[abcfgh]

What's the deal with slotted rotors? What about drilled? Are dimpled any better than drilled? I've read way too many different things online. Just hoping to dispel some myths and find the best performing rotors.

The only thing I'm sure of so far is that drilled rotors tend to crack and not hold up as well particularly when they go suddenly from cold to hot very quickly, like with a long freeway drive and then an emergency brake. I would also guess that blank rotors have more heat capacity just due to more material.

True or False?

1. Slotted/drilled/dimpled help a brake pad when at it's thermal limit and outgassing
2. Outgassing doesn't occur anymore with newer pads or #1 doesn't do anything.
3. Slots help shave the brake pad down to keep it consistent and remove deposits and debris. They do this at the expense of brake life.
4. Slots provide a better bite at the expense of brake life.
5. Slotted/drilled/dimpled help with wet braking performance. Wouldn't any water just vaporize right away when brakes are hot?
6. Slotted/drilled help with cooling.
7. Slotted/dimpled are purely for looks and that's why Porsche, Ferrari, etc. do it and F1 racing cars do not.

Enlighten me with your knowledge!

[dradernh]

Originally Posted by abcfgh

What's the deal with slotted rotors? What about drilled? Are dimpled any better than drilled? I've read way too many different things online. Just hoping to dispel some myths and find the best performing rotors.

The only thing I'm sure of so far is that drilled rotors tend to crack and not hold up as well particularly when they go suddenly from cold to hot very quickly, like with a long freeway drive and then an emergency brake. I would also guess that blank rotors have more heat capacity just due to more material.

True or False?

1. Slotted/drilled/dimpled help a brake pad when at it's thermal limit and outgassing
2. Outgassing doesn't occur anymore with newer pads or #1 doesn't do anything.
3. Slots help shave the brake pad down to keep it consistent and remove deposits and debris. They do this at the expense of brake life.
4. Slots provide a better bite at the expense of brake life.
5. Slotted/drilled/dimpled help with wet braking performance. Wouldn't any water just vaporize right away when brakes are hot?
6. Slotted/drilled help with cooling.
7. Slotted/dimpled are purely for looks and that's why Porsche, Ferrari, etc. do it and F1 racing cars do not.

Enlighten me with your knowledge!

AP Racing can enlighten you better than I can. On the following page, see the 'Exclusive AP Racing J Hook Slot Pattern' section discussing slots in general and describing AP's slot:

https://www.essexparts.com/essex-des...p9562380mm-F80

[Sail Boat]

Originally Posted by abcfgh

What's the deal with slotted rotors? What about drilled? Are dimpled any better than drilled? I've read way too many different things online. Just hoping to dispel some myths and find the best performing rotors.

The only thing I'm sure of so far is that drilled rotors tend to crack and not hold up as well particularly when they go suddenly from cold to hot very quickly, like with a long freeway drive and then an emergency brake. I would also guess that blank rotors have more heat capacity just due to more material.

True or False?

1. Slotted/drilled/dimpled help a brake pad when at it's thermal limit and outgassing
2. Outgassing doesn't occur anymore with newer pads or #1 doesn't do anything.
3. Slots help shave the brake pad down to keep it consistent and remove deposits and debris. They do this at the expense of brake life.
4. Slots provide a better bite at the expense of brake life.
5. Slotted/drilled/dimpled help with wet braking performance. Wouldn't any water just vaporize right away when brakes are hot?
6. Slotted/drilled help with cooling.
7. Slotted/dimpled are purely for looks and that's why Porsche, Ferrari, etc. do it and F1 racing cars do not.

Enlighten me with your knowledge!

This topic is debated on every car forum in existence, so I'll answer it a different way.

From my track experience:
1. Not that I have noticed
2. Not that I have noticed
3. Not that I have noticed
4. Not that I have noticed
5. Not that I have noticed
6. Not that I have noticed
7. I think they look cool.

I don't mean to be smart, just being honest. When I was tracking my E30, I tried 1) slotted and blank rotors with the stock floating calipers, and 2) wilwood fixed calipers (F/R) with two piece slotted and later blank rings. The greatest difference came from switching to fixed Wilwood calipers in terms of feel with incremental improvement coming from pad compound. Regardless of caliper or rotor type the pad compound had the most noticeable effect on overall stopping performance, meaning distance and temperature threshold.

With an M235i, my experience dictates that the only worthwhile improvement to an already excellent braking system (fixed calipers, powerful master cylinder, and massive rotors) is the pad compound. Hardcore track enthusiasts are either using blanks with aggressive pad compounds and track fluid or aftermarket kits that can only really be spec'd with slotted or drilled rotors. I'm sure if blank rings were a cheaper configurable option, they would use them instead.

German made Zimmerman blank rotors can be had for < $100 a corner. In my opinion, the only valid argument for spending an additional $50+ per corner is looks.

[abcfgh]

Originally Posted by Sail Boat

This topic is debated on every car forum in existence, so I'll answer it a different way.

From my track experience:
1. Not that I have noticed
2. Not that I have noticed
3. Not that I have noticed
4. Not that I have noticed
5. Not that I have noticed
6. Not that I have noticed
7. I think they look cool.

I don't mean to be smart, just being honest. When I was tracking my E30, I tried 1) slotted and blank rotors on the stock floating calipers, and 2) wilwood fixed calipers (F/R) on two piece slotted and later blank rings. The greatest difference came from switching to fixed Wilwood calipers in terms of feel with incremental improvement coming from pad compound. Regardless of caliper or rotor type the pad compound had the most noticeable effect on overall stopping performance, meaning distance and temperature threshold.

With an M235i, my experience dictates that the only worthwhile improvement to an already excellent braking system (fixed calipers, powerful master cylinder, and massive rotors) is the pad compound. Hardcore track enthusiasts are either using blanks with aggressive pad compounds and track fluid or aftermarket kits that can only really be spec'd with slotted or drilled rotors. I'm sure if blank rings were a cheaper configurable option, they would use them instead.

German made Zimmerman blank rotors can be had for < $100 a corner. In my opinion, the only valid argument for spending an additional $50+ per corner is looks.

No worries, I know you're not being smart. You just hear all this stuff thrown around and there aren't any definitive tests or testing done. It all seems like marketing material to make you feel better about your "performance rotors".

I ended up buying 2-piece blank rotors (aluminum hat) from Zimmermann which actually cost less than the stock OEM Genuine BMW 1-piece blanks. They cost $80 more for the front rotors and $50 more the rear rotors. Overall, cost an extra $270 to get the 2-piece on all 4 corners. Who knows if it's worth it. I'll be curious to see if there's any weight savings. Even just one pound less of rotating mass would be a welcome change and a marginal improvement in heat dissipation. I also got FCP Euro to add the dimpled/slotted version of these 2 piece as I know Zimmermann makes them, but they weren't on FCP Euro's website. Those start to make less sense cost wise though.

[abcfgh]

If anyone is curious about Zimmermann discs, I posted the part numbers along with style and corresponding weights. Spent a good 45 minutes tabulating these lol....super annoying to compare on their website.

All are Zinc coated, high carbon, and ventilated.

Front:
(listed for M235i, M240i, 335i, 340i, 340mm discs)
150.2903.20 - Blank, 1-piece, 10.90kg
150.2903.32 - Blank, 2-piece, 10.00kg
150.2903.52 - Drilled, 1-piece, 10.90kg
150.2906.20 - Slotted & Dimpled, 1-piece, 10.90kg
150.2906.32 - Slotted & Dimpled, 2-piece, 10.10kg (not a typo on my end, maybe theirs?)

(listed for 335i, 340i w/ 370mm discs which I assume can be used on 2'ers that have retrofitted 370mm discs)
150.2911.20 - Blank, 1-piece, 13.20kg
150.2911.32 - Blank, 2-piece, 12.00kg
150.2911.52 - Drilled, 1-piece, 13.20kg
150.2905.20 - Slotted & Dimpled, 1-piece, 13.20kg
150.2905.32 - Slotted & Dimpled, 2-piece, 11.90kg

Rear:

(listed for M235i, M240i, 345mm discs)
150.2921.20 - Blank, 1-piece, 9.20kg
150.2921.32 - Blank, 2-piece, 8.70kg
150.2922.20 - Slotted & Dimpled, 1-piece, 9.10kg
150.2922.32 - Slotted & Dimpled, 2-piece, 8.70kg

(listed for 335i, 340i w/ 345mm discs, has different inner bore diameter than M235i, M240i which is why the weight is different)
150.2919.20 - Blank, 1-piece, 9.50kg
150.2919.32 - Blank, 2-piece, 8.60kg
150.2920.20 - Slotted & Dimpled, 1-piece 9.40kg
150.2920.32 - Slotted & Dimpled, 2-piece, 8.60kg


For shits and giggles, the M2/M3/M4 brakes:

Front (380mm, true floating):
150.2940.70 - Drilled, 2-piece, 11.20kg Left
150.2941.70 - Drilled, 2-piece, 11.20kg Right

Rear (370mm, true floating):
150.2942.70 - Drilled, 2-piece, 9.30kg Left
150.2943.70 - Drilled, 2-piece, 9.30kg Right


If you're looking at the weights, it appears that the Zimmermann website goes in 0.1 kg (0.22 lb) increments. I'm guessing slotted/drilled make a difference of roughly 0.05 kg (0.11 lb) or slightly less if anyone was wondering.

For the front, going from 1-piece to 2-piece saves 0.9 kg (1.98 lb) on the 340mm discs and 1.2kg (2.65 lb) on the 370mm discs.

For the rear on the 345mm discs, going from 1-piece to 2-piece saves 0.5 kg (1.10 lb).


Now for OEM Genuine BMW rotors:

Front 340mm discs:
34116792223 - Blank, 10.275kg according to BMW
34106797602 - Drilled 10.272kg according to BMW

Rear 345mm discs:
34206797598 - Blank, 8.840kg according to BMW
34206797605 - Drilled, 8.829kg according to BMW

If the weights listed on RealOEM and Zimmermann's website are to be trusted, then just by switching to Zimmermann 2-piece blank rotors you save 0.275kg per rotor (0.55kg per axle) in the front and 0.240kg per rotor (0.48kg per axle) in the rear. All in all, if you swap all 4 rotors as I am currently doing, you save a combined weight of 1.03kg or 2.27 pounds, which is cool. Plus these are high quality, zinc-coated, and a tiny bit cheaper. I've had a good experience with Zimmermann rotors, so they seem like a no-brainer.

Also, after looking at RealOEM, I discovered that the diagram for the M235i/M240i shows 1-piece blank 340mm rotors whereas for the 335i/340i the diagram shows 2-piece blank 340mm rotors. Yet part numbers are the same. This really begs the question....are the OEM rotors a 1 piece or 2-piece? Can anyone confirm? I believe they are 2-piece stock.

Links to RealOEM diagrams:
340i Front:
https://www.realoem.com/bmw/enUS/sho...diagId=34_2353
M240i Front:
https://www.realoem.com/bmw/enUS/sho...diagId=34_1949

[WolfGTI]

Wish AP had a kit for our cars that allowed you to keep 17's.

Originally Posted by dradernh

https://www.essexparts.com/essex-des...p9562380mm-F80

[dradernh]

Originally Posted by WolfGTI

Wish AP had a kit for our cars that allowed you to keep 17's.

That would be nice, wouldn't it. I think our cars are now in their ninth(?) year of production, and I wonder if they'll ever create a kit for us. Maybe if enough 2 Series' are turned into track cars at some point.

[dradernh]

Originally Posted by abcfgh

I'll be curious to see if there's any weight savings. Even just one pound less of rotating mass would be a welcome change and a marginal improvement in heat dissipation.

TLDR; skip to the last paragraph.

Heat dissipation will be maximized by moving as much air as possible into the center of the rotors and out past/through the vanes. Due to our cars' suspension design, that's difficult to do. This post details my attempt to minimize heat gain in my braking system by moving more air along that pathway:

https://www.2addicts.com/forums/show....php?t=1670372.

If you read the post, you'll see that I was able to achieve only a marginal difference in a system that can easily get excessively hot. Besides weight, if you're going to lean on the car, it is also seriously handicapped by limitations designed into its braking system.

So what do excessive temps create? Cracking dust boots, for one, which I succeeded in doing on the single front boot I examined. I've assumed that the rest of my front boots are cracked as well. I expect that if I'd kept going to track days I might well have ended up discoloring my calipers, too, as other drivers have. I'm done with the hobby now, so it's no longer an issue.

I attended 16 track days with my M240i, and on average I ran 2.5 sessions a day for an average of 7 laps per session. This is a video showing a representative lap: https://www.2addicts.com/forums/show....php?t=1769652.

It may be difficult to see without a brake pressure bar showing psi values, but looking at the max speed into each braking zone and the speeds at each turn-in will show that Mid-Ohio is a track with only moderate braking demands. Yet the system on my 2 Series was more than maxed-out by those demands.

I'm easy on the brakes (less braking = a more-stable car, esp. at turn-in, and faster laps), and yet the temps of my rotors and calipers were much higher than the system was designed to accommodate. If I'd continued tracking at that moderate level, I would have needed to refresh my dust boots and piston seals at least every other year.

Please, forgive the digressions!

What I've been leading up to is this: a rotor with more mass combined with a pad with more mass (i.e., one with more metal in it, which the more heat-resistant pads have) will decrease the heat available to be sent into your caliper pistons and therefore into your dust boots, piston seals, and brake fluid. The longer it takes to heat-up the rotor and pads, the longer it takes to transmit that heat to the system's upstream and less resilient components. If you're demanding a lot of your braking system, a lightweight rotor may not be something worth pursuing.

[abcfgh]

Originally Posted by dradernh

TLDR; skip to the last paragraph.

Heat dissipation will be maximized by moving as much air as possible into the center of the rotors and out past/through the vanes. Due to our cars' suspension design, that's difficult to do. This post details my attempt to minimize heat gain in my braking system by moving more air along that pathway:

https://www.2addicts.com/forums/show....php?t=1670372.

If you read the post, you'll see that I was able to achieve only a marginal difference in a system that can easily get excessively hot. Besides weight, if you're going to lean on the car, it is also seriously handicapped by limitations designed into its braking system.

So what do excessive temps create? Cracking dust boots, for one, which I succeeded in doing on the single front boot I examined. I've assumed that the rest of my front boots are cracked as well. I expect that if I'd kept going to track days I might well have ended up discoloring my calipers, too, as other drivers have. I'm done with the hobby now, so it's no longer an issue.

I attended 16 track days with my M240i, and on average I ran 2.5 sessions a day for an average of 7 laps per session. This is a video showing a representative lap: https://www.2addicts.com/forums/show....php?t=1769652.

It may be difficult to see without a brake pressure bar showing psi values, but looking at the max speed into each braking zone and the speeds at each turn-in will show that Mid-Ohio is a track with only moderate braking demands. Yet the system on my 2 Series was more than maxed-out by those demands.

I'm easy on the brakes (less braking = a more-stable car, esp. at turn-in, and faster laps), and yet the temps of my rotors and calipers were much higher than the system was designed to accommodate. If I'd continued tracking at that moderate level, I would have needed to refresh my dust boots and piston seals at least every other year.

Please, forgive the digressions!

What I've been leading up to is this: a rotor with more mass combined with a pad with more mass (i.e., one with more metal in it, which the more heat-resistant pads have) will decrease the heat available to be sent into your caliper pistons and therefore into your dust boots, piston seals, and brake fluid. The longer it takes to heat-up the rotor and pads, the longer it takes to transmit that heat to the system's upstream and less resilient components. If you're demanding a lot of your braking system, a lightweight rotor may not be something worth pursuing.

Thanks for the response! I'm sorry to hear you stopped tracking. If you want to do anything serious, it gets real expensive, real quick! I already know this so if I were going to start tracking seriously, I would just get a dedicated track car. Something much lighter where consumables are cheaper and aftermarket upgrades are able to better cope with the lighter weight. While not exactly a track car, I still think the 228i feels more sprightly and better balanced than the M235i. You can get them to under 3000 pounds without too much work. I wanted to pick one up, but they're hard to come by in the spec I wanted.

I've definitely heard the argument that more mass can hold more heat and that makes sense. At the same time, I have to wonder why 2-piece rotors (any serious performance aftermarket brake system or the M2/M3/M4) are better since they always weigh significantly less. My guess is that the aluminum hats act as a heat sink to dissipate heat more effectively, which outweighs the benefits of having a heavier (more heat capacity), 1-piece rotor? What are your thoughts? Everything seemed to point to 2-piece rotors being better so I just bought those.

On the topic of seals, where would one purchase the brake piston seals? On RealOEM I only saw a "Brake Repair Kit" listed which only contains the dust boot. Do you have an aftermarket brand you would recommend?

Also curious, what pads did you use? Pretty sure some solid track pads meant for heavy cars that can handle sustained higher temperature would work well. Just need to replace pads/rotors often. FCP Euro sells the entire Hawk DTC line-up. The DTC-70's have a maximum operating temperature of 1600 fahrenheit, which is pretty darn high. I'm not doing track though, so the best pad I could find were the Ferodo DS2500, which should help a little bit.

It also has to be said that the brakes on these cars are honestly pretty decent especially compared to prior generation BMW's. I'm pretty sure the blue calipers on our 2 series are exactly the same calipers used on the M2/M3/M4 (without carbon ceramic brakes) but with different mounting tabs so they can fit properly to the knuckles and to accommodate the larger diameter discs. Similar type of deal with the 370mm calipers in the 3 series people retrofit. Only the M2C uses the 6-piston front and 4-piston rear calipers.

[ggggbmw]

Originally Posted by abcfgh

Front:
(listed for M235i, M240i, 335i, 340i, 340mm discs)
150.2903.32 - Blank, 2-piece, 10.00kg
Now for OEM Genuine BMW rotors:

Front 340mm discs:
34116792223 - Blank, 10.275kg according to BMW
34106797602 - Drilled 10.272kg according to BMW

Also, after looking at RealOEM, I discovered that the diagram for the M235i/M240i shows 1-piece blank 340mm rotors whereas for the 335i/340i the diagram shows 2-piece blank 340mm rotors. Yet part numbers are the same. This really begs the question....are the OEM rotors a 1 piece or 2-piece?

Links to RealOEM diagrams:
340i Front:
https://www.realoem.com/bmw/enUS/sho...diagId=34_2353
M240i Front:
https://www.realoem.com/bmw/enUS/sho...diagId=34_1949

The diagram on the RealOEM site appears to be wrong. The stock part #: 34116792223 , description says "Brake disc, lightweight, ventilated". The 'lightweight' part indicates it's a 2-piece riveted aluminum hat rotor. And that part # on any other site does show the riveted aluminum hat version.

Except for the 'looks cool' factor, I haven't found a better/lighter rotor for these cars than the stock blank lightweight version. Especially for the price. $300+ seems pricey compared to some typical sedan, but these are way better than something off an Accord.

The AP Racing J-hook slotteds with directional vanes would be cool, if they actually made one for our car, but they are stupid expensive. $1200+ for a pair. At 4x the price I'd just keep buying the stock rotors.

[jb-slow]

On another car I've had the drill holes crack, I just stick to slotted now

[mikeysosa]

I recently watched this video as I was wondering the same thing:

It's a small sample size, but it's worth considering. Also I have heard that drilled rotors are less durable. Personally, I would spend my money on something that has guaranteed benefits.

[BEM-S4]

FYI on the OEM dimpled rotors, they make a very annoying ticking noise. Maybe an engineer can give you a more detailed explanation but there's some airflow vortex whatever thing going on. It's worse if you're driving next to the middle divider on a highway and not as bad if you're out in free air, but it's definitely there.

[dradernh]

Originally Posted by BEM-S4

FYI on the OEM dimpled rotors, they make a very annoying ticking noise. Maybe an engineer can give you a more detailed explanation but there's some airflow vortex whatever thing going on. It's worse if you're driving next to the middle divider on a highway and not as bad if you're out in free air, but it's definitely there.

I don't think it's there when you drive with the windows up; or, if it is I haven't noticed it. I'll try lowering a window so I can experience this noise for myself.

[BEM-S4]

Originally Posted by dradernh

I don't think it's there when you drive with the windows up; or, if it is I haven't noticed it. I'll try lowering a window so I can experience this noise for myself.

Full disclosure I don't have them on my F23, just my F31. I ran that car with both stock 18" wheels and then 19" Vorsteiner 105s (pretty open design) and ticking noise either way. It's not the worst thing in the world, but it was loud enough that I noticed immediately after the brakes were installed and thought something was wrong until I figured out what it was.

It may just be the BMW dimple depth vs dimples in general. BMW also does not do directional so if you look one side of your car is pointed the opposite of the other. Bothers my OCD but was told for the BMW stuff the slots and dimples are mainly cosmetic.

[dradernh]

Originally Posted by BEM-S4

Full disclosure I don't have them on my F23, just my F31. I ran that car with both stock 18" wheels and then 19" Vorsteiner 105s (pretty open design) and ticking noise either way. It's not the worst thing in the world, but it was loud enough that I noticed immediately after the brakes were installed and thought something was wrong until I figured out what it was.

It may just be the BMW dimple depth vs dimples in general. BMW also does not do directional so if you look one side of your car is pointed the opposite of the other. Bothers my OCD but was told for the BMW stuff the slots and dimples are mainly cosmetic.

Yeah, I'd noticed that the rotors weren't directional. I'm guessing the sounds they make vary slightly depending upon the side of the car in which the listener is sitting.

[Maynard]

I get this too, VERY hard to hear with windows up, really only apparent if there is a barrier or guard rails to bounce it back with windows down. Not really an issue if you can keep from worrying that it is something wrong (sounds similar enough to a pad-drag rub that it took me a little while to acclimate). And I've put some serious wear on my rotors (factory MPerf dimple/slotted) without any cracking. Tracked them and ran my Ferodo's down to the nubs in the rear, so thinking they'll need replacement for next season, but no cracks. For me, I wouldn't go with drilled unless they were really high quality, like AP - if the drill holes don't mesh with a directional veining, they are just a cosmetic weak-spot waiting to fail. I've also hears that hard street driving is even worse than tracking for this, since it produces more extreme heat cycles (street brakes are seldom kept up to temp, so they go from cold to full hot with each hard stop).

[aerobod]

With "drilled" rotors, the only type that are functional and can tolerate hard use as opposed to just being for cosmetic purposes, are the ones where the holes are cast in, not drilled after casting at all. The ///M cars have had this type in the past, but they are typically multiple times the replacement cost of plain rotors.

[overcoil]

Originally Posted by WolfGTI

Wish AP had a kit for our cars that allowed you to keep 17's.

do u know AP and Brembo ar owned by the same parent company ?

was researching the BMW motorcycle Hayes brake caliper (leaking recall) and found out. Hayes had sold their 4+ wheels vehicle brake business to Brembo also.

[AleksanderSuave]

Did you end up seeing any benefit in switching to the zimmermann 2 piece blanks?