Wheel Tapers and Flange Profiles for Small Radius Curves

This information was provided by L. H. Kombrink of Empire, Colorado.  Mr. Kombrink runs a steamer and an RS-3 diesel on a backyard railroad which features 15 foot radius curves.  Many thanks to Mr. Kombrink for his sharing of this information with us.

When operating rail equipment on sharp radius curves, some problems develop with regard to proper tracking of the wheels.  Ignoring for the moment the problems associated with operating rigid-wheelbase steamers on such curves, simple 0-4-0 steamers, rolling stock, and diesels with 4-wheel trucks may experience derailments and excessive flange wear under such conditions.  Typically, the derailment occurs when the outside wheel flange contacts the rail head and climbs over.

One might argue that larger wheel flanges are in order for such curves.  Images come to mind of the large flanges on seen on tinplate toy trains and on mining equipment.  Put into practice,  the larger flanges only seem to lead to further derailments and wear.  The reason is that the wheel flange engages the rail for a distance ahead and behind the point of contact with the rail head.  Increasing the flange depth serves to increase this engagement distance.  On a sharp curve, this means the flange is sticking out ahead of the wheel and tends to bite into the outside rail which is curving into the flange's path. A heavy loco or car can actually cut ribbons off of aluminum rail head.

Reducing the flange depth does not automatically make things better.  On sharp curves, the wheels will still tend towards the outside rail, and the reduced flange does not present much resistance to it climbing the rail head.

The key is to keep the wheels centered between the rails as they negotiate the curve.  This is accomplished by modifying the wheel taper.   The  IBLS guidelines for wheel taper specify a 2-degree 50 minute taper on the wheel tread.  While this reflects prototype practice, it is not adequate for the purpose of  centering wheelsets on what amount to 40+ degree curves.  The purpose of taper is to make differing wheel diameters available to the inner and outer wheel paths so that the curve may be negotiated with minimum resistance.  Increasing the wheel taper to 5 degrees provides enough differential wheel tread diameter to handle such sharp curves.

Mr. Kombrink uses a 5 degree taper coupled with a reduced wheel flange ( approx .08" deep) on all his equipment.  He reports that his rolling stock negotiates the curves with almost no flange contact to the railhead, practically eliminating flange wear.  His former problems with derailments have been eliminated, and his equipment still performs properly on club tracks.

It should be noted that it is prudent to increase the track gauge slightly on curves, this is to allow the tapered wheels room to find their optimal 'groove' as they track through the curve.  This increase in gauge may have to be exaggerated somewhat on really sharp curves.

It is possible to modify 1" Scale Railroad Supply wheelsets to this profile on a lathe.  I recommend using a 3/8" oilite bearing in the tailstock chuck to support the unchucked end of the wheelset, and use a 3/8" collet to hold the chucked end.  Use a fairly slow speed and take light cuts to alter the tread, and flange profile if desired.  Taking too heavy a cut can wrench the pressed-on wheel casting free of its grip on the smooth axle.

Russ Decho of Decho Scale Models adopted the 5-degree flange profile for the RS-3 road switchers he produced.  This model is now produced by Britton Scale Models ( see links page ) and continues to carry this feature.