Problems with Loco Bogies

[ArthurK]

As you may know I have a thread "Arthur's Workshop". In that I have outlined the etches available from my efforts in that direcction. One of these is th NER S2, otherwise known as the LNER B15. What I would like to know is the feelings amongst you as to the best solution to this problem.

 

The NER on their earlier designs used a large diameter wheel for their bogies. Normally these were 3' 7 1/4" but the D20 used 4' 0". However the frames were not cut away to allow passage of the wheels below the frames. As an alternative the frames were inset at the front. The B15 for example had a parallel piece of frame at the cylinder location tapering outwards behind to achieve the normal frame spacing. In front of the cylinders the frames tapered inwards to a distance apart of 3' 7 1/2". The main portion of the frames was 4' 0" apart. This worked fine in the real thing but in models it is unlikely that we can achieve anything approaching the equivalent track radii.

 

There are three possible solutions:

1. Reduce the bogie wheel diameter to pass under the frames.

2. Cut the frames away to allow 'near' scale diameter (say 3' 6")

3. Inset the frames much more the the prototype to obtain the necessary sideplay.

 

The first I personally would dismiss the difference in diameter is too great and would alter the front end appearance too much.

 

The second is more attractive because the problem goes away but the frame cutouts would be quite big and the changes would be visible on close inspection.

 

The third does work (I used this on the Chivers D20), but the inset is severe if you wish to traverse curves of less than 40" (1 metre). It also poses the problem of what do you do with the front guard irons. If they are attached to the frame then splaying the ends to the track gauge begins to look rather odd. They could of course be attached to the buffer in the normal location. Then the side view might be a litte odd. In P4 the distance over frames is normally 16mm. On the S2 the distance over the frames at the front woul have to reduce to no more the 11mm.

 

I would like your opinions of which solution would be best for your own purposes. I especially want to hear from the P4 modellers. Your requirements are probably the most severe os looks and 'true to scale' take precedence.

 

Please feel free to add you comments and argue amongst yourselves. This problem is not going to go away. I might add that in their later designs the NER used a smaller wheel which did pass below the frames. The Class V was in two parts the first was similar to the S2 whilst the V09 variant used parallel frames and smaller wheels.

[Miss Prism]

Hmmm. Yes, that's quite a conundrum. As a P4 modeller, I would strongly favour your option 3. Having said that, I think any prospective P4 purchaser should not expect these close tolerance prototype locos to be capable of going round 3' radius model curves. I think that demand is unreasonable. I think your design radius should be 4' 6". Accordingly, I don't think your 11mm frame front frame spacing will apply, and something more generous could be provided. Secondly, you don't have a crankpin to crosshead clearance problem, so the front axle could be given generous sideplay. If this invokes front wheel to slidebar clearance problems, then offset the slidebar axis to the cylinder 0.5mm wider than it should be - it will be impossible to spot, only you will know about it, and it will allow the front driver that generous sideplay. The next problem is the bogie wheels hitting the back of the cylinders when the bogie sideplays. Suggest don't include the rearmost bit of the cylinder, and/or increase the bogie wheelbase by say 1.5mm - again, no one will know except you. You'll need every trick in the book to get that 4-6-0 around sharp curves. Guard irons? - give people a choice, parts for either on the frame or on the back of the bufferbeam. Let them choose their poison.

[ArthurK]

My solution so far is as I said above to narrow the frames by 1mm at the cylinder location (they were parallel here). The 11mm over frames at the front is based on a design radius of 40" in P4. However having said all that the front frames could be set to any distance that the builder feels is correct for him. There is no spacer ahead of the cylinder-block. A "custom" spacer could be added to stop them flapping about. Pehaps I should incorporate a spacer the simply slides though the frames to set the required taper for the front frames.

 

The point of offset slidebars has already been incorporated into the masters for these. They were built to the prototype then filed from the rear by rather more than 0.5mm, that coupled with a flat rear to the crosshead has probably saved almost 1mm in total. This is vital on the Q5 which used the same slidebar/Cylinders as on the B15. Full scale clearances on the Q5 was 1/2" !

[Flymo748]

My preference would also be to narrow the frames slightly to accommodate more lateral movement. Looking at prototype rolling stock (as I do every day on my commute) one of the things that strikes me most about the difference between the prototype and models, particularly 4mm RTR, is how they "sit" on the track.

 

It's the space that exists between most models' bogies and the underneath of the body that is one of the biggest reasons for loss of realism, IMHO.

 

If I can get the "sit" of my models right, and the corresponding lack of daylight in that region, I'll be very pleased.

 

On the subject of guard-irons, is it possible to include them separately, with a half-etched fold line so that you can use a small extension to make an L-shape, which can be located in the appropriate place under the running plate? Perhaps you could also include an etched guide marking on the underneath face to aid location?

 

HTH.