Drilled rotors on rear wheels of CSi

Catbert

Catbert

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My 1975 CSi has disc brakes on all corners-- a good thing. I'm curious why only the rear rotors are drilled. My understanding is the 1975 CS cars all came with 4-wheel disc brakes. This seems like an aftermarket upgrade. Just curious. I posted a pic of the rear drilled rotor and the front non-drilled rotor.
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Does seem odd. Rotors are drilled to allow gases formed at the pad-disk interface to escape, so the pad doesn't sort of float on the gas layer instead of touching the disk. Slots serve a similar function. It is commonly understood that front brake son a car do about 60+ percent of the stropping work, so it seems unusual to drill the rears and not drill the fronts.

Could be that the previous owner liked the look (I do) but either never got around to getting drilled front rotors, or couldn't find any.

Stock is non-drilled. Walloth sells them in both styles.
 
Lots of people think they are an improvement so lot of auto parts stores carry them. When inventory of other parts gets tight and the auto parts store sees nobody is buying the fancy drilled ones, prices fall and they become the least expensive option.
 
sfdon said:
drilled rotors are great until they crack and you can’t stand the noise they make.
Completely unneeded on 200 hp car driven on the street.

Just one man’s opinion!
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Completely agree. Solid rotors are more robust and dissipate heat better. Form follows function... most of the time. ;)
 
sfdon said:
drilled rotors are great until they crack and you can’t stand the noise they make.
Completely unneeded on 200 hp car driven on the street.

Just one man’s opinion!
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My 360 hp heavy Audi SQ5 has solid rotors... So the threshold for real added value is probably pretty high!!

The RS6 Avant with 550+ HP has YUUUUGE drilled rotors... So the threshold for Audi is somewhere between those two...

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Drilled rotors are garbage! They should never be drilled after casting, the drilling weakens the rotor and makes them prone to warpage and stress cracks. Done properly, they will be cast with the holes in them and later machine finished with the holes being chamfered. As @ScottAndrews mentioned, there main purpose is to eliminate the gassing off of the pads in high heat and stress situations (race). Almost all high end rotor manufactures now slot or groove their rotors.
 
Agree a bit unnecessary on E9's. I used the original supplier to BMW called 'ATE' for the Calipers, Servo, Master Cylinder, Brake Discs & Brake Pads front and rear on my E9 with no noise or issues and it has really good stopping power.

Completely off topic - Here's what the front of my 600bhp 12cylinder Bentley Bentayga brake discs looks like which was at the time the largest brake disc fitted to a mass produced passenger car.

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Here's what the front of my 700bhp Mercedes S63 brake discs looks like

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The Bentley Brakes are always noisy and the stopping power terrible, the Mercedes Brakes are silent and stopping power is excellent. o_O
We trust Mercedes & BMW more than the Volkswagen VAG cars in regards to everything. Never go with aftermarket anything on brakes, only what the manufacturer uses - you should then never have a problem. Last time I checked, it was all still available from BMW.
 
Drilled rotors have their place in this world: primarily on race cars and super high HP street cars that actually need them. On a CS coupe, its overkill. But it looks cool! So there's that :)
 
I am the technician, but my wife is the real hot rodder and owns one of the very last 2026 RS6 Avants. It came with the factory lightweight 22" wheels and these massive brakes. I just took the wheels off to ceramic coat them since the brake dust is horrendous. The calipers are ridiculously huge!
I am not looking forward to the brake job in 10 years. I think(?) that there are 8 "mini" pads on each front caliper.
 

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CamayroZ said:
I am the technician, but my wife is the real hot rodder and owns one of the very last 2026 RS6 Avants. It came with the factory lightweight 22" wheels and these massive brakes. I just took the wheels off to ceramic coat them since the brake dust is horrendous. The calipers are ridiculously huge!
I am not looking forward to the brake job in 10 years. I think(?) that there are 8 "mini" pads on each front caliper.
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Hard to tell without a comparative scale, but that’s about a 22 inch wheel. The calipers are about 16-18 inches in diameter
 
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