STS Chassis

The chassis, common to all car types, carries the motor, axles and guide assembly.

The chassis is an approximately rectangular moulding with an open top. It has clips for the axle, contact arm and motor. The motor fits diagonally in the chassis and its shaft has worm gears at each end. The worm gears are made of brass and these mesh with plastic gears on both axles so all four wheels are driven. The worm gear means that the cars stop very quickly when the power stops and they will not free wheel at all. The motor is small and is probably only rated at 6 volts, hence the lower voltage power supply and high resistance controllers. The size of the motor and low gearing means that the cars are not fast but they are able to climb steep inclines in the track. It is not possible to service the motor.

Only one type of axle is used in sts cars. This is very simple with a black plastic drive gear towards one end and two white wheels with black, treaded tyres.

The chassis is formed with a keyhole shaped cut at each end in line with the ends of the worm gears. These were designed to accommodate pins formed on the end of prototype plastic worm gears so that they were held firmly onto the axle gears. Sadly, this facility does not seem to have ever been used in production cars. The result is that as the motor and chassis wear with use, the worm and axle gears are not held together so firmly and they can start to slip. The worm gears in the plasticard mock-up Mercedes are plastic and do have these pins. For this reason, the car still works better than some others, which have had less use.

There are two types of chassis. The difference concerns how the contact arm (see below) is retained. Type one chassis have open clips (similar to those that retain the axles) so the contact arm can be simply pressed in. Type 2 chassis have closed clips. These require the right hand pin (around which the spring is mounted) to be located first then the left hand pin is pressed home past a bevel into the hole opposite.

The guide assembly is fairly complex. The guide is on the forward end of a sprung drop arm (also called a Contact Arm), which is hinged at the rear of the chassis. This means that electrical contact between the guide and the rails can be maintained when the cars are driven over high obstacles. The guide rotates about a vertical axis as normal; but it is located in a holder, which can rotate about a horizontal axis in line with the cars’ length. This allows the guide to remain flat on the rails when one side of the car is raised (for example, with the see-saw or centre obstacles). The guide also has projections (or “stabilisers”) on each side. The obstacle tracks have grooves to accommodate these stabilisers so that the guide is kept firmly in the slot as one or both sides of the car are raised. The pick-up braids come out of the top of the guide and are curved down the front and under the guide. This allows contact with the rails to be maintained when obstacles lift the car.

The drop arm is a flat piece of plastic holding the guide at one end and hinged at the other where it attaches to the chassis. Power is carried from the pick-up braids to the motor by two thin uninsulated braids, which are retained on the upper side of the arm. These braids then form small loops before passing through the chassis to the motor.

There are two types of contact arm and guide. On earlier arms, if the thin braids came loose from the contact arm, they could touch the casing of the motor (which was exposed through the bottom surface of the chassis) causing the power to short-circuit and the car to stop. Later (type 2) contact arms differ by the addition of a flat plastic cover to clamp the braids and prevent them coming loose and touching the motor casing. This cover is sometimes missing. Type 2 contact arms, which should have the cover, can be identified by a small rectangular hole immediately to the rear of the “sts 4x4” logo, which the cover clips through. Later guides differed in two areas. The peg holding type 2 guides into the holder was about 2mm nearer the back of the guide. This meant that the guide was held further forward in the vehicle. Also the size of the holes (slots) used to hold the braids differed. Early (type 1) guides have smaller holes for 3mm braids; type 2 guides have larger holes for 4mm braids.

The changes in the contact arm and guide seem to have been introduced at different times so do not appear to coincide. I have not looked at many combinations but those I have noticed seem to indicate that the changes to the guide were made after the new contact arm was introduced. The most important point to notice about these differences is the size of replacement braid to be used. 4mm braids will not go into 3mm holes and, whilst 3mm braids will go in to 4mm holes, they could be loose or misaligned.

The spring pressing the drop arm down also pushes the chassis and wheels upwards, away from the track surface, so reducing traction. This effect is counteracted by two substantial weights fitted in the chassis. These also help to lower the centre of gravity of the cars and keep them more stable in the slot. Some of the cars have metal winches on their front bumpers, which, apart from their decorative function, also add weight to the front to improve traction. Cars without these metal winches have extra weight added inside the front of the body.