Sunday 14 March 2021

The Positive Stop Gear Change

Gearboxes on vintage motorcycles evolved rapidly during the late 1920s and early 1930s. The first positive stop foot change mechanism is attributed to Harold Willis at Velocette and was first fitted to the 1928 TT winning KTT bikes. Up to that time, gear changing was by hand and, depending on the manufacturer, each gear had to be selected by either engaging the tank-mounted lever into slots on a gate or, in the case of the hand change mechanism fitted to Nortons using a Sturmey Archer gearbox, by feeling the engagement of a notch in a groove.

Many people believe that the advantage of a positive stop foot gear change is the speed of changing up, ie getting into the next gear faster and "getting the power on" – and, indeed, that is one of the advantages. However, from a riders perspective and as a regular rider of vintage bikes without a positive stop, there is another significant disadvantage of a non-positive stop hand change, namely that it is not possible to simultaneously change down a gear and apply the front brake and this results in the necessity of slowing the bike much earlier when entering a corner. I was not around during the late 1920s when bikes transitioned from non-positive to positive stop but I would imagine that it would have been clear to an observer at the TT at this time that the latter had a significant competitive advantage in their ability to out-brake the former.

During a period of less than 10 years, motorcycle gearboxes evolved from being non-positive stop hand change to positive stop foot change with the entire positive stop mechanism incorporated into the gearbox. However, there were a couple of important evolutionary steps along the way. The first was to incorporate gear indexing within the gearbox itself instead of allowing the gears to take any position on the layshaft and mainshaft (it is possible with a gated gear change to have partial engagement if the adjustment is set up incorrectly).  This consisted of 2 parts: the first is a series of notches, referred to as the part “LS 166 Plate” in the illustration below taken from the 1931 Sturmey Archer gearbox parts list and the second is a spring-loaded plunger that engages with the indents on the plate and has been oriented to show how it engages.

This, in itself, does not provide a positive stop as it would allow a single rotational movement of the selector to go from 1st gear through neutral and 2nd gear to top gear in one go, but it does provide the means of selecting unique and discrete positions of the gears on the shafts.

The second ingredient that is required is the positive stop mechanism itself whereby the movement of the selector (above) is restricted to only that required to move the gears into their next position – either shifting upwards or downwards.

The Sturmey Archer gearbox that I had chosen to use on the V-Twin already has gear indexing incorporated into its construction but there was no positive stop mechanism. I had 3 options of how to do this: 1) Make one from scratch; 2) Use a Velocette positive stop mechanism (I have 3 in my workshop) or 3) Use a positive stop mechanism from a Sturmey Archer/Norton Dolls Head gearbox (I had 2 available). I chose the last of these.

These Sturmey Archer positive stop mechanisms started as a “bolt on” extra located on top of the gearbox, as shown below,


but soon ended up in the “head” of the Dolls Head gearbox

As I had already used the entire gear cluster from this gearbox to refurbish another and not wishing to destroy the casing by decapitating the “head” from the dolls “body” I decided to make another casing to hold the mechanism. Firstly, the entire mechanism was extracted - the collection of parts is shown below:

And all the important dimensions for a new housing were obtained by reverse engineering the existing components.

Firstly, the inside of a 100mm diameter EN3B steel bar was bored.

This was then mounted on the milling machine and the internal details were machined

And the final machining operation on the main body was to form the boss for the spring holder on the rear

The other main part to be made was the bracket that supports the main body just above the gearbox and bolted to the right-side engine plate. This was machined from a piece of 75mm x 75mm x 6mm angle iron.

and the main body of the housing was machined at the bottom to “sit” neatly on the bracket


The 2 parts were then silver-soldered into a single component.

2 trunnions and a short length of threaded adjuster rod were machined and the complete housing with the mechanism inside could then be mounted on the bike for the first time

With an aluminium cover plate and a gear lever put on for testing purposes I found the positive stop worked perfectly, but only after removing a bit of material from the lower part of the engine plate to allow bottom gear to be selected.

Like everything else on the bike, it is a tight fit!

No comments:

Post a Comment