These are the first set of AP's Radi-Cal calipers that are presently being used in the DTM (German Touring Car) Series. They weigh 6.2 pounds each, have 25mm thick pads and have an air bridge that goes over the top of the caliper and cools the outside face of the rotor and caliper. I will post more later when they are mounted on my car. The red caliper shown is the big (8 pounds) Brembo. A version of this was used by Tony Stewart to win at the Glen.
Benefits:

AP Radi-Cal vs. Brembo and other 6 pot calipers:

1. Radi-Cal Design

a. Arrangement of caliper mounting points much closer to rotor centerline = massive stiffness increase

b. Smaller, more square box where pads reside = less deformation under load than a long pad box

c. Integrated diagonal support bridge

d. Offset caliper body design decreases deflection

e. Material removed around piston bores to increase cooling and lower weight

f. Article on design concept: [url]http://www.autofieldguide.com/columns/1207euro.html[/url]

2. Provision for integrated carbon fiber air bridge

a. Increases airflow to outboard rotor face, caliper body, fluid, and pads, dramatically reducing total system temperatures

3. Forged monobloc vs. cast monobloc (Brembo) & two-piece squeeze-forged (StopTech)

a. AP is Stiffer/stronger

b. AP is Lighter- 6.4lbs. vs. roughly 8lbs.

4. Internal crossover ports

a. Less places for caliper to leak

b. Less chance of corrosion

c. Less weight

d. Less chance of damage during wheel install

5. No dust boots, only internal seals

a. They just burn up anyway on the track

6. Raw anodized with machined logo vs. painted caliper with silkscreen logo

a. Very little color shift- calipers look good even after heat and abuse

b. All parts look same, no uneven paint, chipping, flaking, shrinkage, or problems with brake fluid

c. Logo won’t smear, fall off, or yellow

7. Pad is roughly 25mm thick, vs. 14-17mm thick

a. Far superior pad wear, less pad changes, more heat capacity in pads because of greater pad volume

8. Stainless Steel Pistons

a. Much less heat transfer to caliper and brake fluid than aluminum pistons

9. Anti-knockback springs standard

a. Knockback big problem with uprights. More confidence when pedal is there= faster lap times…pedal is there when you need it, every time.

10. Stainless steel abutment plates

a. Brembo has none- pad will eventually wear away caliper body, “knock” around in caliper

11. Racing/Testing

a. These are exact same parts used in DTM, and very similar to those in NASCAR, F3, etc.

[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/P5250402.jpg[/IMG]
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/P5250403.jpg[/IMG]
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/P5250404.jpg[/IMG]
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/P5250405.jpg[/IMG]
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/P5260414.jpg[/IMG]
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/P5260412.jpg[/IMG]

I'm going to need to lock my checkbook in a safe and give someone else they key!

Seems like a 25mm deep pad would put more twisting force on the pad and increase tapering. Obviously the different piston sizes help that. I assume by running a pad that thick, you're not changing them every race.

I like the bolts being closer to centerline, but I'm not seeing the added rotor cooling.

There's an airbridge piece that goes on top.

Quote:

It takes air and routes it over the top of the caliper in a sealed duct, 1/2 of which is the caliper itself, helping cool the center of the caliper. Then it blows down between the outer pistons, cooling them and then exiting on to the outer rotor face. The outer face of the rotor gets very little air as compared to the inner face and center. This helps that issue. Structurally, it is brilliant since the air bridge is not only cooling, it is a very wide stiffener as well. Caliper stiffness is not just about spreading due to the piston force. There is a lot of dynamic deflection in a caliper where the outer side of the caliper is trying to go with the rotor rotation causing a shearing action. The Radi-cal has the wide air bridge half plus the diagonal stiffeners and outer ring that makes this the stiffest caliper possible. This is much more than a trick caliper, it is a system solution that optimizes the entire braking system.

I've been waiting to see them, they don't disappoint.

here is a comparison of the pads.
L to R, Brembo, AP 7778 for this caliper, Stoptech ST-60

I've always been impressed with AP, the Formula SCCA I'm driving in my avitar has AP's.
We bled the calipers every race but aside from that, never had to touch the calipers, pad taper was almost nonexistant.
It's so nice to get a race part, like APs, tha you don't have to constantly tweek.

AP makes brakes for F1. They also make clutches for F1. Serious products.

How much do these things cost? $3k or so?

They also make brakes for Tony Stewart, he won the Glen with a bigger version of this. The price is TBD, but less than that. Since I am waiting on the pads, I keep inspecting it and noticing clever things. Most 4 or 6 piston calipers have a balance tube on the bottom and 2 bleeder screws. This caliper does not have the tube, it is all internal and has only one bleeder screw.

I figure 2 bleed points makes it easier to bleed, but obviously less reliable.

The cross over on the bottom end is just a way to reduce manufacturing costs.

I think the only concessions they made to cost on this was to invest in a forging tool and use stainless steel instead of titanium for the pistons. The internal crossover is mainly a reliability and durability feature.
Here is a detail pic of the "castleated" thin wall stainless steel pistons, and the SS pad abutments. The pistons are cut this way to reduce heat transfer into the fluid.
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/DSC_0132.jpg[/IMG]

Quote:

That's really interesting.

I wonder if carbon fibre would work as a piston material?

carbon fiber yes,

the resin no

The point is, carbon is an excellent thermal conductor, and even in a carbon-carbon matrix, the material properties do not offer any advantage. A ceramic perhaps, if you can get the seals to work, would be worth some thought, but Ti is so good in this application and really, so is SS.

Quote:
Minimize the contact area, minimize the heat transfer path.

Meh. If you need big brakes like this you must be slowing the car down too much...

Nah, braking between the 2 & 1 mark instead of at the 3.

All day long...
500 HP...
Lots of grip...

Need to post that Video link

Quote:

I can top that, I'm gonna cross drill my pistons!

Actually, that would help as long as it was in the part of the piston that is external to the caliper seals

Nah, I was really taking it easy in that because I had Erica Newhall in the car.

Q=KAdT where:

Q= conductive heat flow
K= thermal conductivity of the material
A= cross sectional area
dT= difference in temperature hot to cold

So, if you reduce the area and use a low "K" material you reduce brake fluid temperature.

It looks like that mod could be done on other calipers as well.

Ever see tha CanAm car that had a waterpump to throw a mist into the brakeducts everytime the pedal was hit?

Exactly, very easy to do with a Dremel tool on stock C4 brakes. On my C4, I added 2-3 thin (.003 inch) SS shims between the pads and the pistons. It helped to keep the fluid cooler.

Here are a couple of pad pics:
Left to right, AP, PFC zR33, Wilwood SL6R (both are used, so they have had a couple of mm's less than new)
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/DSC_0136.jpg[/IMG]
Same order, top to bottom
[IMG]http://i67.photobucket.com/albums/h287/gshoffman/Brakes/DSC_0138.jpg[/IMG]

Just as an FYI, Kasey Kahne and Tony Stewart used the bigger version of this caliper at the NASCAR race at Infineon yesterday. They finished first and second. I was just informed that Marcos Ambrose and Jimmy Johnson used them as well so that they were on the first through fourth finishers.