H0 back-to-back
Track gauge, back-to-back and flange on H0 model railways
On an H0 wheelset the inner faces of the wheels are 14.4 to 14.6 mm apart. The inner edges of the rails are 16.5 mm apart. These are two different measurements. Set the wheelset to 16.5 mm and it is too wide. Lay the rails at 14.4 mm and the track is too narrow. At the frog, neither figure decides on its own. The check gauge does.
Source: NEM 310 and NEM 110, 2009 edition. Rail profiles: NEM 120, 2010 edition. Special systems: NEM 340, 2008 edition. Frog: supplement to NEM 110, 310 and 311, 2009 edition.
The upper arrow is the track gauge, 16.5 mm, between the inner edges of the rails. The lower arrow is the back-to-back, 14.4 to 14.6 mm, between the inner faces of the wheels. The distance between two parallel tracks is a third measurement: H0 track spacing.
Four measurements that get mixed up
- Track gauge, dimension G: one track, inner edge to inner edge. For H0, 16.5 mm. On straight track that nominal value is also the minimum. Maximum 16.8 mm. NEM 110.
- Back-to-back, dimension B: distance between the inner faces of the wheels. For a gauge of 16.5 mm, 14.4 to 14.6 mm. NEM 310. Not the track gauge, and not the spacing of two tracks.
- Flange: width T 0.7 to 0.9 mm, height D 0.6 to 1.2 mm. Wheel width N is 2.7 to 2.9 mm.
- Check gauge, dimension K: back-to-back plus flange width. 15.1 to 15.3 mm. You may not add the limits of B and T freely. 14.6 mm plus 0.9 mm would be 15.5 mm, above the check gauge.
Why the frog needs the check gauge
At the frog point one guiding edge of the rail is missing. The check rail and the wing rail guide the wheel on its inner face instead. Two cases derail. If the back-to-back is too small, the wheelset jams between check rail and wing rail. If the back-to-back is too large, or the flange is too thick, the flange climbs the frog point.
The 2009 supplement to NEM 110, 310 and 311 sets two conditions. The largest check gauge must not exceed the smallest check distance C. The smallest back-to-back must not be smaller than the largest distance S between the guiding edges at the check rail and the wing rail.
| Condition | H0 under NEM 110 and 310 | What that means |
|---|---|---|
| K no larger than the minimum of C | 15.3 mm against 15.3 mm | The two limits meet. Above that, the flange climbs. |
| B no smaller than the maximum of S | 14.4 mm against 14.2 mm | 0.2 mm of clearance. Below that, the wheelset jams. |
Check distance C at the check rail is 15.3 to 15.5 mm in H0. Distance S is 14.0 to 14.2 mm. Flangeway F at the frog is 1.1 to 1.3 mm. Flangeway depth H is at least 1.2 mm.
Code 70, 83 and 100 are rail heights
The code is not a gauge for the wheelset. NEM 120, 2010 edition, sets the profile number as ten times the nominal height in millimetres. The code is the comparable name for that rail height. For H0 the standard names these three profiles:
| Profile | Height | Code | Named for H0 when |
|---|---|---|---|
| 18 | 1.8 mm | 70 | Modern main line, also older periods and branch lines |
| 20 | 2.0 mm | 83 | Modern main line |
| 25 | 2.5 mm | 100 | Modern main line |
Where several profiles are named for one scale, new work should aim for the smaller one. The back-to-back stays 14.4 to 14.6 mm. What changes is the room for the flange height, which may reach 1.2 mm and still has to run clear under the rail head. A lower profile is less forgiving of a flange that is too high. It does not ask for a different back-to-back.
NEM 310 and NEM 340
NEM 310 is the binding standard for wheelsets on track to NEM 110. NEM 340, 2008 edition, documents different figures for special systems, in H0 also for centre-rail track. Same words, different millimetres. A wheel gauge must not swap the two sheets.
| NEM 110 and 310 | NEM 340 | |
|---|---|---|
| Track gauge | 16.5 mm, at most 16.8 mm | 16.6 mm, at most 16.7 mm |
| Back-to-back B | 14.4 to 14.6 mm | at least 14.0 mm |
| Check gauge K | 15.1 to 15.3 mm | at most 15.0 mm |
| Flange width T | 0.7 to 0.9 mm | 0.9 to 1.0 mm |
| Flange height D | 0.6 to 1.2 mm | at most 1.35 mm |
So 14.0 mm is not "a little under NEM 310". It is the lower limit of a different sheet. 14.4 mm is the lower limit of NEM 310, not a generous target to undercut.
Checking and resetting a wheelset
- The track first. With the gauge for wheelset and track, check the 16.5 mm gauge, not the distance to the neighbouring track.
- Then the wheelset. Inner face to inner face. On track to NEM 110 that must be 14.4 to 14.6 mm.
- Add the flange width and compare with the check gauge. 15.1 to 15.3 mm. 14.6 mm and 0.9 mm together are too much.
- If the wheelset is off, the wheelset tool presses the wheels together or apart. It does not measure.
- After resetting, measure again with the gauge or a caliper before the axle goes back into the vehicle.
- The marks on the wheel gauge are tool marks. They are not automatically the NEM 310 span.
What you need in your hand
Gauge for wheelset and track
H0 version for the 16.5 mm gauge, made to NEM 310 and 311. Checks wheelset and track, not the distance between two tracks. 14.50 €
Wheel gauge for H0
Checks the wheelset, not the 16.5 mm of the track. H0 version: marks at 14.3 and 13.8 mm. H0 NEM version: marks at 14.50 and 14.00 mm. 15.40 €
Wheelset tool for H0
Presses the wheels to the measurement. It does not replace the gauge. Measure again afterwards. 39.70 €
The same parts, collected: Wheels and axles and Track.
Typical mistakes
- Treating the back-to-back as the same thing as the 16.5 mm track gauge.
- Reading code 83 or code 100 as a different back-to-back. The code is the rail height.
- Reading 14.0 mm as NEM 310. That is the lower limit in NEM 340.
- Taking 14.3 mm or 13.8 mm as the standard because the gauge carries the mark. 14.3 mm is 0.1 mm under 14.4 mm. 13.8 mm is under 14.0 mm.
- Adding 14.6 mm back-to-back and 0.9 mm flange width. The check gauge is then exceeded.
- Using only the wheelset tool and not measuring afterwards.
Common questions
What is the H0 back-to-back?
The distance between the inner faces of the wheels. For the 16.5 mm gauge of NEM 110, NEM 310, 2009 edition, sets 14.4 to 14.6 mm. That is dimension B, not the track gauge and not the spacing of two tracks.
What is the difference between track gauge and back-to-back?
You measure track gauge on the track, inner edge to inner edge, 16.5 mm in H0. You measure back-to-back on the vehicle, inner face to inner face, 14.4 to 14.6 mm. The gauge for wheelset and track is meant for both. The H0 wheel gauge checks the measurement on the wheelset.
Which measurement applies at the frog of an H0 point?
The check gauge, back-to-back plus flange width, 15.1 to 15.3 mm. It must not exceed the check distance, at least 15.3 mm in H0. The back-to-back must not be under 14.4 mm. Distance S ends at 14.2 mm.
Does code 83 or code 100 change the back-to-back?
No. Code 83 is about profile 20, so 2.0 mm rail height. Code 100 is about profile 25, so 2.5 mm. NEM 120, 2010 edition. The back-to-back stays 14.4 to 14.6 mm. A lower profile is more sensitive to a flange height up to 1.2 mm.
Can I set the wheelset with the wheelset tool alone?
The tool pushes the wheels. It does not show the standard. Afterwards you check with the wheel gauge or a caliper. The marks 14.3 mm, 13.8 mm, 14.50 mm and 14.00 mm on the gauge are not the span 14.4 to 14.6 mm from NEM 310.