BSA Motorcycles 1920s

Today in Motorcycle History

4¼ h.p. B.S.A. 1921

The 4¼ h.p. B.S.A., which has a newly-designed engine, gear box and clutch, and cast aluminium chain cases.

B.S.A. 4¼ hp 1921 Sidecar Mounts

A spherical connection helps to eliminate strains when lining up the sidecar.

BSA 4¼ hp 1921 Sidecar Chassis

A simple triangulated sidecar chassis, with double axle tube and a single "cross" spring for the 4¼ h.p. B.S.A.

B.S.A. 4¼ hp 1921 Front Fork

A single central fork spring and large link bearings arc features of the new B.S.A. spring fork.

BSA 4¼ hp Engine 1921

Entirely new timing gear has been applied to the 4¼ h.p. B.S.A. for 1921. This allows a neater cover to be used,

Three B.S.A. Models for 1921.

A New 4¼ h.p. Engine. Re-designed Transmission, and All-chain or Chain and Belt Drive.

SILENCE and serviceability are obviously the watchwords of the B.S.A. Co. in introducing their new 4¼ h.p. motor cycle. Reliability and a capacity for hard work have long been associated with the firm's productions, and the regular performances of these machines - in all the big reliability trials are now taken as a matter of course.

Though the general layout of the new models follows standard B.S.A. practice and embodies all the well-known features, two points have received very special attention in the redesigned machine. First, the question of silence has been carefully studied, and external cams mounted on a common shaft and operating the tappets through long rockers are now employed, and the whole of the valve gear is most carefully manufactured. The new cam gear has obviously been designed with a view to silent action, and a large silencer is also fitted, which, in conjunction with a long tail pipe, reduces exhaust noise to a minimum without causing any appreciable back pressure, since no baffles are used.

A Much Desired Improvement.

The second point consists of a very considerable improvement in the controllability of the machine, brought about by shifting the clutch to the primary shaft of the gear box. The clutch is also fitted with a handle-bar control.

With the exception of the timing gear already mentioned, the engine has been altered comparatively little. An improvement has been made in the shape of the combustion head and valve ports; the interchangeable valves are made of a chromium steel in order to prevent breakage and pitting, and easily adjusted tappets are fitted.

A slight offset of the cylinder centre minimises the wear on the cylinder walls during the power stroke. The crankshaft is carried in extra large ball bearings, while the connecting rod has a caged roller bearing big end.

A very simple form of decompressor, acting on the exhaust valve, facilitates starting; the kick-starter mechanism is now entirely enclosed in the gear box.

The well-known B.S.A. spring shock absorber is mounted on the engine-shaft, and thence a chain transmits the power to the new clutch and gear box. This clutch is of the dry plate type, having seven plates, and the gear box is similar to that of the 6 h.p. machine; that is to say, all gears are constantly in mesh, the ratio changes being effected by sliding dogs. The secondary transmission is also by chain, and both chains are entirely enclosed in oiltight, but detachable, aluminium castings. Except for the more general adoption of steel lugs, the frame has undergone but little modification; but a new spring fork is now used having one long barrel-shaped spring housed between the fork blades, and giving an exceptionally long "travel" without bumping. This fork, combined with the B.S.A. spring saddle-pillar, provides excellent insulation from road shocks. Internal V blocks are used on the front and rear wheels, the heel-operated back brake being fitted with a parallel link motion to ensure even and easy engagement.

A Serviceable Finish.

Details include 26x2½ tyres, sight feed drip lubrication, armoured tool bags and B.S.A. carburetter, while it goes without saying that the finish is superlative. It is, however, perhaps worthy of mention that fork links, springs, hubs, brake rods, etc., are heavily enamelled, thus providing a more serviceable finish than the usual mass of plated parts.

For the benefit of those who still prefer "a belt somewhere in the drive" - and these are not a few - a chain and belt-driven model will be marketed, but except for the transmission the above description stands. Mudguarding on both types is excellent.

A New Sidecar.

Specially designed for the machine is a new sidecar, the chassis of which is of peculiarly simple construction. Briefly it consists of a double tubular axle member, and two side members converging forwards to form a triangle with the axle. This construction is extremely light and strong, and since it is attached to the machine by special lugs and fastenings, forms a rigid whole which cannot get out of line.

The rear attachment is formed in an ingenious manner with hemispherical seatings for the attachment, which render fitting easy and automatically take up any slight variation in centres.

Unusual Method of Springing.

Mounted on the chassis is a luxurious body with deep back and cushions, and having a locker of really useful proportions; The top of this locker is fiat and acts as a platform for additional luggage. An unusual type of springing is employed, the front of the body being supported from the apex of the chassis by a single coil spring, while at the rear a single cross spring, having eleven leaves enclosed in a leather gaiter, is attached to the body in the middle, and to the double axle at either end, One end is, of course, shackled to take up the stretch of the spring.

The 6 h.p. Twin.

The 6 h.p. model, which was exhibited for the first time at the last Olympia Show, has undergone but slight modification.

Our readers will remember that this machine is fitted with a V twin engine of 76x85 mm. bore and stroke (770 c.c.), lubricated by an ingenious type of oscillating plunger oil pump. Transmission is by chain throughout, a three-speed constant mesh gear box being carried in the usual position. Both chains are entirely enclosed in aluminium castings, which, though oiltight, are easily removable. One of the most striking features of the 6 h.p. B.S.A. is the excellent mudguarding. Both guards are exceptionally wide, and enclose the forks and chain stays. For 1921 these guards have been further improved.

The Motor Cycle November 25th, 1920. Page 644