What size calipers (pistons) should I buy?

[zep]

I'm upgrading my front brakes from a sliding setup to 4-pots, but having trouble figuring out which Wilwood calipers I should buy in terms of piston size to ensure it works with my current setup. Right now it feels good under foot.

Current setup:

Front calipers: 2 piston sliders 38mm x 2
Rear calipers: 1 piston sliders 29mm x1
Master: 1"
Boosted
Bias valve for the rear
Pedal ratio: 5:1.

Wilwood have a few options for the piston sizes, with all four being the same size: 44.5mm, 35.1mm or 25.4mm.

Some things I've read say to only calculate the area of 2 pistons (one side) in a 4pot setup, but surely all pistons need to move?

What will be the result if I size up or down piston size?

 

Cheers

[mjrstar]

For measuring clamping force you use the area of one side of the pistons (a pair on 4 pots). Imagine pads coming up against a fixed object it's like one set of Pistons is the fixed object.

For fluid displacement remember that a sliding piston also has to take up the air gap on the "fixed" side pad so the piston travel will be more when measured than one side a set of opposed 4 pots.

 

A 1 inch master cylinder is a good start, to compare with an Evo with brembos, they run a 17/16 cylinder and have 40 and 46mm pistons..

To maintain a high solid pedal I'd be looking at the 35mm pistons, but as always DYOR.

 

Real question is why go for the 4 pots? Is it to make it look cool, as there isn't too much wrong with a well set-up 2 piston sliding caliper from a performance perspective.

 

 

If you went for the 25mm you may have not enough force at the pad, so pedal would be high and hard and require more leg effort for same amount of braking performance.

If you went for huge pistons you'd end up with long pedal travel before the bite point and potential for making the braking pressure more difficult to modulate.

[zep]

22 minutes ago, mjrstar said:

Real question is why go for the 4 pots? Is it to make it look cool, as there isn't too much wrong with a well set-up 2 piston sliding caliper from a performance perspective.

 

You caught me! Nah, the setup I had on there before, while working well, needed to have the steering lock reduced so the calipers didn't hit the LCAs. It also pushed the track out 10mm each side, which is cool if you want wide stance but I'd rather my wheels sit inside the stock guards nicely.

I also need to find a disc to make these clear my wheels, but that's another issue all together.

[sr2]

The main 2 reasons manufacturers introduce sliding calipers was ease of manufacture (i.e. Price) and that floating calipers show less tendency to convert rotor runout ( both lateral and thickness variation) into pedal pulsing; the reason being is that a sliding design uses mechanical movement to compensate for runout whereas a fixed design will use fluid movement. That said a fixed multi piston design will allow for more consistent pressure control over far larger pad area and will also  enable more pressure to be applied to the pads leading edge with the use of varying piston diameters; there is also better heat dissipation with increased pad/piston contact. 

The science behind  the relationship between piston size and clamping force (with both fixed and floating calipers and all hydraulics) is based around Pascals Law. The result is that for a fixed caliper to have the same clamping force as a floating caliper it will need  X2 pistons of the same size for each single piston in the floating caliper.

[zep]

Interesting stuff - thanks! I'm going to go with the 35.1mm pistons. Can adjust MC to suit if necessary.

[sr2]

13 hours ago, zep said:

Interesting stuff - thanks! I'm going to go with the 35.1mm pistons. Can adjust MC to suit if necessary.

Hi, sounds like a great project – a few words of advice from a boring old fart if I may.

Dropping from 38mm x2 floating to 35.1mm  x4  fixed will reduce the clamping figure on your front callipers.

Swept volume being πR squared (remember your old mate Pascal). Your standard configuration (π x 38/2 squared x2 x2 –i.e. floating) is 4,536 square mm. For the Willwoods we have (π x 35.1 /2 squared x4 – i.e. fixed) giving a figure of 3,870 square mm. This equates to a loss of front wheel clamping of around 14.7 %.

How this reduction in front calliper clamping equates to the effect it will have on your cars braking is also dependent on pad composition & area, suspension design, and a myriad of other factors; braking systems have a bad habit of getting f***ing complicated quick!

Over decades of playing with brakes on standard cars, modified street cars and all out race cars I’ve learnt one basic rule; unless you can quantify the pre and post braking performance of any modification you make you will always be floundering in the dark (if not farting against thunder!).

Step one is easy and a shit load of fun. Set your car up with the right tyre pressures, the average load (gas, passengers, firearms, blow up sex dolls, accumulated junk,…. etc.) and find an empty carpark to do some very, very hard stops to zero from 60K. Have a mate spot for wheel lockups (front, rear, left right?) and get a feel for how she pulls and how controllable it feels. Next hit the open road and see what happens with at least 3 consecutive hard stops from 120K to 50; (look for brake fade, directional stability, wheel lockup and most importantly look in your rear vision mirror!).

The next thing to try is even more fun – repeat the above in the wet!

If you can achieve these simple before and after tests you’ll be streets ahead of the average car nut who tries to modify their brakes.

Sorry for the long reply (i.e. rant) feel free to contact me if there’s anything else I can contribute.  

[zep]

Cheers @sr2! I guess I should clarify that the standard car has solid rotors and twin piston calipers with a 48.1mm piston: 1,817sqmm. I upgraded to the 38mm floating and am going back down slightly to the 35.1mm fixed. So relative to a stock car the Wilwoods are up 213%, and the VT sliders were up 250%. I reckon the actual pad contact area will be relatively similar - hard to know as measurements for pads are not the actual material but the overall size!

My thoughts re the loss of clamping area - I was probably over braked before, if that's a thing. I know of a few actual race cars that run the same rear brakes as I have, and front 4-pots with 36.1mm pistons (Rx-7 calipers). If they can stop a car at race speeds (albeit in a car that is probably lighter than mine), then I think the 35.1 pistons should be ok. Hopefully any discrepancy will be small enough that changing pad compound and making adjustments to the bias will be enough to compensate.

[mjrstar]

If it's boosted and you have good pads, in a lightish car that size caliper the braking ability will out perform pretty much any tyre you can fit on it.