Classic UK Minitrix Models  -  Britannia
I've come across a number of these models where the owner has complained of the engine wobbling its way around the track. Mine certainly did.

I reckon that there are a number of possible causes, and mine had most of them. I'd fix one cause, but it only reduced the wobble rather than remove it completely. But after much perseverence I removed each problem and now its gone. Here's what to look for.

Click the thumbnails for a more detailed picture.



Remove the valve gear and the conrods (see instructions on separate page). Balance the weight on the chassis and give the chassis a run. If there is no wobble, the wheels and the gearing are not the problem. Look elsewhere.

Tyres and build up of crud on wheels.

Check out the wheel surfaces, Use finger nails or match sticks to scrape off the compacted debris that accumulates.

Inspect the tyres - they don't last forever, and need replacing every now and then. Contrary to some posts I have seen, they are not hard to get hold of. Trix still make model engines, and many use the same tyre. The Trix part number is 72087800 Put this number and the word 'Trix' into Ebay (or Google) and you will find a number of suppliers. (Make sure you search WorldWide). Some are in Germany. Dont be put off - some have ebay sites which you can display in English. And if you don't understand the German phrases, copy and paste them into Google Translate. Not perfect, but it gives a good idea. However, check that the dealer accepts PayPal. Some don't, and require direct bank transfers which would cost around £30 from UK. Better to find a supplier that does.

Fitting the tyres is a bit awkward, but it is possible to do this without removing the body. You need a 2mm nut spinner. Expo make them, and many model shops have them. Google it. The hex nut on the front and rear wheel is very tiny, and can be removed gently with pliers, but a nut spinner makes much less of a mess.

Additional note from recent experience. The nut in the wheel is very tiny. If the loco has been tucked away for ages then there could be some corrosion on the threads - caused by damp in the air or by the effects of 2 different types of metal in contact with each other. Turning the nut could cause it to sheer off !
So use some penetrating fluid and let it stand overnight. When you turn the nut, apply A little pressure one way, and then back the other way, barely turning the nut at all, but 'rocking' it free. If you feel movement, use small movements, and turn back again - it helps clean the threads. I can't guarantee that this will work, but it will certainly reduce the chances of it breaking.

Get 3 sewing needles and a block of balsa. Lay the Mallard on its right side on the balsa (try to protect the valve gear underneath). Remove the nut of the rear wheel and feed the tyre under the con rod. Thread two needles into the tyre and stick them vertically into the balsa so that they are positioned at the 4 oclock and 8 oclock position of the wheel. ie as close to the wheel and to the base of the chassis as possible, with the tyre stretched between them. With the 3rd needle (or a small screwdriver) work the tyre around the top of the wheel. The needles will keep the bottom part of the tyre in place while you are doing this. Then work the tyre over the bottom part of the wheel and slowly, carefully remove the needles. A strategically placed thumb prevents the tyre from flying off. Replace the screw. It needs to be tight enough not to come loose, but do not overtighten it.

Now try the same technique with the front. Its a lot more fiddly, because the valve gear gets in the way. You may wish to operate the motor to position the screw in a convenient position before you start.

Wheel Synchronisation.

Easy to get wrong, easy to fix - especially if the motor and the valve gear are not installed on the loco.
The drive wheels are connected by gears. The conrod is there just for show, but if the wheels aren't all facing in the same direction, then the con rod can fight against the gearing, resulting in wobble and ultimately, damage.

Look at the detailed version of the photo (click the thumbnail). The balance weights on the wheels are all in the same position (between 9 and 10 oclock). The holes for the screws are all in the 3-4 o'clock position. But crucially, when you fit the conrod as shown, the square hole in the centre wheel is directly underneath the centre of the slotted hole in the conrod, and the conrod is horizontal.

Turn the engine over to inspect the other side. Unless the wheels have spun on their axles, the same alignment should be observed, although the holes will be in a different position - typically rotated 90 to 120 degrees.


It is unlikey that the quartering is out. Each axle should have a wheel in one position and the opposite wheel ratated at about 90 degrees. The actual angle is irrelevant - as long as all 3 wheels have the same alignment. This needn't be precise - the amount of freedom offered by the slots in the conrod allow for a fairly large margin of error. When all 3 wheels are in place and the conrod is fitted, the wheels should rotate smoothly without any tight spots. If the engine is not fitted, the chassis can be rolled around a track. It is difficult to check this with the centre wheel without fitting the valve gear - but a visual inspection of the alignment of the conrod slot with the square hole in the centre wheel as the wheels are turned will confirm whether or not there is a problem.

See the section in Fix & Clean on how to check and adjust the quartering.

However, the wheels are fitted quite tightly to the axles and it is unlikely that they have shifted. Moving them increases the chance that they will move again in the future.



Intermittent electrical contact will cause a wobble as the motor rapidly stops and starts. This could be caused by failing motor brushes or by other bad connections. With a hand lens, you can see the brushes from the right hand side of the motor. Again, Trix still use these motors, and parts are easy to obtain from suppliers in Germany. The brushes have part number 40015400 and the motor is 41207610. Search eBay for this number with the word 'Trix'. The german for brushes is Kohlebürsten. Or see the Useful Links button for sources.

If the brushes are OK and you still suspect that the motor isn't getting continuous power, then try powering the motor directly. Get two long lengths of fine wire and tuck a bare end between the brush spring and the end of the brush. Be careful to keep slight pressure on the brush as you move the spring to accomodate the wire. Connect the other end to your 12v supply. Check the motor runs smoothly. Then try the loco on the track as you guide the long lengths of wire above the engine.

See the section on 'Electrical' in 'Fix & Clean'. I discovered a built in electrical problem in the post-Hornby Flying Scotsman that i have had since new.


Valve Gears Damaged or Badly Positioned.

This is easy to check, and easier to do with the motor out so that the wheels can be turned slowly by hand.


The valve gear in the picture is wrong. But when the valve gear 'falls apart' - perhaps after taking off the engine body, this configuration is the easiest to reassemble. It looks and feels correct, but it isn't and will definitely cause the loco to wobble.

Note the various components - particularly, the plastic square peg for the centre wheel, the conrod and the piston rod. The piston rod is round, has a 90 degree bend at one end and a flattened head. This fits into the hole below the fragile, plastic slider. The small hole in the conrod slides down the length of the piston rod and round the 90 degree bend. The eccentric rod is attached the rear of the slide bar at one end, and has a linkage which includes the square plastic pin at the other. The linkage ocillates slightly off centre from the main axle,

Labelled Parts

The correct configuration. Note the position of the slider, compared to the last photo. Note also the square plastic pin is now inserted through the larger hole at the end of the conrod, ready to fit into the centre wheel. This is the assembly is for the left hand side. The photo above is wrong because when the hole in the centre driving wheel is in the 3 o'clock position, the left end of the conrod is too far away for the right end to reach the hole. But if it is assembled like this, every time the wheel passes the 3 o'clock position, the gearing is 'stretched' and one of the ways it manages this is to pull the wheel forward against any 'play'. This has the effect of making the loco 'skip' around the track. With the slider in its correct position as shown opposite, the left end of the conrod is closer to the middle wheel. The slider is VERY fragile. Take extreme care if you have to reassemble the valve gear. Move the components gently and never apply force. It will break, and is impossible to fix. The valve gear becomes useless.

Note the damage to the plastic peg caused by using pliers to pull it out of the wheel. Don't use pliers. A better method is described in the dismantling section.

From underneath. Note the distinct bend in the conrod - enough to clear the nut on the front wheel, but not so much that the conrod catches on the eccentric rod attached to the square plastic pin. Note also the position of the end of the conrod on the round piston rod. The engine will certainly wobble if one part of the gear catches against another part. There is a lot of play - particularly where the rods are connected to the wheels. Make sure that nothing catches when the parts are positioned at the other end of their extreme.

The offset brass screw on the baseplate is a pin to hold the front bogie in place. The original minitrix baseplate has a plastic pin moulded in this rather odd offset position, only visible when the baseplate is lifted off.


Causes of Damaged Valve and Drive Gear.

Picking up loco with finger and thumb piching gear towards centre.
Dropping loco
Lineside obstructions
Station platform too close to rail (vlave gear sticks out a bit - particularly on 9F locos.)
Point motor drive pins protruding too far through tie bar. Pins should be no higher than the top of the rail.
Careless use of tools or use of incorrect tools when 'fixing'.
Using even the slightest force on some components when 'fixing'.


Wheel Play

Not as you may think, too much play causes the engine to wobble. It doesn't. Rather, not enough play may be the culprit. Minitrix has a very tight 1st radius curve and all Minitrix Models negotiate them easily. Peco have 9" as their smallest radius. Minitrix have 7.66 inches (194.6mm) as theirs. The only reason that Minitrix models can get round these tight curves is because the main drive wheels have a considerable amount of lateral play designed in. The axles need to move freely within their brass sleeves. I estimate the movement to be in excess of 1mm - say about 1.2mm. If this movement isn't available, then as the loco negotiates a corner, the wheel will start to 'bite' in the outside rail of the curve and try to climb out of the track. The same is true on the straight as the engines moves around slightly between the tracks.

Check also the freedom that the bogie wheels have. If you ever try to run the engine without a front bogie, the same 'climbing out of the tracks' effect can be observed. The bogies carry very little weight, but they do go round the corner first. In doing so, they lead the chasis around the bend, just enough to pull the main driving wheels away from the outside rail. If the front bogie cannot move freely, it cannot perform this function.

Causes of reduced lateral movement.

Fluff between axle and brass sleeve
Fluff between chassis and wheel
Base plate is not seated centrally over the chassis - effectively reducing the amount of lateral movement.
Wheels have been pulled off their axles and have been replaced with too small a gap
Electrical pickups are too springy (because they have been replaced with an unsuitable substitute, perhaps.
Motor wire is loose and is somewhere between the chassis and wheel.
Connecting rods between the main drive wheels are bent on both sides and are effectively forcing the wheels to adopt a particular lateral position on their axles.
Valve gear mechanism has jammed.

Maintaining Classic UK Minitrix Locos
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