Thompson BZ 6 wheel van

Julian Roberts
Posts: 761
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.
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Andrew Ullyott
Posts: 166
Joined: Fri Aug 08, 2008 2:31 pm

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
Posts: 1282
Joined: Mon Aug 10, 2009 7:49 pm

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
Posts: 761
Joined: Sat Jan 09, 2010 11:33 pm

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
Posts: 1306
Joined: Wed Feb 11, 2015 3:39 pm

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
Posts: 87
Joined: Fri Jan 18, 2019 3:48 pm

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


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