Caliper piston size/efficiency difference query

[SB118]

Right, i'm in the process of sorting a rear disc conversion on my SJ, the usual method is to fit a set of vitara calipers on the front and use the original SJ calipers on the back.

BUT, while i was fitting them i noticed the vitara calipers have a smaller piston then the SJ units. Brain fart moment. Does the smaller caliper mean for an X amount of fluid it will try to move out further giving more braking effect. OR does the larger piston mean it moves a smaller distance for X amout of fluid, but with greater mechanical advantage (i know mechanical advantage is more pivot/pulley based, but my knowledge of fluid dynamics is "slightly" lacking)

The only thing that makes me think the two calipers work on different principles is that the vitara caliper has a servo to do all the work for you, whereas the SJ is man powered.

I want to be sure that i'm getting the right caliper on the right end of the car to keep the braking bias ok.

Cheers

 

[Jonny69]

Smaller pistons will mean it needs higher pressure but lower volume to apply the same braking force. Volume is only needed to move the calipers, then it's down to the pressure in the lines to actually stop the car. What this boils down to is if you swap a caliper with smaller bore you'll need more pedal effort (because the force on the pad is lower) but the pedal won't move as far.

Unfortunately you now have a misbalance front to back. For the original system you'll find that for X pedal pressure you'll put something like 75% of that to the fronts and 25% to the rears but because you've reduced the piston size on the front you have reduced the proportion of braking force that goes to the fronts. You might need to put a proportioning valve in to reduce the braking on the back or you'll be locking the rears up prematurely.

I have the opposite problem on my Anglia because I have much bigger brakes on the front than on the back. It locks the fronts up before the rears and because it relies on the front brakes more than it should do it's hard work on the pedal. It's a short car though so it's not too far out and it doesn't really adversely affect it.

Edit: just to sum up if it's discs all round and all four discs are the same diameter then the smaller bore calipers go on the back.

 

[SB118]

Unfortunately you now have a misbalance front to back. For the original system you'll find that for X pedal pressure you'll put something like 75% of that to the fronts and 25% to the rears but because you've reduced the piston size on the front you have reduced the proportion of braking force that goes to the fronts. You might need to put a proportioning valve in to reduce the braking on the back or you'll be locking the rears up prematurely.

That's exactly the simple explanation i was after, no big words on a sunday morning, cheers Looks like i'm putting the original calipers back on and saving the vitara calipers for when i cut the drums off the back.

I'm also considering and anchor & skyhook method for improving the (lack of) braking performance.