xschop

Cool. How much of that caliper bridge had to be removed? And I'm not building spindles but billet hubs as these 928 hubs are cast alloy like the 944 hubs and I've seen a few broken/cracked hubs on those. Avoid the 86T hubs/spindles like your mother-in-law LOL...

mark kibort

the stock 928 stuff is steel (cast iron and) its not indestructable, as anderson has cracked/broken one, but that was ONE time in 20years of his racing. (500hp, running 335 slicks, etc) will billet be stronger.

now, the caliper bridge is another challenge. the good thing is , that from your picture, it looks like just steel fingers that guide the pad ends. But, if you look at it and understand we are talking about a rotor that is 330mm vs 282mm, thats near 50mm!! so, the radius is 25mm greater! (1") got to do a little math here to figure out how much further out the arc has to be at about 75degrees of coverage over the rotor. (the same calipers on the new larger rotors). so, on the S4, we only needed to grind down 2mm at the edges about 1" inward. I would suspect, that going from 304mm to 322mm, it would be at least double what I did on the S4.

xschop

Post pics of the caliper bridge mod when you get there. The large Diameter piston can be cross-drilled like I said and you have yourself some performance out of that caliper.
The Spindles are CAST steel and the hubs are all CAST alloy. Did that guy break a spindle shaft or the alloy hub? The T6061-T6 hubs will be at least 6 times stronger than the cast hubs and take alot more heat without expansion like the factory hubs are prone to do. Racer's Edge builds a billet hub for the 87-up 944/968's because of this reason.

mark kibort

I still have a little work to get the thing mocked up. I dont have the S, and have to use Scots to build up the parts. what do you mean about cross drilling the pistons. what does that do?
hubs are cast alloy, that is right. I was thinking of the spindles. Im trying to remember MA's broke . i think it was around the alloy area, all the way around.

xschop

Drilling the perimeter circumference past the piston seal will act as a radiator and heat stop from the back of the pad into the piston which boils the fluid. If you have a 90* indexer on your mill I have found that 16 to 20 holes evenly around the perimeter does the trick. I've personally tested a Hayabusa with pistons done this way at 190-0 without any brake fade using off the shelf pads.
I'm not a track racer, but I have cracked a front hub on an 88 944 that I owned being aggressive on the streets. Luckily I was only doing about 60 around the corner and the wheel instantly went into wobble mode and I was stranded for an hour. I would hate to think what 100+ would have done. Check out the posts of the 944/968 racers and there are some hub failure pics there, scary.

xschop

I'm not at the shop today. I was going to double check the 928 piston with a magnet to see if it was stainless. I would think so. Here's the best pic I could find of a piston done this way....

http://www.k-bikes.com/forums/showth...3&page=2&pp=10

http://www.bing.com/images/search?q=...M=IDFRIR&qpvt=

How do you post a thread link here? These are a few hubs that have failed. Just like mine the stress fracture seems to always start at the inner bearing race seat. Cast alloy and heat cycling are bad ideas, couple that with bigger braking forces...bad juju.......

mark kibort

Sure, all good ideas, but hardly nessesary. Like I said, no on on the planet is racing more than Mark A, Joe F , Scot and I with some pretty powerful 928s. Anderson is the only one that has had a failure, and we dont know what caused it. it could have been an impact, which is more likely than a heat cycle issue. and yes, Anderson broke the hub, just like you have pictured. it was vibrating like a warped rotor and he came in off the track to see it almost ready to completely fall apart.

the S brakes done boil fluid, neither do the stock S4 brakes, which the same calipers and pads are used in F4 systems. sure, every manufacture has a different design, but with racing fluid, and a skilled foot, you will never boil fluid, unless you are driving a track like road america in the summer. The point is, Laguna is notorious for burning up brakes. I laugh driving around the paddock, especially with a novice group with all the cars running 10 seconds slower than us, but having "boiling fluild issues". I can boil fluid, or get fade if I want to. All I have to do is abuse the system.
The key point here is that the S brakes are very capable, but have been fitted with much too small rotors, that put the heat (near 500- 700f) much too close to the hub. once you bring the heat outward a couple of INCHES!! the heat can dissipate better. you get better leverage, more of the energy goes up in stopping the car and less in heat. much more efficient. the key thing about making brakes not fade or in extreme circumstances, boiling fluid (which personally i havent done in over 15 years since my DE days ), is to use them in a way that doesnt over heat them. I raced Scot 82 5 liter at laguna with S brakes with big tires and wheels. the brakes were a little less grabby, and faded a little more than my S4 brakes, but for a 30min race it was driveable. certainly, with much larger rotors, the system would work much better. the key thing to do to fight overheating and fade , but more importantly, uneven wear, is venting the braking system with air duct work. this will pay HUGE dividends and is certainly much more effective than radiator'ing the pistons.
race pads that are not used down to the backing plates, also help! (I toss my pads at 5mm)

anyway, Ill start mocking up a bracket soon and have one made.

xschop

If you know anything about metals, sand cast alloy is the worst of the worst as the sand pores left after casting are riddled with micro fissures. Not only heat cycling, which exaccerbates the grain structure wekening, but everyday fatigue cycling is what destroys cast aluminum parts. I've machined replacement parts for cast alloy brackets that wear out due just to static loads. It's amazing what these thin cast hubs endure, especially when under heavy load in a turn and then you apply the brakes. At any rate, I will have billet ones available for the 928 soon.

slate blue

If that is for the early hubs that will be excellent! Also can make sure that the O.D where the ABS ring goes on the later hubs is the correct diameter so these can be fitted. Early being 84 MY that is.

Cheers

Greg

mark kibort

dont know much about metals. (Just the high level stuff).
are the hubs any different on the S4 vs the early cars?

xschop

What year did the 928 start ABS? I assumed 86 from all the research I've done here. I have a verified 85 at the CMM and there is no ABS ring. I decided I will start with the 79-85 hubs because the spindles are stronger because the outer bearing lands are wider O.D. I asked a question weather the 79-85 years change offset. This I'm not 100% sure of but 944 series are all the same for the Early hubs, then in 87 the hubs changed offset and rotor mounted on front, 86T being the red-headed step child that Porsche haphazardly threw together to get off-the shelf big brakes to fit and made them the weakest of all.

smiffypr

UK 928s got ABS for the '84 model year.

slate blue

There is early and late ABS and they are different, although I haven't seen the early ones. My point being that on the early non ABS hubs they are not of a sufficient diameter to machine to then fit an ABS ring, they are just shy of the right size. So my point being while your in there why not make able to fit a ring easily, this will also be good for traction control too. The hubs between early and late are quite different and different in height also.

Cheers Greg

xschop

*****My point being that on the early non ABS hubs they are not of a sufficient diameter to machine to then fit an ABS ring****

You answered my next question and confirmed my suspicions about the majority of the Late 944/ 968 M030 hub failures. I had an in depth conversation with a friend (master machinist 36yrs) and we agreed that the Late 944/M030hub failures were happening at the inner bearing race seats due to the weakening of the cast hubs where an off-the-shelf hub is taken and machined on it's outter perimeter for the ABS ring land. We came to the conclusion that the failures wouldn't be as frequent if a new hub was recast with a larger diameter base and then machined for a larger I.D. ABS ring. I don't want to hijack this big brake thread anymore than has been so I'll leave it at that. I will start another thread soon and get more input. The ABS ring land can be retro-fitted. I need to know it's I.D. and they are an interference fit so I would want a hub and ring in hand. Maybe it's the same as the 944/968 one I have on hand. Thanks guys.

mark kibort

so, to summarize. I need to make a squared off piece of steel. (any grade suggestions) that has the bolt spacing of 104mm and those bolt holes that meet the existing spindle holes need to be 12.5mm diameter. the thickness of the plate will be 9mm to adjust for the rotor offset for the cayanne rotor vs 928 early rotor. then, the caliper bridge will mount to it through some threaded M12- 1.5mm pitch holes. the plate will have to have a big curved area cut out to make room for the caliper back piston area.

anything else?