AA (M20 & M21)


Make Model

AA (M20 & M21)

(M21 specifications recorded. Click here for M20 specifications)


1937 - 61


Four stroke, single cylinder, side valve


591 cc / 36.1 in

Bore x Stroke

Up to 1938: 85 x 105 mm

After 1938:  82 x 112 mm

Cooling System Air cooled
Compression Ratio 5.0:1
Oil Capacity 2.4 L / 5 US pints
Carburetor 1937-38: Amal 76, 1939-45: Amal 276, 1945 onward: Amal monobloc
Exhaust Single, chrome




Battery 12 V
Starting Kick

Max Power

11 kW / 15 hp @ 4200 rpm
Clutch Multi-plate, dry


Final Drive Chain
Frame Twin cradle

Front Suspension

Up to 1947: Girder

After 1947:  Telefork

Rear Suspension

Up to 1947: None, spring loaded saddle

After 1947:  Plunger type

Front Brakes

Up to 1955: Drum, 7 in

After 1956:  Drum, 8 in

Rear Brakes

Drum, 7 in
Wheels Steel, laced spokes

Front Tyre

3.25 x 19 in

Rear Tyre

3.50 x 19 in


Length:  2180 mm / 85.8 m

Width:     740 mm / 29.1 in

Dry Weight

167 kg / 369 lbs

Fuel Capacity 

11.4 L / 3 US gal

BSA motorcycle combinations were the mainstay of the AA's breakdown fleet in the 1950s and the early 1960s. The AA used over 2000 of the M20, and later M21 models, and was the last purchaser of the M21 when production ended in 1961.

The M21 combination was an unsophisticated but robust machine, powered by a 600cc single-cylinder sidevalve engine producing a modest 15 bhp, which propelled it to barely 50mph.

The AA ordered a different specification from standard, which included a stronger front brake, a 12v alternator instead of a dynamo to power the two-way radio, and higher, 'western-style' handlebars.
600cc single-cylinder sidevalve engine

600cc single-cylinder sidevalve engine

Various types of sidecar, leg-guards and fairing were produced.

During the 1960s the need to carry more equipment (not to mention the desire of AA patrols to keep out of the weather) meant that the motorcycle combination was gradually replaced by the Mini van. It was the end of an era – the traditional AA patrol salute was abandoned in 1961, and the last BSA was retired in 1968.

There are 4 BSA M21s on the AA's heritage fleet, looked after by various AA breakdown patrols around the country.

I'd never ridden a motorbike and sidecar before. 'There's nothing to it,' grinned the cartoon artist Nick Ward cheerfully - but then he would, wouldn't he, because a) he ran BSA M21 outfits as his primary transport for many years, and b) had sold the test outfit to his neighbour Dave Watson. And as I got ready for a first go on the BSA and sidecar, Nick and Dave were exchanging looks with a gleeful edge not too far below the surface. 'Nothing to it', hell -- I know I'm in for trouble.

The AA outfit in question was the earlier kind, with just a plain box sidecar, not the 'plastic fantastic' streamlined fibreglass type introduced from 1961-on. And this M21 bike didn't have the later 12-volt, crankshaft-mounted alternator to supplement its 6-volt Magdyno and help with the auxiliary equipment, which by then included radios. It also didn't fit the higher 'Western' handlebars with which these models were catalogued, or the metal legshields often featured. But there had been plenty of variations among the 2000-odd RSOs (Road Service Outfits) during the Fifties, and it did have the fuller front mudguard and Avon half-fairing; its 8 inch sls front brake had by then been made standard on BSA's remaining big side-valve slogger. There were no AA logos remaining on the tank or fairing, but it was unmistakably ex-AA.

This engine proved an easy starter, and once fired up, the motor also kept going with a wonderful steadiness, no matter what Nervous-Nellihood it was subjected to. So - no more putting it off -- here we go. Boot the gear lever up into first, let out the clutch, and...

And why had I let them park it on that patch of shingle?! It was like trying to drive off a pebbly beach. The rear wheel dug in and spun, the sidecar wheel tried to overtake the bike, and for the first time but far from the last, the Norfolk air turned blue as I panicked. There was a deep gouge in the shingle by the time we'd spat a lethal shower of stones backwards and lurched onto the tarmac.

Where things were no better! Sidecar outfits wobble! In desperation, before leaving home for this foolishness, I had consulted my 40 year old copy of 'Teach Yourself Motor Cycling', a little blue and yellow book which had served me well while I was learning to ride. On the subject of sidecars, author Dudley Noble (for it was he) observed briskly 'When first driving a sidecar machine a rider naturally tries to balance himself... as with his solo machine. The result is that the machine promptly runs into the ditch...' Thanks, Dudley. 'It is only,' Dudders continued, 'when the rider appreciates that he has got to 'drive' in similar fashion to driving a car, that he will steer a straight course.' As a pal had put it, you had to treat the handlebars as a steering wheel.

All right, all right, but meanwhile ... the sensation of lacking control was genuinely terrifying! Either I or the infernal machine, when braking or changing down, pulled hard to the right, ie. straight out into where the oncoming traffic would have been, if it hadn't been for that blessedly empty Norfolk road. Expletives Deleted! A lot of them! Then coming to a halt to re-group, I kept putting my feet down -- completely unnecessarily, obviously, this was one bike that was not going to topple over -- but down went the feet anyway, before the bike had come to a halt, thus risking getting an ankle trapped and torn in the tubework - but habit was too strong, you just couldn't not. More expletives!

Underway again, the sidecar wheel was lifting, not on left-handers, but just bouncing up on the inevitable bumps and irregularities of a country road. This was, ah, unnerving, and the bouncing of the springy saddle, counterpointing the minimal movement from the bike's plunger rear suspension units, added to the overall sensations of wibbly-wobbliness. Though it has to be said that, once accustomed, this set-up did provide a surprisingly comfortable ride. I motored down the road towards a couple of cottages on a gentle right/left downhill bend, all tensed up, which only added to my woes. But there were no real problems - remembering some more advice from Dudley N, I accelerated up to the right-hander then slowed around it, and tried to motor round the left bend. The wheel stayed on the deck. At 30! Hey, this was ... OK. So far...

Gently downhill for half a mile, and then there was a T-junction with a busier road. Gingerly, slowly, and with my feet staying firmly up on the rests, I turned the outfit round through 180 degrees and set off uphill again. Hey, that was definitely fun - how often do you get to turn a bike on full lock with your feet up? I negotiated the bends and throttled on a bit down the straight - I was realising that things were better at 'speed' than when attempting a slow cruise, or slowing down. In fact I was feeling confident enough to try giving a twisted version of the AA man's salute as I approached the others and began to pull to a halt, changing down... AAARGH! BIG Expletive! As the braking and gearchanging again sent us half way across the road, from which the chaps prudently leapt for safety. When I did stop it was a couple of hundred yards further on...

But I did another circuit, and another, cautiously growing in confidence. This time down at the T-junction, doing my feet-up turn with real enjoyment, an old boy in a modern Tonka Toy 4WD lowered his window to stare, and then said 'Keep 'er going!' encouragingly, though when I blurted out that it was my first time on a combination, he accelerated rapidly away.


The final reckoning on the Sidecar Experience? Terrifying, but do-able. I'd need a lot more practice before I felt OK in traffic, because stopping was the most hair-raising thing. A sidecar wheel brake, as found on, say, Panther's proprietary chairs, might be a distinct advantage here.

But I did come to appreciate the pleasingly unified BSA outfit, its unstoppable chugging poke; the details like the period triangular-shaped handlebar grips which made life a lot easier when you had to remove one hand from the bars; and particularly the motor's sidecar gearing, which meant the power was always there where you needed it - very welcome, and spot on for the work. There was nothing to do about the lack of speed - I did see 40, but 30 for the first time on the M21 outfit was more exciting than 80 on a modern solo!

Article by Steve Wilson