Bill Bedford Frames for GWR 56xx

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Bill Bedford Frames for GWR 56xx

Postby MartinBurgess » Mon Apr 20, 2020 1:56 pm

I’ve recently put together one of Bill Bedford’s (Mousa) frame kits for a 4mm GWR 56xx class but have run into the problem laid out below and would be interested in discovering if this has been reported before and of any comment you may have.

The problem is the overall width of the frames.
Although the frames are a precise scaling down of the prototype ‘over the frames’ dimension the practicalities of model building make them too wide to accommodate the scale wheel back to back setting.
I’m building in P4 so my BB measurement is even greater than the EM gauge the chassis is designed for.

The dimensions, prototype and scale are laid out here
Prototype, Scaled to 4mm, P4 Standard mm, Inches, Frame to wheel gap
Back to Back 4’ 5.675” 17.875 mm 17.63 0.696
Over Frames 4’ 3.5” 17.166 mm 0.676 Only 0.010”

Mousa Chassis
Dimension over frames. Measured from assembled kit 0.676” Precise for prototype
Bearing flange (external to frame) 0.012 X 2 0.024” Unavoidable addition
Total width dimension 0.700” Oversize by 0.024”

Added to this is 0.030” which is made up of the rear hub boss of each wheel which is 0.015 proud of the tyre.
These dimensions added together is still just for a tight fit so I would suggest a float of 0.005” making a total of 0.735”

Assembled model frame Overall, including wheel bosses 0.735”
P4 Standard Back to Back 0.696”
Frame oversize 0.039”

I would therefore suggest that the bare frame dimension should be reduced to –
P4 Standard Back to Back 0.696
Minus Bearing flanges 0.012” X2 -0.024
Minus wheel rear hub excess 0.015” X2 -0.030
Minus end float 0.005” -0.005
New bare frame width 0.637”

Please excuse alignment of table info. It looked OK when entered but has lost alignment on being published. I haven't sussed that yet.

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Re: Bill Bedford Frames for GWR 56xx

Postby billbedford » Tue Apr 21, 2020 7:43 am

The frames are correct. The wheels you have used have unnecessary, and un-prototypical bosses on the backs.
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Re: Bill Bedford Frames for GWR 56xx

Postby PeteT » Tue Apr 21, 2020 9:11 am

Hi Martin,

I started some discussion on the dimension over frames on my 3P topic recently:

While looking into this I came across this topic, which includes (halfway down) a calculation by Russ Elliott to determine the sideplay requirement based on wheelbase and the minimum radius.

Edited to add:
from my topic, I specifically did the maths from scale frame dimensions (based on the 3P 8' + 8'6" wheelbase):
[Just for fun, doing the maths the other way - ie prototype frame setting of 17.167mm, and B2B of 17.75mm - wheel bosses thinned to 0.1mm each side gives V of 0.1915mm. With a 32+34 = 66mm wheelbase, radius = (C*C)/8V = 9'3".]

This is based around High level hornblocks, with hornguides mounted inside the frames so outside of blocks level with outside of frames.
Last edited by PeteT on Tue Apr 21, 2020 11:48 am, edited 1 time in total.

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Re: Bill Bedford Frames for GWR 56xx

Postby Philip Hall » Tue Apr 21, 2020 10:22 am

The wheels we mostly use (Ultrascale and Alan Gibson) do have a boss on the back which is actually very useful. It provides a greater surface area for the axle to grip the wheel, never mind whether we choose to pin or not to pin.

To attempt to use a prototypical frame width on a working model to 4mm scale begins to take us down the route some ScaleSeven people took many years ago, and there were quite a few articles about how to gain a few thou here and there to get engines around any kind of a bend.

Martin’s dimensions indicate to me that the frame width is unnecessarily wide here, but then I belong to those amongst us who like about 1.5mm under the b-b dimension to allow room for adjustments. I did also get a bit confused by the mixture of imperial and metric figures; we are used mostly to metric (apart from colloquially referring to ‘a few thou’ and on odd occasions when I only have imperial measuring tools), and I find it easier to judge these kind of dimensions in metric alone.

Granted the end result will look very nice, but
we just can’t scale these kind of dimensions down and my possibly heretical opinion is that it’s bonkers to try!


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Re: Bill Bedford Frames for GWR 56xx

Postby bécasse » Tue Apr 21, 2020 11:29 am

Philip Hall wrote:Granted the end result will look very nice, but we just can’t scale these kind of dimensions down and my possibly heretical opinion is that it’s bonkers to try!

I can but totally agree with Philip Hall on this. It is, perhaps, worth remembering that the originators of P4 carefully developed standards that were as accurate as practical but which also took into account the tolerances required to produce reliable working models, in terms of both the manufacture of the ensemblage of components and the general ability to assemble those components. While many things have moved on in the last fifty years, I would suggest that those founding principles are as relevant to the majority of finescale modellers today as they were then.

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Re: Bill Bedford Frames for GWR 56xx

Postby grovenor-2685 » Tue Apr 21, 2020 12:37 pm

Most of the originator's efforts can be found here
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Re: Bill Bedford Frames for GWR 56xx

Postby zebedeesknees » Tue Apr 21, 2020 2:31 pm

As if my workbench wasn't cluttered enough, this thread has caused me to bend up some more frames..

I have a couple of early ones, one may be the first test, as it is a different colour material to the others, and it didn't come with the overlays. That, and the next made-up one with wheels, are to the same design as the .pdf on the Mousa site, ... ames4.html having cuboid bearings with fixed anchors soldered to them.

On reading Martin's post, I looked at the two new versions on the shelf of shame and realised that Bill had changed the design to his curved top of bearing and frame hooks idea. Hence one came out to be folded and measured. This measures 0.630" across the basic frame before overlays. The first one above measured 620, the second 630, but.. because the bearings dangled in the slots flush with the outside of the basic frame, there were no flanges nor overlay in the way, saving the 15 thickness of the bearing flange and the 15 thickness of the hornguide each side, bringing the distance between the bearing faces up to 690.

The distance between the bosses on the backs of my Gibson wheels, set to S4 b-b, is 685...

Amongst the workbench clutter there happens to be a Mousa 3F frame nearly done, to the same design, and the basic frame width before overlays measures 0.595". Such conclusions as you may wish to draw are your own!


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