RcTek : Information & Resources for the Model Car Racer

Two Speed Transmissions - How They Work

This article aims to explain the design and function of the automatic two speed transmission, which is an increasingly common fitment on remote controlled model racing cars. Whilst different designs have been tried in the past, the design used to illustrate this article is representative of current designs and is not based on any particular companies product.

This article is part of a collection about Model Car Transmissions.

Basic Understanding

Two Speed Gears Arrangement

Shown right is the layout of the gears in a two speed assembly. The smaller gears on the right hand side are the pinion gears and are fixed in relation to each other (They rotate as one unit).

Many people will look at this assembly and wonder how it could possibly work, which is understandable, given that the gears are apparently fixed in relation to each other.

This is where the spur gears on the left come in, as they are held onto the transmission shaft, or as it is generally referred to, the layshaft, in two totally different ways;

The uppermost (Second) spur gear is mounted onto the shaft with a ball raced metal housing, the inside of which contains a centrifugal clutch system, but is otherwise free to rotate on the shaft.

The lower (First) spur gear is also mounted on a ballraced metal housing, but the bearing is of a special type called a one-way bearing.

Identification of the Parts

Shown left is an interactive diagram that shows the various parts of a typical transmission.

Place your mouse pointer over a part to see a description.

How The Transmission Assembly Works

Once the engine speed has risen sufficiently for the clutch on the engine to engage, the car begins moving forward with the smaller of the two pinion gears driving the larger (first) spur gear through the one-way bearing, which automatically locks onto the shaft.

Acceleration increases the speed of the car to the point where centrifugal force overcomes the resistance of the springs and allows the clutch shoes to move outward to contact the first gear outer housing. At this point the driving force of the pinion gear is transferred to the second (smaller) spur gear, which then is driven by the larger of the pinion gears.

This creates a situation where the second gear is rotating at a higher speed than the first. The one-way bearing then automatically disengages its drive of the first gears and allows the shaft to be driven by the second gears.

When you Play the animation on the left it shows what happens when the gearbox changes into second gear.

There are three stages;

  1. The initial movement, where the first pinion gear (not shown for clarity) is driving the transmission.
  2. The point where the clutch engages the second gear and is then being driven by the second pinion.
  3. The point where the speed of the car has decreased and the clutch shoes move back in and release their hold on the outer housing.

You may Stop the animation if required.

This diagram is not quite a correct representation as the whole assembly is rotating as the gear change takes place.

But There’s More

Having studied the diagram above, you will hopefully now have a better understanding of what happens to make the gearbox change from first to second gear. With this design of gearbox though, there is an added element that sets it apart from earlier types.

In the above diagrams, you will notice that there are grub screws and ball bearings that are positioned above and below the adapter in the centre of the gearbox assembly. These not only act directly onto the adapter to set the distance of the clutch shoes from the clutch housing, but they also provide a means for the adapter to lock the shoes against the clutch housing.

If you Play the animation on the left, that shows a cutaway view of the gearbox, you will see what happens when the clutch shoes contact the clutch drum.

Please note the adapter rotating relative to the clutch shoes.

This rotation is brought into effect by the shoes contacting the outer housing, the braking effect of which makes the adapter rotate. The flats on the adapter produce a cam effect that pushes the shoes outwards, which locks them into position and provides instantaneous gear changes.

You may Stop the animation of required.

Note: deliberately emphasised movement for clarity.

And Finally…

When the car decelerates, the forces applied to make it change into second gear are removed and the gearbox automatically changes back down into first gear.

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© 2001, 2009 by Darren Burnhill