And the Indian on the left looks to be a c1914 two-speeder - a couple of years before the side-valve Powerplus appeared. The Rover is similar vintage, and the 3 1/2 Doug seems to have a Bosch magneto, so probably not later than 1915. All-in-all, you'd be thinking the photo was WW1 era rather than 1920s.
Are there names for the riders?
Leon