How is S Scale curved track measured

I would think all track would be based on centerline or the geometry would be skewed.

i don't know.

This was originally posted 6/23/15 by me:

SHS rail by design is 3.45mm in height, the tolerances are -0.00mm and +0.10mm.   That works out to the manufactured rail being anywhere between 0.1358264” and 0.1397634”.  Or in our language; somewhere between code 136 to code 140.  Over my years of working with SHS rail I found it to be 0.138” and only one batch came in at 0.1375”, which to me was pretty close to 0.138”. 

I couldn’t agree more about honesty in advertising from our train manufacturers.  I don’t know what the advertised radius of SHS S-Trax was but the design radius was 18.55”.  A far cry from Flyer at approximately 19.125” radius… that is why I put SHS S-Trax in the Flyer category along with their closing frog switches.

Then we have Circle L with a stated radius of 20” for their track system.  However, that 20” radius is to the outside edge of the roadbed, leaving the center line radius around 18.5”. 

I don’t know what MTH is claiming, but my guess would be they will follow suit.

Tom Stoltz

in Maine

Tom,

Is the design radius for SHS track measured from the center of the circle to the center of the track between the rails?  

PHM0  

<Is the design radius for SHS track measured from the center of the circle to the center of the track between the rails? >

No, the 20” radius is measured to the outside edge of the roadbed.  The centerline radius is 18.55”.

Tom Stoltz

in Maine

I appreciate the replies and am not surprised that the actual radius is measured differently by various suppliers. Does anyone know the radius (R20 and R27) for the Lionel S scale Fastrack and how it is measured?

thanks.

jim r

Given that Lionel's Flyer FasTrack is virtually identical to S-Trax in form and function (and I recently measured my R20 S-Trax at 40" to the outside of the roadbed,)  I would suspect it's measured from the outside of the roadbed.

Only Lionel could really give you the precise answer, assuming anyone there truly knows.

Rusty

Let me amend my numbers for the R19 FasTrack and add to that my measurements for R24 and R29 S-Trax.

Methodology: tracing an actual piece of track roadbed, both inside and outside radius as well as the ends of each. Drawing the chord of each arc, then measuring the length of the chord and the height at chord midpoint.

An online calculator was used to obtain the radius of both edges of track roadbed. Finally adding the two radii to find the average radius which is assumed to be the centerline radius.

Results:

R19 FasTrack OD R=20.616”; ID R=18.067”; CL R=19.3415:

R24 S-Trax     OD R=24.393”; ID R=22.7”;   CL R=23.5465”

R29 S-Trax     OD R=29.733”; ID R=27.782”; CL R=28.7575”

I am sure the numbers could be a little fuzzy due to pencil line width and eyesight, though I did use a magnifying visor, but probably fairly close. I used a machinist’s scale marked with 1/64” increments for the measurements.

I find the results puzzling and realize because of my measuring techniques we should probably round the numbers to the nearest 1/10. So maybe we are actually dealing with 19 3/8”R, 23 ½”R and 28 ¾”R? Who knows? But even with that the centerlines for parallel tracks look strange.

I hope someone will try to duplicate this to see if my findings are anywhere near consistent with reality. Please feel free to shoot this full of holes

Tom Stoltz

in Maine

Tom, I have a 12'x12' L shaped layout set up on the carpet in my home office using 29"R SHS track. Within the accuracy provided by my track assembly skills I measure 58 3/4" diameter to the outside edge of the roadbed. Since my track is on the carpet and not fastened down it may not be precisely assembled but it looks like it is. I would say this confirms your calculations for the 29" sectional track.

The same measurement for the 20" R track is about 40 5/8". Again I think this confirms your data.

Let me amend my numbers for the R19 FasTrack and add to that my measurements for R24 and R29 S-Trax – again.

Methodology:

Using the curve track in question to create an S-curve connected to a piece of straight track at each end to obtain edge to edge measurements.  Edge to edge being the same as center to center.  To ensure accuracy, a 4 ft straight edge was used to measure for parallelism and as a way to extend the edge of the first leg of the S up to the other leg where a measurement from edge to edge could be made.  Then the edge to edge dimension was used in the formula R=.25oc ÷ (sin15°² to obtain the radius, with R being the radius and oc being the ‘on center’ or edge to edge dimension.

Results:

R19 FasTrack; oc= 5.125”,     R= 19.2676039” … call it 19.125” for everyday use.

R24 S-Trax;     oc= 6.46875”, R= 24.14170” . If the oc is 1/32” different, that is 6.5”, the R= 24.2583305” , close enough to 24.25” for me.

R29 S-Trax;     oc= 7..6875”, R= 28.690145831”… a little less than a 1/16” from 28.75”.

I do believe this second method of measuring for the radius to be much more accurate, it is also much easier to do.  Also the R19 FasTrack radius matches the original American Flyer radius and oc spacing, which is what one would expect.  As a bonus, the R24 S-Trax is also 5.125” from the R19 allowing for the same spacing between straight sections as well as curved sections.  However the R29 does not follow suit being 4.5” from the R24.

These are the only samples of track with roadbed that I have.  I hope someone can give us the result to the different FasTrack radii.  It does appear to me that what they call it and what it is are two different things.  I don’t care what they call it, though I wish they would give us the actual centerline dimensions so we have something to work with.

And again, please try to find errors in my thinking.

Tom Stoltz

in Maine