carriage length is governed by the curve radius in track, then are we not still limited to that in LEGO? so although the train could be scaled down, we have to adjust it to navigate our LEGO curve radius, so we have to change the dimensions of length, or add a bogie to get it to be compatible to the radius we have to work with, or if we change the radius to match the real life example that train carriage could navigate, then the real life carriage should work if scaled down?

or would you stil also need to change LEGO wheel shape?

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4 hours ago, bradaz11 said:

When I say buffer, I mean the entire buffer brick, so magnet and buffer. I was refering to your bunching issue when pushing the carriages. If the buffer bricks were mounted to the chassis of the carriage, the connection wouldn't bunch would it?

Correct. The reason why the cars were "bunching" was because there were too many pivots between the fixed point which meant that the couplers would "fold up" when the cars are pushed. You are right to say that a rigid mounting to the chassis would prevent this from happening.

4 hours ago, bradaz11 said:

Or would it cause issue because the axle wouldn't align itself to the track, and so although it may not bunch, it may still have the wheels at weird angles or worse, derail?

This is the real problem. The axles need to be steered because they cannot self-steer themselves, and the best (only...?) way to do this is through the couplers. Without this there would be nothing to align the axles with the track whatsoever.

I did come up with a solution to the "bunching" issue, which was to add a small elastic band to the coupler magnet itself to keep it centred. This was sufficient to keep the magnets correctly aligned when the train was pushed which also kept the wheels correctly aligned with the track.

4 hours ago, bradaz11 said:

thanks for explaining the real wheel relationship to the rails, that does help clear up what is going on IRL.

the thing I'm trying to ask is, if a real single axle train carriage length is governed by the curve radius in track, then are we not still limited to that in LEGO? so although the train could be scaled down, we have to adjust it to navigate our LEGO curve radius, so we have to change the dimensions of length, or add a bogie to get it to be compatible to the radius we have to work with, or if we change the radius to match the real life example that train carriage could navigate, then the real life carriage should work if scaled down?

or would you stil also need to change LEGO wheel shape?

I'm not entirely sure that a re-profiled LEGO wheel would have sufficient effect to ever make it fully compatible with LEGO curves and make them work in the same way as real train wheels, but it might possible if the curve radius is generous enough.

I think you've asked a very valid question. Very few model railway systems ever truly mimic the operations of real railways in terms of vehicle dynamics, but the LEGO system is almost comically unsuited. No real railway has curves as freakishly tight as the LEGO system and so no vehicle designer ever has to consider how to make their vehicles traverse them.

I guess the answer for us depends on personal preferences, and this is often a factor that gets touched upon whenever the question of scale comes up. Do you design your cars to scale with the track with dynamic characteristics to suit or do you look at ways of modifying either the track or the cars to work together in a more harmonious way? There is no right or wrong answer to this question. Some people like playing with the scale to make shortened LEGO-friendly caricatures of real trains while others like to push the boundaries of what is possible with LEGO by creating correctly scaled behemoths and then engineering them to work on LEGO track geometry.

Personally I don't think we need to be limited by LEGO track geometry. I believe that there are ways of engineering solutions to most track-related problems, and this investigation was one example of this approach. There's no reason why a two-axle vehicle needs to use fixed axles with all the limitations this brings when there are alternative methods we can use to achieve compatibility between the train and the track. LEGO is traditionally seen as merely a toy and not worthy of serious consideration amongst modellers because of it's perceived crudeness, a perception that TLG seems unwilling to refute. However, this does not mean it is impossible to create some truly stupendous creations that, at first glance, really shouldn't work but somehow do. I like stuff to scale correctly if at all possible and try to get as close to realism as I can. This is one aspect of this hobby that I find incredibly attractive. The chance to show someone something and have them say "Is that really LEGO...??" and to undermine their preconceptions is priceless.

But those are just my thoughts. As I said before, there are no right or wrong answers to this question and it's up to everyone to make their own judgements. Often it's simply a question of space that dictates the size of everything.

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