mark kibort

well if anyone remembers, the hub is just a disc looking thing with studs poking out of it. it does have little tabs on the edges for aligment, but the gap on the inner diameter of the rotor looks to be pretty substantial, so its not really centered by it. (2mm+ gap). So, it looks like the rotor is centered by the studs in actuallity. the Mercedes rotor ID is 2mm narrower than the porsche rotor, so it either wont fit, or will be centered better than the stock rotor. I just look and there is at least a screwdriver flat head thickness of gap between the outer edge of the hub and the I.D. of the rotor. in measurements, the I.D.s are roughly 153mm vs 155mm for the porsche rotor.

more proof that it not hub centered, is I just remembered doing a brake job and was rotating the rotors and thought they had some major runout, but it was just that that little phillips head screw was not tighten down, angling the rotor. If it could angle that much, its not hub centered by the flat , narrow hub edge.

so, it looks like it will work with minimal modifications.

the real question is, do the mercedes vents work as well as the porsche vents which are larger and less numerous?

mk

mark kibort

just took some more measurements. I was measuring the S rotors, not the S4 hub centering ID. the S4 rotor is even larger and I noticed a little machined raised area too upon closer examination. 153.5mm for S500 mercedes, 154.5mm for the 928S rotor and 156.5mm for the 928S4 rotor I.D. so, about 3mm smaller for the Mercedes rotor. However, there are little tabs on the hub that do the supposed, "centering". I was able to put a feeler gauge in there of near .060" no problem. There i at least 2mm of space on the edeg where this centering is supposed to happen, thats near 4mm of diameter larger than the hub. So, the S500 rotors being only 3mm smaller should fit.

are the hubs different for a S4 vs an S? (as far as diameter goes?)

regardless, i thin the only issuses will be drilling the new holes, and is the cooling vent set up a optimal a the normal porsche S4 style? Also, is this design strong enough to race with? Looks pretty beefy, plus, I like the design where the holes are not a numerous and further from the rotor edge.

Lizard928

Every S4, S, and non S rotor that I have ever removed has shown that the raised tabs on the hub have been in contact with the rotor offering a form of centering.
If you are saying the ID of the mercedes rotors is 3mm smaller this might be too much without shaving down these centering tabs alittle. This though will mean that one will have to always run the mercedes discs from this point forward. Centering via the bolts is not a good option for a street car as it could lead to the disc being slightly offset which could lead to vibrations at speed.

mark kibort

well, as it looks now, the tabs are NOT in contact with the tabs. (by about .060") BUT, im using the 965 rotors (964Turbo) , which may be different. I'm getting some GTS rotors in today or tomorrow and will be able to see, definitively, if there is contact. however, if there isnt, that is not such a big deal as the phillip screws locate the rotor where it needs to be, and once the lug are tight, the rotor is not moving so im really not too worried about centering from that perspective. the lug studs are 14.5mm and the rotor stud holes are near 15.5mm, so its not really centered on the lug studs either.

I certainly might not be looking at the exact point of contact. I wont know this until the rotors come off this next week. I do kind of remember during the brake jobs if the rotor is flush, it is very centered. so, im leaning to that you are probably right. But, it might have been after I tightened down the phillips head screws. Either way, Ill know soon enough.

Are the hubs the same diameter on all 928s?? I would think S4 and GTS would have the same diameter hub.

mk

heinrich

Easy .. just pull your wheel and rotor and try the Benz one.

heinrich

Btw is it any cheaper than a GTS rotor?

mark kibort

That can happen only after I drill some new holes. The good news is that Ill have some dummy benz rotors after tomorrow AM, so Ill scribe out the holes and I have a 15mm drill bit to cut the stud holes in the used benz rotor.

Cost? OH YEAH! MUCH cheaper!!!!! maybe half the cost! AND, 330mm rotors with better looking holes (and less of them)

mk

heinrich

you're not able to push the thing on there by hand?

mark kibort

Us 928 guys have studs! You must have a BMW or mercedes too!

Its not a big dea to drill some holes in the old rotors. Or, i can put some 1mm shims on there to see if there really is tab of the rotor to hub contact for centering. all some has to do is measure the hub face diameter. (at the tabs)
Now im starting to think those little phillip head screws are more valuable than we all gave them credit for.

jmccabe

Hey Mark,
This is exactly what I did. I welded up and changed the bolt circle dia. My "Electric" runs 190E Mercedes wheels

heinrich

ohhhh yeaaahhhh.....

svp928

Mark, 928 rotors ARE centered by the outside diameter of the hub- If the mbz rotor is smaller in the center, you will have to machine them out to fit the 928, or machine 928 hub to the mbz rotor...if you currently have a large gap there, then your rotors are being centered by the studs, and this is not good.
And, Cobalt is right- there are a lot of different types of cast iron- what Zimmerman uses may be softer than OEM..

mark kibort

I think that phillips screw would fix any misalignment. especially if there are two of them. There is no runout in my rotors, if they are not hubcentric. Its interesting, a you know I have the 964T turbo rotors. maybe they are not exactly the same as the GTS? Anyway, Ill check and see if the aligment screw is what is aligning the rotors. If not for the screws, the studs wouldnt even get the alignment close with the 1mm space for the studs, and you are right, that would be bad. Im probably not fully seeing the contact I bet. which means, these rotors will not work, unless they are modified slightly or those tabs are cut off the hubs.

I just found out something else with the rotors from mercedes. they have a a-symetrical vent system, yet there is NOT left and a right. Looks like porsche is not the only company with some bone head designs. You get these rotors OE or zimmerman in a set of the same part number. so, this wont work unless we can find the other directional side for this rotor, even if we could make it work.

Looks like its GTS rotors from now on until we get some hats made!!

mk

heinrich

What sort of hats do you run Mark? I have 2 sets of those but the offset is way wrong for the S4/GTS.

mark kibort

Im running stock GTS rotors, solid, no hats. I want a set of those Steve specials with the abiltiy to run the replaceable coleman rotors. (322mm)
Actually, I started with the hole'd version of the GTS rotors which are porsche OE, 964turbo rotors from 1991my

mk