AP Caliper Differences

[mikey]

AP calipers sure are puzzling to me.. As an automotive engineer, I try to not create more unique parts than necessary.
Somewhat a continuation of my other topics on front and rear brakes on my Lola T91/50 here, but this info isn't necessarily specific to the car so I am putting it in a new thread.

As previously mentioned in the other thread, all 4 calipers have the p/n "3344" cast into them. The rears also have a "3345" cast into them, across from the "3344".
As I contemplate a common brake pad for front and rear, I took a closer look at the calipers today.
Derek had mentioned the rear calipers are p/n CP3567 and fronts are p/n CP4152. The 4152 is stamped into the front caliper. I finally found the 3567 on the rear caliper - it is stenciled on a small spot on the back. So then I did some measuring. What I found is that all 4 calipers are nearly the same, however the fronts are 0.10" wider.

Additionally, the mounting bolt spacing is wider in the front than the rear.

So, knowing that my rotors are both 1.0" wide, and the pads are the same thickness front and rear, that means the front caliper pistons are extended an average of 0.05" further each side of the rotor than in the rear. Whether or not this matters, probably not.
But I then proceeded to more carefully inspect the brake rotors front and rear. While they are both the same width and diameter, the fronts are actually offset by 0.10" on their mounting tabs. The eight mounting tabs are the same thickness as on the rear (0.25"), so I believe the front rotors were custom made by cutting a 1.2" rotor down to 1.0", removing 0.1" from each side while not removing any material from the eight mounting tabs for the hat.

Given the caliper width differences, my previous goal of reducing rotor part count to two (using identical rotors front and rear on each side), cannot be done unless I have new rotor hats made for the front which create the proper rotor offset for a 1.0" rotor.
I think instead, going forward, I will make use of the extra width of the caliper by having a 1.2" wide rotor cut down to 1.1", removing 0.05" from each side and leaving the mounting tabs sticking up 0.05" also. Then the caliper pistons will be saved that extra 0.05" movement each side mentioned earlier.

So aside from all this, I also noticed that AP calipers left to right are mirror images of each other. Not sure why they didn't make them fully symmetrical so that you could simply rotate it 180 degrees to mount from left to right. Maybe you can anyway. The only difference I can discern is the larger of the pistons are on the bottom in the rear, both sides. So if you rotated it 180 degrees one side would have the larger of the pistons on top.

Coincidentally, the front calipers, also mirror images of each other, have the larger piston on top, both sides. Strange!

If I was designing the uprights from scratch, I'd make the caliper mounts the same dimension and use the same caliper part number for all 4 corners. With fluid parts on all 4 corners of each caliper and behind both pistons, you'd simply have to switch the cross-over tube to the bottom position for each corner and the brake line to the lower backside piston for each orientation of each corner.

I digress, I will resign to buying 4 unique rotors and am still hopeful that at least the brake pads might be able to be common all around.

Cheers,
Mikey

[mikey]

Question (afterthought):

Could the necessary front rotor offset be created with washer shims 0.1" thick instead of cutting the offset into the rotor and avoiding making new hats? Or are washers between the hat and the rotor a bad idea? It is a floating rotor with about 0.01" of float.

[Garey Guzman]

Although I'm not a Brake Design engineer, I am responsible for brake systems for an automotive company. I've learned some stuff from the Design guys but I don't claim to be an expert.

Here's what I've learned - Caliper pistons are sized for a particular application. Thier size/diameter, and therefore pressure amplification and force distribution, are designed for optimized force application. Obviously, 2 piston calipers, like FF, are the same size on both sides. For 4 piston calipers, the leading piston will be smaller than the trailing piston to achieve this.

Since the forces are transferred rotationally and not into or out axially, I see no issues with shimming to position the rotor optimally.

[mikey]

Gary,
Is the leading piston the one that sees the rotor first? i.e. (assuming forward motion) the leading piston would be bottom on the front and top on the rear?

If yes, then that explains why the calipers are mirror images of each other left to right and the larger pistons are on top in front and bottom in rear..

I guess you want less force at the leading piston to mitigate tapered pad wear?

[tlracer]

As Gary explained, handed calipers are common and necessitated by having differently-sized pistons. This (piston sizes) helps to reduce uneven (tapered) pad wear. The smaller (leading) pistons are the lower ones, yes.

Typically, again, front-biased braking necessitates larger front brakes. This can be discs, calipers or both. From the information you have posted, I would suspect the slightly wider front calipers (and, presumably bigger overall, given that you say the mounting bolt centres are greater front than rear) were used to allow similarly thicker discs; this would be logical given that front brake wear would be greater than rear wear.

With regard to disc machining, dereklola explained elsewhere that he had a machine shop face AP blank discs to suit.

As a thought, have you contacted AP regarding your calipers, etc? They should be able to confirm the correct pad fitment for the calipers, together with disc sizes, clearances, etc.

According to the online AP catalogue here: https://catalogue.apracing.com/ the correct pad profile for CP3344/45 and CP3567 calipers is their CP2340 family. There are 4 variants shown, all with the same width of 113.5mm and thickness of 15.9mm, but differences in pad width between 43mm and 52.1mm.

Indeed, selecting the correct pad width will - in all probability - solve the issue of the pads contacting the disc bells.

CP4125 (as you quoted, above) doesn't show as a caliper in that catalogue, even though the pad list includes current and obsolete ones. However, looking at one of your pictures, the caliper is stamped "4152" not "4125"...

...and CP4152 DOES appear, again with CP2340 as the correct pad family.

[mikey]

thanks TL!

sorry for the typo above, I called it a 4125 and in the next sentence a 4215. Now it is corrected to 4152. Note although the mounting bolt pattern is further apart than the rear 3567, the caliper itself only seems to differ in the 0.10" width, as well as the position of the leading piston of course (bottom instead of top). Piston sizes are the same on both calipers.

I reviewed the AP catalog that you sent the link for. That syncs up with my belief that the PFC740 pad will work. It seems to be identical to the CP2340D43 pad. Page 4 of the brake caliper section also discusses the "handedness" of calipers, although those questions have been fully answered by everyone here.


The catalog didn't show the two calipers in question here, so I will have to contact them and hopefully get more data on those calipers. It would be nice to see if they have different pad rest bits or get the full dimensions of their backing plates for the pad I'm interested in.

[tlracer]

Mikey,

Yep, assuming your rear calipers are on the front of the uprights the leading (=smaller) piston will be at the top.

The online catalogue covers only current parts, but I'm sure AP will be able to supply details of the calipers you have.

Happy Easter!

[J Leonard]

CP3345 caliper

[mikey]

CP3345 caliper

Where did you find that? Is there a sheet for 3344, 4152 or 3567?

the curious thing about that sheet is it says rotor diameter 280mm. I’d have to space the caliper out 2.5 mm more for that and Don’t think it can happen because there isn’t that much clearance inside the wheel. Likely stuck with 275mm.

[mikey]

Well I believe I stand corrected. I have measured and re-measured and then measured again to be sure. I believe now that I can fit standard 280mm rotors on the rear. It requires stacking two 0.062” washers on each caliper mounting bolt and leaves me with 2.5mm radial clearance between the caliper and the wheel hoop. Is this enough?

the fronts require stacking three 0.062” washers on each mounting bolt. And according to my feeler gauges leaves me with 2.3mm clearance inside the wheel hoop.

Is is there any issue with moving the caliper this far out on the mounting bolts? Are the bolts ok with the lever forces at 0.186” away from the upright? I can’t seem to find any notes from Brembo or AP about maximum allowable shims. Also the retaining nut will be on the very end of the mounting bolt, but fully engaged.

[tlracer]

Mikey - p.31 of the AP catalogue gives some good information on installation, disc pathway clearance, etc. This advises that, on radial-mount calipers, clearances can be adjusted using shims to achieve the correct pad positioning on the disc.

Again, you might want to ask the question of AP, or Essex Parts Services as their US distributor, since they would be in a better position than most to advise on best practice for their products.

[mikey]

Sent an email to AP and Essexparts - the 800# on the bottom of the tech sheet isn't theirs anymore. I called Essex but they said it was easier if I email - I guess the right guy wasn't available via phone today. Will update with what I find out.