ST1100 Front calipers brake piston diameters

[Simmo]

Why are the brake pistons 22mm, 25 or 27? Why not the same sizes?

RH 3 different sizes, LH two @ 25 and a 22.

No problem, just curious. No doubt it is something to do with ABS/CBS/TCS system.

 

[sennister]

I haven't looked into them much but keep in mind that there is the ABS that you mentioned that would change the brakes as they also use linked brakes. Plus things change with model years. That may change what size they are.

It happens in other places also. I know on my cage there are two differnt types of rear rotors. The best I can tell is that the two braking surfaces are closer together (smaller vents between the braking surface) on the non-locking differential version and bigger venting on the locking differential version. The locking of the rear diff is done with an Eaton G-80 mechanical locker so it doesn't have anything to do with the brakes or anything that far out on the axel but they are different. You would think it would be cheaper for the manufacturer to just have on standard part across the board.

EDIT: Oh and another thing that comes into play most likely with the differnt sizes one the Right side vs left is that a smaller piston pushes out quicker than a bigger one. The bigger one provides more braking power. So this is why you see different size pistons. Look at high performance brakes like those from Brembo. They have 6 piston calipers for this reason. Some are smaller to give a quick bite then the bigger pistons come into play the harder you step on the brake to provide more braking power. What happens on our bikes is that one side (the one with the smaller pistons) will begin to slow you first then as brake pressure rises the other side kicks in. They could have done the small and big pistons in the same caliper but then it would have to be much bigger. Heck even on my moutain bike there are multiple piston sizes on the disc brakes. It gives you better brake response and modulation.

 

[Uncle Phil]

SIMMO - Definitely to do with the the LBS. You probably noticed there are two brake lines into each caliper also. The linked option engages a 'different' piston from when the lever is applied, IIRC.

 

[Simmo]

Thanks, it got confusing for a while when I had a bench full of different sized seals!

 

[Simmo]

OK, calipers rebuilt and system bled. ABS CBS TCS all work.

This is my first bike with ABS and it really is a weird feeling. Grab the front brake and no diving or locking up, just a controlled slow down. I thought I'd messed up the bleeding until I remembered that I was on a bike with ABS!

With the bleeding order are you chasing the air through the system? Lots of air out of the front left upper and lots more out of the rear rear. Lots less air through the other nipples. Does this sound right?

I forget which post I read it on but using a broom to hold the back brake on while you open the bleed nipples with your other hand is a stroke of genius!

Ian