Converting a Bachmann N Class to as-built U Class in EM Gauge.

[Stevers]

This appears to have been a well trodden path in terms of speculation, but a bit short on finished engines. It is absolutely not about creating one of the Rebuilt River U Class - such as my long running project based on a DJH kit, with scratchbuilt cab and running plate, with SEF twin beam compensated chassis. This engine now runs and is waiting for valve gear and of course the correct pattern wheels as well as a lot of detailing.


Only the Bachmann boiler and valve gear would be of any use in such a conversion as the running plate height and width is different, and the River Class based cab width and shape is completely different to the N Class

So this thread is solely in respect of the as-built U with the high narrow running plate with tiny splashers. I have the SEF U Class chassis kit, and was going to build the Ex Wills whitemetal kit to go on it, but I also have an early Bachmann model of 31813 with a slightly dicky mazak running plate. I've now sourced a new Bachmann body for it making the old body available for conversion to a U Class, so I thought I'd start with a list of what would need to change.

The N Class coupled Wheelbase is wrong. Addressed by building the SEF U Class chassis to EM Gauge with twin beam compensation, and incorporating the Bachmann valve gear.

The wheels on a U are 6' not 5'6". This means that frames, valve gear, running plate and boiler are 3" higher on a U Class than an N, but that the cab and buffer beam are the same height. The chimney and steam dome also end up 3" higher, but still within the loading gauge. The plan here is to build a new running plate that will mount to the front of the Bachmann running plate where it's vertical (thus retaining the Bachmann buffer beam), the new running plate being carried through to the rear where the steps are mounted.

Changes to the Cab. The handrails on the cab side are 3" higher on the U so that they align with the boiler hand rails. Inexplicably the U running plate comes back further under the cab despite the coupled wheel base being shorter. The increased height of the boiler compared with the cab required a different cab front. I have a DJH cab front etch that might work, if not I'll scratch build something suitable. The Bachmann cab sides will need the hand rails moved and the bottom of the cab re-shaped to fit the new running plate.

The intention is to use as much of the Bachmann N as possible. Once my Crownline Q is finished, I'll do the conversion in tandem with finishing the Rebuilt River. So, have I missed anything?

 

[Stevers]

This is a trace (using QCAD) of a Eastleigh Weight Diagram of an as-built U Class overlaid on the Weight Diagram for an N Class, both taken from Southern Locomotives by JH Russell. Both drawings had to be scaled to 4mm scale from known dimensions for both height and wheelbase length, before they could be compared. The difference in running plate height at 1mm is less than the depth of the chunky Maunsell running plate. The N Class has a rebuilt front end making it longer. These are just Weight Diagrams, and the smokebox on one of them can't be quite right.

A trace of the GA from the Book of the Moguls Part 2, planted the running plate bang on top of the N Class Running Plate, so I think I'll trace the N Class GA that I recall being in the MRJ for the Bachmann P4 conversion to see how that compares.
[Edit] The drawing in MRJ is just for the 4000g tender, not the loco.
[Edit2] The footplate rise on the N Class model is 5.5mm and that on the scaled U Class GA drawing 6.5mm. Current thinking is that the Bachmann N Class is 0.8mm longer than the U Class as drawn in the GA. The position of the rise in the running plate at the front of the N tallies with the U Class GA, and if this tiny disparity in overall length is correct, it will be lost under the cab evenly spread fore and aft of the running plate drop - certainly not something to worry about anyway.
[Edit3] The Bachmann N overall length tallies with the sum of the dimensions on the Eastleigh Weight Diagram so any error is in my tracing of the GA or in the multiple scans of the GA.

 

[Jay Addley]

This appears to have been a well trodden path in terms of speculation, but a bit short on finished engines. It is absolutely not about creating one of the Rebuilt River U Class - such as my long running project based on a DJH kit, with scratchbuilt cab and running plate, with SEF twin beam compensated chassis. This engine now runs and is waiting for valve gear and of course the correct pattern wheels as well as a lot of detailing.

View attachment 189656
Only the Bachmann boiler and valve gear would be of any use in such a conversion as the running plate height and width is different, and the River Class based cab width and shape is completely different to the N Class

So this thread is solely in respect of the as-built U with the high narrow running plate with tiny splashers. I have the SEF U Class chassis kit, and was going to build the Ex Wills whitemetal kit to go on it, but I also have an early Bachmann model of 31813 with a slightly dicky mazak running plate. I've now sourced a new Bachmann body for it making the old body available for conversion to a U Class, so I thought I'd start with a list of what would need to change.

The N Class coupled Wheelbase is wrong. Addressed by building the SEF U Class chassis to EM Gauge with twin beam compensation, and incorporating the Bachmann valve gear.

The wheels on a U are 6' not 5'6". This means that frames, valve gear, running plate and boiler are 3" higher on a U Class than an N, but that the cab and buffer beam are the same height. The chimney and steam dome also end up 3" higher, but still within the loading gauge. The plan here is to build a new running plate that will mount to the front of the Bachmann running plate where it's vertical (thus retaining the Bachmann buffer beam), the new running plate being carried through to the rear where the steps are mounted.

Changes to the Cab. The handrails on the cab side are 3" higher on the U so that they align with the boiler hand rails. Inexplicably the U running plate comes back further under the cab despite the coupled wheel base being shorter. The increased height of the boiler compared with the cab required a different cab front. I have a DJH cab front etch that might work, if not I'll scratch build something suitable. The Bachmann cab sides will need the hand rails moved and the bottom of the cab re-shaped to fit the new running plate.

The intention is to use as much of the Bachmann N as possible. Once my Crownline Q is finished, I'll do the conversion in tandem with finishing the Rebuilt River. So, have I missed anything?

Lovely model in the photo, did you ever finish it?

 

[Stevers]

Thank you for your kind comment! Unfortunately it's one of a number of interesting (to me at least) projects that I have on the go. This last week I've been back in the workshop after a long break with a quick job to fit my not very 4mm scale Mercian Models Armstrong Whitworth Diesel with a 4mm scale 3D printed body that I've designed having been blessed with a lot of 3D CAD time recently.

In terms of messy modelling, there's an etched brass Q Class to finish, and then a W and the Rebuilt River that will need valve gear built from the SEF chassis kits (started but not finished). We've been full time carers for the last 18 months, and that looks set to continue. Right now I think any progress on anything is good! The CAD time has proved compatible with caring and most of that time has been spent on 3D modelling the coaches and vans of the 1884 Downton Train Crash, and it would be better to finish some other projects before I cut metal on that. The plan from now on is to try to do a little bit in the workshop every day, and that way I shall eventually get to my Rebuilt River and this as-built U.