Thompson BZ 6 wheel van

Julian Roberts
Posts: 900
Joined: Sat Jan 09, 2010 11:33 pm

Thompson BZ 6 wheel van

Postby Julian Roberts » Sun Nov 10, 2019 11:46 am

Guy Rixon wrote:Things likely to fall off, from observation:
...................
* Six-wheeled coaches, on plain line. Happens each time anybody farts in the same postcode, on both P4 and EM. .


Such a memorable thought has stayed in my mind challenging me to see if it's true. This nice BZ kit was on sale at Scalefour North in April and is just the job as something appropriate for 1950s/60s exCR/LMS BR Calderside. The plan was to make it on wet days during caravan holidays where a steam locomotive is too complex. But there haven't been many so it's still only half built.

The kit design which is for 00/EM envisages the centre wheel assembly sliding sideways as it enters the curve and, being linked to the outer wheels, steering them to thereby stay more or less at right angles to the track. As I don't intend anything less than say 4ft radius I thought there is no need for the outer wheels to steer. Indeed I have a SR 4 wheel van of the same wheelbase that goes fine. So the centre wheel assembly is fixed and the wheel given enough sideplay to be able to slide laterally within its bearings, which is no problem with the design being for the narrower gauges.

The design sees a piece of NS wire folded to provide a downward spring force on these centre wheels, with one of the outer wheels being able to rock.

I made up the basic chassis and basic bodyshell to test out Guy's assertion here. I found that the downward spring force was not sufficient for the centre wheels to be made to slide laterally. They preferred to mount the rails. So I replaced the wire with steel wire as used for AJs. Much 'tuning' or fiddling was required to subtly modify the shape of the curving fold to exact an appropriate spring force. What is required is to balance the force against weight added at each end. Too much force and the outer wheels are not firmly enough on the track. The more the weight the more the force can be. The weight must be as far as possible towards each end. I managed to find what I think is a medium balance where the vehicle weighs 90g while the centre wheels feel satisfactorily positively sprung. Perhaps a bit heavy but fine for me.

Lubrication of the centre axle is of course vital. Below is a picture of the centre wheel assembly and one of a test stage. The kit is very nice to make and the designer very kind and helpful with subsequent questions I had.
Attachments
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Andrew Ullyott
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Re: Thompson BZ 6 wheel van

Postby Andrew Ullyott » Sun Nov 10, 2019 1:42 pm

I shall watch with interest as I've one of these kits to do as well. I was wondering about somehow steering the outer axles.

Philip Hall
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Re: Thompson BZ 6 wheel van

Postby Philip Hall » Sun Nov 10, 2019 5:45 pm

I have almost standardised on the Brassmasters Cleminson underframe for 6 wheelers, and it works. OK there are a few intricate bits and pieces, fitting the the odd cylinders and brake gubbins around the moving bits, but generally the vehicle just rolls around steadily. One wheelset has to be truly concentric (but I've banged on enough about this before) but the others are not so critical.

I have also - on a short wheelbase 6-wheel S&D brake van - adopted the 'rocking axle one end, middle one on a downward spring' arrangement and that too works, but that has been a bit of a fiddle to set up, as I think Julian has found.

My final observation relates to a 6-wheel commercial brake van (Oxford GWR) and this is completely rigid (albeit with a bit of slop and dead true wheels) and this runs brilliantly...

Philip

Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Sun Nov 10, 2019 6:11 pm

Andrew I just wonder what the point would be? I don't think the real thing had any steering(?) My SR van goes round a 2ft curve no problem. Possibly an advantage we have in P4, the great big flanges in 00 might need steering more than we do?

Edit...whoops!.... I've just seen Philip's post! David Franks showed me his mechanism for EM 6 wheel vans which looked great, can't remember the detail now but basically 4 wheels pivoted as a unit and one pair at the other end were rigid I think.

Worth pointing out my testing was on my 'challenging' test track with humps and bumps etc.

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Le Corbusier
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Re: Thompson BZ 6 wheel van

Postby Le Corbusier » Sun Nov 10, 2019 9:54 pm

There was some discussion relating to 6 wheel clayton coaches on my thread a while back. This post by Mark Tatlow might be relevant ?

viewtopic.php?p=61333#p61333
Tim Lee

davebradwell
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Re: Thompson BZ 6 wheel van

Postby davebradwell » Mon Nov 11, 2019 4:50 pm

I just copied the prototype on my BZ - sprung the axleboxes at the ends normally and used swing links to suspend the ends of the centre springs to give an almost friction-free slide using plain bearings made from 2/1mm tube. Centre spring positioned above floor to give reasonable length links. It can be whizzed round 3ft rad. I took it to Scalefour North a few years ago. I think centre springs are slightly longer so lower rate to avoid it tottering on this axle.

Didn't Mark Tatlow describe a rather elegant arrangement recently with centre wheels mounted on 2mm tube siding on 1mm ale?

Can't see why you would want to steer the outer axles as there's plenty of 4w parcels vehicles with longer w/b that run perfectly happily - it all adds slop and friction in equal measures. Also any application of a spring that has to be carefully tweaked to avoid 2 conflicting situations is bound to end in disaster.

DaveB

billbedford
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Re: Thompson BZ 6 wheel van

Postby billbedford » Tue Nov 12, 2019 1:56 pm

Le Corbusier wrote:There was some discussion relating to 6 wheel clayton coaches on my thread a while back. This post by Mark Tatlow might be relevant ? viewtopic.php?p=61333#p61333


Yes, but going round by China to get to Clapham.

If you have a GW wheel press, and if you build locos and don't why not? the pins are reversible, 3mm and <2mm on the other. So coach wheels can be mounted on the sliding brass tube simply by using the press.
Bill Bedford
Mousa Models
http://www.mousa.biz

Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Tue Nov 12, 2019 3:55 pm

A pic to clarify centre wheel arrangement. I stayed with the kit middle wheel design as far as possible for an easy life. But I think springs outside the wheels rather than inside as here might exert the force required rather more effectively and therefore might not need to be so strong....thus there might not need to be so much weight at each end....
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I doubt I need all this sideplay. I just filed the pinpoints off. There is a tube to link the bearings partly so that the springs won't act directly on the axle. (The brass rod links the 3 wheelsets)

I have to add what a joy this kit is to build. Everything fits absolutely perfectly. Like the bars of the end window frames.
Attachments
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Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Sun Nov 24, 2019 2:29 pm

Van pretty much ready for painting.
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The J hangers took me two modelling days. I can't praise Ian Macdonald enough for his support, sending me extra castings to replace the one I broke, and for the quality of this kit and, no less, the instructions.

My only quibble with the build sequence was, finding it easier to use solder to attach the J hangers castings, it was easier without the batteries in the way, so I removed them.
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Easier without batteries in the way. I always reinforce footsteps.


I'm sure others building this won't forget as I did to push out the door knobs before soldering the bodyshell together :?

Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Sun Jan 05, 2020 5:56 pm

With another project running concurrently progress has been slow. But that allows paint to dry.

Van ready for testing on Calderside. Runs fine on my test tracks. Terrible paint job?....or realistic weathering?....glazing remains to be done once I've decided when to stop.... Couplings to do still.
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Base colour


The weight of the completed van was 108g. Probably quite sufficient in view of my earlier finding 80g was needed but I added 12g weight at each end before testing it, to be sure the weight was sufficient over the ends and not simply 108g of seesaw effect over the middle wheel.

Weathering was done with these paints Dark Grey Leather Reddish Brown and 170 (dark earth?)
Attachments
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Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Thu Jan 09, 2020 9:34 am

Testing on the group layout Calderside the van ran over most of the complex trackwork perfectly but at two locations near or on points derailed occasionally . The non-compensated wheelset was the one jumping the rails. I have put in two more 10g sheets of lead flashing, evenly distributed, so the weight is now 152g, rather than fiddle around reducing the thrust of the springs on the middle wheel, which is quite strong.

I've realised my guitar E string is too thin for my AJ coupling construction jig. The instructions specify 0.011" which is 0.28mm, while the string is 0.19mm. Finding I had some correct thickness wire I then found the jig easier to use and remade one of the couplings. It was while doing this I saw the uncoupling loops were too long. They could have been slightly fouling on the pointwork.

The jig for the uncoupling loops has two settings and I had used the longer one probably erroneously. I made short ones, so the next test if successful won't tell me which of these possible faults was the problem.
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davebradwell
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Re: Thompson BZ 6 wheel van

Postby davebradwell » Thu Jan 09, 2020 8:37 pm

Ah, but will your Jackson uncouple on a magnet, Julian? Almost anyone can make one that "ties on" but uncoupling in a proper manner is the clever bit.

DaveB

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Will L
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Re: Thompson BZ 6 wheel van

Postby Will L » Thu Jan 09, 2020 11:32 pm

Julian Roberts wrote:Testing on the group layout Calderside the van ran over most of the complex trackwork perfectly but at two locations near or on points derailed occasionally . The non-compensated wheelset was the one jumping the rails. I have put in two more 10g sheets of lead flashing, evenly distributed, so the weight is now 152g, rather than fiddle around reducing the thrust of the springs on the middle wheel, which is quite strong.

Interesting that it's the uncompensated wheels that tend to jump ship. The issue with compensated wagons, particularly long wheel based ones, is that the CofG can be fairly close to the edge of the stability triangle, see the explanation of this on the CLAG website. This can lead to compensated wagons tipping enough to derail (I know some people find it hard to believe this is true) and it will be a non compensated wheel that lifts. Despite its 6 wheels, your van is essentially a long wheelbase 4 wheel compensated vehicle.

Of course, compensated wagons inclined this way don't just fall off when stationary. It happens when they are being pulled or pushed (mostly pushed). The strength of the tendency to tip depends on how far the vehicle CofG is above the compensated axles pivot point, how strong any off centre buffering or coupling forces are, and, for completeness, though I doubt its relevance in this case, how fast its going round a corner. It is most common when you consider buffering forces and propelling movements, the off-centre buffering forces pushing produces go a long way to explaining why propelling movements are much more inclined to cause derailments.

In your vehicles case you are using AJs which the picture shows are attached off centre near the middle of the vehicle and the geometry of this will also produce significant off-centre coupling forces when being pulled, particularly round corners. I'm not at all sure adding extra weight will really improve matters much, and, depending on how high in the vehicle it’s put, it could actually make matters worse. It is hard to sure if the sprung centre axle is in any way relevant, I would suspect not.

Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Sat Jan 11, 2020 11:07 am

Hi Dave and Will.

Dave the electromagnetic uncouplers on Calderside are pretty macho. I'll see whether the hoop is now too high to be pulled down. You said earlier you have one of these vehicles - is it a kit of yours, or whose?

Will I refer you to my OP where I experimented with the chassis, weights, springs. Really I have never found this 'problems of compensation' business is what actually happens. I've compensated all my wagons and coaches and had no problem whatsoever. But I do put in extra weight, always as low down as practical. However the 6 wheeler possibly has a heavier roof than the plastic SR van: -

I have an SR parcels van that is a good comparison, being pretty much exactly the same dimensions but with only 4 wheels, this too has never had any problem, though I don't think I've ever hauled it round the Calderside Exchange Sidings which is where I tested the 6 wheeler. These sidings are for coal trains, mostly A6 points, so probably more of a challenge than the van's likely normal operating zone.

I'll test them both at our next meeting next week and put an update here. Attached pictures are of the weight added (the two in the middle are added since this track test), two of Calderside Exchange Sidings. and of the two vans, showing the interesting difference in loading gauge. The SR van must be made for Hastings Gauge and the two together remind me of the Class 206 3R demu units used on the Reading to Tonbridge line, where I were a lad.

Going back to the possible reason the fixed end is the one that derailed, I may try to add a video showing the way the vehicle has much more movement as the fixed end goes into the track dip. Thus the centre axle will have to move further when the fixed end goes into the dip than when the non fixed end does. As the spring force increases as the wheel moves upwards, there will be less weight on the non compensated wheel when it goes into the dip, than on the compensated wheel when it goes into the dip. Anyway that's just a thought to be proven or otherwise on the layout. FWIW on the kitchen scales the spring force is around 70g around mid travel, this very difficult measure accurately.


The non compensated wheels are first and last.
Attachments
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RTR height is a little lower than BZ van which is correct according to buffer height gauge jig.
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Uncoupler visible
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Perth2018 - DAS - IMG_2820-1-1-1.jpg
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davebradwell
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Re: Thompson BZ 6 wheel van

Postby davebradwell » Thu Jan 16, 2020 10:49 am

I prepared a comprehensive reply, Julian, but it disappeared and I can't face repeating it. Essentially I suggest that the springs for your centre axle are too close together so the unit tries to tip rather than slide. It's the same with Cartazzis. Also I can't understand what the central piece of wire is doing as it would appear to fight the springs and be doing no good. Suggest you replace with Mark Tatlow's arrangement with springs outside wheels. Essentially, if it won't stay on the track at 1 oz per axle there's something wrong.

Suggest also you get buffers at correct height on all vehicles as these must be playing a part in holding it off the track.

DaveB

Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Thu Jan 16, 2020 1:50 pm

Thanks Dave - yes it's so frustrating when some thoughts disappear like that.

davebradwell wrote: Essentially, if it won't stay on the track at 1 oz per axle there's something wrong.


The thing does stay on the track! This was a test to destruction on much worse trackwork than it will normally go on.


I tested it again this week. There were three places it derailed last week, the most interesting one being on a triple point which main line locomotives travel over bringing the coal trains in to the sidings. When taking the right hand route there is quite a severe elbow where the set occurs.

This week it derailed again here the first two times it went over it but thereafter it wouldn't derail on 10 repeated traverses, each way round, propelled and pulled. I hadn't lubricated the centre wheel, and I reckon the oil that was there got spread along the axle by its first moves and thereafter was more free to move sideways within its sideplay allowance, and in the sliding up and down part.

The second place it derailed last week was completely surefooted. The third place causes nearly everything to derail and is simply a track fault (this is new trackwork since the photos were taken and not yet fettled and proved.)

I think you misunderstand Dave about the centre wheel unit - it does not move at all, only the wheel moves, up/down and sideways within the unit. The design is for the unit itself to move but this is what I've modified. The wires as you call them are meant to steer the outer axles from the movement of the centre unit - but I've soldered them so they cannot move except to allow one end to rock, and they simply align the centre unit in its place.

Yes springs acting outside the centre wheels would be better, they could be lighter for the same effectiveness I'd have thought, so the overall weight could be reduced. But thinking out how to actually do it yet allowing all the sideplay needed wasn't worth it as this is how the kit is designed - and it works. :thumb

You didn't answer my question about whether your van is one of your kits....? I haven't tested the uncoupling yet.

Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Mon Feb 24, 2020 4:27 pm

Van is finished now, with numbering and weathering advice from friend and guru Allan Goodwillie. Apparently however dirty it became the number would always be wiped to keep it legible. (I'd hoped to get away without bothering with numbering...)
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Here it is at a previous stage being tested on 'Kettlewell'. With no derailment on the Model Rail Scotland show this last Saturday I think it's passed Guy's test (see OP)

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Noel
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Re: Thompson BZ 6 wheel van

Postby Noel » Mon Feb 24, 2020 6:34 pm

Like most passenger luggage vans, PMVs, CCTs, full brakes and brake/passenger coaches in BR days, BZs carried "Load 'X' Tons Distributed" close to the bottom of the side at the van end, RH end both sides for full brakes. 'X' is 5 for Thompson BZs, and the two photos I have seen show it almost directly under the number. It was usually reasonably legible as well if the vehicle wasn't absolutely filthy.
Regards
Noel

Julian Roberts
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Re: Thompson BZ 6 wheel van

Postby Julian Roberts » Tue Feb 25, 2020 9:35 am

Yes Noel I remember that generally on passenger guards coaches. The model kit comes with reference photos of the preserved BZ at the Great Central Railway. Here is one of them. Seems your message needs to be told there too. Or maybe they hadn't quite finished when the photo was taken. Thank you for raising the bar!
Attachments
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Screenshot_2020-02-03-22-54-49-1.png (674.92 KiB) Viewed 904 times

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Noel
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Re: Thompson BZ 6 wheel van

Postby Noel » Tue Feb 25, 2020 4:08 pm

I should perhaps have pointed out that my comment only relates to the later BR practice of right hand numbers, as you have used; the load limits seem to have appeared more or less at the same time as the change from left hand numbers [officially early in 1951, but it took until about mid-1952 for some trade builders to comply].
Regards
Noel


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