Hawg Halters' Four-Piston Differential Bore Calipers
By Brian Moore
If you own a '99 or earlier Harley-Davidson, your motorcycle probably still has the stock single-piston brake calipers to slow you down. The year 2000 brought an across the board (except for the front brake on Springers) upgrade to a four-piston style of caliper, used on the front and rear of the motorcycle. Anyone with experience riding a motorcycle with both types of brakes can surely attest to the improved performance of brakes on the newer Harley models.
So, other than three more pistons, what's the difference between a single-piston design and a four-piston design? To find out, we called up Mark Thompson at Hawg Halters Inc. (HHI) and ordered up a set of four-piston, differential-bore calipers to fit a '99 Fat Boy. Once the components arrived, we opened up the boxes and checked things out. Our first impression was that these pieces were well made, with no visible machining marks, and had an excellent chrome finish. The components are also available in polished aluminum and black anodize. The most outstanding feature was the clean design of all the pieces. From end to end, the components have smooth curves transitioning from one surface to another. The bleeder valves, two per caliper, are threaded into the rear of the caliper, towards the wheel, to keep from cluttering the look of the piece. To top it off, the calipers have no HHI logos displayed on any of the surfaces, a big plus for many guys.
OK, back to the difference between single- and four-piston calipers. Due to the design of a single-piston caliper, when the brake lever or pedal is depressed, the movement of the brake fluid in the brake line causes the piston (on the outboard side of the bike) to move inward towards the brake rotor. Once the caliper causes the outer brake pad to make contact with the rotor, the housing that holds the caliper is forced to slide along a pair of pin bolts until the inner pad makes contact with the brake rotor. While this sounds good in theory, it is not the most desirable way to have the brake pads put pressure on the rotor. Due to factors such as wear, age, road grime, and fatigue, this single-piston design may not operate as it was originally designed. Performance can easily suffer due to mating surfaces sticking to each other and causing the pads to put unequal pressure on the rotor. A far better way of putting the squeeze on a rotor is to put a piston on both sides of the rotor. Even better yet, put multiple pistons on either side of the rotor to put greater pressure, spread out over a larger area on the rotor. Adding to the clamping performance of a multi-piston caliper design is the concept of differential bores. Without getting too deep into theory, physics, and mathematical formulas, a differential bore caliper with the same basic design of a standard bore caliper will give better braking performance due to more balanced pressure being placed against the rotor by the pad, while generating less heat, spread out over a larger area. Basically, the way it works is by having two of the four pistons manufactured to a particular bore size, while the two other pistons have a bore of a larger diameter. Differential bore calipers are assembled with one large and one small piston on each side of the rotor, with the smaller-diameter piston at the leading edge of the caliper, the side which the rotor enters the caliper. Another feature of this design is the ability of the pistons to remain more concentric in their bores as they resist cocking in at the leading edge.
HHI manufactures its multi-piston (four and six) calipers to fit just about any Harley-Davidson that rolls out of the factory with disc brakes. For this application, we used a LHSCC500 (front caliper) which retails for $337, and a RKSTCC500 (rear caliper and bracket) which retails for $490. HHI will ship its calipers with your choice either metallic compound brake pads or organic brake pads. Since we were keeping our stock rotors, we decided to go with the metallic compound due to their more aggressive braking performance and higher friction levels.
Hawg Halters calipers and mounting brackets are machined from solid 6061 billet aluminum and come with chrome mounting hardware, chrome banjo bolts, shims, and crush washers.
Here you get a good look at the stock rear caliper. It is sitting upside down and disassembled for clarity. Using a single-piston design, the piston (A) moves to the right (in this example) as the pedal is depressed, causing the outer pad to make contact with the brake rotor. Once contact is made, the entire caliper housing slides along the smooth surface of the pin bolts (B), as it sits in the caliper mounting bracket, causing the inner pad to make contact with the rotor.
A good look at the backside of the rear rotor reveals the size difference of the caliper bores. The nominal size of the bores is 1.18 inches and 1.33 inches. Also present are the two bleeder valves.
With the bike on a Handy lift and a scissor lift raising the rear tire from the table, the rear axle was pulled and the wheel spacers were removed. This allowed for removal of the brake assembly once the banjo bolt and brake line were detached.
Assembly began by bolting the rear caliper to the mounting bracket.
Next, the bracket and caliper were put back in place along with the spacers and axle. The axle nut was tightened with a 15/16-inch wrench.
This step is very important for proper brake operation and even pad wear. The caliper must be centered over the rotor. To ensure this, take a very good look at how the caliper sits over the rotor. There is a pretty good chance it will need to be shimmed with one or more of the supplied shim washers.
After close inspection, it was determined the caliper would need to move inward around 0.100 inches. This was accomplished by stacking a 0.060-inch and a 0.032-inch shim together.
The whole assembly was then removed from the bike, and the caliper and mounting bracket were separated to allow for the insertion of the mounting bolt through the shims (see arrow). To keep from marring the chrome finish of the mounting bracket, we took one of the unused shim washers and placed it between the bolt and the mounting bracket.
With the brake assembly back on the swingarm, the chrome banjo bolt was passed through the stock brake fitting with one crush washer on either side of the fitting.
Next, the banjo bolt was threaded into the caliper, then tightened with a 9/16-inch wrench.
Not only could the performance of the stock brakes be improved upon, the looks also needed some serious help.
To avoid making a mess with the brake fluid, we disconnected the line from the front caliper, wrapped it in a rag, and hung it from the bike with a bungee cord.
Shimming of the front caliper was necessary as it was with the rear. Here, we only needed to use a 0.010-inch and a 0.016-inch shim washer stacked together to properly center the caliper over the rotor.
With the caliper in place, the supplied chrome button-head screws were used to secure the caliper.
A couple of new crush washers and a banjo bolt, and we were almost finished.
HHI recommends torquing the mounting bolts, both front and rear, to 25 lb-ft. All that was left to do was bleed the calipers and brake lines before going for a test ride.
Bleed Brakes Like a Pro
A well-bled brake system is paramount to proper brake function. The hydraulic fluid pressure necessary for effective braking can easily become compromised if even the smallest amount of air enters the system. The only means to remove air is through bleeding. There are several methods available to bleed your brakes, but considering the accessibility to all the brake components on a bike, the traditional method generally works very well.
Brake bleeding should always begin with brake system flushing, unless the master cylinder, lines, and calipers are all new. Brake fluid has a finite life span, and most manufacturers recommend the fluid be replaced at certain intervals of the bike's lifespan. Start with a fresh bottle of DOT 5 brake fluid -- brake fluid ages poorly and that bottle you have had stashed in your garage for the last two years is not going to do the job. DOT 5 brake fluid is silicone based and should never be mixed with DOT 3 or DOT 4.
We attached a length of clear hose to the bleeder valve on our caliper and guided it into an appropriate container (like a glass jar or coffee can). We then cleaned the master cylinder to avoid contaminating the fluid and removed the cap with the handlebars positioned to keep the master cylinder as level with the ground as possible. We opened the bleeder on the caliper about a quarter turn and pumped the brake lever a dozen or so times. As we did this, we watched the fluid level because we needed to keep it at least halfway filled to avoid sucking any additional air into the system. We continued adding fluid until the stuff coming out of the tubing was the same color as the stuff we were putting in. We closed the bleeder valve and pumped our brake lever five times. Next, we depressed the lever and opened the bleeder valve to release the fluid and air. The lever quickly moved through its travel. When it bottomed out, we closed the bleeder valve.
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