This is rather long...read at your own mental risk or if you are having trouble sleeping. Nevertheless, the conclusions may surprise you.

Denny Anspach writes:

Although in theory these wheels will "fall" into the larger frogs, as
a very practical matter in real time they simply do not.

Weeeelllll. I have written rather eloquently [ I thought ] that wheels with treads smaller than those designed for track built to Code 110 standards [ NMRA Standard 3.2 HO ] would fall into the relatively larger gap between the frog point and the point where the closure rail "bends" to become the wing rail. Denney's and the recent message by Dennis Storzek in which he says:

"And
no, Code 110 wheels well never drop in a properly built frog, no
matter how long, or how high a number it is. Neither will prototype
wheels."

made me decide to analyze the relationship between wheel and track a bit more thoroughly...instead of merely rolling a wheel through a turnout's frog. The results surprised me...although, to be honest, they should not have. In short, I was wrong [ gads ]. A wheel will have an increased tendency to "fall" in the gap between the point of the frog and the frog's throat with frog's of SMALLER numbers rather than LARGER numbers. However, as Dennis Storzek notes, NO Code 110 wheel will fall into this gap on any turnout built to NMRA HO Standard 3.2. [ Well...some very knowledgeable folks thought people would fall off the earth at some point. Some probably still do. ]

To start with, what follows is pretty dull stuff...unless you are
intrigued about track. What does this have to do with frt cars anyhow? It has to do
with wheel sizes [ flange and tread size ] and their use on model track. It
does, I believe, fit into
the need for a "close" association with frt cars because wheels with more
accurately sized wheel treads are more accurately sized frt cars.

Taking a close look at turnout sizes, here's a comment from the 1955 Track
and Structure Cyclopedia:

"In yard turnouts the number 8 frog is the most popular although #7's are
used in older yards and station areas." From Freight Terminals and
Trains...pg 46: "It is usual and practicable to equip yards with #8
turnouts." The AREA chart showing speeds associated with straight switch
points:

#6-14 mph, #8-19 mph, #10-21 mph, #12-27 mph, #20-38 mph

For curved switch points [ the NMRA RP 12.1 shows curved switch points ]:

#6-15 mph, #8-21 mph, #12-29 mph, #20-50 mph.

The UP turnout/speed chart for 1978 shows: #10 and smaller-15 mph, #14-30 mph, #20-40 mph. In 1978, UP showed 110 #20 turnouts on the Wyoming Division
and 9 #10 turnouts. #14 turnouts are not listed but are to be used in all
dual arrangements in CTC territory unless otherwise specified [ #20 ].

So, I have not changed my view that, while we modelers may be moving to more
scale sized wheel treads...code 88 treads attached to code 110 flanges, for
example...we may be improving the accuracy of wheels but not
trackage...unless one is using #14 turnouts or higher.

Lets consider three different
frogs...numbers 10, 12 and 20...in HO scale.

A real frog forms an equilateral triangle. The frog's number is the ratio of
the length of a line bisecting the frog's angle to the length of the spread
between the rails at the heel of the frog, the spread line drawn
perpendicular to the
bisecting line. This can be determined by two times the cotangent of half
the frog angle. For small frog angles like the ones we will be working with,
one can "cheat" and assume a right triangle formed by drawing a line
perpendicular to one rail at the point chosen for the spread. In this case,
we need only use the Cotangent of the angle.


There is one very important turnout dimension that the NMRA standards don't
provide....a
quick history will be useful here:

Quite a few years ago a friend of mine...Brad Bradley...was a member of the
NMRA standards committee. In fact, you can still see his name on some of the
standards. I mentioned to him via a phone call that a dimension was missing
from the turnout dimensions...the distance between the position of the
theoretical frog and the point at which the closure rails bend to become
wing rails [CRWR point].
I felt that this was an important measurement for those building
turnouts...as I was. He remarked that the real railroads didn't have such
dimensions in their drawings and, therefore, the NMRA didn't need to either.
The next thing I heard was...click. Brad would never have been confused with
someone who couldn't make decisions.<G>.

So...I had to determine this measurement myself...which...using the right
triangle
method...is simply the Sine of the frog angle. For our number 10, with its
5.72� this is
0.5". For a number 12 frog, the angle is 4� 46', and a number 20 frog is
2.87�. The
measurement to CRWR point for a #12 is 0.625", for a number 20
it is 1". Since we are concerned with "to fall or not to fall", the distance
across the gap between frog point and CRWR point is important.

So...the next point of interest is to determine where the passing of the
tread of a wheel will intersect the wing rail [ at which point it will
receive support ]. Using RP-25...there are some
inherent dangers with this because it is rather difficult to really know the
width of the tread but we'll take that given...0.080"...for Code 110 and
0.063" for Code 88. For the rest of this analysis, lets work with a code 88
tread on a code 110 flange and refer to it as code 88T. The reason for that
is that a smaller thickness of flange introduces other issues at the frog.

By using the triangle formed by the same frog angle and
the width of the wheel tread, we can determine that for a number 10 frog the
line traveled by the
wheel's tread will intersect the wing rail 0.8" from the CRWR
point as measured along the path of the movement of the wheel. This is the
Tangent of the angle = tread size [0.08"]/ distance from CRWR point[ the
unknown ]. Thus, the distance from CRWR point along the line from CRWR Point
to the frog is = 0.08/0.1= 0.8" when the wheel tread intersects the wing
rail. This is 0.3"
on the heel side of the
frog. However, this trail merely traces the movement of the "shadow" of the
tread. If it were simply a rectangular block...no problem. However, it's a
wheel and, therefore, the best support for the wheel is half way across its
diameter. For a 32" real wheel, this is .18". This is only an issue IF the
intersect of the tread path and wing rail is between frog and CRWR point.
For a number 10 frog, the bottom of the wheel tread
will intersect the wing rail at a point 0.3" prior to losing support at the
frog. For a Code 88T wheel, the tread will intersect the wing rail 0.63"
from the CRWR point. This is 0.13" on the heel side of the
frog, thus this wheel still has support from the frog when it encounters the
wing rail support.

For a number 12 frog, the Code 110 wheel now has 0.375" of support from the
frog when it encounters the wing rail. The Code 88T wheel has 0.165" of
support when it reaches the wing rail.

For a number 20 frog, the Code 110 wheel will have 0.6" of support from the
frog
prior to reaching the wing rail. For a Code 88T wheel, the wheel will have
.026" of support by the frog prior to reaching the wing rail.


Assuming the math is correct, this exercise points out the importance of
locating the CRWR point AND the importance of the dimensions of
the flange ways. All conclusions are based on accurate construction. I would
speculate that the NMRA RP was developed to provide for a certain degree of
error in turnout construction. I note that several of my turnouts [ I have
built all of mine ] have somewhat shorter frogs than is called for. I
suppose this COULD be due to bashing the frogs for 20 yrs with large steam
engines [ my excuse ]. It means the gap is a bit greater...sometimes as much
as .025"...than called for. It means that a wheel's flange will be less
likely to "pick" a frog but will be more likely to fall or bounce off the
frog point. This is apparent with code 88 wheel treads. Even with a frog
built close to the standard, 0.13" is rather close to losing the support
point of the frog.

What surprised me was that my original conclusion that larger frog numbers
produced more tendency for a wheel to drop was entirely incorrect. Just the
opposite is true. In fact, when you think about it, the largest frog number
would be a frog with an angle of 0� in which case the wheel flange would
always ride on the wing rail IF the tread size were greater than the
flangeway [ HO: 0.08 to 0.05 ]. At the other extreme, with a frog angle of 90�, the wheel tread
would not reach the wing rail until the center of the wheel had passed
entirely by the CRWR point. In fact, if a wheel diameter of less than the
flangeway [ the distance between CRWR point to frog pt for such a frog ],
were chosen, it would indeed fall into the gap.

So...if the wheel tread is greater than the flangeway, the wheel should
work...IF it is placed on a Code 110 flange. Theoretically a Proto87 tread
would also work...with its .055" wheel tread IF placed on a Code 110 flange.

For a number 6 turnout, here are the distances from the point of the frog
that a wheel flange still has support until it intersects the wing rail:

Code 110: 0.18"
Code 88T: 0.078"
P87T: 0.03"

For a number 8 turnout:

Code 110: 0.24"
Code 88T: 0.104"
P87T: .04"

These numbers indicate that tolerances must be quite good to avoid problems.
As Denny Anspach notes, stiff trucks help to keep wheels from falling if
one's tolerances with the frog point are a bit off. Note that the distances from the point of the frog toward its heel where the bottom of a wheel would be located when the tread's bottom intersects the wing rail INCREASE with larger frog numbers. IOW, when the bottom of the tread of the wheel intersects the wing rail, the bottom of the wheel will be further toward the heel side of the frog for a number 10 frog that for a number 8. So...turnouts with smaller numbered frogs will exhibit problems with wheels with treads smaller than those designed for the trackwork as opposed to turnouts with larger frog numbers.

Using wheels with flanges other than Code 110 on track with flangeways of .050" introduces other issues which are, fortunately for all concerned, beyond the scope of this message.

Mike Brock