tech46

For all of you BSA B50 guys out there I thought I would review a
little of the history of the racing B50 motorcycle as told by Frank
Melling (British journalist from the 1960's).

Brian Martin of the BSA Competition Shop in 1961 determined that the
weight advantage of the little unit single 250's and 350's sometimes
gave it faster lap times then the famous 500 cc Gold Stars. He bumped
the unit single displacement up to 421 cc by 1964 with which Jeff
Smith won the World Motocross Championship. It gave up 10 HP to the
Gold Stars but was a bunch lighter at 228 lbs. The engine cases were
magnesium and would only last for two 45 minute motos. After a race
day the cases were so beat up that the main bearings would push out
with thumb pressure. The displacement went up again to 441 cc and Jeff
Smith again won the championship. 1964 saw the introduction of a
production racer available to anyone willing to pay, not just Jeff
Smith: the B44GP, with Reynolds 531 steel (oil in the frame) frame but
an aluminum engine instead of magnesium. To make the championship a
triple BSA management decided to go to titanium for the next GP MX
season, without the proper engineering analysis. The frame, the
rockers, fork legs and crowns, wheel rims, and the connecting rods
were titanium while the cases, side covers and rocker boxes were
Magnesium. Aluminum was used for the sprockets, timing gears, and the
clutch hub while a chrome lined bore of an aluminum cylinder (now 490
cc) also helped in the lightening. It was a disaster as the season
progressed when the wheel base got longer with every race, the frames
eventually would break, and the main bearings would spin in the
magnesium cases during the race. Going back to the drawing board for
the subsequent season the engine was fixed but remained magnesium
while the frame used Reynolds 531 steel for a weight of 223 lbs. By
1968 the factory GP MXer was at 498 cc which later evolved to the
B50's final dimensions of 84 x 90 mm. The BSA team, which now included
John Banks, toured the US, racing local motocrosses to demo the new
B50. Having attended the races at Saddleback I took pictures as the
guys sped around. In comparing John Banks B50 to the eventual
production B50 I detected that there were a lot of difference from the
design of the frame to many of the details.

As a journalist that also raced MX in about 1970 Melling requested
BSA to loan him a factory B50MX for "testing". The bike delivered was
a black aluminum engined, chrome plated mild steel framed "factory
racer" with all black "Clews look-missing fins" cylinder. His first
race was a revolution to him as he won which was not his normal
finish. He liked the bike so well that after a few months he requested
to buy it. As BSA had closed it's Competition Shop, they, being cash
poor, agreed but requested Melling to bring it back for a "check
over". When returned to Melling it sported a Reynolds 531 frame,
magnesium fork sliders, magnesium brake backing plates, a new top end
and the speedway camshaft developed for BSA's abandoned assault on the
speedway world of Jawa, JAP and ESO.

According to Melling the bike's performance was so hard that only an
expert could ride it and only a knowledgeable big single guy could
even start it. The bike dyno'ed at 33.4 HP at 6,200 RPM. That's right,
a barely rideable version of the B50 (due to its' hard hitting power)
made only 33.4 HP. Modern big singles commonly dyno at 40 to 50 HP. A
lot has to do with tractability. That is, how much power there is
below the maximum horse power peak or in engineering terms, the area
under the power curve. Engineers know a whole lot more about how to
make cam timing work today then they did 30 years ago. That's why
modern big singles with 40 to 50 HP are rideable by even novice off
road riders, unlike Melling's fire breathing B50. Modern big singles
also rev much higher (and RPM is a component of the horse power
equation) thanks to overhead camshaft designs, four valve heads,
straight running ports, and many engine innovations learned from road
racing four stroke engines.

Melling sold the bike and re-acquired it several years later noting
that it was none the worst for wear since so few of the several
interim owners could even get it started let alone ride it.

One of our members races Vintage motorcross and AMA vintage Grand
Prix's on a much modified B50MX. His bikes sports a Megacycle cam,
ported heads, a 38 mm Mikuni (note that the huge carb on a Clubman
road racing Gold Star is 1 1/2 inches or ONLY 36 mm), and laid down
shocks that allow head angle changes for better turning. I wonder how
his bike compares to Melling's fire breather?

What is to be learned from all of this? B50's have practical limits
due to the 1960's design. Remember that the cases and the transmission
have their beginnings in a little 250 cc econo bike. If you try to
pump that little motor too hard it indeed will break, as the BSA
factory found out. Additionally, titanium and magnesium are not
necessarily the wonder metals they are advertised to be. Sound
engineering is important to successfully use all materials. And
finally, the bike the factory races is not necessarily the production
racer they sell you.