As on the prototype, in 7mm modelling there are standard dimensions for buffer centre to the rail head, and for buffer centre-to-centre distance (see the GOG Standards). In considering kits and RTR items, these dimensions should be designed-in correctly but in scratch-building they need to be considered. Since long vehicles can be prone to buffer locking on curves, it is important to ensure that the buffers are centralised over the track when scratch or kit-building. For the same reason, side play in bearings should be kept to a minimum. On the prototype, except on early vehicles, buffers are always sprung to reduce the shock between vehicles on coming into contact with each other. Many kits provide sprung buffers or methods of springing buffers, but some provide only solid castings, which can normally be replaced by sprung buffers if desired. Most current RTR offerings are fitted with sprung buffers but if you consider the springing to be too weak, replacement springs are available through the trade. In addition, modellers have come up with other excellent solutions to produce buffer springing. By searching the ‘Gazette Archive’ on the Guild website some can be found; similarly, many ‘rolling stock building’ books suggest different methods of springing. The main benefit of using sprung buffers are that vehicles do not ‘jolt’ when moving and bumping together, which gives a more realistic appearance.
There are many different types and systems of couplings. The most commonly used system is the prototypical 3-link and screw coupling. The couplings should preferably be sprung (the prototype nearly always was) as the springs take away any shock when trains are set in motion. Although these couplings can be fiddly and frustrating to use, they are prototypical and most RTR models and kits incorporate them. The Kadee automatic coupling is visually similar to the prototype buckeye coupling but it works using under-track magnets. It is ideal for semi-permanently coupling rakes of coaches, or other vehicles, with screw or 3-link couplings at either end of the rake. An alternative to the Kadee coupling recently became available in the form of small magnets that hold the vehicles in the rake together; they look nothing like the prototype and have no prototype equivalent, they are not automatic, but they are effective and very simple to use.. The Dingham coupling, and the Alex Jackson automatic coupling are also quite popular and there is information on all these couplings on the internet, in Gazette articles, etc. Modellers differ in opinion as to which is the best, but all couplings have their pros and cons.
All but the earliest prototype rolling stock has some form of wheel springing system, normally in the form of coil or leaf springs, to allow for undulations in the trackwork, and to reduce the shock being transmitted through to the vehicle from the rail surface. For very short wheelbase models, springing may be considered unnecessary but for longer wheelbases it may become more necessary to allow some vertical movement in the axles to maintain contact with the track. Many kits and RTR vehicles are provided with suspension in one form or another, but some provide only a rigid chassis design. Although it is highly desirable when building a vehicle with a rigid chassis that all wheels should be in contact with the railhead, there are springing and compensation kits available that many modellers avow are easier to fit than to build an accurate, rigid chassis. A benefit of springing or compensation is that a vehicle will tend to glide along the track with less rocking or lurching as it negotiates trackwork, in much the same way as the prototype, and there is less risk of derailments on uneven track.
With the individual bogies on bogie rolling stock having such a short wheelbase, the requirement for individual axlebox springing is not so critical, but some kits provide it as an integral part of the design. However, the bogies should have some method of allowing an element of rock relative to the body, preferably on one bogie only to avoid excessive sway whilst in motion; many kit manufacturers have their own preferred methods, but searching the Gazette archives and the internet will provide a range of options. In the case of six-wheeled bogies, some vertical movement of the centre axle is advisable.
For six-wheeled rolling stock, the three main problems are: -
One solution, the Cleminson system of the prototype, addresses all three problems; details may be found on the internet and in the Gazette Archive, which has examples of 7mm model equivalents.