# Tomy turn radius



## TK Solver (Mar 18, 2004)

I'm at work and don't have a piece of track with me (how silly of me) and need to know where the Tomy turn radius is measured to. For example, if you have a 12" radius turn, is the 12" to the inner, outer or mean radius of the piece? I once knew the answer but I fear that 30+ years of St. Patrick's Day celebrations are taking a toll on some key brain cells.


----------



## Rolls (Jan 1, 2010)

To the outer edge, so a circle made of 9" curves would have an 18" diameter, outer edge to outer edge.


----------



## TK Solver (Mar 18, 2004)

Thanks Rolls!

I was looking at some of the layouts on Greg Braun's site and his lap lengths weren't adding up as I expected. He's using that turn radius directly in computing the distance traveled, which results in an overestimate. I have a program that uses the mean radius of the track pieces for this calculation. As an example, Braun's "International 39", which he claims has 39' lap lengths, actually has lap lengths of 36' and 34.5' depending upon which lane you're in.


----------



## Slott V (Feb 3, 2005)

*geometry 101*

Actually it's the _arc length_ of the curve you are looking for if you want to know the distance around a layout. It can be found easily with a tape measure in the slot or one of those tapeless measuring wheels.

http://en.wikipedia.org/wiki/Arc_(geometry)


----------



## cwbam (Feb 8, 2010)

Most track is 3" wide, there is your starting point.
6", 9", 12",15" length and radius.
A 36" roll up fabric tape (possibly in a Sewing kit) is good for a quick measurement.


----------



## TK Solver (Mar 18, 2004)

I use the arc length formula repeatedly in my program to compute the length of the center line around the track. An 18" radius 1/8th turn has a mean arc length of pi*16.5/4. A 15" radius 1/8th turn has a mean arc length of pi*13.5/4, and so forth. Braun's site uses the outer radius to estimate the track length so it's overestimating it significantly. The more curves in the layout, the greater his overestimation.


----------

