Fine-tuning the Centering of the Opposed Piston Brake Calipers with Shim Ring!

When you install brake calipers on the front forks, the rotors and caliper centers would be aligned with the stock brake system, but sometimes they can be slightly off-center due to miscalculation or other reasons, such as when you replace the brakes or wheels and then switch out the caliper brackets. In such cases, a controlled thickness shim ring will allow you to fine-tune the caliper position.

Pin slide caliper that automatically moves to the center

Since the caliper itself slides, the pin slide-type must use the caliper support. The grease is applied to the slide pin and the rubber boot is visible on the caliper side. If the pins are not lubricated enough, the caliper will not move properly and may cause the pads to drag.

You know that the principle of disc brakes is that the brake pads clamp the disc rotor from left to right. In order for the brakes to work properly, it is important that the brake pads on each side of the discs contact the rotor evenly.

Pin sliding-type brakes, which are commonly used on mopeds to large motorcycles, allow the caliper itself to move in the direction of the thrust of the caliper bolt (axial direction) so that the same pressure can be applied from the pads on both sides during braking.

Since it is automatically centered, it is relatively easy to install even when using calipers for other models for customization or repair. However, it is necessary to install within the range of the caliper pin slide.

The slide pin inserted into the caliper body is greased and the gap between the pin and the caliper is fitted with a bellows-type rubber boot that can be extended and retracted.

These pins and grease are very important for the caliper to move smoothly, but if the pins stick due to rust or lack of grease due to the aging of the boot or lack of maintenance, the pads will be left hitting the rotor and dragging and the brake lever stroke will be deeper.

To avoid such problems, make sure that the caliper itself moves smoothly according to the slide pins when cleaning the caliper and rubbing out the pistons.

Opposing calipers with fixed caliper positions

The caliper was installed via one-off support, and upon closer inspection, the rotor is slightly outside the center of the caliper. I was lucky to be able to pontificate on the flat caliper supports, but there was still room for adjustment.

In contrast to the pin-slide type caliper, the opposing piston caliper, which presses the pads on either side of the brake rotor with special pistons, is known for its higher performance. Since the caliper itself is rigid and has more pistons, many supersport models are equipped with these calipers for increased stopping power.

The caliper pistons are pushed out by the pressure of the brake fluid, and the pressure applied to the pistons inside the caliper is the same if the area of the pistons is equal.

The piston that is pushed out of the brake lever when the lever is released returns to its original position due to the elastic deformation of the caliper piston seal (this is sometimes called rollback) and the brake pads also separate from the rotor.

Opposing calipers, which do not require the caliper itself to move and be centered like pin-slide type calipers, are fixed in a rigid position to the front fork, although they may also have caliper supports. Therefore, there is no need for maintenance, such as greasing the slide pins, and as long as brake dust is removed from the caliper pistons and the pistons are lubricated, they will always perform at their best.

Make sure the brake rotors are in the center of the calipers.

Taking care not to open the pads with the rotors, I removed the caliper and checked the amount of piston protrusion, the upper flange side piston is slightly less protruding. When you remove the wheel, the amount of protruding piston is slightly less than the amount of protruding piston on the upper flange side. Let's see.

However, there are some things you should also check with the opposing piston calipers. That is, "Is the brake rotor in the center of the caliper?".

You would think that an opposing piston caliper that pushes the pads from either side of the rotor would provide the same braking force regardless of the caliper's fixed position. Even if the calipers are offset to either side, with one piston sticking out less and the other side sticking out more, there should be no difference in braking force even if it's not in the center, since the pressure is the same when the pads clamp the rotor.

However, if the amount of piston protrusion is uneven, there may be a difference in piston return. As explained earlier, when you release the brake lever, the brake pads separate due to the rollback of the caliper seal.

If the pistons on the left and right side of the caliper have different amounts of protrusion, the piston with less protrusion will have less deformation of the seal and may have less force to return to the master cylinder. When the brake lever is released, the fluid in the brake caliper will also try to flow toward the master cylinder, so the piston with the larger amount of seal deformation may have more force to return to its original position.

As a result, the pads on the side of the piston that is less protruding tend to drag, although this includes a rule of thumb.

At this point, if you can move the caliper mounting flange away from the caliper support so that the centering is achieved (i.e. the piston ejection is equalized), you can adjust the dimensions by inserting a shim between the flange and support.

Shims and flat washers are similar parts, but some shims are called precision shims, or shim rings with strictly controlled thicknesses. These are available in 0.01 mm increments, so they can be used to offset, say, 1.2 mm.

This is handy when you have a custom brake custom and the caliper support is one-off and the center is slightly off.

However, if you want to move the caliper more toward the center of the wheel, you can't adjust it with shims. If you're installing the calipers through the caliper supports, you can put shims between the front forks and the caliper supports to move the calipers more toward the center of the wheel.

If brake drag is a concern with the oncoming caliper, checking that the brake rotor is centered on the caliper and adjusting its position with a shim ring if possible may eliminate the unpleasant brake squeal.

Parts called precision shims and shim rings can be purchased online at machine tool stores. You can choose them by inner diameter, outer diameter, and thickness, depending on the caliper bolt diameter and the outer diameter of the flange section. Motorcycle parts are sold by Daytona under the name of shim sets. They are available in a range of thicknesses from 0.5 mm only.

There is no need to replace the caliper bolts with longer ones, as they won't shift 5mm or 10mm in the direction of the thrust. Rather than stacking several shims, measure the required offset at the first check stage and use as few as possible to ensure the required amount of movement.

By shifting the calipers with a shim ring, the rotors were settled in the center.  The brake squeal from time to time went away because the amount of protruding pistons on both sides was aligned. The rotor must be parallel to the flange surface that holds the caliper to the caliper support.