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  #1  
Old 07-29-2004, 11:19 PM
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one stop, 75-0, cross drilled vs. slotted vs. blanks

Hi,

I am thinking about zimmerman cross drilled vs. ATE power discs vs. balo blanks for my 91 BMW.

I am curious about your thoughts on this:

cruising 75 mph, need to make an emergency stop, which one will stop the best, assuming the same brake pad and tires (and that stopping power is not tire traction limited)?

I would guess cross drilled, then slotted than blanks. I think this is the case because in a single stop, heat dissipation and rejection (the main purposes of a rotor) are secondary to full friction and prevention of off gassing as temperatures get really hot. The cross drilled rotors will deal with this the most effectively, while cooling the best during the fact.

In auto-xing or similar racing, I believe that cross drilled rotors are the worst, because of their low mass which doesnt allow them to 'store' significant thermal energy from the conversion of the kinetic energy obtained in repeated braking. For repeated stops, I believe that mass is the most important, so that the heat can be adequately dealt with. Heat rejection (cooling) is secondary to being able to convert the kinetic energy to heat, and temporarily store it (in the rotor).

But Im not auto-xing, Im driving fast on the highway, and a tire retread breaks off, and the highway comes to a screeching halt. Braking must be maximzed, and the first thing to heat up is the pad. Off-gassing will occur, and this must be dealt with, so the drilled holes are best. Plus, in moderate city type driving, the rotors never get enough heat to cause the reduced mass to be an issue.

But thats my thought, and I hope that some experts can comment. After all, MB is not putting drilled rotors on their cars, and Id have to guess its for momentary safety in stopping, because most MB drivers arent looking for the 'bling-bling' nor are they looking to race or auto-x their cars.

Thanks for your comments,

JMH

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Old 07-30-2004, 07:59 AM
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For a single panic stop - my guess is that the pads and rotors won't heat up enough to worry about out-gasssing at all. The drilled and slotted rotors probably help during an autocross, where the brakes are continuously being heavily applied and heat becomes a formidable issue to deal with.
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Old 07-30-2004, 09:05 AM
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Ive heard that drilled rotors are poor choices for autocross because they devellop local thermal stresses that lead to cracking. In repeated braking, the rotor has to hold a lot of heat energy, and then release as it spins, until the brakes are applied again. This causes a heating and cooling cycle, that in conjunction with the drilled holes causes cracking.

Thats what Ive heard...

JMH
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Old 07-30-2004, 04:08 PM
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Drilled are far better than anything else, cooling, heat, gas and water evacuation...period. I raced Porsches in enduros for years with drilled Brembos and never had a crack. My blank rear rotors wrapped very fast however. Slots do little more than cut off pad material. For panic high speed stops, drilled will do a better job...especially in the rain. For 50 mph or less stops you can't tell the difference.
BMW, AMG, Porsche and Ferrari all come from the factory with drilled rotors not slots.
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Old 07-31-2004, 02:42 PM
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I believe all the OEM "drilled" rotors are cast with the holes, not "swiss cheesed" after the fact.
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Old 07-31-2004, 04:23 PM
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Quote:
Drilled are far better than anything else, cooling, heat, gas and water evacuation...period. I raced Porsches in enduros for years with drilled Brembos and never had a crack.
Yep, that'd be because they are not drilled. Porsche only uses cast holes. Drilled rotors are bad.....very bad!
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Old 08-01-2004, 03:56 AM
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Ate Power slots make some noise, very mild, but someone told me that they wear through pads quick. We shall see.

the zimmermans that i have for my 560SEL front brakes appear to have cast holes. So far I have no craking and I have about 10k miles on them.

for one stop, I think the blanks are better because they have more contact area with the pad vs the drilled (perforated) rotors. The slotted ones are kind of like razors coming by each time to eat the pad. Thats just what I think.
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Old 08-01-2004, 10:26 AM
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I believe the "blank" will stop the fastest in a 1 stop comparison. Increased contact area's my guess as to the reason. I doubt a lighter drilled rotor would make up for it's weight benefit since it's contact area's considerably diminished. Maybe that's why Porsche favors large brake pads, as does MBZ on their newer, drilled applications.

As to slotted rotors, I've been running them for a while purely because they look good. According to people I've spoken with in the brake business (EBC, Perf Friction, StopTech, Rofren) off-gassing is of little concern anymore.
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Old 08-01-2004, 01:23 PM
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I think the holes and slots still help with water on the brake disc.
__________________
Current Stable:
1994 S500 v140, 210k miles, white with grey.

Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold (rumor has it, that it has 750k miles on it now)
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 "300E" Amg Hammer 88k mi sold (it was really a 200, not even an E (124.020)
1992 500E 156k mi sold
etc.
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Old 08-01-2004, 06:20 PM
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Yep, that'd be because they are not drilled. Porsche only uses cast holes. Drilled rotors are bad.....very bad!


True, Porsche does cast their stock rotors so the holes don't weaken the molecular integrity of the metal. The new Carrera GT and Enzo use ceramic rotors that are drilled, not cast but that's a whole different world. After-market Zimmermans (which I use for racing) are all drilled with chamfered edges to relieve structural stress. As long as the holes don't cut into the veins of the center venting there won't be any cracking. I have seen cheapo (APC) drilled rotors on Hondas that have cracked, but I think that matches the cracked bondo on their bumpers nicely.

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