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Scalefour Digest 1.2

 Issue 2, December 1998

P4 Track and Wheel Standards

by Russ Elliott

Acknowledgements  The P4 standard track and wheel dimensions given in this section of the Digest were produced by the members of the Model Railway Study Group (MRSG) in the mid-1960s. The railhead sections and tyre contour drawings are based, with permission, on the original Protofour track and wheel drawings made by Joe Brook Smith.

Introduction

This revision in the presentation of the P4 track and wheel standards has been produced:

• to provide information for existing and potential manufacturers who wish to produce components to P4 standards (the original presentation of the standards having been drawn up with only one manufacturer in mind);
  
  
• to provide information to modellers on the derivation, purpose and operation of the standards.

It is emphasised that much of the material given in this section of the Digest, particularly that shown in figure 1, is directed primarily at existing and prospective manufacturers of P4 components. For the modeller, the available construction gauges and rail components should result in correctly constructed P4 track without the need to measure anything or the need to refer to the standards, but it is hoped that the information regarding the derivation and operation of the P4 standards will give both modellers and manufacturers some of the founding principles behind the standards, and will therefore be of interest.

In the derivation of the standards, manufacturing tolerances, including the effect of the possible build-up of tolerances on other dimensions, have been taken into account, but the Society welcomes any comments from manufacturers in the application of the standards.

The P4 recommended track and wheel standards are given in table 1, and are conditional on the use of the railhead contour(s) and tyre profile shown in figure 1.

Annexes to this section of the Digest contain information on:

• recommended dimensions at turnout switches
• use of gauge widening
• use of 4mm scale equivalent dimensions
• protototype track and wheel dimensions (for 4' 8½" gauge)
• technical derivation of P4 track and wheel standards

Note  With the exception of figure 1, the drawings are not to scale.

General comments on P4 standards

Some members, especially new ones, become rather concerned at the prospect of having to work to two places of decimals of a millimetre. The Society would like to point out the following:

Table 1    Recommended P4 track and wheel standards, 4' 8½", 5' 3" and 7' 0¼" nominal prototype gauges
Dimension All P4 dimensions are in millimetres		4' 8½" gauge		5' 3" gauge		7' 0¼" gauge
		min.	max.	min.	max.	min.	max.
	TG Track gauge	18.83*	see GW	21.00*	see GW	28.08*	see GW
	GW Gauge widening (at 528mm radius)	-	0.22	-	0.22	-	0.22
	CG Check gauge	18.15*	18.20	20.32*	20.37	27.40*	27.45
	CF Crossing flangeway (see note 2)	0.65	0.68*	0.65	0.68*	0.65	0.68*
	BC Between checks	-	17.47*	-	19.64*	-	26.72*
	EF Effective flange thickness (see note 1)	0.35	0.40	0.35	0.40	0.35	0.40
	BB Wheel back-to-back	17.67*	17.75 (see note 3)	19.84*	19.92	26.92*	27.00
	TW Tyre width (see notes 4 and 5)	1.85	2.00	1.85	2.00	1.85	2.00

Notes:
1	Any deviation from the dimensions marked * will alter the effective flange dimension (EF) from the P4 standard. Deviation from any flange or railhead dimension may adversely affect performance.
2	The tyre profile is based on BS 276 contour A.
3	All dimensions are in millimetres.
4	The tolerances on any contour tools used for the production of tyres or rail are recommended to be: ± 0.0125mm, for all flange dimensions (with the exception of the tyre front edge radius); ± 0.025mm, for railhead dimensions.
Figure 1   P4 tyre profile and railhead contours

Annexes

Recommended dimensions at turnout switches

The following dimensions should be observed at turnout switches:

Use of gauge widening

Gauge widening should not be applied to any sections of pointwork where CG, CF or BC dimensions are specified.

Where applied to non-pointwork curves, prototype gauge widening at 10 chains radius is 0.25", at 7 chains radius is 0.5", and at 5½ chains radius is 0.75" maximum. (In 4mm scale, 1 chain is equal to 264mm, or approximately 10½".) In P4, where BB_max is less than the 4mm scale equivalent, and where adequate sideplay can usually be given to inner axles, gauge widening should not be necessary unless using long-wheelbase stock around sharp curves.

Where required, check rails should be set from the outer rail of a curve, using a CG gauge (in the same manner as the CG gauge is used in turnout construction), and gauge widening should be applied only to the inner running rail if wheelsets are still found to be binding.

Use of 4mm scale equivalent dimensions (4' 8½" gauge)

For a number of reasons, some of the P4 recommended dimensions are not exact scaled-down equivalents of the prototype dimensions, and most modellers use the recommended P4 dimensions as published, with wheels, track components and construction gauges conforming to these standard dimensions. The exact scaled-down dimensions differing from the P4 recommended values are:

Wheelsets built accurately to P4 dimensions will not run through exact-scale pointwork, but 4mm scale equivalent wheelsets (with the increased BB) will run through pointwork built to P4 recommended dimensions. It is emphasised that the Society recommends only the P4 dimensions, whilst recognising that more experienced modellers may choose to use the 4mm scale equivalent dimensions.

The P4 recommended BB dimension is specified in order to allow vehicles with a rigid wheelbase of more than 2 axles to negotiate turnouts and curves appreciably sharper than the prototype equivalents. The 4mm scale equivalent dimensions are used by some modellers to:

• reduce sideplay between wheels and track;
  
  
• improve the functioning of check rails in obtuse ('K') crossings and to improve the running characteristics of wheelsets through certain configurations of turnouts.

The use of the 4mm scale equivalent BB dimension may be found to be beneficial for all 4-wheeled vehicles, and for vehicles of greater than 4 wheels where adequate sideplay can be introduced on the inner axle(s). In this sense, the 4mm scale equivalent BB dimension should not be regarded as incompatible with other P4 recommended values, since, as stated previously, the 4mm scale equivalent BB wheelsets will run through pointwork conforming to P4 dimensions. It should however be noted that the 4mm scale equivalent BB may result in the overall wheelset width exceeding very slightly the nominal prototype equivalent maximum of approximately 21.7mm; Society compensation components are designed to cater for this very marginal difference.

In practice, the only difference required to implement the 4mm scale equivalent dimensions is a slight adjustment of the BB value and crossing flangeway (CF) setting.

Prototype track and wheel dimensions (for 4'8½" gauge)

For information, and comparison with the P4 values, the prototype 4' 8½" track and wheel dimensions and their 4mm scale equivalents are given in table 2.

Technical derivation of P4 track and wheel standards

Tyre and railhead contours as per figure 1

The most important single factor in ensuring reliable P4 operation is the maintenance of the correct tyre and railhead contours, and the successful use of the P4 track and wheel standards is conditional on the use of the scaled-down equivalents of these contours, as given in figure 1. From these contours is derived the effective flange thickness dimension (EF), which is the distance between the rail running face and the back face of the wheel flange when the front of the flange is in contact with the rail, i.e. when the flange root radius meets the radius at the side of the railhead; any further movement of the wheel toward the rail will cause the wheel to ride up and loose contact at the tread. The EF dimension replaces the complex actual dimensions of the flange in the compilation of the P4 track standards. The P4 value for EF is specified as 0.35mm minimum to 0.40mm maximum, and is equivalent to the corresponding nominal 1.125" EF of the prototype flange dimensions.

Many of the dimensions given in table 1 are interrelated, and the following conditions, whose basis applies equally to the prototype, have been used in the formulation of P4 standards.

1 BBmax + 2EFmax + clearance  ≤  TGmin This condition allows wheelsets to roll freely along the running rails, whether through turnouts or on plain track. NOTE. The clearance in the above condition represents the sideplay of the wheelset on the track gauge. In the formulation of P4 standards, a nominal clearance value of 0.25mm was adopted, in order to take account of the less than prototype equivalent track radii used in models, and the accuracy to which the wheelset BB could reasonably be expected to be maintained within the specified maximum. In practice, experience has shown that 0.25mm may be a little too generous, and a clearance of 0.15mm may be regarded as the minimum permissible. See also the notes on Use of gauge widening and Use of 4mm scale equivalent dimensions.

2 CGmin  ≥  BBmax + EFmax The minimum check gauge must not be less than the maximum wheel back-to-back plus one maximum effective flange: this condition represents the checking function, i.e. that the check rail, in making contact with the rear face of the flange of wheel B, will ensure that wheel A cannot take the wrong route through the crossing vee when approaching the vee from the toe of the turnout. (In practice, because of the shape of the root of the flange, the likelihood of wheel A taking the wrong route increases only as BBmax approaches the value of CGmin .)

3 TGmin − CFmax  ≥  CGmin The minimum track gauge minus the maximum crossing flangeway must not be less than the minimum check gauge: this condition is a counterpart of condition 2, and ensures that the wing rail can act with a checking function to prevent the flange of wheel B from taking the wrong route where the running rail R is or is about to become itself a part of another vee or crossing. NOTE. Whilst the above condition is necessary for the reason stated, it is obviously undesirable for the rear face of the flange of wheel A to strike the wing rail too abruptly when approaching the vee from the toe of the turnout. The necessity for a wing rail to act with a checking function in the above circumstances, e.g. in a three-way turnout (where there is a discontinuity in the adjacent running rail) has therefore to be balanced against the desirability of avoiding wing rail checking in other situations, e.g. on the outside road of a curved turnout (where wheel B is naturally hard up against the running rail). This dilemma is shared by the prototype, and care needs to be exercised where specialised pointwork is being constructed. The use of the 4mm scale equivalent BB dimension will alleviate the problem.

4 TWmin  ≥  2CFmax Twice the maximum crossing flangeway must not exceed the minimum tyre width: this condition prevents wheel drop at the crossing nose. NOTE. The P4 value specified for TWmin exceeds twice the CFmax value by a comfortable margin, to ensure that: the wheel is fully supported both before and after the actual point of the crossing vee nose; and an adequate proportion of the specified part of the wheel tread is in contact with the railhead on sharply-curved track.

5 CGmax − CFmin + clearance  ≤  BBmin This condition ensures that there is adequate clearance between the inner faces of the wheelset flanges and the distance over the outer faces of the wing and check rail pair. Since BCmax = CGmax − CFmin , the above condition can alternatively be expressed as BCmax + clearance  ≤  BBmin . NOTE. The clearance in the above condition represents the sideplay between the inner faces of the wheelset and the width over the outer faces of the check and wing rail pair. In the formulation of P4 standards, a nominal clearance value of 0.17mm was adopted, to take into account the less than prototype equivalent track radii used in models, and the accuracy to which the wheelset BB could reasonably be expected to be maintained within the specified minimum.

6 EFmax + clearance  ≤  CFmin This condition ensures that the flange can pass freely through the crossing flangeway.

7 Although it does not affect the correct interrelationship of P4 track dimensions, a value of 2.00mm is recommended for TWmax in order to keep the overall wheelset width from exceeding the prototype equivalent (approximately 21.7mm, in the case of 4' 8½" prototype gauge).

end

© Russ Elliott 2002

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