mark kibort

I cant find the thread that we had discussing the brake upgrade path for the old-buggers. Are there rotors that are near 13" (322mm like GTS) that can be hubcentric and fit over the older 928 hubs (correctly) . I havfe some old GTS, 84, and a 993 rotor and they all seem to have a different hubcentric center. the 993 is much thinner of a rotor.

Im going to have a set of plates made for Scot, as his little OB brake rotors are just a little too small for racing! Im thinking if we find an offset of the rotor that is enough to make a plate to move the calipers in to match the offset, we can help other old buggers run rotors that are 2" bigger diameter!!! (but not have to change the calipers)

anyone know any rotors from porsche that will work on the early car hubs?

atb

I think your stuck with the 993's. Any 928 variant will rub against the ball joint. The 993 rotor that I'm running I'm pretty sure is the same thickness as the 928, the hub just has less offset. Maybe a 993TT?, which would be the same size as a GTS?

S4 is on the left, 993 is on the right (disregard the bottom photo):

mark kibort

adam,

what i grabbed from the shop, might not have been 993 rotors, or maybe the are non-turbo, which are much thinner. But, the inner diameter for the hubcentric fit, seems way off. The OB rotor has a pretty narrow hub that it fits over. are you going to use the hubcentric fit or just rely on the studs on the OB to align the the rotor?

certainly, the 993 vs S4 is an other animal because the axial mount and offset would have to be used, while using just GTS rotors or 965 (964T) rotors only ned the spacer as I have shown.
So, what about the OBs? are there any larger rotors that will fix? is the bottom piture one of the OB rotors on the left?
If so, then a nice thick aluminum plate could be used, or steel to make the adapter.

mark kibort

anyone know about the hub concentricity comparison of other rotors for the pre-85s? since most of them are larger offsets, if there was one with the same hubcentric dimemsions, we could make a brake kit for them REAL easily with spacer bars with 4 holes .

mark kibort

does anyone know of any rotor that will fit the hub on an older 928 (OB's)? the ID of the rotor for the hub-centric rotor to hub match is differerent on an of the rotors Im checkng. since most offsets of porsche rotors are much more than the OB 928s, that should bode well for a bracket on the OB stock brake set up just to move them out and use the bigger, 320 or 322mm rotors.

atb

Where are the rotors different Mark? Are you saying the hole in the center of the rotor is different? I'm pretty sure the hubs are dimensionally the same at the boss around the outer bearing.

davek9

I did the GTS Big Caliper upgrade on Rob Budds '80, and only used the adapters and the rear calipers only required spacers (washers) to center the roters.
Don't know where he got the roters , e-bay I think.
I'll look for the pic's I took.

mark kibort

The hole in the center is off, but that is not an issue. the rotor centers itself by the 4 machined areas of the inner part of the rotor. the studs have a ton a slack around them, and have nothing to do with centering the rotors. the GTS rotors are the same as the S4, as they are interchangeable. But, on the OB hubs, they are a different size and require a rotor with the same inner diameter machined surface.

Interestingly, i found a rotor that is real close. remember that 330mm rotor I was trying to see if we could fit on the S4? It needed bigger radiused stud holes, because it came from a mercedes but the inner hub was way off (too small) But, the OB inner diameter is only 1-2mm difference at the hub, so it could be machined lightly and new holes drilled. Then, a 17mm thick plate could be made with 4 holes, 2 threaded to remount the OBs calipers out the distance of the smaller diameter OB rotor compared to the 330mm mercedes rotor.

GlenL

Mark, you confuse me.

Is this a true OB? Or are you talking about S brakes? Why are you looking into this when a racer can go Big Red or Big Black?

Changing the rotor that requires moving the caliper should be a class violation, if the rule writers or scrutineers are at all clever, if that's what you're looking at.

atb

Mark, can you post a pic of what you're referring to?
Last weekend I ran the 993 rotors on OB hubs all day at the track with no issues.

danglerb

Normally the whats an OB is amusing, but are we talking about 79 hubs?

mark kibort

Sorry, I forget that the OB really means the OB's! (ie 79s) So, yes, scots is a 82, so "S" brakes.

What I have proved is that you dont really need Big calipers, when the S can be quite adaquate with the right pads and properly sized rotors (ie larger)

I think all brake upgrade kits that have adapters are legal, but I havent looked at the rules in that area that closely. BUT, if it is legal, all you need is a steel or aluminum spacer, over .5" thick with the proper holes drilled (4) and tapped (2).

330mm rotors on an older 82 would be quite nice for racing, even with the single piston calipers. by the way, single piston calipers are still on some VERY fast cars only a 100 -200lbs lighter with a heck of a lot more power.

Mk

mark kibort

So,you ran 993 rotors on OB (79) ?? is the hub the same diameter where it fits the rotor as a 82 (S)?? did you know the rotors are hubcentric at the hub?
that hubcentric part is quite a bit smaller on the non S4 hubs and rotors.

wouldnt the 993 rotors (993 street or 993 turbo?) have the same hub sizing as the GTS, but with a different offset?

by the way, if the 993 rotors are different than the 993 turbo, are they also different thickness? Im wondering as I got a set of used 993 rotors for scot to test, and they are thinner (28mm vs the 32mm of S4, GTS, 965T, etc). the hub area, or tollerance area, was not the same as well.

Im wondering if you installed the rotors and just got lucky with the studs centering the rotor.

can you take a few pics??

Ill post a few of what im talking about

GlenL

Righto on S working well.

It's hard for me to wait long enough before braking with Hawk Blue pads on S brakes. The things hauls down so quickly it's hard for me to judge where to brake and to have the nerve to wait and go that deep into the corner.

A bigger rotor would allow the pads to be mounted further out which would get more torque on the wheel. That'd require some sort of adaptor.

mark kibort

first problem is the hawk blues. kind of like pagid oranges. they are good, but not great. HT-10s or pagid blacks would get you more confidence, but the diameter is just tiny compared to most any cars brakes these days.

the adapter would only be as thick as the offset difference and then 4 simple holes in a plate moving the mounting outward to the distance that the rotor is larger in diameter

mk