Garden Railway Forum

Okay so the title doesn't fit exactly but I couldn't think of what else to call this... Basically I bought myself a copy of Peter Jones' "Building Small Steam Locomotives" and inside there is an example of a locomotive I love the look of but as far as I know there are no actual drawing for a real-life full-size prototype, but all the measurements are there to build a model of the loco in 2.5" gauge. How would I convert these measurements into those suitable for 15mm Scale (for example)?

I don't know how to do this but im sure theres a simple answer

Use the "reduce" option on a photocopier? Youd have to work out the %age reduction.........

Unfortunately using the "reduce" option on the photocopier will only reduce the size of the drawing. If only it were that simple I am talking about changing the measurements of a drawing so that they conform to 15mm scale instead of 2.5" gauge. I could work out the percentage I guess if I had a drawing of a full-size prototype, but I don't and even then I may as well use the full-size prototype instead of the 2.5" gauge scale.

I hope I am making sense... This all seems perfectly logical in my head as I type it.

EDIT: Just a thought, but how did Roundhouse and Accucraft decide on the measurements for their models which aren't based on any real-life prototypes (e.g. Ragleth, Katie, Millie, Lawley etc)?

If your wanting to model it in 15mm, then your ratio is 15mm=1'.

Therefore 2'6"=37.5mm

I know all that. That's not what I am getting at.

I have a drawing of a 2.5" gauge locomotive. As far as I know there is no real-life standard gauge prototype to base it on. Someone just created it. What I want to know is how can I convert the measurements on the drawing to measurements suitable to create a 15mm scale locomotive.

HereticUK:90248 wrote:I know all that. That's not what I am getting at.

I have a drawing of a 2.5" gauge locomotive. As far as I know there is no real-life standard gauge prototype to base it on. Someone just created it. What I want to know is how can I convert the measurements on the drawing to measurements suitable to create a 15mm scale locomotive.

Now that's an interesting idea, trying to scale something that doesn't even exist. If you have no existing scale drawing then downsizing of an existing "supposedly" scale drawing can only be done by guesswork and experience. If the only measurement that you have is the gauge then surely only the back to back measurement has any meaning -and only then if it is based on a particular style of loco. As the loco does did not ever exist even that is meaningless.

As to certain manufacturers rubber rulers, the less said the better. If it looks right it is right is about all that can fairly be said.

That's all I needed to know - I was simply curious as to whether it was possible or not, I didn't mean to cause any problems or anything. If I have offended anyone then I am sorry.

Maybe a way for you to proceed would be to try and guess how an average-sized man would fit into the engine in question, and scale your model accordingly. For instance, if you look at the doorways or the height of the cab roof above the footplate on your 2.5" gauge locomotive, and give that a realistic but arbitrary value, for instance you could assume the cab roof in the centre is six feet above the floor. Then it gives you some idea of scale right there. Since there is no prototype in full size, you could just perhaps look at pictures of full-size engines of a similar type and note their size, and make an assumption there too about the size your locomotive should be scaled down from that. This must be what manufacturers have done with freelance engines.

Here's an example. The height of the doorway on my Roundhouse engine (a Billy) is 87mm. Assuming the "Billy" is supposed to be 16mm scale, (which it isn't because it's "freelance" style engine but it is meant to run with 16mm scale stuff) then the height of the doorway works out to about five feet, five and a quarter inches. And the height of the cab in the middle seems to be a scale six foot, eight inches.

In the same way, you could take a picture of a full-size engine that is a similar size to that which you want to represent in 15mm scale, and figure out the height of some common part like the cab doorway. Assume then that cab doorways tend to be relatively similar heights from engine to engine, measure the one on your 2.5 inch gauge engine and use that to figure out what "scale" it is. Then you can figure out how big the engine should be to look right in real life, and use THAT imaginary scale to figure out how big your 15mm scale engine should be. Not exactly scientific, but it should result in a pleasant-looking engine. I would use the size of the cab doorway simply because while engine size is widely variable, the size of cabs and doorways is less so because people need to fit in them!

I hope this makes sense.

Is the freelance loco standard gauge or narrow gauge?

2.5" gauge (Ga. 3 in the UK) usually implies either 1:24 or 1:22.3 standard gauge modelling - even if the precise loco is freelance.  1:24 is roughly 12.7mm to the foot, and 1:22.3 is something like 13.5mm to the foot.  I don't know if that is any help, but it might give your a starting point...

Peter in AZ

HereticUK:90248 wrote:I know all that. That's not what I am getting at.

I have a drawing of a 2.5" gauge locomotive. As far as I know there is no real-life standard gauge prototype to base it on. Someone just created it. What I want to know is how can I convert the measurements on the drawing to measurements suitable to create a 15mm scale locomotive.

Seems like you mix up track gauge and model scale. (?) I don't understand what you actually want.
Do you have a detailed drawing with measurements and all?
Do you actually want to change the scale, or just the track gauge?
What track gauge do you have?

LnBmad:90253 wrote:Is the freelance loco standard gauge or narrow gauge?

This is a good question. If the loco is standard gauge, 2.5" (63.5 mm) track gauge would mean that the loco might be G scale and not so far from the scale you want.

I really don't think I thought this question through at all... Now I've had these questions and comments plus a night's sleep under my belt, I realise that I am a moron and that if I had thought about it a bit more I would have the answer I was looking for. Sorry about this guys, I really do need to learn to think before I post.

I suggest you are confused by scale and gauge.

A drawing of a 2.5" gauge locomotive is meaningless unless you know the scale, or can make a guess of the author's intentions, is it intended as a narrow gauge or standard gauge engine?

If you believe the drawing is of a standard gauge loco, then the then the scale is going to be in the region of 12mm/ft , you want it at 15mm/ft so all dimensions are multiplied by 15/12 =1.25.

This will give a gauge of about 70mm, for a scale of 15mm/ft.

However, if it is narrow gauge, based on 2ft (60cm) gauge, then the scale is about 32mm/ft, 15/32 = 0.46, so all the dimensions are multiplied by 0.42.

This will give a gauge of about 30mm (63.5 x 0.42), for a scale of 15mm/ft.

3ft prototype, scale approx 21mm/ft so scaling factor 15/21 = 0.7, gauge of about 45mm, also for a scale of 15mm/ft.

However, if you are working in a specific gauge, say 32mm, 1.25" then the factor is 1.25/2.5 = 0.5 so all dimensions are multiplied by 0.5.

45mm, 1.75" gauge 1.75/2.5 = 0.7 etc etc.

Thanks for the help guys. I am really sorry about all this; I was getting myself really confused and ended up confusing everyone else.

HereticUK:90259 wrote:I really don't think I thought this question through at all... Now I've had these questions and comments plus a night's sleep under my belt, I realise that I am a moron and that if I had thought about it a bit more I would have the answer I was looking for. Sorry about this guys, I really do need to learn to think before I post.

I think I understand the question well enough. You have drawings of an engine which is made to run on 2.5 inch gauge track. You like how it looks and you would like to make a steam engine that looks just like it, only smaller, so it will look right pulling a 15mm scale train. You want to make a set of drawings for the smaller version of the locomotive, but since there is no full-size prototype to measure, you aren't sure how big to make the smaller version so that it will look like a 15mm scale engine. Am I correct?

The gist of my comment above was that while real-world locomotive sizes vary greatly, in my opinion cabs tend to be pretty standard. So I would measure the cab on the 2.5 inch gauge loco, compare it to the size of the cab on a full-size locomotive of a similar style to yours, and thusly divine the scale. Then it should be easy to work out a conversion factor to make a smaller engine that has the same appearance, but looks right on a 15mm scale train.

Not difficult at ALL!

Indeed not.

Whether it is difficult or not depends on your skills.

I am curious so I had a "look inside" the Peter Jones book at amazon, and in the "table of scales" 2.5" gauge (64 mm) is referred to as standard gauge in various scales. So I assume what HereticUK wants is to regauge the loco to narrow gauge on 32 or 45 mm track and more or less keep the scale. Maybe scale a little bit up.

Questions are never stupid. Only answers can be.

Questions are never stupid. Only answers can be.

Well, you say that, but if I hadn't neglected to mention all of this in the question in the first place... It was late at night and I was in a lot of pain and was unable to concentrate properly. It is my fault entirely.

Despite that, I thank you for your help. I would say that my skill level in this is absolute beginner. I was merely curious as to the possibility.