4 pot versus 6 pot calipers

[Ant]

I am wondering what the advantage is with additional caliper pistons relating to Wilwoods SL4 and Wilwood SL6 calipers, as they both use the same size pad, plus the SL4 is more original on these cars and more acceptable for classic racing, they have 1.88/1.75" differential piston size which should be good on a nice 13" GT Rotor, but I think why not go 6 piston Wilwoods as they are not much more expensive.

Having said all that its good to make an informed decision so I am interested to hear any advice on brakes for an approx 1000KG or less car!

[Excaliber]

Why are 4 pistons better than 1, or 6 better than 4? Braking modulation and feedback are the main reasons. You have more control over 'impending lockup'.

[aldersonjames20]

I wondered the same thing myself... also wondered how such a light car could possibly need 6 piston brakes... is it overkill?

James

[Excaliber]

Heres an example:

Motorcycles often have twin disc brakes up front (mine does) with four pot calipers per side. I can brake very hard with the front wheel and can feel when the wheel is starting to 'lock up', release the pressure slightly and get maximum braking effect. My other bike is single disc and single pot, it is nearly impossible to brake hard without over doing it and locking up the front wheel. I just don't have the 'feel' with a single pot caliper.

A single pot caliper will certainly work, no doubt it's as capable of locking up the wheel as a four or six pot caliper. Do you want to 'lock it up'? Not the best way to brake.

[Ant]

Excaliber,

Your explanation makes good sense to me, one CC member who is into single seaters etc mentioned that 4 pot calipers are probably overkill unless you were using aerodynamic aids! That theory or I suppose modulation would be relative to the speeds achieved!

[JGC]

Everyone is right about the incease in feel. If you think of it as resolution, the greater the more control.

Larger diameter disc and more pistons, more resolution.

The actual stopping force at the wheel is based on piston area and pressure. Given the same pressure, more piston area will give more clamping force. So long as the pads are as large as the pistons the stopping force is about the same as a larger pad.

A larger pad will not wear as fast. If you don't wear your pads out during a race, they are large enough. You don't brake hard enough, frequently enough on the street to really need huge pads.

Getting the overall system to work together is most important. Front and rear balance is critical along with the right pedal ratio so it doesn't require excessive pedal force. Just changing calipers or rotors at one end is a mistake if the car was braking properly otherwise.

All that said, 6 piston calipers and 15" cross-drilled and slotted rotors look REAL COOL!

[Ant]

JGC,

Looking at down to earth price calipers Wilwood they have these two calipers which have identical pads, but slightly different area due to number and size of pistons, so I have just added up the sizes which may be wrong but they come out quite close in area?

6 Pot, 1.62, 1.12, 1.12 = 3.86
4 Pot, 1.88, 1.75 = 3.63

This makes a difference with the 6 pot being slightly bigger in piston area of .23, I wonder whether this would actually be noticed if you changed calipers on a test track!!!

Another issue is cooling some say put the largest pad caliper on your car but this can inhibit the rotor/caliper to cool, so I am wondering whether big is better, or just a bit more than you need is good! I have 17" wheels and could fit 14" rotors, but unless it is a skinny rotor the unsprung weight just goes up.

I looked at AP Calipers and could have got some 2nd/hand from an Aussie Supercar, but they are a bit wide for my application, wilwood seem to cater for different needs at a good price better than their more expensive competitors, I cant see the point in spending USD$3500 on an AP or Alcon when a $400 - $600 Wilwood should give good service.

[Jac Mac]

Ant,
Another problem with large rotor dias is the extra rotating weight which slows the response time of the wheel to changes in speed and direction. This will be noticed as heavier steering effort at high speeds and particularly when racing in the wet if you have a brake lockup the wheel will take longer to respond & regain grip. The Six pot calipers usually have the piston dias increase from pad entry to exit to equalise the effective pad pressure. A few years ago I would have opted for the heavier willwoods, now Im not so sure, the lighter AP etc are probably worth it when it comes to racing. In your type of car and the shorter races it runs in I doubt the need for 6pot's. cooling is the big issue.

jac Mac

[scottj]

The SL6 (1.62/1.12/1.12) piston area = 4.03 sq. in
The SL4 (1.88/1.75) piston area = 5.17 sq. in.
That should give the SL4 more clamping power. Although, the smaller bores in the SL6 may give it the edge in rigidity over the SL4.

I went with the Wilwoods also but I used the 6-piston GNIII caliper with a 13 x 1.38 rotor in front and a Forged Billet Superlite (1.25/1.25) with a 12 x 1.25 rotor in the rear. I also used dual masters and a balance bar. I use PFC 01 pads, Castrol SRF fluid, and air ducting to the fronts. Obviously, the faster you're going and the harder you're braking the more heat you're going to generate. I went through a couple of brake setups before this and found them to be unable to handle the heat after 2 laps. Lighter rotors are of no benefit it you can't slow down. This is the only setup that I've been able to run 30 minutes all out without fade.

Scott

[dlampe]

Scott, what size masters are you running front and back?

[scottj]

Hi Dean- 13/16 fr, 7/8 rr

[RACER X #99]

Scottj,
Are those your Cobra brakes or you dirt car brakes?

RD

[scottj]

RD-

Cobra brakes. The Dirt Car uses 4-piston Superlites, same pad, same pistons on all 4 corners.

Scott

[Ant]

jac mac,

I have done some broad research on weights of calipers and a rough comparison between AP Racing and Wilwood calipers

AP Racing 4 pot calipers Caliper Family No. CP3789 2.2kg
Wilwood 4 pot Superlite calipers 4Ib, (billet superlite 3.75Ib)

AP Racing 6 pot Rally calipers Caliper Family No. CP6750 3kg (circuit calipers are around 2.3kg)
Wilwood Superlite 6 pot 5Ib

Having said that these may not have been the lightest AP's, as their lightweight versions maybe lighter than Wilwood, and obviously for the difference in price the AP has to have some benefits over the wilwood!

[Aussie Mike]

The AP 4 piston callipers are about AU$900 each over here. I'm looking at a set for the front of mine. I might have a look at the 6 pot jobbies too and see how much they are.

One advantage the APs have over the Willwoods is that their pistons have dust seals which will reduce the frequency of caliper rebuilds. It's a requirement for rego over here that the calipers have dust seals so the Willwoods are out. AP are also used in a lot of high performance OEM applications like Brembo. This makes me think they must be up to snuff if the factories are choosing them.

Cheers

[Ant]

Aussie Mike,

Hey big brother has to stipulate all the technical criteria for the masses!

Have you a link to a supplier, as that pricing doesnt sound bad for 4 pots, but I reckon 6 pots will be a marked jump up in $$$.

[Aussie Mike]

Hi Ant,

I've been talking to Racebrakes. Here's a link to their web page: Racebrakes

The real sting in the pricing is when you go to get the brackets to bolt the calipers to your uprights. If you can make these yourself you can save a fair bit of money.

Cheers

[DanElam]

We run the Willwood 6 piston setup on the coupe as a dedicated race car. The advantage is the larger rotor which gives you some advantages in cooling and allows for a bigger pad in braking. Most of the FFR spec racer crowd has moved over this year to a new ST-2 class where brakes are unrestricted. One those cars, most of us have already moved to the bigger brakes and seen a noticeable improvement.

[scott coyle]

I would say the 4 piston set up is probably enough. I run the 4 piston front and rear, castrol srf, balance bar and 3/4 front master, 13/16 rear master. I have cooked a bunch of fluids and pads to determine this set up. I run PF 01's in front and an r4 porterfield in back. This sytem wil stand up to road america and a high altitude airport track. Huge brake zones at both tracks with 650hp pushing the cobra. I run front ducts.

The PF 01's are the best pad I have found. They only work well when braking really hard and the harder you break the better they work. I also run a bunch of rear brake bias. I keep adding rear brake until the rears lock in a straight line, then back it off a little and don't trail brake.

Good luck,

Scott

[Ant]

Scott,

Your setup and experience makes good sense to me, I have just been on the phone to Wilwood in CA, and they recommend the 120-6385-RS for my setup, Narrow SL6R with Thermotech pistons, these run a 16mm pad.

They are their narrow caliper but should be real good, so now the next stage is to make my hubs up, mount the rotors, and do a real world measurement, because I might just be able to squeeze the wider caliper in which runs the 20mm thick pads, and it may be borderline but I could run their nice 4 pot 1.88/1.75" differential piston caliper, but they are lug mount, and I think radial mount would be easier to fabricate for this setup, hence the SL6R.

Aussie Mike,

The AP's, Alcon, and Brembo prices for me are just way out of my logical thinking, and I cant justify the expense, (poor hard up old wage earner) the wilwoods are cheap, one downside is they have no dust seals on any of their performance calipers, but that would apply to the better brands also, the wilwoods are no heavier than these gold plated calipers, wilwood supply thermotech pistons at not much more expense. NZ V8 Supercar type series Holden/Falcon saloons are running the 120-5960-RS, SL6R calipers with alloy pistons, and some Dynalite 120-6985 calipers on the rear, and those guys are really doing it in 1450kg saloon cars, which surprises me a bit as the pad area in the SL4 and SL6R etc is the same and not overly large. I dont know the rotor size for them but would emagine either 13" or 14" which 1" obviously makes a big difference in leverage and heat transfer capability.