RP25 CONTOUR VS RAIL CONTACT

Model Railroader General Discussion (Model Railroader)
1

I decided to bring up a topic that is not mentioned very often, I think?

It is where the shape of the flat of the wheel contacts the rail.

Neither the rail or the wheel is flat, thanks to those of you over the years who begged the manufacurers to be more prototypical.

In fact, the contact of the wheel to the rail head would produce a sharp line around one of your wheels, if you look closely.

With the railhead being kind of rounded, and the wheel with an RP25 contour being at an angle, the wheel would contact the innerside of the rail/ plus whatever the flange would rub against- especially when going around curves.

That being said, one could not possibly eyeball his/her tracks, and declare that they were clean.

The method I have used to clean my tracks is to use a cheapie IHC cleaning “caboose” filled with paint thinner, cut off the horn-hooks, and to push (PUSH) it around the tracks with the same locomotive I was having problems with. And it seems like the pad for the cleaning car is made of felt, so the edges of the rail get cleaned as well.

That way, not only does the track get cleaned, but there is enough thinner on the tracks to clean the locomotive wheels as well. KIll two birds with one stone.

Yeah, I know, I know- mine is only a 5X8, DCC with two feeders, but the principal remains the same no matter how large the layout.

Plus DCC users with sound will draw more current (Heck, it takes a lot of power for the sound) and the rails/wheels will oxidize more than with DC.

2

In regards to the prototype world, everything is engineered so there is minimal contact between the flanges and rail. The angle of the wheels, among other things, keeps the wheels centered on the rails. I suppose in theory, on straight level track (good trackwork and good wheels) a car would stay on the rails without the flanges?

3

Well, not really.

Due to friction, the wheels would end up “walking” up and out.

Nice though, though.

4

The model rail head is not rounded as in tinplate rail. It does have a very small radius corner due to the impractibility of drawing a perfect 90 degree corner through a die (which is how model rail is made). Model rail typically has a web that is way too thick, a wider and higher than scale base, and in the smaller sizes, a head that is usually somewhat wider than scale. Practical considerations of making the die, and the rail being strong enough to take our abuse of our track - being leaned on by non-scale weights - are the normal reasons for a non-scale cross section.

Protoype rail shape is designed to a reasonable compromise of minimal friction, wear, and cost. And it must be strong enough to not be the cause of derailments.

The RP25 wheel spec does not require a tapered tread, but I don’t know of any RP25 wheels that don’t have the 3 degree maximum taper on the tread. The taper makes wheel manufacturing easier, and provides steering on straight track. Model curves are often (but not always!) too sharp for the tread taper to do the steering on curves, so a combination of the flange fillet (not the flange itself!) and sliding on the rail enable the wheel set to go around sharp curves.

The big deal of the RP25 wheel when it was introduced was the incorporation of a flange fillet, which allowed use of a much more protoypical-sized flange. Previously, with no flange fillet, it was believed the bigger the flange the better the track-holding ability of the wheel.

The prototype does make extensive use of the taper to steer the wheel set, avoiding use of the flange if at all possible. Take a look at the wear patterns on a prototype wheel. The tread wears long before the flange. The Brits, and now sometimes in the U.S., lay rail at a tilt so that the rail head matches the tread taper angle. This is typically done with asymmetrical tie plates.

You are correct in that the electrical and mechani

5

But, But, but-

When the Engineers were designing the wheel/vs rail, weren’t they thinking about adhesion?

I mean a sharp edge biting into steel would provide more traction than just smooth metal against metal.

Giving the weight and all that jazz?

I don’t know.

But Fred, you sure gave a lot of input and insight into the matter.

6

7

SqueakyWheels wrote:

Um, but model rail does have a flat surface, at least in HO scale. The prototype has a curved rail head surface (which is another reason why you’re never supposed to stand on it…ever…as it makes it easier to slip off of it).

As for wheels, believe me, you don’t want to have flat (non-tapered) wheels. Wheels with no taper would “hunt” (shift from side to side randomly) constantly. This would lead to greater derailments as wheels would tend to pick points on switches, hit uncoupling magnets, hit flangeways, and so on.

There’s a lot of slop in HO scale track and wheel design to take into consideration the sharp curves we run on. You need tapered wheel treads to better keep the cars on the rails.

Also, a tapered wheel allows wheelsets not to scuff too much going around corners (self steering, as fwright called it). Normally, as in a car, you need a differential because when you take curves the outer wheel travels farther than the inside one. With tapered wheelsets, the outer wheel rides on the rail on a greater radius while the inside wheel rides on a smaller radius. This provides the “differential” for the wheelset.

There’s only one problem…you also would have to clean the flanges of the wheels as well, or you might as well not even bother cleaning the inside edges of the rail head.

[quote]
Plus DCC users with sound will draw more current (Heck, it takes a lot of power f

8

When the engineers were designing the wheel-rail interface (terminology which wouldn’t be used until a century and a half later!) they assumed that ALL the traction would be as a direct result of the almost invisible deformation which makes the wheel/rail contact a chalk line rather than a pencil line. At no time, anywhere, will you find a sharp edge biting into steel under a locomotive - with one glaring exception. That exception - sand.

Note that sand is only applied when needed, and only in the amount needed to assure that the wheels won’t slip.

Chuck (modeling Central Japan in September, 1964)