This is a brief story and where near complete! Some information
is used from articles/books from Bob Currie and Owen Wright.
In 1909 a 499cc sidevalve, the first all BSA model was made, which model
was officially launched by 1911. For 1912 the range was expanded to 4
models, a 3½ hp. model A (single speed), 3½ hp. model B
(single speed / with clutch hub), 3½ hp. model C (2-speed gear
+ clutch in hub) and last but not least the 3½ hp. model D. (TT)
(single speed). These models being easily recognised by the enforcement
of the long steering head.
For the 1913 season the 2-speed, first all-chain model E was launched.
This latter enforcement was last used for the 1914 models, when on request
the models C and E had a 557cc engine, in which case they were named the
models H and K.
The onwards 1915 model D-TT had the frame tube over the petrol tank bent
upwards near the steering head. According to the catalogues this model
remained unaltered with the 500cc s.v. engine and belt driven directly
from the crankshaft until 1920. (I would like to get in contact with any
post 1916-models owners)
The 1915 '20 models H and K, all being 557cc versions, differ from
the later models as they have the front fork spring pointing forwards.
Late 1919 for the 1920 season the first 770cc v-twin was added to the
program.
In 1921 there was the big disaster in the TT, when BSA entered with a
dozen of their first knife-edge ohv-models, of which none got to the chequered
flag (this disaster having a great influence in their racing scene for
the next 4 decades). By that time the 770cc v-twin model E got his "bigger
brother" in the 986cc model F. Also in those days, the gifted genius
Harold Briggs (who had been trained by the Daimler factory) had joined
the company. He initially came to evolve the firm's first real high-efficiency
engines like the 1923 350 and 500cc s.v models and later the first saddle
tank model Sloper.
His study of an old Hotchkiss v-twin engine (also used
in a BSA
Light 10 Motorcar) formed the basis for the first public 350cc ohv
in 1924. Tragically, he later committed suicide as the result of a
doomed love affair.
By 1924 the typical model Round Tank also was added to the program.
For the 1927 season all models were equipped with drum instead of
the rim brakes. I can tell you from my own experience that this indeed
was a big improvement. For 1928 a new BSA was born in the short-life
destined 174cc two-stroke, while the Round Tank was withdrawn from
the program. However its engine was continued in the one-year earlier
introduced wedge-tank model. For 1929 all models, except 2 models
of the v-twins, were equipped with saddle tanks. By 1930 even those
two v-twins had the saddle tank, and brings this episode to a close.
In 1921 there was the big disaster in the TT, when BSA entered with a
dozen of their first knife-edge ohv-models, of which none got to the chequered
flag (this disaster having a great influence in their racing scene for
the next 4 decades). By that time the 770cc v-twin model E got his "bigger
brother" in the 986cc model F. Also in those days, the gifted genius
Harold Briggs (who had been trained by the Daimler factory) had joined
the company. He initially came to evolve the firm's first real high-efficiency
engines like the 1923 350 and 500cc s.v models and later the first saddle
tank model Sloper.