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Affect of changing brake caliper piston sizes

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Firstly, I just want to say that this is the second time I’ve been looking into this question over the years, and frustratingly, answers I got from people – reputable people – were often polar opposites.

The first time I looked into it was when I did a big brake package on the front of my Celica. I ended up with Wilwood calipers of a certain piston size, but my bias was off – it was too far to the rear. I asked the person who sold them to me what I needed to do and he said I needed to go with smaller pistons (more piston area = more pedal travel to operate the piston, so it stands to reason that with a smaller piston, the piston will move more in relation to pedal travel). I mean, it’s logical.

I asked around, however, because my feeling was that I needed larger pistons. After all, to combat pad taper on a lot of aftermarket calipers, the smaller piston is placed at the leading edge of the pad so that there is less force at the leading edge. Theory being more piston area = more force on the pad.

Opinion was certainly divided, I asked brake component suppliers, workshops etc and the mix was roughly 50/50 between people saying larger would give more braking pressure vs smaller. I ended up ordering larger piston calipers against the advice of the person selling them to me, and when I installed them, my bias was better. It came to the front.

Well, I’m looking into this again, for a different reason. Bit of background, I’m racing a 2015 Subaru BRZ in Production Class here in Australia. I’ve fitted 4 piston Alcon calipers on the front, replacing the OEM dual piston sliding calipers. Now, when working out the piston area, I understand for a sliding caliper, you multiply the piston area by 2. The Alcon calipers end up with slightly less piston area compared to OEM. I’m using the OEM single piston sliding calipers at the rear.

I’ve noticed that every now and again, however, that with a big heavy brake application, the pedal goes really stiff and it requires a whole bunch of pedal pressure in order to retard the car. I can approach the same corner 2 laps in a row, brake at the same brake marker, but my pedal would be different between each lap. I never bothered using good pads in the OEM calipers, so not sure if this happened with OEM fronts also. From checking around various forums, etc., and indeed Andre mentioning it in one of the podcasts, it seems that this is a somewhat common issue relating to the BRZ/86, and that it may be caused by the electronic brake force distribution controller in the car. Other than this, the brakes a great. Pretty good pedal feel, great performance and good pad life using the compounds I’m running. But when this happens, it’s quite un-nerving.

Another competitor in my series is running the same Alcon calipers as me, and has changed his rear brakes to the 2 pots from the 2001 – 2005 WRX – theory being that because of changing from a sliding caliper at the front to opposing, it senses that there is more fluid going to the front and can shut off the front circuit and open up the rear circuit to gain balance. Apparently it’s fixed the pedal pressure issue.

This doesn’t make a lot of sense to me. For starters, wouldn’t the fluid volume be the same between sliding 2 pistons and opposing 4 pistons of the same size?

The WRX calipers have smaller pistons and also use a much smaller pad, so my concern is, installing these calipers will actually reduce my braking performance my moving my bias too far forward. I’d like to understand a bit more the relationship between piston size, braking force and also find out hopefully from anyone else tracking the BRZ/86 if this issue is solved by changing the rear calipers.

Thanks all!

Hydraulic systems work on pressure. The pressure is nominally the same at all locations in the system. So, for a given line pressure (defined by the pedal force, pedal ratio and master cylinder size), the force at the pistons will be in direct proportion to the area of the piston. Assuming the brake pads are the same size, then larger pistons will mean more clamping force. Downside to larger caliper pistons, more pedal travel due to the volume required to move the pistons the same amount.

Similarly, using smaller master cylinders will produce more pressure for a given force, as the pedal force acts over a smaller area. The downside is there is more pedal travel required for more pressure.

If your calipers have multiple pistons, it is the total piston area that defines the force produced for a given line pressure.

David summed it up well.

Yes, if using sliding calipers, with piston(s) on just one side, assume the same on non-piston side as the caliper moves so the same, piston, force is applied on both sides. It's the same volume displacement because the caliper body is moving in the opposite direction to the piston, increasing the net piston travel to about the same displacement.

Having a little less piston area may not be that bad a thing - the better the tyre grip, the greater the potential load transfer to the front and so lower rear fraking may be desireable.

If I were you, I wouldn't stress too much about it, and look at the friction curves of the available pads, front and rear - I assume you've already got temp' paint for the discs to get a baseline peak opering temperature - this may be helpful for that, and most pad manufacturers, and maybe suppliers, should have dedicated charts for their products, some are in the below...

https://wiki.seloc.org/a/Brake_pads

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