The Protofour Manual
Section 2. Standard Dimensions.
2.0. Protofour & Preferred Proto Ratios
PROTOFOUR
When the Protofour 4mm/1ft scale
standard track and wheel dimensions were finalised by the Model Railway Study Group in
March 1966, many modellers in other scales expressed a desire to adopt the Protofour
concept of authentic modelling. Investigation by the Group resulted in Proto-standards,
and associated data, being computed for sixteen of the scales in common use throughout the
world. With the exception of the basic Protofour 4mm/1ft, these scales fall into six
groups, each centred upon a nominal track gauge and having variations of scale within each
group according to country of origin. Since there were, even then, clear indications that
the appeal of the Protofour concept was becoming an international one, a rationalisation
of the basic scales was obviously necessary to avoid confusion.
Broadly speaking the situation was
as follows. British scales are traditionally based on metric equivalents of imperial feet,
with track gauges adopted from other countries. As a consequence these track gauges are
generally out of scale. Whereas the American scales use inch fractions or decimal
equivalents of feet with near scale track gauges, the European scales are expressed as
round figure ratios, or proportions, of the prototype and have virtually correct scale
track gauges. Furthermore the European series of scales is more evenly spaced, and
achieves a near consistent (approximately twofold) area ratio for adjacent steps in the
scale series.
In attempting to resolve the
problem of rationalisation many factors were borne in mind; firstly, the obvious need to
retain the Protofour 4mm/1ft scale as the British basis of the Protofour concept. Britain,
Australia and New Zealand are joining 90% of the world’s population by adopting a
metric system, while the United States and Canada are considering similar changes. In a
generation or so imperial-based scales will not only be obsolete, but they will leave an
illogical series of decimalised ratios peculiar to this country.
It seemed logical, therefore, to
suggest the adoption of the European series of round figure ratios, as this also has the
advantage of offering internationally accepted track gauge dimensions which are themselves
virtually dead scale; the worst case of such rounding up error is only 0.35% in Proto-160.
amounting to less than 0.032mm over gauge. Six scales other than the basic
Protofour 4mm/ift scale have been selected for adoption as basic scales and these are
termed the Preferred Proto ratios. The accompanying table is headed with the basic
Protofour standards, followed by the track and wheel dimensions, with tolerances as
necessary, for the six preferred Proto ratios. For consistency these dimensions and
tolerances are directly proportional to the Protofour dimensions. It is important to
emphasise that although the preferred ratios give decimalised metric/imperial feet scales,
the provision of conversion tables, charts and prototype information compiled by the Study
Group completely eliminatesthe often quoted problem of metrication. One very important
aspect of the preferred ratios is that it will be possible for modellers in other than
4mm/1ft scale to continue using their traditional track gauges, but with turnouts and
wheels correctly scaled to the appropriate Proto-standard for improved appearance and
performance. Research and the development of
products to the preferred ratios is continuing within the Study Group and the Protofour
Society. Comments and enquiries on any aspect of the standards and their use will be
welcomed, and should be addressed to the M.R.S.G., c/o The Editor of Precision, the
Journal of the Protofour Society. September 1970 STANDARD DIMENSIONS FOR PROTOFOUR
Standard Dimensions for Protofour
| mm | in | ||||
| 1 | Track Gauge | TG | min. | 18.83 | 0.741 |
| 2 | Gauge Widening | GW | max. | 0.22 | 0.009 |
| 3 | Check Gauge | CG | min. | 18.15 | 0.715 |
| 4 | Crossing Flangeway | CF | max. | 0.68 | 0.027 |
| 5 | Crossing Flangeway | CF | min. | 0.65 | 0.026 |
| 6 | Between Checks | BC | max. | 17.47 | 0.688 |
| 7 | Effective Flange | EF | max. | 0.40 | 0.016 |
| 8 | Effective Flange | EF | min. | 0.35 | 0.014 |
| 9 | Wheel Back to Back | BB | max. | 17.75 | 0.699 |
| 10 | Wheel Back to Back | BB | min. | 17.67 | 0.696 |
| 11 | Tyre Width | TW | max. | 2.00 | 0.079 |
| 12 | Tyre Width | TW | min. | 1.85 | 0.073 |
THE PROTOFOUR MANUAL 2.0.(1)
| PROTO-160 (N-gauge) | PROTO-64 (S-gauge) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| PROTO-120 (TT-gauge) | PROTO-45 (0-gauge) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| PROTO-87 (H0-gauge) | PROTO-32 (1-gauge) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| © THE MODEL
RAILWAY STUDY GROUP 1970 Not to be used or reproduced for any commercial purpose without written permission of the M.R.S.G. |
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