atb

Here's a pic:




I haven't mic'd any 993 rotors, but my impression is that the the N/A 993 is the same as the S4 and the 993tt is the same as the GTS, the only difference being the offset.

It sounds to me like the 993 rotors you got for Scott aren't 993 rotors. Look at my first pic showing the S4 and 993 side by side, they look identical to me.

mark kibort

I suppose you are right that the GTS , S4 and probably the 993 are the same as far as the hub design. also , the rotors I have have a 320mm diameter, vs the 322mm of the GTS or 993 . the point is, there is a much larger hubcentric ring on the inside of the rotor. Do the rotor set screws fit in the same locations? maybe that is your centering, and its working, but not optimal.

Now I wonder what rotors we got from the shop to experiement with. the are 28mm thickness and 320mm diameter. cayman? do you have 993 turbo rotors? diameter exactly the same as the GTS rotors?

atb

Yes the rotor set screws fit. (993N/A to OB hub)
I really think the S4 and 993N/A are the same except for the hub offset.

mark kibort

I agree, the S4 and GTS, and possibly the 993 NA and turbo, along with the 964T are the same, except for the offset, BUT, the inner hub diameter is way different than the 1984 hubcentric diameters. did the OBs change to the "S" as far as wheel hubs? there is about a 1/4" diference between the inner hub diameter fitting of the 84 rotors and the S4, gts,etc. you are aligning the rotors via the rotor set screws. Compression of wheel on the rotor via the lug nuts is not really enough to keep it in place, for long. eventually, that set screw will eventually break and the rotor will be off alignment. the wheel wont change, but the rotor will be off center. I dont know what the effects would be.
However, if it works, it would be a cheap way to run larger rotors on the OBs or the "S" systems.

are the 993 and 993T rotors the same and have the same offset?

mk

atb

From what I've seen, for the US spec cars the hubs are the same until they got ABS. I know the OB and S brake hubs are the same.

Don't know if the 993 N/A and 993tt have the same offset, if I pulled the front wheels off both cars I could measure.

Gotta be honest Mark, I'm still not following the hubcentric fit of the older rotor that you are concerned about. Are you saying that hole in the center of the 933 rotor is larger than the hole in the center of the older rotor?

928SS

I used 83 spindles for my big red conversion. made it a pretty easy job. had some issues getting close to the front rims with the tie rod ends. had to do a bit of grinding on the rotor, but it all clears now.

also used S4 calipers on the rear.

GlenL

Pic?

928SS

kinda fuzzy, not sure if they'll help much. I powdercoated all the new stuff in red, so I'd remember was wasn't stock, hopefully.

the custom front sway bar and mounts are pretty easy to see though.

Dave Roberts at 928 specialists told me what I needed re: spindles. he might have more pics on his site or have better details. also got the big red kit from him.

I used his kit spacers on the front also. seem to fit/work well, so overall I'd recommend it as an option for an OB.

I also used the gts dust shields in front.

GlenL

Looks cool. Ya got a lot of speed parts showing there. I need to get my swaybar mounts reinforced. Much easier before they're torn off, I'm told.

I was especially curious about mounting the Big Blacks to '83 spindles. I thought that was hard to do.

928SS

83's are easier, iirc. seemed to really settle body roll. now the car is flat when I spin through the wall of cones... lol

mark kibort

I dont think the 993 and 993tt have the same offset, but maybe they do, but the TT has the 322mm rotor, like the GTS with a different offset. Hubs are the same as far as inner diameter.

Now, here is the story of the hubcentric fit. Yes, the 84 and probably OB, have 154mm inner diamete rotors for the hubcentric fit. the GTS, S4 and probably all 993 variants have 157mm. this means that the rotors will be centered only by the centering screws. the Mercedes rotor of 330mm, will just about fit on a OB or 84 "S" set up. However its hub is 153mm. it only needs 1mm machining, with the 5 bolt holes to fit on the OBs. (the offset is somewhere between the S4 and the OB.

what happens when the S4 rotors are used on the OBs? does it hit the lower ball joint? I dont know the offset difference of the S4 and 993, but I think its only about .25". dont know if that is thick enough for an adapter plate.

mk

Fabio421

The S4 rotors do slightly touch the ball joint on an early spindle. I have fitted the S4 calipers to the 993TT rotors using adapters that I had made. I will make more if you want some.

mark kibort

Thanks fabio, but I think the question is what rotor to use on the OB spindles so that a simple spacer or plate can be used to just move in and out the stock "S" calipers. Its an EXTREMELY cheap upgrade for the OBs when ONLY performance is considered. question, how think as a minimum, should this plate be to not be dangerous. S4 rotor from OB rotor would be 17mm difference. that thickness plate would be strong enough. But, OB vs 993 being only 10mm, is that strong enough for steel or aluminum? And, if the 17mm offset of the S4 rotors being used on the OB makes them rub, then with 993rotors, there would only be a 7mm clearance to the ball joint. is that ok?

mk

GlenL

Mark, as an old-timer you should be clear on that "OB" is not "S" and that's especially important when talking about brakes.

Swapping rotors without moving the caliper further out from the hub (spindle) won't change braking force. That is a radial move so the pads are grabbing it farther out on the disc. The bigger rotor doesn't do anything by itself. I was curious about this because with the way S brakes mount it'd be hard to move them out from the spindle and really hard to move the caliper further out along the spindle (deeper into the wheel). If the disc had a bigger hat and the disc was further inboard then it might work. I'd still mant to see 1/2" or better so a meaty piece of steel could be used.

mark kibort

I think you totally missed my point. I could be wrong, but let me explain again.

OB or "S" systems use the same spindles, so I was refering to the inner diameter of the brake rotor from a hub-centric perspective. Lets talk "S" brakes.

1. The S brakes have a rotor with a 154mm inner, hubcentric rotor diameter. the part that fits on the hub.
2. The S4 brakes and that entire family of the same hubcentric rotor inner diameters (GTS, 993TT, 964T, S4, etc) have a inner diameter of 157mm.
3. IF we want to increase the rotor size as far as diameter by using S4, GTS or 993TT rotors, OF COURSE we need to move the calipers out to take advantage of the greater leverage. The reason that this mod can be so easy (as it is on the radially mounted S4 brakes going to 322mm GTS) is that instead of using spacers to push out the calipers radially, with the S, we can do BOTH, with the axially mounted calipers. We would do this with a simple plate the width of the two mounting holes axially on the "S" models. use bolts and bolt the plate to the brake caliper mounts . THEN, there would be two more drilled holes spaced radally outward about 1" or so, (whatever the diameter difference of the S brakes and the rotors you want to install). BECAUSE you are using a spacer of some thickness, you make that thickness equal the offset difference. (hopefully greater than .5") Now you have the calipers moved inboard by .5" via the space made of a strong steel or aluminum, and radially outward by the diameter difference of the old and now new , say 322mm rotors)

The only question is how thick that material should be, probably made of stainless steel. the rest is pretty simple. the only hard part is threading only 2 of the 4 holes on the adapter plate. on my post, remember, I said things need to be moved, "in and out". This meant, the spacer moves the caliper inward and outward, radially.

Is that a little more clear?

Mk