tech33

Explanation of the British Transmissions
by Lynn Bennett

Intro.:

All of you have probably looked through your bike's Shop Manual at all
the pretty color pictures of how your bike's transmission work and
skipped to the next section. So I will undertake to tell you as much as
I know about the transmissions used in British bikes during the 1960's
and 70's. Of course this design may have been used before and after but
I expressly will avoid the earlier much cruder British designs since I
know little about them and don't really need to know about them (my
oldest bike is 1966).

Direct vs. Indirect:

Transmissions are termed to be either direct drive or indirect drive.
Gear ratios of the transmission are normally generated by transmitting
the input power across a pair of gears running on parallel shafts. When
the power is input on one shaft and exits on the other shaft, the drive
is said to be indirect. When the power is input on one shaft and exits
on the same shaft centerline, either on the same end as the input or on
the opposite end of the same shaft, the drive is said to be direct. But
that is only one test. The real test of a direct drive transmission is
how the highest gear ratio is generated. It is said to be direct if the
input is hard coupled (directly coupled) to the output. That means that
high gear is a ratio of 1:1 (1 to 1), or one RPM in gets one RPM out.
Other ratios in either direct or indirect gear boxes for example might
be 1: 1.24 for third, 1:1.65 for second, and 1:2.65 for second. In
third gear in this case that means that for every 1.24 revolutions of
the input shaft (engine RPM) the transmission output shaft rotates one
revolution. The other gears ratios work the same. For indirect boxes,
which are not a part of this discussion, the highest (numerically the
lowest number) might not be 1:1 but could be lower (1:1.1 ) or higher
(1:0.75). Each time power is transmitted across a set of gears there is
a power loss of 2 to 5%. If the input and output are directly coupled
together as in a direct box the net power loss in high gear is 0%. In
indirect gear boxes every gear ratio goes through a gear set with 2 to
5% loss, even high gear. But, as you will see from the explanation of
the operation of a direct box, gears selections other than high will go
through two gear sets for a loss in those gear selections of 4 to 10%.
Loss means heat generation.

As for nomenclature, we Americans have called the input shaft of a
transmission the input shaft and call the parallel shaft, to which the
power is transmitted across, the counter shaft since it rotates in the
opposite direction as the input shaft. The British like to call the
input shaft the mainshaft and the countershaft the lay shaft as it
"lays" along side the mainshaft. As an aside, older American manual
non-overdrive automotive transmissions are direct while some of the
latter designed overdrive manual transmissions are indirect. Overdrive
is just another gear with a ratio higher (numerically lower) than 1:1.
Overdrive means that the output shaft is turning faster than the input,
which is the exact opposite of all other gear selections.

Gear Forms:

People make a living for their entire lives designing gear forms
(shapes). For our purposes there are two general classes of gear forms
used in gear boxes. The first is the straight cut gear. The teeth are
cut at 90 degrees to the gear side. The other class is helically cut
gear teeth that are a part of a helix, angling across the gear.
Helicallly cut gears slide (with friction) across each other avoiding
the ringing that happens when straight cut teeth hand the power from
tooth to tooth. Hence, less noise with helically cut gears but even
more power losses than straight cut gears. Most British transmissions
used straight cut gears, basically for the economy of manufacture.
Details of the straight cut gear form may differ inside a gear box from
gear pair to gear pair as the form is changed to allow a specific
numerical ratio not possible using the standard form tooth. The size of
the teeth is what is changed per a set of established gear forms that
are derived mathematically.

Direct Transmission Operation:

The British transmissions we will discuss are termed constant mesh.
That is, each set of gears for every ratio is in constant mesh with
its' mate and never comes out of mesh, even during gear changes. The
trick is that in any gear set pair one gear is attached to its' shaft
while its' mate runs freely on its' shaft. The gear that is fixed can
actually slide along the shaft so that notches on the gear sides (dogs)
connect a free gear to a gear that is attached to the shaft. The power
deliver path is as follows: the power comes in the mainshaft, the gear
selected is spun at the mainshaft speed either through permanent
attachment to the mainshaft or through the gear side dog engagement to
a gear that is permanently attach to the mainshaft, the power then
crosses over to the mating selected gear on the lay shaft, that gear
then spins the lay shaft either through permanent attachment to the lay
shaft or through the gear side dog engagement to a gear that is
permanently attach to the lay shaft. On our British transmissions the
lay shaft then spins the "high gear" on the lay shaft through permanent
attachment. The power then crosses over to the "sleeve" gear or high
gear. The sleeve gear is coaxially mounted, spinning on a long bushing,
on the mainshaft. The sleeve gear provides a means of mounting the rear
chain drive sprocket. (We often erroneously call the rear drive chain
sprocket the countershaft sprocket on British bikes but it obviously
does not reside on the counter shaft. That terminology only belongs to
an indirect transmission). In summary the power comes in the mainshaft,
crosses over to the lay shaft via the selected gear set, and crosses
back to the sleeve gear that rotates coaxially on the input end of the
mainshaft.

For high gear the sleeve gear is lock onto the mainshaft via gear side
dogs for a direct 1:1 gear ratio. All other ratios must go through two
gear sets, the pair termed first, second, third, etc., and the high
gear set that is made up of the sleeve gear and its' mate on the lay
shaft.

Gear Ratio Math

All gears except high must go through the sleeve gear set which act a
multiplier of the selected gear set ratio. A typical sleeve gear set
ratio might be 1.57 (14 teeth on the lay shaft and 22 teeth on the
sleeve gear). A typical gear ratio set might be: low 1.69 (16 teeth on
the mainshaft and 27 teeth on the lay shaft); second 1.05 (21 teeth on
the mainshaft and 22 teeth on the lay shaft); third 0.79 (24 teeth on
the mainshaft and 19 teeth on the lay shaft); fourth 1.00 (mainshaft
locked to sleeve gear). The product of the sleeve gear and each
selected gear are as follows: low 1.57 times 1.69 equals 2.65; second
1.57 times 1.05 equals 1.65; low 1.57 times 0.79 equals 1.24. There
you have it: 2.65, 1.65, 1.24, 1.00. Often times to get a close ratio
gear set only low gear is changed while wide ratio gear sets usually
require all gears to be different.

Engagement Dogs and Selector Plates:

The dogs can be round or rectangular with a matching hole pattern in
the other gear's side. The engagement is made somewhat sloppy so that
the dogs can easily engage during gear changes. In fact, the mating may
be loose enough for up to about 90 degrees of relative motion between
the dog mated gears. Automobiles transmissions use synchronizer rings
or cones that bring adjacent gears up to the same speed to minimize the
wear and tear on the gear dogs. Motorcycles typically don't resort to
synchronizers since the power levels being transmitted are an order of
magnitude lower.

Gear selection, again, is done by sliding gears on their respective
shafts. Shift forks partially encircle the gear to be moved and are
guided by a flat shift plate with a gear change pattern machined into
its' surface. (Modern bike transmission tend to use a rotating drum
with the pattern machined on the drums surface). A ratcheting mechanism
moves the plate to predetermined stop points, one for each gear plus
one for neutral, and is stopped and held by a spring loaded plunger or
a flat spring with a notch pressed into its end. Shift plates take a
real beating especially when a rider looses his patience with a balky
gear change mechanism and stomps on it with his foot full force. The
forks can get bent, the tracks in the shift plate get widened or
damaged, and the gear dogs get rounded from full power shifts. All that
can result in balky transmission shifting.