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A comment at the end of this ChrisFix video (at about 14:42): https://youtu.be/gjqeFEyaUxc?t=882 raised my eyebrows a tiny bit.

Chris says that you can run two brake calipers (two calipers on each of the rear wheels, in this case) off one single, stock brake line (with no modifications to the master cylinder). Is this accurate? I don't have any experience with this, but just from my own intuition it would make sense that you would have 50% of the pressure applied to each brake. I don't know if that would result in the same amount of braking power or not.

What would the effects really be if you did add a second caliper to a stock brake line?

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    Good question :o)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 7, 2020 at 2:20
  • I realized that while attempting to formulate an answer for this question that I'm missing a lot of knowledge required to give a complete answer. Unfortunately I can only speculate. My recommendation would be to not do such a modification as the effect would most likely be adverse or literally do nothing - again not an engineer so I might be completely wrong. My only real basis is that the og engineers didn't design it that way and it might upset the braking balance of the vehicle. I'd be happy to corrected though as I'm extremely curious now.
    – Techlord
    Nov 7, 2020 at 3:53

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Pascal's law says that the hydraulic pressure must everywhere be equal. Pressure is defined as a force devided by an area. P = F/a. Thus the force F is equal to the pressure P times the surface area A.

F = P . A

Consider a simple brake setup with 1 master cylinder and 1 caliper (with 1 pistion). The pressure at the master cylinder must equal the pressure at the pistion of the caliper. If the area of the caliper's pistion is twice the area of the master cylinders pision, the force on the brake disk is double the force applied to the master cylinders pistion.

Now if you add more pistions to the caliper, the total surface area increses, thus the force applied to the brake disk is multiplied even further. Adding another caliper is effectively the same as replacing the caliper with one that has more pistions.

The only practical problem you may get is the volume of brake fluid required to move all pistions. The amount of brake fluid required to move all pistions is equal to the distance a pistion moves before reaching the brake disk times the total surface area. For example:

Each pistion moves 2 mm before both brake pads are touching the disk and you have 2 calipers (1 pistion each) with a total surface area of 25 cm^2, the amount of fluid required to move the calipers = 0.2 cm * 25 cm^2 = 5 cm^3 (cc). Fluid is not compressable, so after contact is made, no more fluid is needed to apply more pressure.

Obviously the master cylinder must be able to move the required amount of fluid in one go for the extra caliper to have effect. Else you have to upgrade to a bigger master cylinder.

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  • Thank you for the insight into the pressure applied to the pistons! The one question I still have is whether there would be a difference in braking force due to the larger surface area in contact with the brake disc. This is where I would've thought it would differ from a system where you just add another piston to an existing caliper--there is now 2x the area of brake pad to brake disc contact so I would've thought this would result in more friction.
    – Goulash
    Nov 13, 2020 at 4:36
  • Well you can't just "add" as pistion to a caliper, you'll need a new caliper with a different size brake pad. I'm sure the bigger surface area of the brake pad will result in more friction, thus more stopping power, but the most significant factor is the total area of all pistions (in multiple calipers or not).
    – user60481
    Nov 13, 2020 at 6:38
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Pressure applied won't change after the pistons stop moving, the pressure during piston movement is down to the friction losses.

The real concern is if sufficient fluid is moved by the master cylinder. I changed brakes and needed to change the master cylinder to increase the delivery volume.

If the vehicle has a split braking system, and depending on age there are a few possibilities, then fitting dual piston calipers to the rear can be done so each split circuit acts on all four wheels, again the delivery volume needs checking before you need to brake for real.

With the newer ABS with a separate connection to each wheel then again the concern is delivery volume - going from small 2-pot calipers on the rear to larger 4-pot calipers does increase the fluid volume to be moved.

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  • Stating something is the way it is doesn't explain why a thing is what it is. Can you explain how Pascal's law comes into play here? If you spread the same amount of input pressure across more output pots (x2 in this case), wouldn't that drop the output pressure (spread it out more) causing an overall pushing force to be equalized? If not, why?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 7, 2020 at 12:39

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