Mike_964

I have some trouble deciding what rotors I should choose.
I was thinking about buying some crossed drilled Zimmerman rotors, but then I stumbled over this post Cracked rotor is that scary or what....
What is your experience with cross drilled discs and in particular Zimmermans?
The price for the genuine Porsche rotors and the Zimmermans cross drilled are the same, and Zimmermans without holes costs about 20% less.
I have been told that a genuine Porsche rotor is in fact just a Zimmerman rotor made especially for Porsche, is that true?

What would you chose? I should perhaps mentioned that I'm on the track 4-5 times a year, and the new rotors should of course be able to withstand the pressure.

Regards,
Michael

C2 1991

Wachuko

Front/Rear 79.00/70.00 OEM from VertexAuto... not much to decide from my end... I got all four...

99.00 if you want the cross drilled... I don't track my car... no sense in spending an additional 80-100 bucks for these...

Others that do will chime in...

Bill Gregory

Michael,

I'll assume you have stock 964 brakes on your 91 C2. First, avoid rotors that are literally drilled, as they crack pretty easily. Typically the failure mode on holey rotors is that cracks start developing, and if not monitored, crack to the edge of the rotor where it can self destruct. Porsche has a spec that cracks should not exceed 7mm in length, after which they call the rotor toast. Also, if the cracks reach the edge, the rotor is toast.

Porsche 'cross drilled' rotors are, in fact, not drilled, rather, they have the holes cast-in during the manufacturing process. These are more resistant to cracking, however, they too will crack over time. FWIW, I cryogenically treat my rotors for longer wear, which my experience has confirmed. Others may have different experience.

Some manufacturers that make rotors for Porsche also sell rotors with cast-in holes. If you wanted holey rotors, those would be the ones to get. I've heard several people say they don't get the longevity out of OEM rotors, as they do out of Porsche rotors, and my experience would mirror that. While Zimmerman, or others, may make rotors for Porsche, we don't know that the same metal or processing is used. OTOH, rotors are a consummable, like brake pads, and especially with track use, you'll use 'em up.

If I were buying stock rotors for street and track use, I'd buy OEM that have been slotted and cryogenically frozen. I use Diviersified Cryogenics, however, there are others that do that. Slotting allows gas/water to escape between the face of the pad and the rotor while maintaining the heat sink capabilities of the solid vented rotor. They aren't inexpensive, however, in my experience, they last longer than non-treated rotors.

pete000

I just did the Zimmermans on my C2 no track time for me, but I think they are also cast in holes and not drilled just like Porsche does it. So far they seem to work great and look cool.

Marc Shaw

I have the Zimmerman rotors too (cast in version as they do have drilled ones too) and, after 4 track days (Green group) and 6000 km, no cracks at all - Ferodo pads.

Marc

Chris M.

I've always heard that non OEM rotors squeak more. Is this true?

PC2

I have 30,000 miles including 14 days of DE on my Zimmermans and hairline cracks have been developing around the holes especially in the fronts. When the cracks gets worst I'll probably replace them with OEM or slotted.

Marc Shaw

I was under the impression it was related more to the type of pads used rather than the rotors.

Marc

Millemiglia

Yeah, me too.

If you use Carbon brake pads, then I guess you will have to get used to squealing brakes...

Alon

Jumping forums for a minute (993 forum is boring today)...

In my personal experience with Zimmerman versus OEM on my 993, performance and longevity seems to be the same (no issues with street and track time on my car) but the Zimmermans that I got for the front of my car sis not have painted hubs and have rusted. If you buy them and they aren't coated/primed/painted, do it prior to install.

Oddjob

Ive told this story a few times, so bare with me if youve heard this one before:

A few years back, I ordered a set of rotors for my 89 944TS from PAP (Porsche Audi Parts) out of Georgia. Used by to owned by a guy named Joe Cogbill, who was an IMSA Racer. Was one of the first discount part sources for p-cars that I know of, but I believe that PAP no longer exists.

Anyway, when I got the rotors, one was a factory Porsche boxed rotor, the other was a Zimmerman boxed rotor. And as mentioned above, the Porsche rotor had the zinc coating on the hat and rotor edge, the Zimmerman did not.

I used the car for DE events and quickly noticed that the Zimmerman rotor did not hold up as well as the Porsche rotor. The Zimmerman started heat cracking much earlier and much more severe than the Porsche rotor. I replaced the set as a pair when the Z-rotor was shot, but the P-rotor still looked useable.

I spoke with Bill Groshen of Cryo/Frozen rotors about this at a track event, and he said that he believed the same: the the Porsche rotors, even if manufactured by Zimmerman, were made to a higher quality spec than the generic Zimmerman rotors. I do not know if it is a casting process difference or a material/metalurgical difference.

Since, then I only will use the factory rotors. I order them from the discount mail order dealers that advertise 20-30% off of List price in Excellence or Pano, so they are not much more expensive than Zimmerman from aftermarket suppliers. I believe that the rotor will last at least 10-20% longer, so that negates the price difference. And to me, its worth the extra few bucks per rotor to not have to deal with rust/painting the hats.

But with that said, my reasoning is based on that single experience, and was several years ago. So I am basically offering unsubstantiated and obsolete advice!

Bill Gregory

Well, until you install 993 brakes on your 964, which require either a 928 front rotor or, if partial to holey rotors and you want the Porsche variety, then you get the 968 M030 holey rotor. Last I checked, at discount, they were running around $350 each! At that price, you consider an OEM 928 cast hole holey rotor with cryogenic processing, and including shipping back and forth, you still have quite a few dinero's left.

chancecasey

- Hey, how do you tell what the heck you are buying at Vertex? Their descriptions are terrible. Do you call in?

-Chance

-oops, I see they are a sponsor - sorry Vertex! What I meant was, the descriptions are very clean and minimalist. I'd also be more than happy to lend a hand with your site

Oddjob

I put a disclaimer on my post, basically acknowledging that my experience is not gospel, so take it for what it was worth.

The rotors I was talking about were for my 89 944TS which all came with the 928S4 calipers and use the 928 rotors. Solid Porsche rotors for that are under $100. And ironically, the larger 928 rotors are cheaper than the smaller rotors for my 87 944T (which are the same size as 964 rotors and calipers).

When I was quoted the 968 M030 front rotors (cross-drilled), which are a 965 part number, last fall, they were around $250 each from Kenn Young at Hennessey Porsche out of Atlanta.

Bill Gregory

Jim, you got my curiousity up, so I checked on the current price, and those rotors are now less expensive than I was quoted (and seeing the list, I may have locked that number in the brain). Although the Porsche list is $325, they can be had for around $225.