Hod Carrier

Vehicle Dynamics Laboratory investigates the Castering Effect

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Thanks for the input. I value your criticisms because it helps to refine the designs and move towards a better understanding of the mechanics of the problem.

You're absolutely right to highlight the weakness of the passive steering design. Although it works quite well in some situations it works less well in others. For example, it is the cause of derailments on points/switches because the leading axle is being incorrectly steered through the gap in the frog when it should really be centred. I would have preferred to allow all axles to move independently of each other, but I was experiencing issues with the last axle derailing when the cars were being pushed through bends. This is where I have found passive steering to work particularly well because it steers this last axle into the bends and prevents such derailments.

The self-centring issue with the last vehicle seems to be caused by friction in the pivot because physics dictates that the trail in the axle pivot should be sufficient to cause it to centre. However, until this can be overcome (I'm currently considering adding roller bearings) the only way to centre these axles is by applying some force to them. I resisted the temptation to add elastic bands because I didn't want to restrict the freedom of the axles to pivot. I was hoping that the force exerted through the couplers would be enough, although it appears not to be sufficient. I would be interested to see how the last axle behaves under the influence of the elastic bands because it is not obvious to me what force would be acting upon it to induce it to steer.

I'm going to modify the VGA vans to use your idea and see how things go. If it proves more reliable than the solutions I have come up with so far then I shall be happy. At the end of the day, my aim was to discover whether or not it would be possible to make long wheelbase vehicles that can operate successfully on standard LEGO track. So far it seems that there are a range of solutions. I hope that others may be spurred on to possibly discovering further solutions to this problem. :classic:

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I keep wondering if the passive steering design would benefit from a special treatment for only the last axle in a train. It seems to work like a charm mid-train, and only the last axle gives problems. Stupid solution, rigidly mount the axle. More sophisticated solution- play with the pivot point or other tricks to get the rear axle in to place.

That said, if the elastic bands could provide a universal solution that would be even better.

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I've carried out a few tests off camera. I hope no-one minds there not being a video but time is tight at present. If there's demand I may find time on another day to do some filming.

I modified the VGAs to have elastic bands for centring and removed the passive steering mechanism. Being a Sunday and having been at work it was only done with table scraps, but I followed the principles laid down by @LEGO Train 12 Volts. Rather than a single band I used a pair of thin bands on each axle assembly (actually each band was a pair of "loom bands" linked together to give the necessary length).

Running both vans together the behaviour of the coupled axles was little different to how they would behave if the axles were free castering. The self-steering was near identical and the axles centred as they would without assistance. In light of this observation I removed the elastic band centring from the lead VGA and continued the test. This was when I noticed that the self-steering was less on the van with elastic bands compared to the van without. Even halving the centring force applied to the axles by removing one of the pair of bands did not noticeably improve the level of articulation compared to the free castering axles. This reduction in articulation did result in an increase in friction between the wheels and rails. Even while running forwards the train slowed marginally, although noticeably, when going through bends.

Where things were different was with the last axle. As @LEGO Train 12 Volts predicted this axle stays pretty well centred but, as I feared, this was mostly because it didn't seem to articulate very much at all. About the only time it seemed to move dramatically was when the wheel caught one of the check rails on the points when being pushed through. There seems not to be any force acting on it to induce a steering effect strong enough to overcome both the friction in the pivot or the centring force exerted by the elastic bands. On the plus side, it did at least keep the axle sufficiently centred to prevent the wheels steering themselves through the gap in the frog when being pushed through points at low speed, although the train did stall due to friction and even derailed a couple of times further through the points.

I then tried @zephyr1934's idea of removing the pivot altogether from the rear axle, but sadly the wheels scrubbed so badly that the friction was simply disastrous.

I'm beginning to wonder if there are any simple answers to this conundrum. My initial thought was to try and find some way to make long wheelbase cars viable without having to worry about having lots of different systems requiring operators to know where in the train they should go or which way round they should run or what types of track or track formations to avoid. However, I now think that this was perhaps too ambitious. I appreciate that I'm probably pushing things quite far with a 24 stud wheelbase, but then if a system works it should work for the longest of cars as well as the shortest.

The idea of having each axle independently steered, as @zephyr1934 suggests, is obviously a good one. But how can it be implemented? How does an axle know which way it should be pointing without reference to some external factor? Those axles that are coupled act under the influence of each other, but the last axle is a problem because it is not coupled to another axle. Perhaps the simple answer is to stick with free castering axles, but don't try pushing these long cars through any R40 points unless you have a conventional non-castering car to keep the last axle under control.

Oh, and make sure you use a BIG loco to overcome the friction due to "bunching" if you ever need to push them. :look: :laugh:

Discuss.

Edited by Hod Carrier

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Hi all, I'd like to show how I tried to manage the castering effect. I follow this thread since I've subscribed, because I like two axles - long bed wagons.

I saw both @Hod Carrier and @LEGO Train 12 Volts solutions, so I decided to try a mixed solution between two independent axles and passive steering. It's a sort of a semi-linking mechanism, involving the Lego rubber joiners as elastic part. So I built a goods (probably BANANAS!!! :laugh::laugh::laugh:) wagon for testing - it is my first all-technic train creation...I think I will experiment more on  this kind of solution :devil:

WP_20180205_16_11_27_Rich

WP_20180205_16_11_41_Rich

 
WP_20180205_16_12_15_Rich

Here's the semi-indepent linking featuring the two rubber joiners. Pivot point of the two axles is centered with wheels.

WP_20180205_16_16_49_Rich

The axle connected to the locomotive turns more than the other one (which is guided by the linking, but keeps a bit of freedom. In this case turning is forced to an unnatural angle to show the link effect. 

WP_20180205_16_15_22_Rich

The rubber joiners allow the linking to go back to central position. I did not film the train in motion, but it works on points, on multiple points, on crazy curves both forward and reverse, as a single wagon, in the middle of train, as first and last wagon. All tested with the little 7720 since I do not have a faster locomotive at the moment (12v test layout was put away). 

WP_20180205_16_19_20_Rich

If needed, one of the two axles can be locked in central position (Technic long pin). The other axle is free to move.

I have to test it more, maybe with a longer bed - this one is 29-studs long. Let me know what you think about it! :laugh:

 

 

 
 

 

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That’s an interesting concept. I’d like to build and test it too and see how it works out, if that’s OK. 

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30 minutes ago, Hod Carrier said:

That’s an interesting concept. I’d like to build and test it too and see how it works out, if that’s OK. 

Go on without any problem, it is an interesting problem to solve...but like you wrote at the beginning..."spaghetti" are the enemy here.

When pushing, it happens that the locomotive moves the copuling on the opposite way it should go, following the wheel. On smaller cars like the Banana, this problem is less evident. On bigger cars I think it is a real pain.

WP_20180205_21_44_58_Rich

This is a longer car. As you can see the pivot point is immediately after the coupling, and there's a stopping pin that prevents too much lateral play on the connecting rod.

But I think I'm near to the limit. I can push two big 12v wagons (extremely heavy on wheels) and it can manage them on points, but without stop pins it derails, sometimes.

And the 7720 is slow...:laugh:

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Oh, I see. So there are still issues even with this set-up.

I can see how the stop pins will prevent the axles wandering off in their own direction, but I can also see how much they restrict the amount of articulation. The way the wheels are scrubbing against the rails in that last photo looks painful. If you had a heavier car the friction that would generate could stop the train dead in it's tracks. It's also interesting to see where you've decided to put the axle assembly pivot point.

You've given me some things to think about and try out.

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few times ago, I mad some test too. What I remember is to always do the test with multiple chassis.. because the last one never reacts as the others. being coupled front and rear give better result.

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I thought a lot on the behaviour of the wagon when pulled, pushed (or both, in case it is in the middle of the train). What I began to think about is the "sensing" of what is required to the train car.

Bogies are smart, the paired wheels know were to go. single axles are ignorant. Coupling are more informed about train behaviour, but only in pulling mode. In pushing mode they are more ignorant than axles.

Probably the linking is good to teach axles where to go, but I'm beginning to think it is not sufficient, since it is driven by couplings, and if they fail, it fails too. :thumbdown:

I have an idea to develop, this evening I will work on it :wink:

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Interesting stuff. You’re right about “dumb” and “smart” axles. It’s an analogy I’ve used too. The challenge is working out how each axle “knows” which way to articulate at any particular moment.

Sadly I’m at work at the moment so I can’t share a little discovery I made earlier today until much later tonight, but it’s something that seems to help. I’m wondering if maybe we’ve both had the same idea. :look: :grin:

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I was idly watching the VGAs going round and round on my kitchen floor earlier today, seeing that taunting Z shape made by the axles and couplers as they failed to centre properly, and wondering what it was that I might have missed. Then it dawned on me. I'd spent so much time concentrating on the axle pivots that I'd completely forgotten that there was another set of pivots at play; the ones on the magnetic couplers. What would happen if I could make them self-centre, and what effect might that have on the behaviour of the axles?

Well, click on the image below and see.

39227046925_4ebce04af3_z.jpg

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Great work!!! We're following the same "pivot" path :wink:!!!

...I forgot the phone at home today, so I cannot post any picture, but I'll try to explain it in a fast way :laugh: - Powerpoint will help me.

There are three possible pivoting points:

  • The magnet one (the one you centered with the rubber band)
  • The coupler one
  • The axle one

so in my opinion they must be separately managed.

The enemy is the "spaghetti effect", which  happens when the pushing car tries to move the pushed car coupler on the opposite side it should go.

Having the coupler pivot and axle pifot on the same point (first case) means that I have a bigger leverage, therefore less force needed from pushing car to move both coupler and axle in the wrong way. 

 

Spaghetti

In case 2, coupler has its own pivot, and so does the axle. There are two rods to transfer steering  to the axle during normal operation. Leverage from coupler to pivot is now shorter. This alone does not eliminate the "spaghetti effect", but forces to create it must be stronger.

Now, a little focus on linkings geometry - it must be analyzed a bit, since it there are two specific cases:

  • Coupler is subjected to pushing and pulling forces, which are quite strong - or no forces at all (free coupler on the first pushed car)
  • Axle is only responding to track inputs, which are reeeeally weak.

So linkings must be configured in order to:

  • allow axle to  move a bit the coupler with less effort (as it is shown, more or less)
  • allow coupler to move the axle , even with a worse leverage.

  Then...let's see where to put the self centering mechanism, I'd say on the coupler lever and on the central linking lever between the two axles (which I'm convinced it is still needed). And on magnet, obviously :sweet:

 

 

 

 

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Some very interesting thoughts. I look forward to seeing the results of your research in action. :classic:

The problem that causes "bunching" on straights is to do with a hinged lever's natural tendency to try and shorten itself when under compression. The forces acting on the coupling to push it out of line only need to be very slightly away from perpendicular to the axles to cause the coupling mechanism to fold up and cause "bunching". Most often just coming out of a bend onto a straight provides enough deflection for "bunching" to take over because the axles are turned.

To overcome this there needs to be a way to impose a straight line on the entire mechanism either through a) centring the axles or b) centring the couplers. Personally I favour option b because any force that is acting on the axle to centre it must be overcome in order to induce it to self-steer. Another possibility might be to try and find a geometry whereby the length of the entire mechanism between the magnetic coupler and the axle assembly pivot is shortest when it's in it's central position.

You're absolutely correct to say that the size of the force will be proportional to the length of the lever, but it will also be proportional to the mass of the object being pushed and the resistance due to friction. I've not weighed the VGAs yet but, at over 700 parts each, I expect they will be pretty close to 1kg. That's quite a lot of weight to push. Add in the friction from the axles themselves and the forces acting through the couplers under compression will be quite high.

I'm beginning to be persuaded that a passive steering link might not be entirely helpful. I put the links back on the VGAs and suddenly I was in a world of problems with derailments and other issues. It might be because of the modification to the couplers, but I'm still trying to understand why the cars now behave this way. In addition to this, I do agree with @LEGO Train 12 Volts regarding the need for axles to be fully independent. When you start getting out to very long wheelbases it's very likely that the passive steering will cause the axles to be turned in the wrong direction at the wrong time, as is the case with my cars when I try pushing them through points. Hopefully you will be able to come up with a better solution than mine that eliminates this problem.

I think I may have misunderstood something you said earlier about the behaviour of your cars. Did you say that the wheels are steering in the opposite direction when being pushed through curves? Is that still happening?

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Hi! I absolutely think is better to work on coupling centering, I still have to try the full solution , yesterday it was too late and I fell asleep looking at the wagon!:laugh:

I try to add weight as well so I can understand how the steering linking between coupler and axles work. I'll try also to keep axles independent. I now have a PF "locomotive" to move the cars at faster speed.

Regarding your last sentence, the rubber linking solved a bit the problem (wrong turning wheels on zigzag curves) shown by @LEGO Train 12 Volts , because it left a bit of freedom to each axle. So it is more a "suggestion" than a real fixed link. But I'm beginning to consider it less efficient than a good axle-coupler solution. :sweet:

Edited by Paperinik77pk

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After a bit of thinking and trial/error procedure, here it is - the multilink.

@Hod Carrier - you were AB-SO-LU-TE-LY right on magnets - the more I stop them, the more the car's behaviour goes better.  So I used a rubber band wrapped around the magnet. It moves a bit, but it does the job. It allows the coupler bar to follow the pushing car withouth any pasta effect we can think about. :wink:

WP_20180207_23_09_08_Rich

Following the coupler, wheels turn as well, guided by the multilink. Wheels have their own pivot, The coupler turns more than the wheel, since it is in an advanced position. This should allow a better alignment of the wheel on the track.

WP_20180207_23_09_20_RichWP_20180208_08_09_06_Rich

It works fine on standard radius turns, both in pushing and in pulling mode.

WP_20180208_08_10_11_Rich

There's no link between the two axles, only a rubber band in common.

Now the "stress test" part. Obviously I had no batteries, and 7720 came back to help. 

WP_20180207_23_08_08_Pro

This is quite a big load, but the good-old little locomotive can manage it (I put two 12v weights over the 4,5v motor to increase traction). 

  • Between two wagons - pulling mode - very good
  • Between two wagons - pushing mode - very good
  • Last wagon in pulling mode - good
  • First wagon in pushing mode - good

Why only good in these two last cases? The "free" axle really tries to turn the coupler too and to follow the track. In some cases it has success, in some cases it fails. With an empty wagon it is not a problem even if it fails. With 1 kg on it it makes really the difference. It does not stop the almighty 7720, but it has some slowdowns.  

I think this part can be improved removing the common elastic band. If the pushed coupler is turning, it is applyng tension to the central band, which pulls the free axle and tells it to go straighter than it should do. So I'll try to go for dedicated rubber bands (but I want to buy better rubber bands, these are really bad).

The inter-axle (pivot to pivot) is currently 23 studs. I think the multilink can turn wheels even with a larger inter-axle, but I do not know the real limit.

I think it is all for today! :laugh:

 

 

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

 

This "bunching" issue may be insurmountable without some means of centring at least one axle, but I'm currently at a loss to see how that might be achieved without somehow restricting the axles ability to free caster.

 

 

There might be no way to actually achieve this, but could you theoretically have a trailing and leading pivot point, and only have one of them engaged, depending if you are pushing or pulling?  Would that address some of the binding issues when pushing? 

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It is an interesting approach I was wandering about before trying the multilink.  By moving pivots you also mean a little movement of the axles too? So - a little axle movement forward/backward to engage the most correct pivot point? :classic:

 

 

 

 

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I thought about this too but abandoned the idea because I couldn't see how to make it work reliably. It would be fine if the axles lined up properly as they should so that they could engage with each pivot in turn, but they don't always.

At this point perhaps we might have a little recap because the thread is starting to get quite long and there are lots of ideas contained within it, some that have been tried and tested and some that have not.

I think we have established that long wheelbase cars are possible and that there are various techniques you can use to make this happen. The consensus seems to be that independent axles are better than those rigidly linked and that self-centring couplers are necessary if you want to push these cars without excessive friction due to "bunching". Where axles are coupled to a neighbouring car the vehicle is happy to go anywhere, forwards or backwards, and even through standard points. The issue still to be resolved is how to control a free axle that is not coupled to another car, especially when pushing the train. @Paperinik77pk, would you agree that this is the current situation?

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Absolutely yes. This is the current situation. A pushed wagon has problems managing the direction of the free axle.

Yesterday I tried to play a bit with axle pivot, trying to move it near the edge of the wagon, to achieve a "shopping cart wheel" effect. More or less like I did in this old example, but applied to one axle the new yellow wagon. Sincerely I do not know...it does not convince me even if I think is correct from a physics point of view.

WP_20180205_21_44_58_Rich

It was late and I did not experiment further. 

See you later! :laugh:

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Oh well... at the end I discovered that a bit of central link is needed in case of wagon pushed on one side and with a free coupler on the other one side. I moved pivots forward and back...problems everywhere. So I got back to the standard multilink.

I tried to keep the axles alignment independent (each one with its rubber band) and it works average. The central link with rubber parts is good for smaller wagons, but has problems on bigger ones like the one I'm using.

The common elastic band connected to the two axles both works as alignment and a bit as link. If I turn one axle, the second one turns a bit aswell. But under heavy load, this auto-turning movement is very very difficult since the rubber band is not hard enough,

I tried to make a video but there was no light and no silence enough.

I used my 7740 converted to PF with standard PF wheels. It is an extremely heavy locomotive, with batteries over PF motor and original 12v central weights. The wagon goes nicely alone through curves with no or small load (three or four 4,5v motors). But with more than one kg of weight, the 7740 had problems pushing and pulling it on simple curves,starting to slip wheels *huh*. But...it worked before with 7720 and now, with a powerful 7740 I get stuck *huh*? Is it possible :angry:? Yes...it is .

I switched again to the good old red locomotive. And it works. The problem with heavy loads, at this point, are the small PF wheels - they do not have any traction (and I have all new PF wheels with very soft rubber bands). 

I think I'm done with the tests for now - I cannot think of another better solution to steer that damn free axle :laugh:. I think my future 2 axle wagons will be shorter than the one I'm using for experiments. Going back to BANANA (version 2.0)!!!:laugh::laugh::laugh:

 

 

Edited by Paperinik77pk

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I think I've reached the end of the developmental line too and hit the bufferstops. While it wasn't possible to hit all the targets I had set for myself I still feel happy with the progress made. Maybe someone brighter than me will come along and solve the free axle problem. In the meantime, I shall be making sure that my VGAs are coupled at both ends.

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Did you try to have the last free axle fixed ? as it is simple wheels, as they are first they will follow curve pretty well as a short wagon. it is then up to mobile and attached one to move.

 

 

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@moustic Yes I tried it but it doesn't work. The problem is that the length of the car is so great that a fixed axle at one end is not actually following the curve but trying to drive across it. This turns the wheels against the inside of the rails and the friction this generates is enormous.

@Ludo I have removed the steering link between the axles. I thought the steering link was a good idea when I was testing the prototypes, but I now think the vans works better without it. As @LEGO Train 12 Volts said, the steering link sometimes turns the wheels the wrong way at the wrong time and I discovered it can cause more problems when running heavy vans like the VGAs. The vans do need the extra regular wagon at the end to control the last axle, though. Add the elastic bands to the magnets too and these wagons will now run anywhere on any track, forwards or backwards.

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