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

5.72ï¿½ this is

0.5". For a number 12 frog, the angle is 4ï¿½ 46', and a number 20 frog is

2.87ï¿½. 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 0ï¿½ 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 90ï¿½, 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