The later BSA B25 is not very often used for trials, partly because it is too new for Pre-65 events, and partly because of the Oil-in-frame design. However the engine is a considerable improvement on the previous C15 (just as the B44 is a much better engine than the B40), and the frame is slimmer at the rear and has the chain adjustment at the swinging arm mount.

    After measuring a Can-Am 350, Italjet 350, SWM TL320, Montesa Cota 349, Suzuki RL250 and an Aprillia Climber I decided on the frame parameters. Settling on a wheelbase of 52", a head angle of 27deg, and footrests mounted just below and behind the swinging arm mount on a level with a line through the wheel centres.

    First I shortened the swinging arm by 1.25" by moving the rear wheel mount forward to the rear suspension mounting. The forks were fitted, as were both wheels, and the bike set on a stand with 10"of ground clearance. The frame was then strapped down around the stand so that both wheels touched the ground, the suspension units not being fitted at this time.  The entire top part of the frame which forms the oil tank and rear mudguard mounting was removed. As the bottom tubes flexed when this was done a cross piece was made and fitted between the front and rear engine mounting points to maintain the correct distance to match the engine dimensions.

    The front down tube was then heated just above the engine mount and given a very slight bend. The tube was then heated just below the steering head and bent down until the desired fork angle was achieved. Another brace was put in across the top of the frame to maintain the head angle. A new top tube made from thin walled 1.34" Chrome-Moly tubing was bronze welded into place, and a short tube to brace the steering head was added to complete the main frame loop.

    The tubes rising from the swinging arm mount were bent inwards and slightly forwards to slim the rear subframe. A strut with the same dimensions as a 360mm Betor unit was made and bolted into the bottom suspension mount, and the new top mount was bolted into its top to hold it in the correct position while it was bronzed into place. When this was done on both sides the suspension top mount was connected to the main frame with 0.75" Chrome-Moly tube. Rear mudguard mounts were bronzed on, and the frame was complete except for a couple of mounting brackets and the footrests.

    Various spacers and brake mounts had to be made up for the Honda TL125 wheels which were used, these having been laced with new spokes. An oil tank of 1 Lt capacity was fabricated and placed across the frame just above the swinging arm mount.

    The engine was left in standard trim with the exception of PVL electronic ignition, and smaller engine and gearbox sprockets. In order to help with the tickover (idle) and to make the engine more responsive at low revs a small bore Mikuni carb was fitted. A new silencer was also fabricated both to quieten the exhaust note, and also to tuck it into the frame and clear of the riders boot. 

    After initial test rides, including a two day trial in Ohio, I am very pleased with the bike. It is, of course, heavier than the trials bikes from the seventies, I think about 220lbs, but it turns and handles as well as those bikes. With some modifications to riding style to account for the lesser ground clearance it seems very competitive. Once you get used to having the rear brake on the left, and the righthand gearchange being 1 up and 3 down that is.