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 frame was then placed in a jig to ensure that all modifications are to required parameters. The entire top part of the frame which forms the oil tank and rear mudguard mounting was removed.
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. A new top tube made from thin walled 1.34" Chrome-Moly tubing was bronze welded into place, and strengthening plates were added around the steering head 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. The new suspension unit top mount was bolted onto the jig 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 Montesa Cota 348 wheels which were used, these having been laced with new spokes. An alloy 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, smaller engine and gearbox sprockets, and a compression plate under the cylinder to reduce the 10:1 compression ratio to a more trials friendly 8.5:1. 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 on a custom manifold. A alloy silencer fabricated in the UK was added both to quieten the exhaust note, and also to tuck it into the frame and clear of the riders boot.
I am very pleased with the bike which performs to a very high standard, certainly comparable with most of the 70s trials bikes. It turns and handles as well as those bikes and weighs only 205lbs. The engine is very responsive with lots of low end torque to provide tremendous grip in all sorts of conditions. With some modifications to riding style to account for the lesser ground clearance it seems very competitive. This particular bike has the standard British pattern of rear brake on the left and gearchange on the right, but for future models buyers will have the opportunity to have the rear brake on either side.
The B25 as made by BSA weighing 275lbs and with a 20" front wheel.
Remade by Britannia Motorcycles.
The modified frame, sandblasted and ready to paint.
Shortened swinging arm with two mounting points for the chainguard, and below that for the chain tensioner.
Here the new rear subframe and footrest mounts can be seen. All joints are bronze welded (this is not the same as brazing).
The modified frame has a much narrower profile.
A number of pieces were nickel plated using one of Casswell's home plating kits.
Various other bits. Some standard, some bought, and some fabricated.
The engine fitted with PVL electronic ignition from Sammy Miller Products in the UK, and a compression plate to reduce the 10:1 ratio to 8.5:1.
The front brake torque arm fitted to one of the fork lugs with no modifications necessary, but a bracket for the cable had to be made.
The rear brake is cable operated on the left side (where it should be).
Chain tensioner is from Sammy Miller Products. Very nice but unfortunately it was kinked to the wrong side for my application so little professional brute force was needed to get it right. The rear brake torque arm combines the cable stop.
The beauty of the PVL system, apart from first kick starts, is the fact that it completely replaces the alternator with only two wires coming up to the coil. These wires were routed up through the front down tube which used to be a part of the bikes oil tank.
The carb is from one of those pit bike things. Nominally a 25mm choke it actually narrows at the throttle slide but with some jet tuning it works really well. As the bolt holes are the standard 1 7/8" apart, and not the Amal 2", a manifold had to be made. This also allows a longer intake manifold which has a noticeably beneficial effect on the engine.
An alloy oil tank of 1 Lt (just over a quart) capacity was made. I did the cutting and folding and Mike Komer of The Tryals Shop did the welding.
The silencer came from the UK. It is light and beautifully made but little loud.
As you can never find a tree when you want one, even in the woods, I fitted a propstand to the Mk II.
It not only looks really pretty but it works well too. Mick Andrews spent a day riding it on a recent visit and he cleaned all of my sections on it.