Brake Caliper Pistons

[Paul Bryant]

You will notice that the Rockets front Brake Calipers have two different size pistons in them.
The leading (lower) piston is smaller diameter than the trailing (top) one.

Why is this ?

 

[Claviger]

The easy answer Paul? I'm not being silly is:

They looked in their inventory at what would do the job on the Rocket and found the Daytona 955i/Speed Triple brakes they already had and used them.

The technical reasons behind why two different sizes... no idea, though I suspect something to do with heat or feel at the lever as the main driver.

... OFF TO GOOGLE I GO

 

[Claviger]

Found these answers:



Vk Gopalakrishnan, Mechanical Engineering Design Consultant
Answered Mar 1, 2018 · Author has 79 answers and 101.2k answer views


The use of a smaller diameter piston at the side where the disc enters the brake pads will ensure more uniformity in the pressure distribution on the brake pads.

When the thickness of the brake pad is large compared to its length, there will be higher wear on the leading end of the pad. The frictional force acting on the disc contact surface and the reaction at the abutment of the pad will result in a turning moment on the pad, leading to higher wear on the leading end. The following figure explains the formation of uneven wear, and illustrates how a smaller diameter piston at the leading side makes the pressure distribution more uniform.

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Benjamin Monroe, studied at LeTourneau University
Answered Feb 4, 2018


By having pistons of differing size in thencaliper, the brake designers are able to tune the distribution of pressure across the brake pad. This can allow them to improve the brake noise, compensate for brake pad wear,make changes in the brake output, or make other performance changes in the brake behavior.

For a simplified example, if you have a caliper with a 32 mm leading piston and a 34 mm trailing piston, the center of the pressure distribution for the pad would be shifted towards the trailing portion of the pad as the 34 mm piston has a greater force output for a given brake line pressure input. If there was an issue with the pads wearing in a tapered manner biased towards the leading edge, having the pistons arranged in such a manner would tend to compensate for the taper, thus equalizing the wear across the pad.

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Marc Gallichotte, Custom Bike Builder (1989-present)
Answered Jan 31, 2018


As the surface of the rotor heats up, the clamping force of the pistons has to be increased to avoid brake fade. If the caliper has multiple pistons (or multiple pairs of pistons), the brake rotor surface is initially heated by the pistons pushing against the brake pad at the leading edge of the caliper, making the rotor surface hotter when it rotates back to the pistons closer to the trailing edge of the caliper. Therefore it helps if the pistons closer to the rear edge of the caliper are larger. Differential-bore calipers use smaller pistons up front, larger pistons toward the back.

 

[Joesmoe]

Now this is interesting.

I have the PreTech 3-piston calipers installed, and I wonder . . .

 

[Claviger]

Depending on how the caliper carries the pad, they may have an alternative fix to the rotational movement of the pads allowing them to use 6 identical pistons.

 

[Paul Bryant]

Hmmm, very interesting, and makes sense.

I'll tell you why I asked a little later. (with photos)

 

[Claviger]

 

[ozrider]

@Paul Bryant in motor racing we have used everything from 4 piston, 6 piston and 8 piston calipers.
The early days of 4 piston being the same size we would have all sorts of problems in long distance races like Bathurst, the brake pedal would go long as the pads would wear in a taper from top to bottom (looking like an axe head when taken out), that would lead to all sorts of problems including pistons jamming in the calipers when doing pad changes during refueling pitstops.
Back in the 90's I did a lot of work with Brembo and we ran a 6 piston front caliper and 4 piston rear with offset bores and different size pistons top to bottom, the end result was we could run the pads almost to the backing plate with no pad taper and the driver was happy as the brake pedal stayed solid the whole time.
Eventually they came up with a brake pad set that could do 1000kms of racing so we didn't require a pad change... from memory they were about $1500.00 per pad 6K a set......

 

[barbagris]

Hmmm, very interesting, and makes sense.

I'll tell you why I asked a little later. (with photos)

Oh I can guess. Stop listening to the voices.