H0 reverse curve

Minimum radius and the straight between reverse curves on H0 model railways

When a curve changes direction, a straight longer than the longest vehicle must lie between the two curves. Otherwise the buffers lock. With close couplers that force a gap between the buffers, the smallest H0 curve is 354.75 mm for group A, 412.5 mm for group B and 495 mm for group C.

Source: NEM 111, 2016 edition, status 7 September 2024. G is the gauge under NEM 110, 16.5 mm in H0. Vehicle groups under NEM 301. Until the 2024 editions that group definition stood in NEM 103.

The arrow lies on the straight between the curves. It is drawn longer than the grey vehicle. The curve in the picture carries no millimetre figure. The radii are in the tables. Parallel-track spacing on a curve is covered by H0 track spacing.

Two ways of running, two sets of radii

NEM 111 separates the cases by whether the buffers can touch. Close couplers to NEM 352 that force a gap between the buffers may use the tighter curves. Once the side buffers can touch, the radii are larger and the straight between reverse curves is required as well. The standard says to use these minimum radii only where you must. Otherwise choose the largest radius you can.

With a fixed wheelbase the angle at which the wheel meets the rail must not exceed 12 degrees. On bogie vehicles the coupler and its side play limit the angle. The recommended radii give an acceptable run through the curve there too.

Smallest curve with close couplers

The table is for H0, G = 16.5 mm. Group A is a model body up to 230 mm, group B up to 278 mm, group C up to 313 mm. That is a prototype body of 20.0 m, 24.2 m and 27.2 m, divided by 87.

	Group A	Group B	Group C
Permitted minimum radius	354.75 mm	412.5 mm	495 mm
Recommended, station siding	412.5 mm	495 mm	577.5 mm
Recommended, branch-line running line	495 mm	577.5 mm	660 mm
Recommended, main-line running line	577.5 mm	660 mm	742.5 mm

21.5 times 16.5 mm is 354.75 mm. 25 times 16.5 mm is 412.5 mm. 30 times 16.5 mm is 495 mm. The recommended values continue the same series: 35, 40 and 45 times 16.5 mm.

Buffer to buffer

If the side buffers can touch, NEM 111 names different radii. Conditions include the minimum mass under NEM 302, buffer heads under NEM 303, sprung buffers, and a loose coupling: the gap between the buffers with the hooks extended is greater than 0.07 times 16.5 mm, so greater than 1.155 mm. A transition curve under NEM 113 may stand in place of the straight.

Group	Length over buffers up to	Minimum radius
A1, two-axle	9.8 m	825 mm
A2	14.6 m	940.5 mm
A3	17.5 m	1171.5 mm
A, bogie	20.3 m	940.5 mm
B	24.5 m	990 mm
C	27.5 m	1386 mm

354.75 mm from the close-coupler table is not enough here. Group C needs 84 times 16.5 mm, which is 1386 mm.

The straight in a reverse curve

A reverse curve is an S: the curve changes direction. Without a straight between the two arcs the vehicle ends swing towards each other, the buffers meet sideways and lock. For buffer-to-buffer running, NEM 111 requires a straight longer than the longest vehicle, measured over the buffers.

In H0 that is the prototype length over buffers divided by 87. Group A up to 20.3 m becomes 233 mm. Group B up to 24.5 m becomes 282 mm. Group C up to 27.5 m becomes 316 mm. If such a vehicle runs, the straight must be longer than that vehicle, not shorter and not the same length.

The larger spacing of parallel tracks must already be complete at the start of the curve. Those values are on H0 track spacing. The spacer sets that distance. It does not set the radius.

Laying the curve

1. Take the group of the longest vehicle. A body up to 230 mm is A, up to 278 mm is B, up to 313 mm is C.
2. Choose the way of running. Close couplers with a forced buffer gap use the first table. Buffers that can touch use the second table and need the straight.
3. Do not go under the permitted radius. On a running line use the recommended value.
4. In a reverse curve make the straight longer than the longest vehicle over the buffers.
5. Set parallel tracks at the start of the curve to the NEM 112 distance. Use the spacer for that. The track clamps hold the flex track while you form the curve.

What you need in your hand

The same steps: Laying flex track.

Typical mistakes

• Running 354.75 mm where the buffers can touch. Then 825 mm and more apply.
• Laying a reverse curve with no straight between the arcs.
• Making the straight shorter than the longest vehicle over the buffers.
• Widening the track spacing only in the middle of the curve. Vehicles still clip at the start.
• Treating the spacer as a radius gauge. It sets the distance between two tracks.

Common questions

What is the minimum radius in H0?

With close couplers that force a buffer gap, it is 354.75 mm for group A, 412.5 mm for group B and 495 mm for group C. NEM 111, 2016 edition. If the buffers can touch, the radii are much larger. Group C then needs 1386 mm.

How long must the straight in a reverse curve be?

Longer than the longest vehicle, measured over the buffers. In H0 that is the prototype length divided by 87. For group C that is more than 316 mm. The same length is not enough. The standard requires longer than the vehicle.

What is buffer lock?

On an S-curve the vehicle ends swing towards each other. The buffers no longer meet face to face. They meet sideways and lock. A straight that is too short, or a radius that is too tight for buffer-to-buffer running, causes it.

Is the close-coupler radius enough when the buffers can touch?

No. 354.75 mm, 412.5 mm and 495 mm apply only when the coupler forces a gap between the buffers. Buffers that can touch need the radii in the second table, and a straight or a transition curve.

Does the spacer set the minimum radius?

No. It sets the distance between two parallel tracks, and that distance must already be there at the start of the curve. You choose the radius from NEM 111 and bend the flex track to it. The clamps hold it.