Relative min. radius difference between O and HO

I believe, if you were to measure the HO wheels and multiply by two, this would result in a fairly heavy, or coarse 0 Gauge wheel.

Back in the day in 0 Gauge, most rolling stock would negotiate 24" radius, 0-48, but Marklin, Hornby, Lionel (I believe) and Bassett-Lowke all developed 36" radius, 0-72, for their grander offerings, and this quickly became the pre-War desideratum for any model railroad with pretensions to reality.   Let us say this 3ft radius became widespread, or at least widely availalble c1935.

Curiously, HO is half British 0 Scale, modelled to 7mm/ft or 1:43.5, still the largest scale employed for modelling.   This came about owing to the relatively small British loading Gauge and the requirement to enlarge the models if the then German commercial mechanisms were to be used inside the locomotive bodies.

But, this resulted in British models actually having slightly too narrow a Gauge, just as the American models run with one that represents 5ft.  The correct Gauge for the British scale should be 33mm, not 32mm.

Now, if you halve the correct gauge you get 16.5mm - HO.

I presume they chose the British scale as a basis since in the 1930s they needed all the space they could get for mechanisms, and choosing the largest version gave them the easiest time.

Anyway, half of 3ft radius is 18".  I do not know how H0 wheels have changed over time in the USA, but with the messy example of the development of 0 Gauge, behind them, manufacturers tried to standardise on something sensible from the start.

Just to complicate matters, when British HO was developed, the old problem that our trains are small came into view again, and the aboption of 4mm/ft scale, 1:76, was used to allow standard mechanisms in the UK market.  This is the established British modelling scale, known as 00 Gauge ever since the introduction of Hornby-Dublo in 1938.

I would welcome contradiction or confirmation of this from anyone who knows different! 

To me 18" (36" diameter) for HO and 27" Diameter for O 27(31" for O) is puzzling. They can make a larger train go around a tighter turn? Don

I think they took advantage of the smaller scale to incorporate greater realism, higher standards if you like.

Once the train set would fit on an ordinary dining table, there was no need to go to the relatively extreme measures adopted to allow 0 Scale to be used in relatively small spaces.

I believe 9" radius, 18" diamater, has been used in 0 gauge in the early days for the most basic outfits,  and some of this track has survived to prove it.

Imagine an HO set employng 9" diameter!

I think HO trains are designed for 18" radius curves since that was the most commonly used sectional track radius throughout much of the history of HO and HO packaged sets, and many modelers still use those curves.

2 rail O scale had no such sectional track history nor requirement, so minimum radius was solely related to lead and trail truck clearance and swing, side-to-side play in drivers, amount of firebox piping and detail that would impede swing, etc., etc.

3 rail proves that you can get almost any engine around any curve if you take enough liberties with realism, for example,  swinging pilots, reduced size of pilot wheels, complete lack of underframe detail in the lead and trail truck area, lack of detail parts that could impede swing, use of blind drivers, truck mounting of couplers vs. body mounting, etc. etc.

While not resorting to the extreme compromises in O gauge 3 rail, many HO manufacturers take just enough liberties to get their engines around 18" radius curves.

The minimum radius of a 2 rail O scale model is also largely determined by how few of these compromises a manufacturer is willing to make, and what the typical consumer is willing to accept. Most 2 rail O scalers seem to prefer fewer liberties with accuracy, and have come to accept the fact that their locos will, therefore, require large radius curves.

Jim

I'm surprised at the high number. My MTH N&W J will get around much tighter curves. I have been told I have good trackwork, though.

Jim

What you wrote chrystalised the situation for me.

0-72 was developed for 0 Gauge to allow good running of the larger models progressively being commercially developed by the principle firms.  They were obviously sufficiently popular that the trade realised that the nacent HO standards should allow models of large prototypes from the start.

This seems to have been the reason why they chose the equivalent to 0-72 when sorting out what they were going to do for HO.   As a child, I bet you couldn't wait to get a big engine, and I don't suppose children who like model trains have ever changed.

If you combine this desire with the fact that 18" radius would fit on a table top, why go tighter?

G scale is strange also in that 48" diameter seems to be the standard. This makes the track just slightly to big for a 4 x8 sheet of plywood . I did see 31" G track on Ebay but it is not common. Don

Hi Christopher. I am a big fan of your layout. What kind of radius will your J take?

Another comparison would be the Sunset/3rd Rail Erie 2-8-4 Berkshire that I would like to have. The 3 rail engine can negotiate O-54(27"r) while the 2 rail version is listed as 56"r. I understand that the 3rail model has blind center drivers, but man that's a big difference. Especially since many HO 2-8-4 models will negotiate 18"r. If you do the math you see why this puzzles me: 18"r * 87(Ho is 1:87) = 1566" (1:1) / 48 (O is 1:48) = 32.625"r in O scale. I suppose you are right Jim, but this one is puzzling because this is the same model with only the blind drivers and tolerances being different I would think. Perhaps that's all it takes?

I'm still not totally clear on what the missing piece here is. Especially when people tell me that their engines will negotiate  much tighter radius than the advertised min. AGHR Matt mentioned that I think most MTH engines up to the 4-6-4s would negotiate 36"r. My own tests with an Atlas F3 coupled to a pair of Atlas gondolas showed that while it doesn't look pretty, they will negotiate 24"r. which I believe is less than what they are listed as.

The whole point of this exercise is to understand what will really run on curves in the 30-48" range that my upcoming layout will have without taking a "Buy it and Try it" approach.

I wouldn't say that the 48" diameter of G is quite the standard, but that is the diameter of track that comes with most all G starter sets.  Most folks, once they get really involved in G, go for a larger diameter/radius, and there are plenty to choose from, including flex-track in brass, stainless steel, nickel silver, and aluminum.

I, too, have always thought it a bit strange that they went with 48" as a minimum because it will not quite fit on a standard sheet of 4x8 plywood, but who can explain some of the things that go on in this hobby in any scale?

Hartland Locomotive Works (HLW) offers G track w/built-in roadbed in a 36" diameter--works well enough with short-wheelbase locomotives--and at one time Aristo offered even tighter curve sections (perhaps they still do).  My garage G layout uses the 48" and some wider curves.  The Marklin track, which I like best, has a 5-foot minimum diameter curve. I have a tone of that track, but no place to use it these days unless I just do a point-to-point pike for my many Marklin MAXI trains.

It's probably mainly a combination of blind drivers and increased swing of pilot and trail trucks. Something in the design had to change in the pilot area to make room for the large flanges - probably more than just wheel size since lead and trail trucks require much more mobility on typically less exacting 3 rail track.

Don't forget the coupling distance between engine and tender, also. 2 railers wouldn't accept the typical 3 rail gap between engine and tender. Then there is the body mounted coupler, etc.

It all adds up.

Last time I was in 2 rail, my 48" radius curves allowed for just about any of the All Nation engines except the Altantic (no rear truck swing). But, my ancient Mountain was OK although that one demanded perfect track laying at that curvature.

I also ran a Scale Craft Hudson and some of the smaller Lobaugh engines. The smaller US Hobbies including the 0-8-0 were good too.

I think I just dated myself with those brand names!

Jim

I am confused.  What was the question?  Geometry is geometry, and the limits have mostly to do with truck swing and coupling distance.  A 3- rail locomotive can turn sharper corners partly because when a wheel strikes a cylinder block it does not blow the circuit breaker.

Consider - if you take an HO model and enlarge it to 187% (or whatever that scaling factor is) you can also enlarge the radius by that same percentage.

What most 2- railers want is lead trucks and cylinders of proper size, tail beams under fireboxes, and cars coupled realistically close.  That works out to about 70" radius or more.

If you buy, for instance, a "scale" MTH. Hudson, you will see no tailbeam at all, seriously undersize cylinders and lead truck wheels, and off- center piston rods.  That is so it can get around 36" radius for 2-rail.  If you run 3-rail, it can probably go O-54, which is slightly less than 36" radius, because the pilot wheels and tender can touch things without shorting.

That's all opinion, and may not track what the original question was.

Something else to bear in mind is that the Mfgs. tend to rate the min. radius for their engines based on the min. radius of their track.  FWIW

So you are saying that those prototypical features do not exist on the HO models? Not even HO Brass?

That strikes me as odd. I don't think I have ever seen an HO brass engine that wouldn't run on 30"r or there about. 30"r scales up to 54"r in O scale. So a scale HO brass model that has all of the features you mention will negotiate tighter radii than it's O scale equivalent. That is what my question is. Why do 2 rail O models of the same prototype require more relative radius than an HO model of the same prototype?

From what I am reading the answer is that the mfgs. probably figure that the HO market in general needs tighter radii because of what I call the 4x8 effect. Where there is no such thing with O scale. It  seems to be that the O market wants accuracy over any modifications for tighter radii. If that is true, then HO is not just a reduced O scale. It would have to have many more compromises.

What I find interesting is that I personally just want a bigger HO train. But that doesn't seem to be what O scale is. I am amazed at the cultural differences between HO, S, and O. What is Priority #1 for one group might not even be Priority #5 for another group. Very interesting.

O Scale (excepting the P:48 folks) also has the biggest compromise: 5' gauge track, requiring the wheels be further out than on an HO model.

Sometimes, that extra scale 1-3/4" per side can be the difference between a pilot wheel rubbing a cylinder or not on some curves.

Rusty

Jonnyspeed,

My mainline minimums are at 56" and 60" and that works for everything I own. Those minimums were determined by some of my more highly detailed locomotives. In hidden storage areas I have loops that are 60", 56", 51.5", 47", 38" and 34".

I've run the MTH J on 47" track with no problems. I've also run the Overland J on 47" track with no problems. When it gets down to the tighter tracks on my layout (38") the MTH engine will get around it but it is really awkward on there and I wouldn't run it fast on there. I think the engine is rated for 40", so I believe that really is a good rating for it. The drivers want to straighten the track on 34" curves. 

I've run the Sunset Y3 and the Y6 engines and the MTH A on 38" track with no problem other than on the Y6 the extended handrail on the back touches the tender. One would have to extend the drawbar or do something with the handrail. All of these tracks are in hidden areas.

I do have a short, 5' or so, 36" radius run of track in a mine area that the Sunset Y3 and the MTH A can negotiate but they really haven't been on that track except for 2 or 3 times in 8 years.

I will say, seeing these big trains on the tightest curves does not look as good but that would apply to 2 rail or 3 rail and H0 I suppose. I think you really should be looking to give yourself the larger radius curves on any visible mainline trackage if you can.

I totally agree with you. My space just isn't going to allow me to run the 56"r+ that I would like to have. Or if I did run that big I would wind up with a giant oval, which is not what I am looking for. I realize I am going to have to give something up. I'm just not sure what yet.