Sidepull Brake Service
Properly adjusted brake systems require attention to small details. This article will discuss sidepull caliper arm adjustment, pad adjustment to the rim, pad centering, and pad clearance. For purposes of this article, the terms "left" and "right" will be from the "mechanic's point of view," not from sitting on the bike.
The side pull caliper rim brakes were once the common road bike brake. Both caliper arms share a single pivot, which is centered over the wheel. The image above shows modern dual pivot, and centerpull brake calipers, and sidepull. Sidepull calipers are also found on some BMX, Freestyle, and some recreational bikes.
The caliper arms pivot off the center bolt or stud. The arms should open fully when squeezed, but should have no play or knocking. There are two basic types of sidepull caliper designs. The double nut type has two nuts in front of the caliper arms. The inner nut is the adjustment nut and is locked in place by the outer locknut. The other design type was commonly used by Shimano® and is called the “safety pivot”. In this design, there the adjusting nut and locknut are behind the brake arms. The brake must be completely removed from the bike to have the caliper arm movement adjusted. The adjusting nut doubles as the spring holding nut.
Both brake pads move downward on an arc as they approach the rim. As the pads wear thinner, they will travel downward even more. Generally for side pull, adjust pads to the top of the rim braking surface, but not so high they would strike the tire. See image below.
Pad Centering and Alignment
- Inspect the wheel for adequate centering in the frame/fork. Correct as necessary.
- Loosen and lubricate threads of pad fixing bolt/nut. Adjust pads so they strike strike upper edge of braking surface, but not the tire (figure 2).
- Check front end and back end of pad. The pad should be square or tangent rim (figure 3).
- Tighten pad-fixing bolts.
- Attach cable (if necessary). Squeeze lever to test pad clearance. Use barrel adjuster to adjust pad clearance. Set clearance as desired. Draw slack from system using inner-wire pinch bolt if barrel adjuster is set out to its limit.
- View pad centering to rim. If pads are off-center, manipulate center pivot bolt. There are different procedures possible.Some models use a flat machined into the center stud. Use the Park Tool OBW-4 Offset Brake Tool or a cone wrench to grab this flat. Place a wrench on the mounting nut behind the brake. Move both wrenches the same direction and same amount to move the pivot. In the image below the wrenches are rotated counter-clockwise from our view, which moves the caliper arms counter-clockwise around the pivot bolt (figure 4 and 5).
If the brake is a double nut type, and has no centering flat, it may accept the OBW-3 spanner. Use the OBW-3 to grab the spring and move the pivot. It may also be necessary to loosen mounting bolt and move brake to center, hold it by hand and re-secure mounting nut (figure 6 and 7).
NOTE: It is not critical or even important that brake pads strike the rim at the same time. It is possible for one pad to reach the rim first. The rim will not be pushed to the side because the pivot of the brake arms is above the wheel. The other pad will eventually strike the rim and the pads will then squeeze the braking surface to slow the bike.
Squealing is the result of a harmonic resonance from a slip-and-stick phenomenon, similar to how a violin bow resonates on a string. The brake pads grab the rim and are pulled forward by the force of the wheel. The arms must at sometimes jerk backwards, but if you are still braking, the arms are flexed forward again. This is repeated again and again many times per second, and this creates the squeal. Because of this, flexible and less expensive arms tend to squeal more than stiff calipers of better brakes. It is possible to reduce squeal by having the leading edge of the brake pad strike first (figure 8). This tends to reduce the back and forth jerking of the arms.
Some brake caliper arms can be toed. Toeing can be achieved in some cases by bending the caliper arm slightly using a small adjustable wrench. Begin with a slight gap at the back of the pads and test the bike before adding toe. An option to toeing is to file the pads so the leading edge strikes first.
Pads will wear out with use and require replacement. Pads will also harden and become ineffective with age. Pads may also become embedded with aluminum or other contaminants (figure 9). Inspect and remove as necessary. Pads that are aligned too low on a rim will tend to develop a lip on the low edge (figure 10). This lip makes correct alignment impossible.
Double Nut Caliper Arm Adjustment
The double nut types can be adjusted with the caliper mounted to the bike. Check arms for play by grabbing each arm and moving back and forth along the axis of the pivot (figure 11). If there is no play and the calipers open when gently squeezed, the pivot adjustment is adequate.
If there is play, the adjustment should be tightened. Hold the adjusting nut secure with a thin wrench. Note position of wrench. Loosen locknut with a second wrench, and move adjusting nut slightly clockwise. Hold adjusting nut and secure locknut fully (figure 12).
Repeat check of caliper arms for play. Repeat as necessary until play is gone and brake open upon gentle squeezing (figure 13). If arms will not fully open when gently squeezed, the adjustment may be too tight. Try to adjust looser a very slight amount. With much use the spring may become fatigued and a good setting is not possible. Replace spring if necessary.
Safety Pivot Caliper Arm Adjustment
Safety pivot caliper arm mount to a single bolt. Below the caliper arms shown apart on the pivot bolt (figure 14).
The locknut and adjusting nuts for the safety-pivot types is located behind the caliper arms. The brake must be removed from the bike to access these nuts. The spring must also be disengaged and flipped back out of the way (figure 15). Use care not to damage plastic spring carriers, if any.
It is easiest to work with the brake in a vise using soft jaws such as the AV-5. If the brake has a 12-point locknut, place the box end of a wrench over the nut, and then grab with the vise. The adjusting nut also holds the spring, which can make the adjustment awkward. Hold the adjusting nut with a thin wrench such as the OBW wrench, and loosen the locknut (figure 16).
Tighten adjusting nut, turning it toward bolt head, only slightly. Hold adjusting nut and secure locknut fully. Test for play, and repeat adjustment if play if felt (figure 17).
To test final adjustment, flip spring back into place and attach spring to arms. Squeeze gently and release. Readjust as necessary.