PART FOUR - Transmission

The Model 40 Petrol machines had an unusual transmission arrangement.
It was a great improvement over the Model 19 Bonmow - to offer dual clutches.
I point out at this juncture that the Model 40 used two petrol powerplants in
its history: originally a Villiers 4-stroke; then a Briggs and Stratton 4-stroke
[that would later be used on the first 14" Model 45].

For reasons - that are not clear - Scott Bonnar decided on not using the 16"
Model 33 transmission design released a year or so before. Instead, the primary
clutch was of a steel drum energised by making contact with a rubber wheel bonded
to a crankshaft boss. This clutch was encased in a safety housing and engaged by a
small lever [see 5a below].

Note that the instructions said that the lever was to be left disengaged when not
in use. I guess this was to prevent 'flat-spotting' of the rubber drive wheel during
storage.

[Linked Image]

Of particular interest, also, is the chaincase arrangement.
Note that there are only two chains. The Model 33 [and Model 45] used 3 chains
[or 1 belt, two chains]. How was this possible?

I feel that this may explain, somewhat, the different preferences of the 14" and
16" designs. It was clever. The drum and rubber wheel ['friction roller'] provided
a significant reduction ratio to the output shaft [5], that permitted the designers
to remove the need for an extra intermediate sprocket and chain, as used on the
Model 33.

The Model 40 did adopt, however, the reel bearing carriers and setting method of the
Model 33. A good system, but a little more awkward for setting than the one that
would be adopted for the Model 45.

Finally, I point out that The Model 33 and Model 40 had a friction plate clutch
mounted axially to the landroll. The Model 40, however, moved the engaging mechanism
into the chaincase - an improvement.

PARTS GALLERY

[Linked Image]

TO BE CONTINUED ...