Comparing single piston OEM calipers to 4 piston aftermarket

When calculating piston area of a 4 pot caliper, do you base it on two or four piston area?

I'm looking to replace an OEM caliper with a single 54mm diameter piston. I need to spend more time getting my head around cylinder ratios etc but I'm working to the assumption that a reduction in slave/caliper piston area will result in less pedal travel.

Assuming you're talking about an OEM single piston sliding caliper, which has 1 piston on one side.

4 piston calipers have 2 pistons per side.

When comparing the piston area it's always on one side of the caliper only.

So only use the area of 2 pistons for the 4 pot caliper.

As Connor said - remember, David, the caliper slides/swings to balance the forces on each side of the disc. Think of as a "G" cramp on a block of wood - you apply the force on one side with the the screw, but the frame applies an equal force to the opposite side.

Thanks for the replies folks. That's where my head was at, so at least I'm on the right path.

I need to go back and watch the brake videos and podcasts again. I've got a braking system that:

1) Feels a bit spongey (even though properly bled multiple times) - maybe an internal leak somewhere?

2) Poor initial bite - even though running DS2500

3) Inconsistent/long pedal travel

I've never lost the brakes, but I've also never had full confidence to lean on them too hard, so probably not really getting up to temperature.

I don't intend to change pedal box from OEM, or install a bias valve, or remove the ABS unit - I just want it performing properly as you'd expect an OEM braking system to do.

1/

What brake fluid are you using? DOT5 is a silicone base and is compressible, and has other issues - 5.1 is a premium "normal brake fluid, although 4 should be fine unless running very high caliper temperatures.

Some vehicle's brake hoses are more elastic than others, and their expansion under pressure will give a spongy feel. You may be able to find a legal braided hose, or heavy duty, set for your vehicle.

Some vehicles, especially with ABS, have specific bleeding procedures - did you follow the book, and if possible cycle the ABS via the OBD2 port?

2/ DS2500 is a compound that should work well even from cold - but correct bedding in has a big affect on how well they will work, and if glazed they will be much less effective. Some pad and disc combinations do sometimes fail to work well together, though, and even cleaning the discs' and pads' friction faces they may never work well. This site has some very useful info', including incompatabilities - https://wiki.seloc.org/a/Brake_pads

3/ See 1/, above. With high brake forces caliper flex, pedal and pedal box flex, pad compressability, etc, can also contribute.

What vehicle is it?

Hopefully, others will comment to add to, or correct, what I wrote.

Gord-

1/ Motul RBF600 Dot 4. Running new OEM rubber hoses. I am not generally a fan of stainless steel braided brake hoses (even though they are popular in the aftermarket). The ABS module does have a specific auto-bleed function, however I am not using the factory ECU and as of yet have not had the time to determine if it's possible to trigger the procedure directly with the ABS module over the CAN bus. Something on the to-do list, though. I have however bled the brakes both manually with the pedal, AND with a pressurised pump through the resevoir. Both have resulted in the same pedal. These cars are often raced and are not commonly known as one that needs to use the auto-bleed procedure.

2/ I've had mixed results with DS2500 over the years. Occasionally on track they do feel that they're biting, but generally not. A quick pre-stab on the pedal at the end of the straight often leads to minimal braking force, and it's not pad knock-back. I think the pads are glazing, even though they were originally bedded in properly.

3/ Vehicle is a 2002-2008 Ford Fiesta, fitted with a turbocharged Honda K24 engine, using the OEM brake servo/master cylinder. No additional vacuum assist, although I do have an electric vacuum pump which I was debating testing out, although if it's no stronger than the manifold vacuum then assumed no benefit.

I think the easiest thing to test first is a different pad compound..

Forgot, a common issue is flex of the brake booster/master cylinder mounting, where the bulk-head isn't stiff enough to hold it rigidly, especially without the booster as higher brake pedal forces are required.

If you confirm you don't want the booster, there are some reasonably priced 'pedal box' kits for the vehicle, that make the conversion much simpler, such as https://www.compbrake.com/product/ford-fiesta-mk123xr2-brake-bias-servo-replacement-pedal-box-kit-hydraulic-dual-cylinder-kit-b/

Alternatively, you may be able to fabricate a means of removing the booster entirely and/or fit a smaller bore master cylinder, and/or modify the pedal for a better leverage ratio.

Gord - I would need to do some testing/monitoring to establish if the brake servo/cylinder is moving under usage. It's not commonly reported as an issue, and you can't readily buy the aftermarket support brackets that are often common in JDM circles. I suspect it's probably not an issue. Also as I mentioned before, these cars have been used in race series' for a very long time and brake performance is not commonly a concern.

As per my original post, I am not looking to change to a pedal box, remove the servo or ABS module.

Interested in your reason for not wanting to move to braided brake hoses? as that would be one of the easiest wins in my opinion.

An inconsistent pedal feel is common with a booster, I wonder if your engine swap is somehow affecting the vacuum.

Do you still have the check valve in place?

Connor-

My reasons are based on observing for many years people replacing old knackered rubber hoses with braided and claiming a massive difference, when replacing with new rubber hoses would most likely have a similar result. I don't doubt that there's _some_ benefit of stiffer hoses but I'd be surprised if it's causing my issue.

Check valve is in place and brake feel was the same with the original engine.

I'm going to try some other pad compounds first I think.

Some pad compounds can be a bit more compressible than other. Not to say this is the case for your pads, but it could be. All these small factors, like compressible pads, swelling brake hoses, pedal box and caliper flex, and brake boosters can add up to be significant.

Also proportioning valves require a small amount of extra fluid displacement which essentially adds to the compliance issue.

Easiest way to check for flexing is to have someone push the brake hard, with it stopped, while you check for visible movement of the assembly. From what you say, it's unlikely (some other vehicles were notorious for this), but if it can be checked off it's a positive step.

Might be a bit of a long shot, but a leaking check valve* in the servo's vacuum line may be giving variable vacuum at the diaphram, as can a clogged air inlet to it. Might be worth replacing/cleaning them, if you can positively eliminate them, rather than assume, it's a good move.

*depending on the type, the higher pressure differential due to boost may have affected it, or cause it to stick. It should be easy to find a replacement designed for boost, just in case.