S7 nominal frame dimensions?

Steph Dale

Western Thunderer
Guys,
I'm currently drawing up a set of loco frames for etching and have got the 'finescale' dimensions about where I need them.
I'm pondering adding a set of frame spacers for S7 to the artwork, but am not sure whether there is a useful nominal overall frame dimension I should use or am I better just drawing it up to the scale dimensions of the prototype?
Steph
 

richard carr

Western Thunderer
Steph

There isn't one to my knowledge, in theory it should be the prototype width but this depends on the prototype and how widening the frames will impact on cylinders, J hangars etc, as widening the frames can mean narrowing these.

Martin Finney kits would include 3 frame spacer widths but even the one for S7 would still give frames less than 29 mm wide so you could use them for finescale if you wanted to.

Richard
 

Dog Star

Western Thunderer
Steph,

Where you "put" the frames is going to be influenced by a number of factors...

* the hassle factor if you are trying to produce a set of frame plates to work with either FS or S7 frame stretchers. Richard has given some pointers to the related problems, you can read Richard's comment as "any casting or etching where the dimensions of the model component are dependent upon overall width of the frame" - for example:- a brake cross shaft casting which is suspended from brackets attached to each frame plate, the issue here is that a casting which works with FS may not be long enough tom work with S7;

* scale equivalent of prototype dimension "over frame plates". If you go with scale width then be prepared for your S7 track to be laid to a radius compatible with the "dead slow" curvature written on the relevant weight diagram;

* scale equivalent reduced to improve running. How long is your piece of string? How good is your engineering in regard to achieving working clearances? How much of the (rest of) the loco are you prepared to compromise?

To give us some idea of the extent of your task, what is the prototype?

regards, Graham
 
Last edited:

Eastsidepilot

Western Thunderer
Steph,
I have built a few S7 loco's and found that you need to be aiming for about 29mm o/a the frames, this will give you enough room for any wanted side play in the axles for tight track curvature as pointed out by Graham. The points he and Richard make are all relevant to designing and building loco's in S7.
S7 ain't complicated it's just working to a set of standards as you know doubt know mate but mixing standards in one kit is sometimes awkward.

If I was designing my own kit I'd draw it for S7 and then add the additional bits for F/S if that is possible, really depends on the prototype I suppose.

I've seen F/S kits with big chunks cut out of belpaire fire boxes to make the wheels fit or the box is fudged and reduced in scale width to make it fit, but then it's useless for S7.
Big dia. wheels, splashers and boilers on the same loco don't work well either in F/S.

Col.
 

Steph Dale

Western Thunderer
Thanks guys, I can see where you're coming from. I'll put a sketch together in CAD and see how much clearance I actually need to get it around a suitable radius and allow for that in the design. The 'finescale' version managed 6ft radius, so tightening the tolerances on that should help me see where I need to get to.

Cheers,

Steph
 

DavidinAus

Western Thunderer
Of interest is perhaps what David Sharp and I have done with the Stanier 8F kit.
I looked initially at the correct scale width of 30mm outside the frames, but decided that this gave me too little room for error/sideplay/whatever, so I asked DS to make up the frame stretchers, etc (there is a full set of "prototype" frame crossmembers in the MOK kit) to allow a 29mm outside dimension.
It seems to have worked well. There are perfect-to-scale subassemblies for the front and rear ends of the locomotive, which represent frames at the ends of the model, and they are not noticeably different from the main frames. In fact the front frame subassembly clamps neatly around the main frames to keep the body aligned.
So I would advocate 29mm as the best compromise.

David
 

rusty

Active Member
Yes I agree - 29mm outside frame measurement has worked for me and allows for a bit of mid axle drift if needed on a six coupled chassis and above. This has got my 28xx to negotiate 6' rad. in S7. However, I see from my notes that my 2251 was built with frames at about 29.5mm outside with no ill effects. The type and size of hornblocks used is, of course, relevant.
Julian
 

Dan Randall

Western Thunderer
With apologies to Steph for the thread hijack, is there any progress to report on your latest layout please Julian?


Regards

Dan
 

rusty

Active Member
Will do a WT 'layout' thread update on my Great Malvern project soon. Should be small bit in the next S7 newsletter.
Returning to frame widths I should have noted that my main problem has been with a large GW 2-6-2T due mostly to the radial axle but reducing the overall frame width a tad and thereby providing an opportunity to increase drift on the mid and rear coupled axles, has helped. I am trying to avoid a temptation to build a 72xx 2-8-2T!

Julian
 
Top