How much front rotor does a track car really need?

[OldRanger]

The brake options out there for 18" wheels include a range of front rotor diameters and caliper types. The predominant front rotor diameter options seem to be:

1. OE, non ZCP: 325x28mm
2. ZCP/CSL: 345x28mm
3. Big Brake (Brembo, AP, etc.) 355x32mm (AP)

How much rotor does an e46 M3 that weighs in at 3,050 lbs. (1/2 tank of fuel and a driver) really need?

The pace is on par with instructor/advanced group

Does adding a 200lbs. instructor change that?

[S54330Ci]

My first question is what is currently on your car? The larger the rotor (both diameter, thickness, vane count) will have better thermal capacity. BBKs also have the advantage of lighter calipers which reduces weight. Do you need a BBK? Depends.

Keep in mind SPECE46 runs factory rotors and calipers with no issues and these guys hammer their brakes.

IMO the key to successfully running an OEM setup is 1) proper pad compound, 2) proper cooling, 3) calipers and guide pins in top condition

Many who occasionally track their car will try and run the same pad for both street and track. This usually results in a pad that is not fully suited for the track (can't handle high temps), or a pad that is not street compatible (noisy, low bite when cold, dusty). If your car is a dual purpose machine, I'd seriously consider you run two sets of pads, one for the street and one for the track, swapping them once at the track. Find a manufacturer that offers pads for both scenarios so you avoid running different manufacturer pads with different formulas. Remember pads will lay down a transfer layer on the rotor (which you want) and using different brands will sometimes force you to re-bed your pads (especially for track use) each time you swap.

Brake cooling is oftentimes ignored, but is a critical element to extending both rotor and pad life as well as getting the best braking performance on the track. Given your car's weight and potential top speed on a given track, I'd encourage you to get a ducted cooling solution that routes air directly to the back of the rotor.

As far as compounds are concerned, for track use find a pad with a high coefficient of friction that doesn't fall off as temps rise. Most manufactures will have charts to help determine the right pad. Finding the right pad is somewhat a trial-and-error sort of process. Every pad has their own set of characteristics (bite, release, temp capabilities) and there isn't one perfect pad. Much like oil discussions, everyone has their favorite, but what works for one person may not work for you.

Before you jump off the BBK cliff, I'd do the following before your next track day:

1) Refresh your rotors to OEM specification. Use a high-quality rotor
2) Make sure your calipers are in top condition (seals replaced, pistons smooth and without pitting)
3) Install reinforced caliper bushings and guide pins Reinforced Brake Caliper Bushings - BMW E30 E36 E46 E28 E34 E39 & More (bimmerworld.com)
4) Install a brake cooling solution BimmerWorld Brake Cooling Kit - E46 M3
5) Use a dedicated brake pad for the track. I'd suggest Colbalt Friction XR2 in front and XR4 in the rear. Cobalt Racing Brakes - High Performance Brake Pad Technology (cobaltbrakes.com)

I think you will find your OE braking system will perform just fine for all but the most demanding tracks.

[OldRanger]

Thanks for your response S54330Ci

My first question is what is currently on your car?

For Track I use these brakes with RE71-RS tires
:
Front: OE Caliper with PFC 331, Zimmerman ZCP 345mm drilled rotor, ECS Caliper Bracket, BW Brass Guide Pins, rebuilt last year. SS BW brake lines
Rear: OE Caliper with PFC 332, Zimmerman 328mm drilled rotor, OE bracket and guide pins, rebuilt last year. SS BW Brake lines

For any extended time on street I'll put OE Textar pads in and Michelin PS4s on.

Brake cooling is oftentimes ignored, but is a critical element to extending both rotor and pad life as well as getting the best braking performance on the track. Given your car's weight and potential top speed on a given track, I'd encourage you to get a ducted cooling solution that routes air directly to the back of the rotor.

I have ordered a HARD Motorsports kit with backing plates from AAF that will fit the 345mm rotor. I have yet to install it, but others recommend that I do this as well with the stock calipers. I had avoided that because it complicates cool air supply to the intake and it seemed to be somewhat of a gimmick to me.

Before you jump off the BBK cliff, I'd do the following before your next track day:

1) Refresh your rotors to OEM specification. Use a high-quality rotor
2) Make sure your calipers are in top condition (seals replaced, pistons smooth and without pitting)
3) Install reinforced caliper bushings and guide pins
4) Install a brake cooling solution
5) Use a dedicated brake pad for the track.

I think you will find your OE braking system will perform just fine for all but the most demanding tracks.

I agree on all points, and had hoped to run OE brakes for HPDE, but the front pads are only lasting about 6-8 track days with the rotor lasting about twice that long. I'm also getting irregular wear on the pads and rotor. This OE approach is expensive even when using the FCP warranty. One contributing factor to that wear rate must be too much friction with 0% brake pressure. I can't seem to get the OE caliper set up without a substantial amount of rubbing. When the car is on the lift and I spin the front wheel as hard as I can, I can only get the wheel to spin about 1.5 turns. This drives me mad, as I know this is inefficient and creates heat. I believe that the better BBKs have an optimal bracket design that addresses some of my concern here.

All of that said: Like you advise, I am pulling the front calipers to rebuild them this week. I know I cooked the dust boots in the first few track days last fall after I rebuilt the calipers in June so I presume the piston seal is a routine maintenance item somewhere around 15 track days. All in, I think I've got about that many track days on the rebuilt caliper with many of them on VIR and Summit Main.

Edit: I should add that I've tried PFC 08 on all corners and didn't like the way the initial bite worked and I think the ABS didn't either. So I changed to 331/332 - which likely do wear out faster.

[S54330Ci]

I definitely think additional cooling will increase your pad and rotor life. You mentioned your cooling kit "complicates cool air supply to the intake." Are you running an Eventuri intake? I know that kit has a scoop to direct air from the area behind the fog light which would interfere with the cooling duct.

Just curious what you think your pad life should be. 6 to 8 track days at VIR in the Advanced / Instructor group sounds about right to me.

When you get ready to rebuild your calipers, you may want to look into Racing Brake high temp dust seals. The feedback is positive from those who track their cars. Unfortunately, dust seals are going to be a "consumable" item. They don't last forever, especially on a track car.

Girodisc may be an option to extend rotor life (and performance). They make a 345mm rotor. Much more robust than OEM, however they do come with a cost ($1,100). Probably not financially worth it given your FCP Euro replacement warranty.

Check out the BW reinforced caliper guides. They are a bitch to install due to the solid nylon reinforcement but will keep the caliper aligned better than the non-reinforced version.

[OldRanger]

You mentioned your cooling kit "complicates cool air supply to the intake." Are you running an Eventuri intake? I know that kit has a scoop to direct air from the area behind the fog light which would interfere with the cooling duct.

On the M3, the brake OE brake cooling duct splits and also supplies air to the bottom of the OE Airbox. In my case with the CSL style intake...it simply dumps air into the 'space' behind the headlight assembly. I don't have it all figured out yet, nor do I even have the Hard Motorsports kit on hand at this point...but using their Brake inlet/foglight delete and their ducting elimnates this source of air. I am considering some alternatives to their inlet, but also in another direction, considering a modified headlight assembly since this car won't have State safety inspections that might fail by removing the high beam.

Just curious what you think your pad life should be. 6 to 8 track days at VIR in the Advanced / Instructor group sounds about right to me.

I honestly have no idea. Maybe it's right on for the pace we're at. I was hoping to get some feedback like this. Thanks.

When you get ready to rebuild your calipers, you may want to look into Racing Brake high temp dust seals. The feedback is positive from those who track their cars. Unfortunately, dust seals are going to be a "consumable" item. They don't last forever, especially on a track car.

Good idea - I looked at their website and will ask them if they have an option that works for the OE caliper. Thanks for all of your advice - I appreciate it!

[S54330Ci]

One final thought with respect to BBKs. I have an AP Racing kit (front and rear) on my S54-powered 330Ci coupe. I am extremely impressed with the performance of the CP8350 / CP5144 calipers. These are 4 piston units (f / r) and fit under 17" wheels (not sure what size you are running). The front rotors are 325mm, so slightly smaller than your 345s but just as capable. I'd wager to say more capable the factory rotor as the AP rotor is thicker with more vanes and mounted on a floating aluminum hat (better thermal capacity). The rear calipers mount using the factory mounting points and are compatible with OE rotors.

Front calipers use an industry standard pad shape available in a wide range of compounds. Pricing will be equal to or less than race compatible OE shaped pads. A simple one bolt bridge holds the pad in place, so pad swapping is literally a 60 second process. Pad swapping for the rear caliper is exactly the same. These calipers do not have dust boots which is great for track use but may require extra maintenance if you put a lot of street miles on your car.

And the best of all is the pricing. You can purchase both front and rear kits for the pricing of a front-only 6 piston AP kit.

Here are some links for your review:

Front
Essex Designed AP Racing Competition Brake Kit (Front CP8350/325)- E46 M3 | Essex Parts Services, Inc.

Rear
AP Racing Competition Caliper Kit BMW e46 M3 Four Piston Rear (Caliper Only) | Essex Parts Services, Inc.

[OldRanger]

For perspective, S54330Ci

I appreciate your feedback on the 325mm AP Racing option. Personally, I'm guessing that is one of the best options. Part of what I'm seeking is more regular pad and rotor wear. These pads have 4 track days on them. 2 at VIR and 2 at Summit Main. This is my second set of Zimmerman rotors in 15 track days, so my rotor change interval is about 2,500 miles and 8 track days.

And the caliper design in general...I'm getting uneven wear with pad material thickness ranging from 4mm to 7.5mm. The dust boots are toast, the seals are likely overdue for a change...and those pistons were new last year.

[S54330Ci]

A couple of observations:

1) I've never liked the floating caliper design for track use. In every car I have setup for track use (Acura NSX, BMW E30 M3, BMW E46) I've replaced the stock brakes with some variation of a radial-mounted caliper (StopTech, AP Racing). With the floating caliper I experienced exactly what you have described. You are always going to see uneven wear with a floating caliper. It is just inherent in the design. Racers who race in classes that require factory brakes (SPECE46) rotate their pads (inside to outside, left to right) to even the wear oftentimes between events. Even doing this pad wear is going to be uneven.

2) Drilled rotors are not up to the task for track use. Like you, I have also experienced cracking around the holes. A slotted rotor is really what you want. Ideally you want a slotted rotor mounted to a floating aluminum hat like AP or Girodisc. The AP site explains why better than I can. You will need to scroll down to the section that talks about disks. Essex Designed AP Racing Competition Brake Kit (Front CP8350/325)- E46 M3 | Essex Parts Services, Inc. While the OEM BMW rotor design is marketed as a two-piece floating rotor, it is not a true floating design. OK for the street, marginal for the track.

3) For true track use dust boots are a no-no for exactly the reasons you show in your pictures. Almost all true racing calipers do not have dust boots.

4) Cooling is a must no matter the type of brake system used. You would be surprised how much additional life you can get from a set of pads just by dropping the rotor temps 50 - 75 degrees. I'd strongly recommend finding a way to get a brake cooling setup on your car.

5) Pad life is really dependent on driver and track. I don't have much to offer here other than to say your example above is interesting. I would recommend next time you are at these tracks, talk to other drivers who have like or similar cars to get feedback on their experiences (maybe you have already done this). If you are open to trying another pad manufacturer, I'd give Cobalt Friction a try. I've had great luck with their products.

[bigjae46]

If you're at the track 6-8 times or more per year, running faster lap times and using stickier tires then a BBK makes more sense. I think I posted the economics in another thread. A 355mm rotor can fit under a 17" wheel, depends on the caliper. Stoptech ST40 fits.

IMO, the best setup economically is still a ST40/355 4 wheel BBK. The rotor discs and pads are the same for both axles. I swapped the discs front to rear once the fronts got close to min thickness. A rear disc cracked so I installed new discs on the front and the old discs back on the rear. It's getting close to time for some new rotors. And pads...my spare pads are the rear brakes. If it gets close during a weekend, I swap pads front to rear. The rears last a long time so no worries.

Also cooling will help the life of the pads and rotors. I'd guess that cooling adds about 33%-50% more life on the pad, about another 2-3 days. My stoptech ST40 rotors have lasted a hell of a long time. My data is old. I'm about 400lbs lighter so I don't even run brake cooling anymore. My current set of pads are on day 8 and aren't even half worn. I love it...my E46 M3 has miata level consumable costs and I get to keep my dignity.

[S54330Ci]

Unfortunately availability of most of the StopTech products is declining. As I understand it on December 21, 2023 StopTech announced the closure of their USA manufacturing facilities, effective immediately. StopTech plans to build a new factory in Juarez, Mexico but warns that new orders may not begin shipping until 3rd quarter 2024 or beyond. At this time, no orders are being accepted for StopTech components labeled as unavailable or backordered. There was a sale and consolidation back on 2022 which began the eventual decline of StopTech kits and replacement parts in the supply chain.

While StopTech is a very good BBK, I'd look at other manufactures as parts for ST BBKs will become increasing harder to source.

[bigjae46]

Unfortunately availability of most of the StopTech products is declining. As I understand it on December 21, 2023 StopTech announced the closure of their USA manufacturing facilities, effective immediately. StopTech plans to build a new factory in Juarez, Mexico but warns that new orders may not begin shipping until 3rd quarter 2024 or beyond. At this time, no orders are being accepted for StopTech components labeled as unavailable or backordered. There was a sale and consolidation back on 2022 which began the eventual decline of StopTech kits and replacement parts in the supply chain.

While StopTech is a very good BBK, I'd look at other manufactures as parts for ST BBKs will become increasing harder to source.

Not really true.

The calipers use the same off the shelf parts as a brembo caliper. The pads are a standard and widely available pad shape - a Porsche OE pad shape - which is also a shape that Brembo uses. The rotor discs are standard 355x32mm discs which are a common size. The only thing proprietary might be the caliper body, rotor hat and caliper bracket. I think Girodisc can source the rotor hats, not sure. I guess you do lose the patented aerorotor design...not sure what's so special about it.

Replacement parts shouldn't be the deterrence. In fact, I think it is still a very strong selling point. If anything, it's the pricing that makes less sense since competitors aren't that much more.

[S54330Ci]

Not really true.

The calipers use the same off the shelf parts as a brembo caliper. The pads are a standard and widely available pad shape - a Porsche OE pad shape - which is also a shape that Brembo uses. The rotor discs are standard 355x32mm discs which are a common size. The only thing proprietary might be the caliper body, rotor hat and caliper bracket. I think Girodisc can source the rotor hats, not sure. I guess you do lose the patented aerorotor design...not sure what's so special about it.

Replacement parts shouldn't be the deterrence. In fact, I think it is still a very strong selling point. If anything, it's the pricing that makes less sense since competitors aren't that much more.

Perhaps all that you say is true, but for someone looking to purchase a BBK the availability of full ST kits are severely limited **** that was my point ****

I checked all the major web sites and NONE have front ST40/355mm kits available for the E46 M3 chassis and do not know when they will have them available. Most vendors drop ship from Centric's (or whoever owns them now) warehouse and do not physically stock them due to all the variations of rotor configs and caliper colors. If their web site indicates "in stock" all that means is, fingers crossed, the warehouse has them.

If you already own a ST BBK (like I do on my 1994 NSX) then you would need to do as you outlined and purchase replacement parts from other manufactures such as Brembo.

Honestly, if you are considering a BBK for the E46 then StopTech should NOT be a consideration. AP Racing is a sure bet given their racing heritage and overall involvement in motorsports.

[mrgizmo04]

Perhaps all that you say is true, but for someone looking to purchase a BBK the availability of full ST kits are severely limited **** that was my point ****

I checked all the major web sites and NONE have front ST40/355mm kits available for the E46 M3 chassis and do not know when they will have them available. Most vendors drop ship from Centric's (or whoever owns them now) warehouse and do not physically stock them due to all the variations of rotor configs and caliper colors. If their web site indicates "in stock" all that means is, fingers crossed, the warehouse has them.

If you already own a ST BBK (like I do on my 1994 NSX) then you would need to do as you outlined and purchase replacement parts from other manufactures such as Brembo.

Honestly, if you are considering a BBK for the E46 then StopTech should NOT be a consideration. AP Racing is a sure bet given their racing heritage and overall involvement in motorsports.

Literally everything that you need for a Stoptech kit in terms of consumables from pads, rotors, pressure seals, dust boots, even pistons, etc can be sourced from multiple places and they are interchangeable with Brembo 355x32, so Brembo, Girodisc, Coleman, etc make what you need. I haven't heard of people needing new hats or calipers or mounting brackets, except in super rare circumstances. Even if new kits are not available, used ones are. Shameless plug - I'm selling a front 355x32 kit.

Sent from my SM-S911U1 using Tapatalk

[OldRanger]

A couple of observations:

1) I've never liked the floating caliper design for track use...You are always going to see uneven wear with a floating caliper. It is just inherent in the design. Racers who race in classes that require factory brakes (SPECE46) rotate their pads (inside to outside, left to right) to even the wear oftentimes between events. Even doing this pad wear is going to be uneven.

I have started rotating the pads and it seems to be helping prevent the uneven wear. At first, I was only rotating them left to right, now I'm rotating them left to right and inside to outside. This requires removing the spring clip that holds the inside pad into the piston, but with PFC pads they usually fall off anyway. Simply prying on the pin with a screwdriver will easily remove it.

2) Drilled rotors are not up to the task for track use. Like you, I have also experienced cracking around the holes...

4) Cooling is a must no matter the type of brake system used. You would be surprised how much additional life you can get from a set of pads just by dropping the rotor temps 50 - 75 degrees. I'd strongly recommend finding a way to get a brake cooling setup on your car.

I agree, these drilled rotors are not optimal. As you recommended: the brake cooling system I've installed seems to help with this. It's certainly delaying some of the cracking on the rotors I've seen in the past, and appears to be extending pad life. With that installed, I'm now using all of the airflow from the foglight and OEM duct opening for brake cooling.

This eliminated a partial supply of air to the air box/engine intake that was provided by the OEM duct next to the driver side fog light. So, I've installed a headlight duct to supplement the refresh air supply to the engine intake. Note: if you do not put screening in front of the brake cooling duct, routinely remove the rotor and clean out any rubber marbles/chunks that might find their way into the rotor hat.