Ok, maybe conspiracy is a bit much, but after doing a lot of research for some Fast Tracks curved turnout fixtures I have discovered that curved turnouts aren't always what they appear to be. For one thing they are a deviously complex bit of trackwork that may not look all that difficult to design, but believe me ? they are.
There also seems to be two very different approaches to building curved turnouts. And as with most things to do with model railroading both are compromises on the ideal.
Commercially produced curved turnouts are sold as #6, #6.5, etc., and usually state an inside and outside radius such as 24"/20". Now anyone who has tried to measure the radii of commercial curved turnouts (which can be very difficult) have probably discovered that the radii stated on the package are
not the same as the radii of the actual turnout. (See Image 1) Something I was very surprised to discover.
The outside radius of commercial curved turnouts is always what is claimed, as this is the easy size to maintain. The inside radius on the other hand is usually not what the manufacturer says it should be. In fact, it can be
significantly different.
Here is an interesting test. Measure the outside radii of a full line of standard curved turnouts, a 6, 6.5, 7, 7.5 and 8 and you will find that all of them are accurate. You will also note that the outside radius of one turnout is the same as the claimed inside radius of the next size up.
Now place the outside radius of one turnout onto the inside radius of the next sized turnout, and you will clearly see that the sizes are quite different. (See Image 2) In fact you will discover that the inside radii are always smaller than what is claimed by the manufacturer and are actually a compound radius. (A single curve that is made up of different radii.)
So what is going on here?
I think the reason that there is a discrepancy between the stated inside radius and the measured radius has to do with the way that curved turnouts where originally designed by the manufacturers. I suspect that they simply took a straight turnout and bent it into the curve that they needed.
The only problem with this approach is that the inside radius ends up being much too small to operate correctly. For example, an HO #6 curved turnout with an outside radius of 24" must have a minimum inside radius of 16" in order to work properly. If you simply bend a #6 straight turnout so the outside radii is 24" while maintaining a #6 frog angle, then the inside radius through the frog will also be 24"R and the radius from the points to the toe of the frog will be around 15" ? not 20?R. Since a minimum inside radius of 15" is required for the curved turnout to work geometrically, something must be adjusted to get a larger radius.
To do this it appears that most manufacturers have come up with a
fudge factor by using two different radii for the two routes through the frog while maintaining an almost constant inside radius thereby achieving an 18" inside radius - a little more acceptable and much closer to 20"R.
Now how the commercial manufacturers of curved turnouts came up with the fudge factors that they are currently using remains a mystery. But I believe they where developed over a long period of time. That, in addition to a bit of truth stretching with radii claims and you end up with what we see today -- commercial turnouts with a stated radii of X"/X" that actually measure X"/Y".
The end result is a turnout that is reasonably short and that will work under most circumstances, but also a turnout that I am sure has left many modelers scratching their heads trying to reconcile the radii printed on the packaging and the radii that they were measuring. I know it had me going around in circles for quite some time!
Now the other approach to constructing curved turnouts is usually used by modelers whom are hand laying their own track. First the centerline for the outside radius is drawn out then the inside centerline is laid tangent to this and the turnout is built around these two radii. This will produce a nice looking, smooth flowing turnout.
But like commercial turnouts, there are some downsides to this approach, namely turnouts that are very long and take up a lot of space on your layout. Especially if the two radii are close in size, like a 30"R and a 28"R which works out to the equivalent of a #15 frog and would need to be over 26" long for HO scale. It can also be very difficult to maintain a consistent frog angle with this approach, although generally the more important feature is maintaining the two radii.
Neither of these approaches is right or wrong, they are simply making different compromises. The commercial method results in a turnout that is reasonably long and will work within the space limitations of most modelers, and still work reasonably well. While the hand laid method will produce a more accurate and more reliable curved turnout, albeit requiring a lot of layout space.
A curved turnout can actually be built to specific frog angles with any size of outside radius as only the inside radius will vary. So it is possible to have a #6 curved turnout with a 24" outside radius, or even a 90"R and still have a #6 frog angle. This method lends itself well to working with Fast Tracks fixtures, as the standard Frog Point Grinding Jig can be used to build curved turnouts.
When I first thought about adding curved turnout assembly fixtures to my line of track tools I assumed that it would be a simple matter to design each fixture to suit each customer?s unique needs. But after doing the research I have discovered that designing curved turnouts that have a reasonable length means you must use compound radii and do a lot of ?fudging? of each design to come up with a functional geometry, which is simply not very practical. So instead I have decided to maintain the status quo and base our upcoming line of curved turnout assembly fixtures on existing standard commercial curved turnout sizes.
I also plan on developing a line of Fast Tracks curved turnouts in scales and radii that are not offered by other manufacturers to help fill in some of the gaps.
Of course matching printable tie templates will be available for all of these fixtures, so you will be able to test fit the curved turnout on your layout before deciding which size will work best for you.
Login
FAQ
Search
