By Evan WilcoxBy Evan Wilcox
Most of us interested in old bikes have had the satisfaction of going to the garage and making a successful modification to our bike. I made a few good modifications recently to my BSA B40 trials bike.
I knew that going non-standard, in addition to degrading originality, also makes stock parts no longer fit in some cases. But after studying Classic British Trials Bikes by Don Morley (Osprey Books), I noticed that without exception all the famous bikes were non-standard. Sammy Miller’s Ariel and Gordon Jackson’s AJS are extreme examples. The latter was built by Brian Slark and company at the AMC comp shop. They moved the motor back 1 inch in the frame, shortened the subframe and increased fork travel, among other things. There is a precedent for taking a hacksaw to our classics.
On my BSA I had previously fitted the B40 trials footpegs, which went down and back a few inches. I shortened the subframe, which jacked up the rear end, steepening the fork angle and improving the steering. It also made the original oil tank, seat, muffler and kickstarter not fit. There was no turning back now. I had to make or modify substitutes for the original parts. The B40 was now looking and acting like a real trials bike.
AHRMA member Laf Young was visiting at Christmas, and we were discussing his Triumph Cub project. In particular, rake and trail of the front fork. He rode my B40 down in our sand pit and found the front wheel would plow in a tight turn. Even weighting the outside footpeg didn’t help the wheel to complete the turn.
After calling Matt Hilgenberg at Speed & Sport, it was discovered I had too much trail. I needed to reduce trail from 5 inches to around 3 1/2. Matt said the Bultaco forks I was using did not have enough offset in the triple clamps.
At that point I should have bought his aftermarket triple clamps that reduced the trail 1 inch. Instead, that evening I put blocks under the rear wheel to see if longer shocks would solve my problem. Yes, 14-inch shocks would give 3 1/2 inches of trail, but they would also look silly.
My solution was to take off the gas tank and cut through the backbone of the frame. With blocks under the motor, I was able to spread the tube with a crowbar, leaving a gap that would later be filled with a slightly pie-shaped piece of tubing. With both tires touching the ground, I measured the trail using a long straight edge from the steering head to the ground and another from the front axle to the ground. The distance between the two points showed 3 5/8 inches of trail could be had with a 2-inch-long tube spacer welded back in the backbone.
Back to the sand pit, I was now able to make a tight
turn and the wheel was more reluctant to plow. The steering was
more precise on loose off-camber turns as well.
"Good fix," I thought to myself as I headed down a steep bank. I tried to steer around some rocks, which grabbed the front wheel, wrenching the handlebars out of my hands and sending me out the front, boots over hat!
It turns out I now have extreme negative trail when going downhill, which requires strong shoulders to hold the bars when rocks or tree roots try to deflect the front tire. However, the pinpoint steering was a fair trade once I got the descent thing sorted out.
The following weekend I had snow to practice in—like trying to find traction in a bowl of Cream of Wheat! After close examination in the Morley book, I saw that my footpegs were still about 4 inches too far forward. The sweet spot seemed to be under the swingarm spindle, about even with the rear axle.
I shaped a piece of angle iron with a grinder and welded it in place with the pegs bolted on. Good fix, except the brake pedal had to go, and the kickstarter hits the footpeg again. With my homemade brake pedal and re-bent kickstarter, I headed back out in the snow.
This time, in a taller gear and with a whiff of throttle, I could chug along in the snow. My weight bias was on the rear wheel. Good fix!
Postscript
The author writes: As you might expect, there was more
to the story. Much of the steering trouble was really
point-of-balance trouble. The big, heavy motor too close to the
front wheel was making it plow when turning. I ended up
shortening the swingarm and fitting triple clamps with more
offset from the steering head.
This effectively moved the motor back, making the front wheel steer without plowing. The greater offset triple clamps reduced the trail further, so I was able to take out most of the spacer I previously fitted to the frame backbone under the tank. This made the fork rake less steep and improved stability over steep banks, roots and rocks. Sometimes it is good to go too far and then back up some to find out what the design limits really are.