There are two types of disc brake calipers, the opposed-piston type, and the single piston type, and the latter is often used on mopeds to 400cc class models. The function of the slide pin in the single-piston type caliper is important, and the lubrication of this area determines how well the brakes work. If the pads drag after you let go of the brake lever, check the condition of the slide pin.
A single-push caliper that draws the pads together when pushed by a piston
An example of a single push-type brake caliper. The caliper pistons are on one side only (left side in the example in the image) relative to the rotor, and on the other side only the pads. Attached to the front fork is a caliper bracket, and the caliper has two pins on the bracket that are inserted into the caliper. The caliper slides along these pins.
The principle of disc brakes is that when you grip the brake lever, the caliper piston pushes on the brake pads and pinches the brake rotor to control the speed. If you look at the relationship between the brake rotor and the brake pads, the rotor is pinched by the brake pads from the front and back at the same time.
Next, if we look at the calipers, the opposing piston-type has the caliper pistons literally facing each other so that the brake pads can be pushed out of the front and back at the same time. In contrast, a single-piston caliper has the pistons on only one side. When you look at the motorcycle from the outside, you can see the caliper body where the pistons fit, but on the inside of the wheel, you can see the brake pads directly in contact with the backside of the rotor. That means you're not pushing the pads directly from behind the rotor.
Still, the pads can pinch the front and back of the rotor, thanks to the work of the sliding pins. The calipers on the single-push pistons are always secured to the front forks via the caliper brackets, and the calipers and brackets are floated together by two pins. These pins are in a parallel relationship with the caliper bolt, allowing the caliper to move thrust-wise against the bolted caliper bracket.
With this combination, the caliper pistons try to come out more and more when you pull the brake lever, but on the front side, the pads hit the rotor and can't move forward anymore, so the caliper itself moves outward, which creates a force that pulls the pads on the backside toward the backside of the rotor.
The literal representation makes it seem like there is a time difference in the movement of the pads on the front and back, but in reality, the pad strokes are so slight that they are sandwiched between the front and back of the rotor at the same time. Strictly speaking, even if there is a time difference, it is so small that there is no problem with the effectiveness of the brakes.
The Honda CB750 Foer, the first production bike to use disc brakes, was the first to use single-piston brake calipers, and the Kawasaki Z2 followed suit. On the other hand, the Yamaha XS650E, released around the same time in 1971, had opposed-piston brake calipers, which were touted to be superior to those used on racers at the time.
Although opposed brakes give the impression of being more elegant and higher performance than single-sided brakes, single-sided brakes have the advantage of being easier to install in spoked wheel cars because they do not protrude to the inside of the wheel. In addition, it is common for automotive disc brakes to have the piston side facing the wheelhouse to avoid interference with the inner surface of the wheel.
While opposing piston calipers have become a necessity in motorsports and racing, one push is not decidedly inferior to the other in general use, such as city driving and touring.
POINT
- Point 1 - One-sided brake calipers pull the pads on the opposite side of the brake caliper pushed by the piston to brake.
- Point 2 - The caliper slides along the pins of the caliper bracket fixed to the front forks.
Insufficient lubrication of the slide pin is the cause of the trouble.
Pull out the brake pad pins (already removed) and push the caliper bracket to the caliper side to remove the brake pads. At this point, check to see if the caliper brackets are moving smoothly. If there is the right amount of grease between the slide pin and the caliper, the movement should be sticky, but there should be no dragging or rubbing.
The pads on the caliper piston side fit into place on the caliper bracket, and the pads on the backside are set into the caliper body using the pins on the caliper bracket as a fulcrum. Since there is no need to disassemble the caliper and caliper bracket for normal pad replacement, it's easy to neglect to check the lubrication of the slide pins.
One-sided brake calipers are the most popular type of brake caliper on many motorcycles, but proper maintenance is required to get the best performance out of them.
It goes without saying that the most basic and important part of maintaining your disc brakes is to check the remaining pads. Without pads, your brakes won't work. Don't forget to check the brake shoes on your drum brakes as well as your disc brakes.
Cleaning and lubrication of the caliper pistons are also important as they are exposed to more and more pad wear. The pistons protruding from the caliper body are covered in brake dust that has been scraped off by friction, and also get wet when driving in the rain.
If left unchecked, the hard chrome plating on the pistons' surfaces can rust. Also, if pistons contaminated with brake dust are pushed back into the caliper, the rubber dust seals and caliper seals inside the caliper could be damaged.
If the pistons are dirty, not only when replacing worn pads, but also during a routine visual check, remove the calipers from the motorcycle body, remove the pads, give them a full wash with a mild detergent and brush, and then apply a metal rubber lubrication spray such as MR20 to the pistons. A few rubs in and out of the pistons will improve the response to fine lever action.
And one more thing, you should also check the operability of the slide pins.
The two slide pins on the caliper brackets are inserted into the caliper and the surfaces of the pins are coated with grease. Some models have a rubber boot attached to the caliper and the base of the pin.
As mentioned earlier, in a single push type caliper, the piston pushes on the front pads and pulls the pads on the backside of the caliper. The caliper also exerts a large force in the direction of tire rotation when it pinches the brake rotor, and the caliper bracket bolted to the front fork also exerts a large force perpendicular to the pin.
Lubrication between the slide pin and the caliper is essential for the caliper itself to move smoothly without friction loss in such an environment.
But in reality, checking the slide pins is often neglected compared to pad replacement and piston rubbing. If there are pad springs in the caliper brackets when you replace the pads, you might just touch them to grease the springs.
Even if you have a waterproof boot, if the caliper pistons have been exposed to enough rainwater to rust, water might poke through the gaps in the boot and seep in. They might be torn with age.
If the pins become poorly lubricated or the surface rusts in such a state, the caliper will naturally move less and less. When you apply the brakes, the pads are pushed out by the strong force of the oil pressure, but when you let go of the lever, the force of the piston back into the caliper is not as strong as when you push.
If the caliper pins are sufficiently lubricated, the frictional force applied by the backside pads to the rotor will be gone as soon as the pressure is released on the caliper pistons.
However, if there is a large friction loss between the pins and the caliper, the caliper may be reluctant to move and the pads on the backside may drag on the rotor.
When you grip the brakes on a bike that has been left unmaintained for a long period of time, the brakes can lock up and you can't push or pull them, but often the cause is not just the caliper pistons, but the slide pins. Even if it doesn't get to brake lock, it can cause a dragging condition where the pads are still in contact with the brake rotor.
POINT
- Point 1 - Pay attention to the slide pins as well as the caliper pistons when performing brake maintenance.
- Point 2 - Poor lubrication of the slide pins and calipers may cause dragging.
Does it slide smoothly during caliper maintenance?
The caliper brackets will pull out if you pull on them, as the calipers are just plugged in. The caliper in the picture is new, but some brakes that look like they've been left unmaintained for years have rusted or stuck to the slide pins. The condition of the rubber boot on the caliper side is also important, and water can get in because it wasn't seated well in the stepped area at the base of the pin.
If you are a maintenance enthusiast who regularly flushes and rubs out caliper pistons, you may have to attach and remove the caliper brackets when cleaning the pistons. It's important to check the lubrication status of the slide pins at the same time.
If the pins are inserted into the calipers without pads and move smoothly, there is no problem. If the grease on the surface of the pin is dirty, clean it and then grease it up. Cleaning is not only done on the pins, but also on the caliper side. Old grease may be stuck deep in the holes where the pins stick, so wipe off the dirt with a cotton swab. Be careful not to use a thin flathead screwdriver or a pick tool as this can tear the rubber boot.
If the rubber boot on the caliper side of the caliper contacts the entire surface of the pin, then silicone grease suitable for lubricating the metal and rubber would be a good choice. It is also important to make sure that the grease coated slide pin is inserted deep into the hole in the caliper because if the air is left between the pin and the boot when the pin is pushed in, the air pressure will cause the entire caliper to rebound and the pads on the backside of the rotor may drag as the entire caliper tries to move outward.
Maintaining the calipers with this type of slide pin lubrication in mind will improve brake response and reduce friction loss.
The service manual may include instructions for replacing the pads, but may not mention lubrication of the caliper bracket slide pins. However, it's an important part of the brake maintenance process for one-sided calipers, so make sure the caliper moves smoothly along the pins and grease it up if necessary.
Grease the slide pins and caliper pinholes and assemble them. If the caliper bracket and slide pin are not at the right angle, the caliper and brake rotor will be at an angle. When the brakes stick, you may kick the caliper or hit it with a hammer. Remove the bolts on the caliper brackets and gradually slide them outwards to avoid bending the pins and discs.
POINT
- Point 1 - Remove the dirt on the caliper side as well as the slide pins when cleaning.
- Point 2 - If the slide pins are well lubricated, brake drag will be reduced.
