A model locomotive usually consists of two basic items, the body and the chassis that transports it. This section is a compilation of the processes recommended when constructing a steam locomotive chassis in order to ensure good running.

These notes are based on the use of chassis parts that are assembled from etched or profile milled components, though the same methods can be applied equally to scratch built parts. It should be noted that there are many different methods of constructing chassis, and the final choice will depend on the builder’s inclinations, skills, and ability and willingness to purchase special jigs and tools. These notes cannot reflect all of the various methods employed by different modellers, but do endeavour to emphasize tried and tested methods that do not require high levels of expertise or extensive workshop resources, and that, properly applied, allow good chassis to be constructed.

The articles on chassis design and compensation give some notes about other techniques.

Most model underframes rarely conform to the true profiles of prototype frames. The top edge is often reduced to a straight line that conforms to the edge of the running or footplate (the true top edge of the underframe often being reproduced on the running plate at the correct width rather than the often narrow frame width of the model). This is an advantage to accurate construction as it provides a datum from which to assess the trueness of the frames as the frames can be placed with these edges on a flat surface.

Underframes constructed from etched frames and spacers can become distorted during assembly. To avoid banana shaped frames, initially fit the frame spacers, one to the left-hand frame and one to the right-hand frame, as shown in Figure 1. This will distribute the heat during soldering, as it is the unequal expansion that creates the distortion.


Figure 1. Frame spacers fitted to alternate sides avoid distortion. Check regularly with square and straight edge using a flat surface as a datum during assembly.

Where frames are joined with turned spacers, tightening the fixing screws into the spacers will often distort the frames. This is especially true if countersunk screws are used. The countersink head gives a wedging action that pushes the frames around. To avoid this, substitute cheese-head screws and ensure that the holes in the frames have a good clearance around the screw.

If there is a matching countersink in the frames provided, either substitute left to right to put it on the inside or, if there are features that prevent this, then blind the countersink with a washer under the screw head. Gently tighten the screws checking all the time that the frames remain vertical and straight. Once this is done solder up the frame spacers and discard the screws. The holes on the outside can later be filled with Milliput.

Note that because the turned frame spacers have a large thermal mass, a different technique is required. Solder both ends of the first spacer at once to the frames. Let that spacer cool, and then solder the next, and so on.

If a split chassis pickup is to be used, the frames must be electrically isolated from each other. This requires spacers in Tufnol or plastic blocks. These will be screwed to the frames or bonded using epoxy or cyanoacrylate adhesives, or initially screwed to achieve the correct location, then bonded. Where screw fixing is used, the same remarks concerning countersink heads apply.

There are several types of bearing in common use. Plain bushes and hornguide systems are described here. The former are used in totally rigid frames or in conjunction with the latter for a compensated chassis. A sprung chassis requires an all-hornguide system. Both types are illustrated in Figure 2.


Figure 2. Bearing bushes and hornguides. The compression spring is required to hold the hornguides in position.

Another way of mounting bearings in a compensated chassis is to mount the bearings directly into the compensation beams rather than hornguides.

These usually take the form of ‘top-hat’ bearings and it is important that the bearing head seats firmly against the frames. To ensure this, break the edge of the hole in the frames by gently running a drill about three times the hole diameter against the hole to remove the corner. Spinning it in the fingers is sufficient. This will enable the head to seat firmly against the frame. The small radius between the head of the bush and the body outside diameter sits in the chamfer created by the drill.

Before securing the bush, check that the axle running in it is square in two planes to the frames. Using a small engineer’s square, sight the blade against an axle fitted in the bushes. As the square cannot be placed directly against the axle, hold the parts up against the light and sight the blade against the axle, rotating the whole so that a diminishing gap between the blade and the axle can be seen. It is possible to spot any taper in the gap if the axle is not square to the frames. Correct the position of the bush that is considered to be out of position.

Do not try to elongate the hole, enlarge it concentrically with a broach by a small amount, refit the bush and axle and, sighting along the square again, move the bush in the appropriate direction to get the axle square (Figure 3). Mark the required position by striking a line across the bush head and frame with a fine felt tip pen or a scriber. If the hole has not been enlarged sufficiently then repeat the process. Once the bush is correctly positioned, solder it in place.


Figure 3. Marking the position for soldering.

Having established the position of the first axle, the coupling rods can be used to position the remaining bushes. Alignment jigs with jury axles, such as supplied for assembling hornguides (Figure 4), can be used for this. In this way you will ensure that the axle holes are to the same pitch as the crank pins. Alternatively, chassis alignment jigs are now available from the trade, and provide a ready method of assembling and aligning a chassis. Some modellers dispense with jigs entirely and check the axle spacing on both sides of the chassis using vernier callipers.


Figure 4. Jury axles used to locate hornguides.

In some respects these are easier to get right than plain bearings due to the movement in them. It is necessary to ensure that the two hornblocks supporting each axle are correctly aligned to each other across the frames.

Set up the first pair of hornguides with either a jury axle or an alignment tool. A set of compression springs that fit over the jury axles and push the hornguides and bearing assembly onto the frames are ideal for this (Figure 2). The hornguides can be juggled around, sighting the axle against the square until they are positioned accurately. Then, depending on the type of hornguides used, either solder or glue them in place. The same applies when using a chassis assembly jig.

The remaining hornguides can then be positioned using the coupling rods as guides (Figures 4 and 5) or by using one of the chassis alignment jigs that are available from the trade.


Figure 5. A 7mm chassis showing the springs holding the hornguides in position.

A compensated chassis is simply a combination of the two systems and is treated in the same way but combining the methods. Always start by fitting the plain bearings.

Once the bearings have been fitted, the wheelsets should be fitted and the free rolling or otherwise should be checked. If all is well the chassis should roll down a short length of track with one end lifted an inch or so. If the chassis is reluctant to run the axles are most likely tight in the bearings. The axle can be polished with a fine grade of abrasive such as “wet and dry” paper. In many cases this is successful if the bearing is made to size. If necessary the bearings can be opened out. Most standard axles are 3/16 in. diameter, and reamers to suit are unlikely to remove any material if the bearing bores are already to size. It is possible to purchase reamers larger than the basic size. A 4.8 mm reamer is 0.189 in. diameter or 0.002 in. larger than the basic size. This seems to work well.

For smaller bores a roll of wet and dry paper, 360 grit will do nicely, can be inserted into the bore and twiddled around in the fingers (Figure 6). This should remove the necessary few thou’ to free up the axles. For 3/16 in. axle bores, where a suitable reamer is unavailable, this tool can also be used. It may be mechanised by fitting the wet and dry into a slot in a short length of brass tube and the whole rotated briskly in a modelling drill. Beware the brass dust and particles of abrasive that can fly out. Eye protection is essential for this operation.


Figure 6. Tool for easing the bore of a bush.

A problem with hornblocks is that, with time, they will wear, which results in a loss of wheel positioning (the same happened on the prototype). Not only is this important for smooth running, but it can cause problems with some prototypes where wheels are very close to motion brackets and brake gear, because this may result in electrical shorts. Whether this becomes a problem will depend on how intensively the model is run.

Another option, where the chassis is compensated, is to mount the bearings directly into the compensation beams. If the compensation beams were perfectly rigid, this would mean that the bearing bores on opposite sides of the chassis remain parallel to one another when one bearing is displaced vertically relative to the other. In practice, the beams are usually thin enough that they will flex a little to allow for differences in height. It is, however, important that the beams on either side of the chassis are square because they will not flex longitudinally. It is also helpful if the beam is in the middle of the bearing, or the beam will twist under the weight of the locomotive, which can lead to binding and wear. See Locomotive compensation for more detail.

The first essential is that all the joints in the motion have freedom of movement. Although some parts may only move through a small arc, nevertheless it is necessary that the parts should move freely, one around the other. An example is shown in Figure 7.


Figure 7. The combination lever and union link of a Walschaerts’ valve gear. The union link must move freely on the combination lever.

Similarly, the crosshead should move freely and smoothly through the slidebars. The sliding surfaces of the crosshead should be smooth and parallel. The slidebars themselves should be parallel in both planes and especially parallel to the piston rod (Figure 8). The easiest test for free movement here is to see if the crosshead will fall smoothly through the slide bars under the influence of gravity. The manufacturers who provide cast brass crossheads and slidebars make this particularly easy. It is usually just a matter of cleaning up the castings and checking the alignment.


Figure 8. Checking for parallelism.

The joints between the components are critical to the performance of the motion. The jointing methods that are usually provided by kit manufacturers are either rivets, pins, or small bolts. Dealing first with the riveted joints, the first step is to ensure that the hole is just large enough to fit the rivet supplied. If the hole is too small then open it out with a broach. Do not drill it, because, when the drill breaks through the opposite side of the component, it often snatches, picks up material and tries to rotate the part, usually severely bending it or breaking it. With a broach you are in control.

When riveting the motion, it is preferable to fit the rivet with its head at the back of the assembly. It is fairly simple to correctly set the tail of the rivet and to expand it without seizing the whole assembly solid, but an essential tool is a small ball pein hammer. This is a hammer where the end opposite the flat part of the head that has a spherical shape. The ball end is used to set the rivet. To set the rivet properly it needs to have about one rivet diameter protruding through the parts. Tap the rivet lightly with the ball and the edges of the rivet deform into a dome (Figure 9). The hammer should be steered around the rivet to obtain an even set. Riveting this way will leave the two components free to move but unable to come apart. The spherical end to the rivet looks neat and tidy too.


Figure 9. Riveting with a ball pein hammer.

Like all things, successful riveting is a technique that can be learned. If you are unsure of your ability, it is as well to practice first on some scrap material. If you are still uncomfortable with the idea, other methods of fixing the motion are available.

A pinned joint can also be made by first soldering a pin on one side of the joint. The pin is then passed through the second component and a washer soldered behind. The first stage is to fit the pin. Generally 0.9 mm diameter brass or nickel silver wire is very suitable for the pin. Cut an initial length of about 3 mm and, using a piece of MDF to support it, solder it into the first component (Figure 10).


Figure 10. The stages of assembling a pinned joint.

Remove it from the supporting MDF and coat the pin and the back of the component with Carr’s solder mask. By thinning the masking product with isopropyl alcohol, it can be painted on. The masking product should then be dried in situ by applying gentle heating with a soldering iron.

The joint surfaces of the mating component should be similarly treated. Slip it over the pin, followed by a 14BA brass washer. Some kits provide an etched part in place of the washer. Polish the washer on one side on some fine wet and dry and apply a small touch of a paste flux to it. Then apply the iron and introduce the solder. This should run around the washer and onto the exposed pin.

Reverse the components, trim the pin to just above the outer component and file flat just proud of the surface. This then looks like the pin of the real joint. Reverse again and trim the pin flush with the washer, filing down to create a thin head.

Finally, the motion can also be joined using nuts and bolts. Generally, these do not look as prototypical as other methods (there are a few exceptions where nuts were visible on the prototype, particularly on the crosshead), and care must be taken because the head of the bolt and the nut can take up the clearances so that the motion binds. However, it is certainly the easiest method to use. Care must be taken not to over-tighten the nuts or the joints may not move freely. To prevent them working loose during operation it is recommended to secure the thread with a touch of nail varnish. Should it be necessary to undo them at a later date, nail varnish remover (acetone) will soften the grip to allow easy removal. Solder may be used for the same purpose, but care must be taken to prevent it penetrating the threads and into the motion parts themselves.

Once the wheels and motion are installed in the chassis, the whole should be as free running as when it was tried with the wheels alone, without a motor or pick-ups installed. Unless split frames or some other form of current collection that avoids running pickups is used, the pickups of whatever type will introduce a braking element to the chassis. However, if done well this can be minimal.

Figure 11 shows a six-coupled underframe on a tight model curve and illustrates the amount of sideplay required to negotiate that curve.


Figure 11. Sideplay needed on a simple 0-6-0 chassis to negotiate a tight curve. An exaggerated illustration.

The sideplay required also depends on the fixed wheelbase of the locomotive, the track radii that the locomotive will have to negotiate, any gauge widening on curves, and on the wheel and track standard adopted. These should all be decided before construction begins. There is advice available on minimum curvature (but these are guidelines, not unbreakable rules).

0 fine standard wheelsets have a maximum over-flanges dimension of 31 mm. When standard 32 mm gauge track is used there is a ‘built in’ allowance before sideplay is needed. For example, a six-coupled locomotive with an 18 ft (126 mm model) wheelbase on a six-foot radius curve requires a sideways displacement of the centre axle of 1.1 mm to follow the rails. 1.0 mm of this is taken up by the built-in allowance of the standard, leaving only a very small sideplay of 0.1 mm in the centre axle – a quantity so small that it will be allowed by the build clearances between the backs of the wheels and the bearings. Additional sideplay is only required with longer wheelbases and/or tighter radius curves, or other wheel and track standards.

The Guild standard for wheel dimensions does not specify a maximum over-flange dimension. If using wheels that have unusually thick flanges, additional sideplay may be required. If in doubt, the over flange dimension should be checked with callipers and compared with the values noted there.

If at all possible, the sideplay should be limited to the centre axle while the outer axles have the minimum necessary for free running. This will minimise the amount of throw-over that occurs at the buffer beam when negotiating curves. Side clearance on the leading and trailing driving axles can cause crabbing, buffer locking, and the effect of snatching and lurching into curves is non-prototypical. It can also bring the wheels into conflict with brakes, sanding gear, crossheads, crankpins and outside valve gear. Sideplay requires a considerable degree of thought.

To maximise the clearances behind the crosshead, the small end joint at the end of the connecting rod should also be considered. The arrangements here will depend largely on the kit. If possible, make the joint with a countersunk screw with the head is on the inside and the nut on the outside (particularly if the nut matches prototype practice). It will usually be necessary to limit the side motion of the leading wheel and set it to a minimum. It is also possible, and often prototypical, to cut a counterbore into the coupling rod at the leading wheel position and fit a recessed crankpin (Figure 12).


Figure 12. This shows the recessed crankpin on the leading driving wheel necessary to clear the connecting rod.

The gearbox should not be fixed rigidly to the chassis but allowed to float to prevent undue stress on the gearbox and chassis bearings. Such an arrangement means that the axle driven by the gearbox must have no sideplay and cannot be allowed to move vertically under springing or compensation. If the gearbox is mounted properly on the driven axle, the only consideration this unit has in side clearance is the clearance between the gearbox in the chassis and the motor in the body.

One of the best ways to improve the way in which the locomotive will negotiate curves is to provide side control springing in the bogie or pony truck. This applies to single axle and twin axle bogies, both leading and trailing. Figure 13 shows the full size arrangement on a LNER bogie. The pair of springs can be clearly seen either side of the pivot. Note that in this instance part of the locomotive weight is carried on the hemispherical pads on the outside the bogie frames.


Figure 13. LNER bogie side control springing.

On the model this can be duplicated in a number of ways. In Figure 14, from left to right, the side control springs are: a spring wire, leaf springs and coil springs. It is important that if using a spring wire, it is allowed to deflect as shown in the sketch at top left. This means that one end is rigidly attached, and the other has a point location that has freedom of movement. A simple wire loop to locate the spring will do. The other two versions speak for themselves and a certain amount of ingenuity will be required to incorporate them, that will depend very much on the parts provided.


Figure 14. Alternative methods of spring control of bogies.

Side control springs may help to steer the fixed driving wheels into a curve, as illustrated in Figures 15 and 16. The amount of side control required depends on the weight and speed of the engine, and the radius of the curves that it has to negotiate. It is particularly so for prototypes such as a 2-4-2 or 4-4-2, where crabbing on straights can be as much a problem as keeping the truck on the rail through curves.


Figure 15. Free running bogie has no effect in guiding the locomotive into the curve.


Figure 16. Simple sideways springing of a leading bogie.

Side control springs on bogies must centralise the bogie. The amount of side control required depends on the weight and speed of the locomotive and the radius of the curves that it has to negotiate. Experience shows that it helps to put stops on the springs to prevent them acting part the centre point, as shown in Figure 16. This gives the bogie a definite centre position and improves side control. Two springs working against each other may reduce the side control.

Side control springs only work is the bogie is taking sufficient weight for the strength of the springs, i.e. it is not simply floating freely. Ideally the locomotive should be designed from the outset to distribute sufficient weight to the carrying wheels for them to work properly, while keeping enough weight on the driving wheels for traction. With many kits this can only be done to the extent that the design of the kit allows. A simple downward acting coil spring, if it can be included, is usually sufficient (Figure 17). Weight can also be added selectively to the body of the locomotive (rather than as a single lump of material wherever is convenient). Some trial and error during the construction stage may be necessary.


Figure 17. Downward pressure on a bogie bolster.

The side control spring should act as low down in the bogie as possible as this helps prevent the bogie from lifting its wheels off the track on the inside of the curve. The position of side control springs is also important for a pony truck as explained in Figure 18.


Figure 18. Effect of the position of side control springs on a pony truck.

This figure shows a pony truck moving on a curve. Force F1 is the force created by the outer rail acting on the wheel, and F3 is the force on the pony truck pivot point, that acts on the fixed chassis. Force F2 is the reaction force exerted by the side control spring. When the side controlling spring in located above the line shown in the figure connecting the two other reaction points, the result is a clockwise rotation of the truck causing the left hand running wheel to lift. Since it is this wheel providing the reaction force F1, it will derail, unless sufficient vertical force is provided. This takes traction load away from the drivers.

The lower part of Figure 18 shows how to avoid this problem. The side control is applied below the line connecting the reaction points. As a result, the truck will attempt to rotate in the counter-clockwise direction, causing the left wheel to press more strongly onto the track, and restrain any tendency to lift. As the lateral force F1 increases (as the curve gets sharper), this counter-clockwise effect increases, which is what we need. There is no reduction in traction because there is no additional requirement for vertical load from the fixed chassis onto the pony truck.

Wheel arrangements that can cause problems on tight model curves are the 0-4-4T, and to a certain extent the 4-4-0, and single driver locomotives like the Stirling single. In the former case, a possible solution that has been tried successfully is to fix the bogie frame so that it can rotate and tilt about its pivot but with no sideways movement. The front driving axle is also constrained and the second driving axle given sideplay. This reduced the overhang at the bunker end by about 50%. Figure 19 illustrates the principle. See the comments above concerning driving axle sideplay.


Figure 19. A method to reduce buffer beam displacement for an 0-4-4 tank locomotive.

Make a line sketch to scale of the underframe and mark the position of the leading axle and the bogie pivot. Draw a curve representing the minimum radius through these points. The amount of sideplay on the rear driver and buffer beam displacement can be measured from the sketch. The upper curve shows the overhang with sideplay on the bogie and no sideplay on the driving axles. The lower curve shows the method of reducing the overhang by allowing the bogie pivot no sideplay but allowing it for the rear driving axle. The amount of sideplay needed on the rear driving axle can be found by making a line drawing as shown here.

The leading bogie of single driver locomotives is not given sideplay but can swivel. In the case of the Stirling single the presence of splashers on the bogie wheels can be a particular source of trouble. This can be overcome by ‘modeller’s licence’ by making the bogie frames integral with the main frames so that the locomotive becomes, in effect, an 0-6-2. A small amount of sideplay on the second bogie axle gives the steady running required while retaining the illusion that the locomotive is a 4-2-2. The idea is illustrated in Figure 20.


Figure 20. Single wheeler with bogie fixed and sideplay on second bogie axle. Using ‘modeller’s licence’ to reduce body movement on a single wheeler by making the leading bogie a dummy. The line sketch shows the sideplay required on the secpnd axle and the rear bogie axle, which can be measured from the sketch.

See the Guide to motor and gearbox selection.

This article was written by Bob Alderman and Ken Sheale for the Gauge O Guild Manual. It was adapted for the GOGWiki by Nick Baines.