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?

[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.

[D-CEL]

With similar piston area, and caliper mass, performance will be identical. The only true advantage of a 6 piston vs. 4 piston is control of pad taper. The leading edge pistons are smaller to reduce the clamping force so that the pads will wear evenly. On race cars with complete pad and rotor wear over the course of an event, this is important.

The key is to use the smallest / lightest caliper and rotor package, that can absorb the energy (heat) your car produces, then shed it prior to the next input cycle. The system must have sufficient mass to absorb the heat while keeping the pads from exceeding the maximum temp (fade) or causing the fluid to boil (fade).

6, 8, 10 piston calipers look great (to me) but they offer very little performance advantage over a properly sized, differential bore, 4 piston unit unless you are in a 10/10ths application.

Wilwood vs. AP vs. Brembo……
Once upon a time I was a manager @ Wilwood. During my time there, I was afforded the opportunity too play with every bodies toys. I can say without question that the high end Brembo and AP offerings are fantastic parts. Beautifully machined, and assembled. They perform as a very expensive caliper should, perfectly.
However in blind testing, there is no perceivable difference between them and the Wilwoods.
I am asked on a regular basis “Why are Wilwoods so cheap?”
My response is always: “Your question should be, why are the other guys so expensive?”
Are you getting a great caliper from AP for $900.00? Absolutely. Is it $500.00 better than a Wilwood? Not to me. Its nice to be able to buys pads for $50-$60 a set and replacement rotors for $70.00 -$100.00
Many people perceive value in the higher cost item “its more expensive, it must be better” Also a discussion we had many times @ Wilwood.


Hence the name
D-cel

Jason

P.S.
Still a Wilwood dealer if anyone needs parts

[rdorman]

What D-Cel (Jason) said. Although I seem to recall some staggered bore 4 pistons as well. Either way, Wilwood is the best bargain in performance brakes today and their full disclosure of technical data such as the temp vs cf charts on their pads make it, at least for me, my first stop if I am designing a brake system for some one. My oldie but goodie Unique uses an MG front end and the wilwoods don't fit... at least until I was done machining their hubs they didn't. It was worth the time. The difference from the Girlings is nothing short of miraculous.

Jason... did you ever get my brake work sheet? It is in Excel and you can enter all the paramaters (everything from aero CG's and specs down to tire CF) and play with the combinations on paper before you try it in real life. You would probably have fun playing with it. Email me and I will send you a copy. Also, The wilwood kit I modified to fit the MG spindles was a 70 chevy mid size kit as I recall with 10.75 rotors. I want to change those out for the 12.19 so I will be in touch!

[Ant]

Jason, Scott

Thanks for your advice, and I agree a nice 4 pot setup will do a good job, once all the right pad compounds etc is sorted!

I have decided to go to the Wilwood SL6R #120-6385-RS caliper, as it still runs 20mm common pads, and has a narrow outboard body, which after having another measure up the SL6R gives me a bit more badly needed clearance. This problem arose as the Compomotive wheels have less offset on the hub mount than the Trigo which reduced my caliper clearance, Compomotive told me that the wheels were the same..........!?!
For the rear of car I am trying to find some calipers, from a hi-perf road car which have a handbrake incorporated, plus on the rear to keep it simple I have access to rotors/hats all one piece factory eg BMW etc that I can adapt to the rear!

[D-CEL]

Ant,

Wilwood now has a hydo-mech rear caliper as you are describing. They call it the "Combination Parking Brake Caliper"

Here is a link: http://www.wilwood.com/Products/001-...-cpb/index.asp

The only problem I have found with this type of rear caliper is its lack of mass and surface area. In a Hi-perf/track application, you must run a lot of front bias in the system to keep the rears from boiling out the fluid.
This is a common problem on early Vipers. When the owner does a pad-line upgrade (which they desperately need) The rears get hot so fast that they boil the fluid, burn the seals. Then fluid squirts onto the red hot rotor and bursts into flames!!
Cars with good rear weight bias (Cobras &Vipers) benefit from a little more rear caliper. I like 1” bore Dynalites or NDL’s and then use the Combo caliper as the P-brake only.

Jason

[scott coyle]

I have a set of PBR rear calipers, they are a big single piston floating caliper. They come with the E-brake attachment. I would sell them to you if interested.

On boiling fluid, I have boiled 4 or 5 different brands. The castrol SRF is a must, it will not boil.

Scott

[Ant]

Scott,

Will keep that in mind, as you can gather I am not sure on how far I want to go with rear calipers, as I normally tend to go overboard, but the $$$ add up!

Castrol SRF, thats a good recommendation, I have always liked Castrol products, and used their brake fluid in the past, but I think that was only Castrol LMA.