Question - why stock caliper pistons are not the same size?

[IG.]

I am rebuilding the front calipers, and noticed that out of three pairs of pistons within a caliper, the first pair of pistons (in the direction of a disk rotation) is slightly smaller than the other two.

Anyone care to explain what's the reasoning behind it?

Sorry, no pictures. When your hands are in gloves covered with brake fluid and other crap, it's pretty challenging to convince yourself to go grab a camera.

BTW, it's amazing how well brake fluid preserves all the seals. It's a 10-year old caliper, and all inner seals look like new. I am not goona re-use them, but will save just in case.

 

[afterhours]

it has to do with the the actual force applied to the brake pad. they use different sizes to apply the presure evenly to the brake....then to the disc.

 

[BIKE DUDE]

The smaller piston will move faster and further but not ultimately have as much power or modulation and the larger pistons will move slower or less but ultimately give more stopping power or modulation. I may not be explaining it exactly but......The diameter of the caliper pistons in relation to the diameter of the master cylinder piston and how much fluid it moves. The two different sizes give a combination of modulation with one size and power with the other. This is my understanding but I may be a little off 0n the modulation but I tried.???

 

[IG.]

What you are saying makes a lot of sense. What I don't understand is why?

As I look at the piston positions, they press at the pad at even intervals. As correctly pointed out, the leading smaller piston will tend to engage sooner at slight lever pressure, while the other two trailing pairs of pistons will engage later with higher lever pressure.

My concern is that when full pressure on the lever is applied, the trailing part of the pad is squeezing the disc with higher force than the leading part of the pad. This uneven pressure can't be good, and may cause the overheating of the trailing part of the pads while underloading the leading part of the pads.

Another concern is when using wave rotors with cut outs in them. They are designed (I think) in such a way that the brake pad always has the same area of contact with the metal. Another words, as the disc moves and let's say the leading edge of the pad moves over a cut out area, the trailing edge of the pad moves from the cut out area to a solid are on the disc - thus the overall contact area is constant. The reason for this is to make sure the braking effort does not change as the pad moves over the disc - otherwise the brakes will be pulsating.

It all works well if the entire surface of the brake pad applies the same pressure on the disc. But if not (due to different size of leading pistons), then it's not possible to avoid pulsating brakes when using wave rotors!

I need to look more carefully. Maybe the pistons are spaced in such a way that the leading smaller piston takes care of the smaller area - then we are OK. I was also thinking that maybe forcing the leading edge of the pad to engage first prevents possible brake pad chatter at light braking effort.

 

[Tufbusa]

The smaller piston will move faster and further but not ultimately have as much power or modulation and the larger pistons will move slower or less but ultimately give more stopping power or modulation. I may not be explaining it exactly but......The diameter of the caliper pistons in relation to the diameter of the master cylinder piston and how much fluid it moves. The two different sizes give a combination of modulation with one size and power with the other. This is my understanding but I may be a little off 0n the modulation but I tried.???

Sounds logical but that's not exactly the way it works.

 

[IG.]

OK, I think I figured out how the different size pistons create equal pressure across the pad.

I carefully looked at the trace of the pistons on the pad. Guess what? The pistons don't press on the pad at symmetrical points, they are slightly shifted, and I assume to exactly compensate for different forces generated by different size pistons.

As to the reasons why the leading piston is smaller than the other two, aside from my guess above, I sent an email to EBC (they have been helpful to me in the past) and not Suzuki, and hope to get an answer from them soon.

On the side note, what a difference the rebuilt calipers made!!! Why I waited for so long? The feel, the sensitivity of brakes, and the compliance of the bike is just amazing now!

And the brakes engage at the earliest lever travel, but not until I did something special. After I reinstalled everything, including the new fast bleeders (those look like regular bleed screws, but have a one way valve in them), I notice that the lever had to travel a lot before I felt the pressure. Turned out, after I release the lever and try to wiggle the pads, outer pads stayed solid - I couldn't move them with bare hands. However, the inner pads on both a calipers had noticeable play. For whatever reason, the seals were pulling the pistons back after I release the lever.

So, I pulled inner pads out - one at a time, and squeezed the lever once or twice to move the pads in. Barely fit them back in, and from that moment on, the lever engages right away.

BTW, I had to use Mity Vac to pull the fluid initially while the system had nothing but air. Otherwise, the fluid just wouldn't go down from reservoir. Once the fluid was in the system fast bleeders worked great.

...on to changing oil/filter, checking the valve clearance, and installing the air horn...