# Kato unitrack geometry question



## Da&Co

I am NEW to this hobby and have spent just enough time researching online to be very, very dangerous. I've started with a basic Kato HO unitrack oval set and it's working great, but I'm thinking about expanding. And I'm not understanding something very basic about how a curve's radius is measured.

I have repeatedly read that HO curves should be as wide as possible, with a minimum of 18" (that's 18 *inches*) and preferably greater than that, especially for longer locos and cars. This part makes sense to me.

Although the Kato unitrack curves come in many different lengths, they all are described with the same radius: 22.5. https://www.katousa.com/PDF/HO-Unitrack.pdf It looks like it's degrees, although my understanding is that a radius wouldn't be measured in degrees, but an angle would. I'm not sure why they call it a radius.

My 16 year old daughter is in precalculus and explained, with a mixture of amusement and condescension, how a curve (or arc) can be measured in degrees, and so I understand that two curves (or arcs) can have the same degree and yet be of different sizes or lengths. And it looks like a number of longer pieces of curved track will essentially be a wider turn than the smaller pieces of track, even though they have the same 22.5 degree arc.

So what am I missing? And, without using trigonometry, how do I convert the information about curves on the Kato website to measurements in inches that I can apply to a layout design? I know I'm missing something very basic, but I don't know what.

Thanks!


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## cid

Hey Da, those 22.5 are degrees, the radii are given in mm. Divide the mm by 25.4 to get inches. Kato make a lot of different curves, and different length and radii can be combined to get you just about any curve you
would like!


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## Da&Co

cid said:


> Hey Da, those 22.5 are degrees, the radii are given in mm. Divide the mm by 25.4 to get inches. Kato make a lot of different curves, and different length and radii can be combined to get you just about any curve you
> would like!


Okay, THANK YOU, I'm getting there. Stay with me. When Kato describes a curved piece of track as being "730mm (28 3/4")," is that the radius or the length of the piece of track? It can't be both, can it? Huh, maybe it can ...?

In the end, I'm trying to figure out how to move from their website measurements to the layout...

And, by the way, are they technically wrong to say "Radius 22.5" degrees?

Anyway, I really appreciate the help.


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## cid

730mm (28-3/4") radius, 22.5 degree curve track.
That's what they're trying to say. The length isn't given. If you put eight of 'em together, you'll get a semicircle (180 degrees) 730mm across center to center.


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## cid

Yes, they would not be correct to say 22.5 degree radius!!


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## Howard1975

With Kato Unitrack, the 22.5 degrees represents how many pieces will fit to make a full circle. Please remember from math, 360 degrees is a full circle. 180 degrees is half of a circle, while 90 degrees is one fourth of a circle. In HO scale with the Kato Unitrack, It takes 4 pieces of curved kato pieces to make a 90 degree curve. Follow me, 22.5 x 4 equals 90. Or you can say, 22.5 + 22.5 + 22.5 + 22.5 = 90. I hope you understand how Kato is measuring their curves. 

It's measured in a degree of the full curve. It takes 4 curved pieces to make a 90 degree turn, while it takes 16 curved pieces if you wanted to make a complete circle. 

Other brands can be different. For instance I know with the common track from Atlas in HO scale, --- most curves are 30 degree of a circle ( which is true for both 15 or 18 inch radius), while 22 inch and larger radius sizes, are 22.5 degree of a circle. 

The other part of the equation is the curve radius question. For most model train scales (including HO scale), the curve radius is measured BETWEEN the 2 rails, on a curve. Image an invisible line drawn on your layout, directly between the 2 rails. 

For instance Kato 730mm (28-3/4") radius, is measured 730 millimeters between the 2 rails, when you make a curve. Which means, to build a full circle, it will measure twice that size, or 1,460mm (57.50 inches) which would be the diameter of the complete circle, again measured between the 2 rails. Then you need to add a little for the outside edge of the ties and plastic roadbed ballast. To use the 730mm Kato Unitrack to make a complete circle, you would need a table at least 60 inches wide, at a bare minimum. 

For most model train scales, curved radius pieces are always measured when you are making a complete circle, from the center of the circle, to the middle of both rails. NOT how long the individual piece is. Only straights are measured based on long an individual piece is.

Also concerned HO scale, the larger locomotives, freight cars and passenger cars, really like wider curves. 18 inch curves are fine for short equipment, and diesel locomotives with 4 axles (8 wheels). And 18 inch radius works fine with smaller steam locomotives, up to about a 2-6-0, 4-6-4, and similar. For longer diesels with 6 axles (12 wheels) and larger steam (like 4-8-4, 2-10-2, etc), it's really best to go with larger curves. Generally, the common advice is to go for the biggest curves you can fit, so you can run larger equipment reliably. 

Naturally if the only equipment you run are small 4 and 8 wheeled trolley, you can get by with much sharper curves, like 8 inches or even less. 


Lionel type of track (and other toy train tracks) are measured a little different. They are often measured from the outside edge of the plastic roadbed ballast, or of the outside rail. And usually measured in diameter, such as O-27, or O-31, etc. 

Hope that helps some.

Howard


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## Da&Co

Cid and Howard, thank you both for taking the time to help. I think that part of my confusion is that everything I've read about curves has been in inches, the radius, and when I pulled up the Kato website with its mediocre reproduction, it looked like it was saying that all their pieces formed 22" radii when connected--I didn't distinguish the degree sign from the inches sign--and of course it said "radius." I was trying to understand, if they all had the same radius, how to use small straight pieces to extend the curve, and I now see that was entirely wrong.

It also looks--and correct me again--that the straight pieces are measured from end to end in inches while the curved pieces are measured by the degree of their arc (22.5) and the implied radius of their curve (e.g., 730mm). 

In any case, your explanations have been enormously helpful and I can now move forward ... with the master plan (hehehe). 

More questions to come!! E.g., what's all this about wiring ... ?


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## cid

Ha-ha Da, you'll have a ball figuring out the wiring! Really, it's not so bad, but helps a little if you already know how to wire a light switch! Just keep in mind, there's a positive side and a negative side, and don't let them touch each other! Always must be some load in between. (Engine, light, switch machine, etc.)

AND BTW, nobody caught my screw-up in answering your question before!! I'll just leave it as is, maybe you will see it!


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## DonR

This thread is the best promotional piece for using Flex track that I've seen yet.

I can't add 2 and 5 and get 8 every time so trying to calculate the radius
of a curve using degrees would have me pouring another vodka.

Don


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## CTValleyRR

DonR said:


> This thread is the best promotional piece for using Flex track that I've seen yet.
> 
> I can't add 2 and 5 and get 8 every time so trying to calculate the radius
> of a curve using degrees would have me pouring another vodka.
> 
> Don


I agree with Don in that I would much prefer flex track myself, and would even recommend that a beginner give it a shot if he hopes to progress to larger and more complex layouts.

That said, the human brain loves to assemble things. Look at the popularity of LEGO toys. Roadbed and to a lesser extent sectional track, just clicks together... rather like LEGOs. This removes the need for the more abstract thought required to deal with flex track and it's virtually unlimited geometric possibilities.

I think this is the appeal of those products to first-timers.


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## Da&Co

DonR said:


> This thread is the best promotional piece for using Flex track that I've seen yet.
> 
> I can't add 2 and 5 and get 8 every time so trying to calculate the radius
> of a curve using degrees would have me pouring another vodka.
> 
> Don


I hear what you're saying, but is another vodka really so bad?


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## Lemonhawk

Sometime at a garage sale or used book store, pick up a CRC handbook (Chemical Rubber Company), has all the formulas in. Of course, you can also search the internet, its buried in there also.


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