Not much is heard these days about Douglas motorcycles, but their racing machinery in the 20's was among the best and most innovative in the world. They built the first 500cc motorcycle to record 100mph in Britian (1921!), won many Isle of Man TT's, and developed the first disc brakes. Shown in this photo are two 'RA' models, after a race on a banked circuit. The 'RA' was so named after the Research Association, a group attempting to advance motorcycle technical development, and which created the disc brakes on these two machines. Hydraulic calipers, as used today on discs, weren't developed yet (not until the late 30's), so they used mechanical leverage on a steel 'shoe', which pressed on the disc, which was made of friction material - the opposite of today. Still, there were much better brakes than the 'dummy rim' type on my motorcycle - basically a flimsy secondary wheel rim attached to the spokes, onto which a shoe of friction material is pressed (a design cribbed from horse-drawn carriages!).
These RA's would have run on alcohol, and in full flight would have probably hit 110 mph; this is 1924. Amazingly, these 1920's racing Douggies are still used in competition today, albeit in vintage sprint meetings, where they win regularly against bikes from the 60's and 70's, pulling 1/4 mile times in the 11 sec range.
I love the old one-marque sweaters which were fashionable in the 'teens and twenties, probably knit by a sympathetic mother. If you look closely at the lettering on the right rear mechanic, it's a different font from all the other 'Douglas'. Also, the American rider is wearing a football helmet, which was a common sight on board track and dirt track racing in the USA.