E92 M3 AP Racing BBK Review (long)

[AP Racing - Chris_B]

Originally Posted by mastek

Thanks Chris,

Thanks for the info.
Local support means a great deal ---

I'm still on the fence between the StopTech Trophy 4-pot 355mm kit..... and yours
I had a chance to look @ the StopTech Trophy kit at the race track last weekend ... it was very impressive, super-lightweight & forged.
Your kit wins in the rotor size, rotor design & 2 extra pistons .......

Now if I could only have the best of both worlds.... hmmmm

OK, help me understand what is missing. Why is a forged caliper important when stiffness, not strength, is the primary issue? Forging does not improve stiffness one bit. What does that caliper weight (fully loaded with pads)?

[mastek]

Originally Posted by AP Racing - Chris_B

OK, help me understand what is missing. Why is a forged caliper important when stiffness, not strength, is the primary issue? Forging does not improve stiffness one bit. What does that caliper weight (fully loaded with pads)?

lol - I sense tention -

I was @ a race track & did not get a chance to weigh the caliper.
It had no pads - and it was 4-piston.
I was able to carry and handle it with one hand without tiring - and compared
to the many Brembo and StopTech calipers I've handled, the weight difference is noticable (I'd say 5-10lbs. noticable).

As far as forged vs. cast == I guess it's like wheels ... There are many great cast wheels that I use and dont mind and they dont seem to bend or deform because their cast -- so it might not be as important as people make it out to be -- but it's a nice plus.

[AP Racing - Chris_B]

Originally Posted by mastek

lol - I sense tention -

I was @ a race track & did not get a chance to weigh the caliper.
It had no pads - and it was 4-piston.
I was able to carry and handle it with one hand without tiring - and compared
to the many Brembo and StopTech calipers I've handled, the weight difference is noticable (I'd say 5-10lbs. noticable).

As far as forged vs. cast == I guess it's like wheels ... There are many great cast wheels that I use and dont mind and they dont seem to bend or deform because their cast -- so it might not be as important as people make it out to be -- but it's a nice plus.

No tension here, but there certainly are questionable marketing claims out there -- nothing new!

The forging process has many advantages for parts such as wheels, pistons, connection rods and many, many others. Most caliper designs (including any street/track day caliper I've ever seen) do not benefit from forging at all. But the marketing guys like to print that word! So, no plus like a wheel would have.

The reason is that a part's stiffness is based on (primarily) two things: 1) Part geometry, and 2) Modulus of Elasticity -- not strength. Since brake calipers are a stiffness-based design (they are far stronger than they need to be because we design them for stiffness over strength) and forging does not raise a material's modulus, there is no real advantage. Forging should be considered when lightweight designs are limited by material strength. When buying parts that need to be as light and strong as possible and stiffness is a secondary consideration, like connecting rods, forging is the best way to go.

When you take weight out of a caliper, you usually can't avoid reducing its stiffness, making the brake pedal less responsive and more vague. The recent exception has been the revolutionary Radi-Cal race caliper line.

NASCAR road course/short track front (a couple of designs ago):



This caliper was around 15% stiffer than the heavier caliper it replaced. Also, running temperatures were reduced by almost 100°C compared to the previous, symmetrical version due primarily to the increase in surface area. It's easy to understand why almost 90% of NASCAR teams are using these on the heavy braking courses.

F3 front caliper (last year) and very similar to the current NASCAR rear speedway caliper:



The above 4-piston caliper feels like it will float away if you dare let go of it! One entire F3 front corner brake system (including caliper, pads, rotor, hat and floating hardware) weighs 9.63 pounds!

Also, there are certain racing applications where caliper cross-sections are impossibly thin due to things like wheel size restrictions. Some of those are forged because strength is a viable concern. Of course, there are other ways to achieve the needed strength, but some advanced materials have been outlawed in certain motorsports series' due to cost and/or concerns over hazardous materials.

But for street cars also used on the track, forged calipers offer no tangible benefit -- except for the ad guys. Stiff calipers rule!

Chris

[mastek]

Originally Posted by AP Racing - Chris_B

No tension here, but there certainly are questionable marketing claims out there -- nothing new!

The forging process has many advantages for parts such as wheels, pistons, connection rods and many, many others. Most caliper designs (including any street/track day caliper I've ever seen) do not benefit from forging at all. But the marketing guys like to print that word! So, no plus like a wheel would have.

The reason is that a part's stiffness is based on (primarily) two things: 1) Part geometry, and 2) Modulus of Elasticity -- not strength. Since brake calipers are a stiffness-based design (they are far stronger than they need to be because we design them for stiffness over strength) and forging does not raise a material's modulus, there is no real advantage. Forging should be considered when lightweight designs are limited by material strength. When buying parts that need to be as light and strong as possible and stiffness is a secondary consideration, like connecting rods, forging is the best way to go.

When you take weight out of a caliper, you usually can't avoid reducing its stiffness, making the brake pedal less responsive and more vague. The recent exception has been the revolutionary Radi-Cal race caliper line.

NASCAR road course/short track front (a couple of designs ago):



This caliper was around 15% stiffer than the heavier caliper it replaced. Also, running temperatures were reduced by almost 100°C compared to the previous, symmetrical version due primarily to the increase in surface area. It's easy to understand why almost 90% of NASCAR teams are using these on the heavy braking courses.

F3 front caliper (last year) and very similar to the current NASCAR rear speedway caliper:



The above 4-piston caliper feels like it will float away if you dare let go of it! One entire F3 front corner brake system (including caliper, pads, rotor, hat and floating hardware) weighs 9.63 pounds!

Also, there are certain racing applications where caliper cross-sections are impossibly thin due to things like wheel size restrictions. Some of those are forged because strength is a viable concern. Of course, there are other ways to achieve the needed strength, but some advanced materials have been outlawed in certain motorsports series' due to cost and/or concerns over hazardous materials.

But for street cars also used on the track, forged calipers offer no tangible benefit -- except for the ad guys. Stiff calipers rule!

Chris

Interesting --

My car weighs more then a NASCAR -- I do want - how much??:

[AP Racing - Chris_B]

Originally Posted by mastek

Interesting --

My car weighs more then a NASCAR -- I do want - how much??:

A NASCAR brake system runs up around $30k, not including pads, brake lines, hardware or trailer and pit crew. These would not be very fun on the street as they tend to be pretty harsh and noisy. Also keep in mind these are very wide and made for 42mm rotor thickness and around 32mm pad thickness, so you would definitely need very special custom made wheels for an M3! And don't bother asking for sponsorship -- AP Racing does not sponsor any race teams anywhere. They all must purchase their goods.

[STALKER]

Aren't stiffness and strength related?
Also, with forging, doesn't the material used need to be of a higher caliber?

I have already ordered the AP kit, but the Stoptech trophy was really high up on my list. I wish they made a kit that was similar in size to the AP kit.

[rzm3]

Without going into too many details...

...stiffness (characterized by the Modulus of Elsticity) is the initial slope on the stress/load vs. displacement graph. It basically tells you how much load a material can take for a fixed amount of displacement.

...strength, or yield strength in particular, is defined in terms of stress (force over area, i.e. pound-force per square inch). It characterizes a level of stress when a material transitions from elastic deformation to plastic deformation. On the plot, it is a point short after the initial linear portion of the curve (typically using .2% offset method)

There are materials that have high stiffness but not strength (think glass), and there are materials that have high strength but not stiffness (think composites). So, as far as I know, they are independent parameters.

[AP Racing - Chris_B]

Originally Posted by STALKER

Aren't stiffness and strength related?
Also, with forging, doesn't the material used need to be of a higher caliber?

I have already ordered the AP kit, but the Stoptech trophy was really high up on my list. I wish they made a kit that was similar in size to the AP kit.

Yep, rldszhao is on the mark. I had read your post and thought -- "How do I condense several college courses into a few paragraphs?" It was done well.

The relationship between stiffness and strength is material dependent. For most metals, the harder you make the material (by heat treating, cold working, etc.), usually it becomes stronger and more brittle. But it does not get stiffer. Only a geometry change can do that for a homogeneous material.

Consider a suspension coil spring. Better materials and heat treatment will make it stronger and more resistant to fatigue. But to make the spring stiffer, you must change the coil diameter, wire diameter and/or the number of windings regardless of the material callout. There are tons of books on the topic of metallurgy if you ever can't get to sleep at night...

As far as forging materials, they are not automatically better or worse. Each can be specified for their strength properties, but must also flow well when hit to avoid extreme internal stresses or breaking the forging dies. Some forging aluminums are high strength, but so are some cast and billet aluminums. While you can align grain structure for some parts by forging (like the connecting rods mentioned earlier), calipers need directional stiffness on three different vectors to offset the three main deflection modes. This is not so easy to improve with a forging as grain alignment tends to favor only one direction.

Sorry to bore you with details, but it is important to understand there are decades worth of engineering data, materials research and testing that go into a modern, top-shelf brake system. If forging were better for this particular application, AP would offer it that way.

Chris

[STALKER]

Originally Posted by AP Racing - Chris_B

Yep, rldszhao is on the mark. I had read your post and thought -- "How do I condense several college courses into a few paragraphs?" It was done well.

The relationship between stiffness and strength is material dependent. For most metals, the harder you make the material (by heat treating, cold working, etc.), usually it becomes stronger and more brittle. But it does not get stiffer. Only a geometry change can do that for a homogeneous material.

Consider a suspension coil spring. Better materials and heat treatment will make it stronger and more resistant to fatigue. But to make the spring stiffer, you must change the coil diameter, wire diameter and/or the number of windings regardless of the material callout. There are tons of books on the topic of metallurgy if you ever can't get to sleep at night...

As far as forging materials, they are not automatically better or worse. Each can be specified for their strength properties, but must also flow well when hit to avoid extreme internal stresses or breaking the forging dies. Some forging aluminums are high strength, but so are some cast and billet aluminums. While you can align grain structure for some parts by forging (like the connecting rods mentioned earlier), calipers need directional stiffness on three different vectors to offset the three main deflection modes. This is not so easy to improve with a forging as grain alignment tends to favor only one direction.

Sorry to bore you with details, but it is important to understand there are decades worth of engineering data, materials research and testing that go into a modern, top-shelf brake system. If forging were better for this particular application, AP would offer it that way.

Chris

Thanks for the response gents.
Glad I went into medicine an not engineering, lol.

[AP Racing - Chris_B]

Originally Posted by STALKER

Thanks for the response gents.
Glad I went into medicine an not engineering, lol.

Just remember to be nice to engineers when you can as they are not usually a social bunch. Well, except for racers! And some even design medical equipment, too!

[swamp2]

Originally Posted by AP Racing - Chris_B

Since brake calipers are a stiffness-based design (they are far stronger than they need to be because we design them for stiffness over strength) and forging does not raise a material's modulus, there is no real advantage. Forging should be considered when lightweight designs are limited by material strength. When buying parts that need to be as light and strong as possible and stiffness is a secondary consideration, like connecting rods, forging is the best way to go.

Technical content post of the week (or month or whatever)! Make perfect sense and well presented. Keep up the good fight for engineering facts over marketing hype. Each application needs to be carefully matched to a design and a manufacturing process as well to deliver the most product for a given cost.

[aus]

Are those Nascar caliper's forged since there is a lot less material there?
They're pretty sexy looking. Reminds me of something out of a Terminator movie.

[sparkyg]

Great info Chris, I wish I knew more of this last year when I got the Brembo BBK

Still this is great stuff, next car for me I guess

[fe55]

Since this a thread about AP Brakes I thought I would chime in and ask Chris whether they got the pads to fit the CP 6602 rear kit.
I am running PFC 01 pads custom made for the rears because the CP6602 kit is a europeon kit and pads are not available yet in NA. At least they weren't last month.

Glen ; your Brembo's are fabulous . Nothing to be envious over.

[STALKER]

Originally Posted by sparkyg

Great info Chris, I wish I knew more of this last year when I got the Brembo BBK

Still this is great stuff, next car for me I guess

Brembos are amazing.
I ran them on all my previous M3s.

[Radiation Joe]

Originally Posted by AP Racing - Chris_B

Just remember to be nice to engineers when you can as they are not usually a social bunch. Well, except for racers! And some even design medical equipment, too!

... and nuclear reactors.

Thanks for the technical fix. I had to be educated on this myself a few months back by Chris. What's embarrassing is that my degree is in material science (forgot some of it over the years, apparently).

[AP Racing - Chris_B]

Originally Posted by sparkyg

Great info Chris, I wish I knew more of this last year when I got the Brembo BBK

Still this is great stuff, next car for me I guess

No sweat -- see you next time!

[AP Racing - Chris_B]

Originally Posted by aus

Are those Nascar caliper's forged since there is a lot less material there?
They're pretty sexy looking. Reminds me of something out of a Terminator movie.

The current NASCAR calipers are not forged. They keep changing more quickly than forging toolmakers could possibly keep up! They are made from specific materials which have properties that must fall within the rules.

I'm looking forward to when some of the features start showing up in street calipers. Some of the lessons learned already have.

Chris

[Lemans_Blue_M]

Originally Posted by AP Racing - Chris_B

The current NASCAR calipers are not forged. They keep changing more quickly than forging toolmakers could possibly keep up! They are made from specific materials which have properties that must fall within the rules.

I'm looking forward to when some of the features start showing up in street calipers. Some of the lessons learned already have.

Chris

Just to make sure I understand your logic here Chris...

What material construction is AP Racing using for the calipers in the current 368mm/356mm street kits?

Aren't they CAST 2-piece brake calipers with steel bolts to improve the overall stiffness?

Also, there is a discrepancy in your description about the Radi-Cal forged calipers earlier...

I was told by AP Racing (UK), that the reason they selected a forged monoblock racing caliper was to reduce the weight while also maintaining the stiffness and strength they wanted. (similar to their 2-piece caliper designs). Since they wanted to build a slim caliper that fit inside smaller racing wheels they were forced to build the Radi Cal calipers out of forged material. Cast monoblocks or 2-piece Forged calipers would have required too much MASS, which would have defeated the entire purpose of making a strong low profile racing caliper. They were able to CNC machine lightening pockets into the body of the forged caliper, because it was cut from a high-tonnage forged aluminum billet. (that was properly heat treated)

The grain flow of that forged material (and subsequent heat treatment) was essential in achieving the goals they were trying to achieve with the Radi Cal monoblocks.

Heat-treated 2000, 6000, and 7000 series aircraft quality forged aluminum billet has improved tensile and yeild strength properties. (not to mention improved fracture toughness, fatigue strength, hardness, shearing strength, etc.)

I was also told this was necessary due to how much strength a brake caliper loses as it heats up under heavy braking loads. (more than 70% of it's strength at high temperatures)

So it sounds like it was pretty important to their engineering dept., or why else would they even bother using forged heat-treated billet aluminum material. (if it doesn't matter in regards to brake calipers)

A contact at BMW Motorsport told me essentially the same thing, when I inquired about the availability of the OE AP Racing BBK used on the M3 GT4 race car sold in Germany. (too expensive and NO dust seals for street duty ruled those out for me )

BTW: The StopTech engineers also said the same thing when I contacted them in July. (so that's 3 different sources)

I'm just curious to know why you made it seem irrelevant...

Are you saying that AP Racing's engineers got it wrong?

Or maybe they used forged heat-treated material for no good reason?

Following your earlier logic, AP Racing could have just made a CAST 7075 aluminum monoblock caliper (like BREMBO and ALCON) and called it a day.

That would have been much cheaper (and less time consuming), since it would not have been quite as difficult to CNC machine after it came out of the mold.


I'm asking for a clarification to your earlier statement, because your opinion doesn't match what I was told by AP Racing in Europe.

Someone is wrong (or is misinterpreting) what is or isn't relevant in regards to the build construction methods required to build these lightweight racing calipers...

[AP Racing - Chris_B]

Originally Posted by Lemans_Blue_M

Just to make sure I understand your logic here Chris...

What material construction is AP Racing using for the calipers in the current 368mm/356mm street kits?

Aren't they CAST 2-piece brake calipers with steel bolts to improve the overall stiffness?

Also, there is a discrepancy in your description about the Radi-Cal forged calipers earlier...

I was told by AP Racing (UK), that the reason they selected a forged monoblock racing caliper was to reduce the weight while also maintaining the stiffness and strength they wanted. (similar to their 2-piece caliper designs). Since they wanted to build a slim caliper that fit inside smaller racing wheels they were forced to build the Radi Cal calipers out of forged material. Cast monoblocks or 2-piece Forged calipers would have required too much MASS, which would have defeated the entire purpose of making a strong low profile racing caliper. They were able to CNC machine lightening pockets into the body of the forged caliper, because it was cut from a high-tonnage forged aluminum billet. (that was properly heat treated)

The grain flow of that forged material (and subsequent heat treatment) was essential in achieving the goals they were trying to achieve with the Radi Cal monoblocks.

Heat-treated 2000, 6000, and 7000 series aircraft quality forged aluminum billet has improved tensile and yeild strength properties. (not to mention improved fracture toughness, fatigue strength, hardness, shearing strength, etc.)

I was also told this was necessary due to how much strength a brake caliper loses as it heats up under heavy braking loads. (more than 70% of it's strength at high temperatures)

So it sounds like it was pretty important to their engineering dept., or why else would they even bother using forged heat-treated billet aluminum material. (if it doesn't matter in regards to brake calipers)

A contact at BMW Motorsport told me essentially the same thing, when I inquired about the availability of the OE AP Racing BBK used on the M3 GT4 race car sold in Germany. (too expensive and NO dust seals for street duty ruled those out for me )

BTW: The StopTech engineers also said the same thing when I contacted them in July. (so that's 3 different sources)

I'm just curious to know why you made it seem irrelevant...

Are you saying that AP Racing's engineers got it wrong?

Or maybe they used forged heat-treated material for no good reason?

Following your earlier logic, AP Racing could have just made a CAST 7075 aluminum monoblock caliper (like BREMBO and ALCON) and called it a day.

That would have been much cheaper (and less time consuming), since it would not have been quite as difficult to CNC machine after it came out of the mold.


I'm asking for a clarification to your earlier statement, because your opinion doesn't match what I was told by AP Racing in Europe.

Someone is wrong (or is misinterpreting) what is or isn't relevant in regards to the build construction methods required to build these lightweight racing calipers...

I have summarized a lot of information here for the sake of brevity. A lot was left out as it was not really pertinent to the street/track day caliper discussion. Again, street/track day calipers are stiffness-constrained designs, not strength constrained (usually, anyway!). You may or may not have noticed that I regarded the Radi-Cal as an exception to my street/track day caliper summary regarding stiffness versus strength.

AP Racing makes a huge number of calipers for a large variety of applications. Some do require special forgings. Some are made from forged blanks and machined thoroughly. Some are die cast (sometimes referred to as pressure casting). Some are machined from billet.

Forged and billet are two different things, but sometimes the language differences between the UK and the USA do confuse the issue. My comments were simplified to include forged blanks along with billet -- even though they are very different. Billets (in our language) are continuously cold formed through a die. Forged blanks are hammered into shape in a forging die one at a time, but they still require a ton of machining. Grain structure is optimized for certain load paths, but not all of them. What I call a "forging" is a piece that comes out of the die in a shape that requires minimal machining. The grain structure is better aligned (along with a few other advantages) than with a forged blank that has been machined. AP cannot use near-net shape forgings for the Radi-Cal or else they would never get them to release from the tooling -- they would be trapped! Pretty much 100% of the surfaces are machined, so they are made from forged blanks. Semantics, I suppose...

As far as aluminum alloys go, all of them must be heat treated -- cast, billet or forged. Since calipers are exposed to high stresses while hot, 7000 series alloys do not make good candidates as their strength nose dives at temperatures over 350°F. There are some very good casting and/or forging alloys that don't suffer as much, even though their strength at room temperature might be a bit lower. STIFFNESS varies less than 1 or 2% across all of the aluminum alloys (E = 10.6 x 10^6 PSI), at least until exotic alloying elements are introduced (MMC, beryllium, etc.). Since many racing series' have banned such materials, we are back to using some of the better aircraft alloys or specific designer alloys. Most of the better ones used in Europe don't fall perfectly into the ANSI (2000-7000) series designations.

The E9x M3 kits we are discussing here are made from near-net shape die cast halves, heat treated and artificially aged, finished machined, hard anodized, painted, assembled, pressure tested and leak tested. They go through the same quality gates that AP Racing's OE programs must go through.

My point was that forged calipers do not improve 99% (or more) of street/track day calipers, but it certainly doesn't hurt. However, it often requires the customer for pay more for something they are not likely to make use of. Solutions for pure-bred racing cars are often different for many reasons -- and are much more expensive as a result. Plus, you wouldn't want to live with them on the street anyway. Be wary of any company offering to sell you brakes for the street that are the exact same ones they are racing with. The phrase "different horses for different courses" definitely applies to brakes if looking for the best results.

Chris

[STALKER]

AP kit arrived today.
Box with the front kits was a little wet, but everything seems to be OK. Just have a little rust on one of the rotors, but should scrub off with first application.

I do have a question though about the front rotors.
Upon inspection, I noticed that on the outsides of the front rotors, where the cooling veins end, it mostly is coated black, smooth and well machined. But there is a 3" to 4" section on each where there is no such coating, the area is LOWER then the rest of the rotor, and the material is much more rough and not at smooth.
I have attached some pics. Its only on the front rotors.

I have used Brembos on all my last cars, and never noticed this on Brembo rotors.

Second last pic is where the box was moist. Should I worry about it, or will that all be gone after I use the brakes?

Thanks.

[Jaypod]

Congrats! Cant wait to see them installed and hear your review!

Too bad about the moist boxes - I am sure the rust isn't an issue - Hopefully Chris will chime in.

Congrats though, you must be pumped with the BBK and RAC's!