Bodging the Brakes Safely
I recently bought a second (or third or fourth or fifth) hand Linear recumbent bicycle.
It’s an interesting design, consisting of a single extruded aluminium alloy beam with all the elements – seat, steering, drive and wheels hung from it. Mine dates from the mid 1980s or early 1990s. This is suggested by the fact that it was fitted with a triple front chainring (probably not original), anodised chainstay and it still has caliper brakes, though I’m not sure if they’re the originals. And it’s the caliper brakes that are a safety and performance issue for me. You see, someone upgraded the shifters at some point. They used lovely Shimano click-shift controls – the thumb and index finger ones - with integrated brake levers. And, of course, these levers are designed for linear pull cantilever (or ‘Vee’) brakes. So what? Well, linear pull cantilevers need to travel twice as far as caliper brakes. So the levers pull twice the length of cable, but only transmit half the force to the caliper. So, when you try to stop a bike with a ‘Vee’ brake lever on a caliper brake what happens…?
Not… a… lot…!
So I needed to reduce the amount of cable pulled and increase the force pulling it. Remember school days - physics class lessons on levers? To get twice the travel you move the lever twice the distance from the pivot. Yes? That’s how vee brake levers work. Trouble is, that halves the force when it doubles the travel, doesn’t it. That’s what’s happened with the new levers. So, how do I reduce the travel and increase the force in this situation? Given the opposite scenario – vee brake with caliper lever, there’s a device to fix the problem. It’s called a travelator or some such, and apparently consists of an eccentric pulley to increase the cable travel to vee brake requirements. But it fixes to the brake lever, I believe. (I haven’t actually studied one.) So it isn’t ‘reversible’ for the opposite scenario.
My solution isn’t so high tech, but it does spring directly from those physics lessons. Remember basic pulleys. You can calculate the mechanical advantage of a pulley system by counting the number of lines on the tension side of a pulley system. And I only want to double the force and halve the distance, so it’s the simplest of pulley systems. A fixed end to the cable pulled by the lever passes over one pulley. This gives two tension lines and a mechanical advantage of two to one (2:1). And it halves the movement of the output cable which is fixed to the pulley block. Exactly what’s needed.
Here’s the finished prototype. The lever pulls on the upper cable on the right. The pulley travels to the right, drawing the attached assembly and cable with it. The caliper brake is on the left, actuated by the left hand cable.
the practicalities weren’t too complicated. i found a suitable pulley on a lady’s frame. it had been used to direct the brake cable upward to the caliper after it had followed the sloping top tube down to the base of the seat tube (a 1:1 scenario).
the pulley block i fabricated from tubing which i cut from another scrap bike and flattened, shaped and drilled for the pulley screw and bushing and the barrel end of the output cable.
The tension side cable stop was fabricated from more of the same material.
It works beautifully on the bike. The brake operation is progressive and quite positive considering the amount of slack the system. I’ll eventually dismantle it all, wire brush it and paint it with black hammerite so it ‘disappears’ into the side of the bike, and grind a bit off the back side stop nut so it doesn’t touch the frame. But for now I’m thinking how to squeeze the second one in behind this one so I can have a powerful rear brake on the bike too! I think the pulley assembly and cable stop could come forward. This would probably leave enough space for another. This modification is probably not so easy to do on a bike with a round tube frame, but for the Linear it’s a breeze.
-- Denis Buckley