07-01-2011, 04:24 PM
This controversy comes up from time to time.
There are 3 wheel standards currently in use:
- Code 110/RP-25 the recommended HO standard, wheel tread is 0.110 inches wide
- Code 88/RP-25 semi-scale wheels with RP25 flange, wheel tread is 0.088 inches wide
- Proto87 wheels scaled very close to exact prototype, flange is thinner and shorter than RP25, tread is approx Code 64 (jsut over 1/2 the width of an NMRA wheel.
Track needs to match the wheel. NMRA studies have shown that the wheel tread width needs to be at least twice the flangeway width for the wheel to be properly supported at a turnout frog. HO NMRA flangeway standard is 0.040" to 0.050". Proto87 is considerably smaller.
As one can see from the dimensions, code 110 wheels will work well with the 0.050" flangeways. The wheel is fully supported, and does not drop at the frog when both wheels and track are correctly gauged.
Commercial turnout manufacturers tend to favor the widest possible flangeway for several reasons. The actual track gauge determines the flangeway width if the check gauge and wheel back-to-back is held constant. Fast Tracks has a video that explains this. A slightly wider track gauge permits use of a smaller minimum radius; conversely, track gauge right at minimum spec pushes minimum radius up. Wider flangeways also let slightly out-of-spec wheels roll through without binding or climbing the frog point. So a commercial turnout with wide flangeways will pass more rolling stock without derailing, at a cost of perhaps some ride smoothness. That said, ME flex track definitely has a slightly narrower track gauge than Atlas; I would assume ME turnouts - I don't have any to compare - match their flex track.
Going back to the dimensions - the code 88 wheels are less than twice the maximum flangeway in width. Which means the wheel runs out of support from the wing rail as it crosses the opposite path flangeway. Which means the wheel drops into the frog flangeway unless held up in the air by the rigid frame of the truck. The drop is more pronounced with larger number frogs. The solution for running code 88 wheels is to use the minimum spec for the flangeway, which means the track gauge is also set to the narrow spec. Then the wheel tread is over twice the flangeway width, and there is no drop at the frog. Unfortunately, the only way to get flangeways at the narrow spec is to build your own turnouts, or custom order them that way. FWIW, Fast Tracks standard dimensions are the wide spec. Fast Tracks tries to minimize the drop by using very sharp frog points, and pushing the frog point further into the frog throat than normal. All that said, most code 88 wheels will not derail in a commercial turnout, but they will "rattle" and suffer wheel drop, especially in #6 and larger turnouts.
Proto87 uses a prototypically thin flange, and a different back-to-back dimension on their wheel sets. These will typically derail in an NMRA turnout because the guard rails don't function correctly with Proto87 wheels.
Wheel tread width is much more visible on models of 19th Century rolling stock than on more modern. And the oversize tread width is much more apparent on the smaller diameter wheels of narrow gauge. Which is why HOn3 typically uses the code 88 wheels.
The ideal is to match track and wheel specs. Less than ideal will produce less than ideal performance.
Fred W
There are 3 wheel standards currently in use:
- Code 110/RP-25 the recommended HO standard, wheel tread is 0.110 inches wide
- Code 88/RP-25 semi-scale wheels with RP25 flange, wheel tread is 0.088 inches wide
- Proto87 wheels scaled very close to exact prototype, flange is thinner and shorter than RP25, tread is approx Code 64 (jsut over 1/2 the width of an NMRA wheel.
Track needs to match the wheel. NMRA studies have shown that the wheel tread width needs to be at least twice the flangeway width for the wheel to be properly supported at a turnout frog. HO NMRA flangeway standard is 0.040" to 0.050". Proto87 is considerably smaller.
As one can see from the dimensions, code 110 wheels will work well with the 0.050" flangeways. The wheel is fully supported, and does not drop at the frog when both wheels and track are correctly gauged.
Commercial turnout manufacturers tend to favor the widest possible flangeway for several reasons. The actual track gauge determines the flangeway width if the check gauge and wheel back-to-back is held constant. Fast Tracks has a video that explains this. A slightly wider track gauge permits use of a smaller minimum radius; conversely, track gauge right at minimum spec pushes minimum radius up. Wider flangeways also let slightly out-of-spec wheels roll through without binding or climbing the frog point. So a commercial turnout with wide flangeways will pass more rolling stock without derailing, at a cost of perhaps some ride smoothness. That said, ME flex track definitely has a slightly narrower track gauge than Atlas; I would assume ME turnouts - I don't have any to compare - match their flex track.
Going back to the dimensions - the code 88 wheels are less than twice the maximum flangeway in width. Which means the wheel runs out of support from the wing rail as it crosses the opposite path flangeway. Which means the wheel drops into the frog flangeway unless held up in the air by the rigid frame of the truck. The drop is more pronounced with larger number frogs. The solution for running code 88 wheels is to use the minimum spec for the flangeway, which means the track gauge is also set to the narrow spec. Then the wheel tread is over twice the flangeway width, and there is no drop at the frog. Unfortunately, the only way to get flangeways at the narrow spec is to build your own turnouts, or custom order them that way. FWIW, Fast Tracks standard dimensions are the wide spec. Fast Tracks tries to minimize the drop by using very sharp frog points, and pushing the frog point further into the frog throat than normal. All that said, most code 88 wheels will not derail in a commercial turnout, but they will "rattle" and suffer wheel drop, especially in #6 and larger turnouts.
Proto87 uses a prototypically thin flange, and a different back-to-back dimension on their wheel sets. These will typically derail in an NMRA turnout because the guard rails don't function correctly with Proto87 wheels.
Wheel tread width is much more visible on models of 19th Century rolling stock than on more modern. And the oversize tread width is much more apparent on the smaller diameter wheels of narrow gauge. Which is why HOn3 typically uses the code 88 wheels.
The ideal is to match track and wheel specs. Less than ideal will produce less than ideal performance.
Fred W