Here at HOT BIKE we take what's going on in the motorcycle industry seriously. As part of our top-secret (until now, that is) anecdotal subjective study of the current trends of the two-wheeled variety, we noticed that big is in. "Big" as in large displacement, additional gears, fat tires, wide and high bars, more, more, more. OK, we're just like you-guys riding and wrenching, trying to squeeze every tenth of a horsepower possible for quick launches, bragging rights, and tire-shredding acceleration.
That's great-we love it, you love it, and the industry loves it. But (you knew that was coming) how about slowing down that 1/2-ton bagger loaded-toward-the-sky two-up as the "I didn't see him" cage driver pulls out to ruin your moment of pleasure? Obviously, this is leading toward a story about the opposite of speed; we're talking the oft-overlooked deceleration duties that we should be just as concerned with. Brakes for the American V-Twin have evolved to levels only seen on the racetrack just a short time ago.
Even Harley-Davidson decided to enter the 21st-century world of better braking by equipping all of its bikes (except the Springer models) with a fixed-mount four-piston front brake caliper for the model-year 2000, which replaced the single-piston floating caliper.
OK, before we proceed, a few terms should be loosely defined. The piston (or puck) in the caliper is the cylinder that pushes against the brake pad when activating the brake lever. On the H-D floating single-piston caliper, only one piston on one side of the disc (rotor) is activated by brake-line pressure. As that one outboard-lying piston pushes on its brake pad, the caliper and the opposite brake pad move ("float") as a unit to clamp down on the rotor. It worked but was often overworked due to the small pad area and flex within the caliper and mount. The newest H-D incarnation (also made from cast aluminum) contains four equally sized pistons-two on each side of the rotor. All four of the pistons are hydraulically controlled to squeeze the pads against the rotor. Another big difference from the earlier calipers was that the new brakes were solidly affixed to the fork leg. A much better unit for sure over the previous model, but still prone to flex and heat buildup.
High-performance brake calipers from makers such as Performance Machine (PM) are manufactured from a block of billet aluminum that practically eliminates caliper flex. This means more of the braking energy is transferred to the disc where it belongs. Another significant advantage to PM's calipers is the inclusion of differential-bore pistons within the caliper. What this means is that the pistons on the same side of the caliper are of different diameters. The theory is that the majority of heat buildup in the brake system occurs at the leading edge of the caliper where the spinning rotor enters. Using a smaller piston at this entry point, compared to the exit point, not only reduces heat buildup but also balances out the braking force, leading to better control and feel at the brake lever. An added bonus is more even pad wear and longer pad life due to the equalized pressure being applied across the entire pad.
The FLHX with Stage 3 PM brakes featuring dual 13-inch rotors and six-piston calipers. |
We disassembled these four calipers to illustrate the number and size of the pistons. From top to bottom are the H-D single-piston floating caliper, H-D's current four-piston caliper, PM's differential-bore four-piston unit, and PM's differential-bore six-piston caliper. |
One brake pad from each of the previous calipers is shown here. The take-home message is that the pad surface area increases with more pistons. |
The first step in swapping out brake calipers was removing the brake line from the caliper... |
...followed by the removal of the two caliper retaining bolts. |
Here is a view of PM's (left side) four-piston caliper. All mounting hardware comes with each caliper, as well as a new banjo bolt to attach the brake line. |