'79 930 - Brake refresh in progress

[jwasbury]

I decided its time to address the brakes on my car. Normal everyday braking is fine, but with hard use (spirited street driving or on track), I'm getting a lot of pulsation felt through the pedal and steering wheel. My '79 has the original 917 style floating rotors upfront. The old front rotors mic'd at 30.6mm (new spec is 32mm and wear limit is 30mm). Question: Do you think its possible to have these cut on a lathe and get some more life out of them or is there not enough material left? Another fun fact about the rotors is that they weren't floating anymore (due to brake dust, or perhaps corrosion build up). I had to knock out the ring with a mallet. Perhaps cleaning up the mating surfaces of the old rotor would allow the assembly to float properly again? If the old ones can't be reused, no biggie...I purchased these replacement rotors from VCI: Bought a set of Pagid Street/Sport pads to replace the "metal-masters" that are in there now from PO. Another question: does anyone reuse the lock tabs for the rotor bolts? And another question: I noticed on at least one caliper that the pad thickness was different on either side. I suspect this is a sign of a sticky caliper piston. Visual inspection shows the pistons look fairly clean, no sign of corrosion. I've not noticed any unusual braking behavior in the few years I've owned the car. Is a caliper rebuild required, or could I "exercise" them a bit and see how things go? Rear rotors mic'd at 27mm (new is 28mm and wear limit is 26mm). Considering running these of should I just go for freshness all around? Finally, some picture of my solution for pulling off those pesky dust covers on the front hubs using my handy craftsman puller and a 2" muffler clamp:

[cellison]

timely post for me as i may be doing the same thing soon. regarding the pad wear, i had the same thing on another car a while ago. the caliper/piston looked fine so i just stroked it a few times, cleaned everything well and made sure the pads were seated properly. the new rotors yo uare puitting, on,arethey ready to install out of the box orr ned some >prework"?

[jwasbury]

Charles,

The VCI rotors will bolt up to the "floater rings" just like the factory ones.

[fredmeister]

Typically when a rotor is resurfaced they remove from 0.010-0.020 inches per side to clean up the surface and insure flatness again. It really depends upon how badly the surface was gouged and warped. Assuming they removed 0.010 inch per side then you reduced thickness by 0.5mm or 0.020inches which puts you pretty close to the wear limit on the front rotors.
Plus you need to check to see how bad the cracking is between the vent holes in the stock rotors. Once they connect between adjacent holes or especially to the outside rotor diameter surface they are finished. This did mine in before they wore too thin. I asked for it with all the track days I was doing, not looking for excuses.
The VCI are nice. Slotted is a better approach to longevity.
And I reused my lock tabs....seen no reason not too if they still bent around the nuts.
What are you using to lube the mounting tabs on the floating hub when you reattach the rotor?
Uneven pad wear is hard to avoid, but theoretically should not happen, cycling the pistons will definitely help things.
Do you know if they sell rebuild kits for the 930 calipers (ie seals and what would be a dust boot)? I have not seen it on Pelican.

[jwasbury]

Originally Posted by fredmeister

What are you using to lube the mounting tabs on the floating hub when you reattach the rotor?

Are you supposed to lube? I would think putting lube there would be a bad idea...it could get on the rotors. Also it would attract a lot of dust/dirt to build up so that eventually they would stop floating properly, no?

[fredmeister]

Originally Posted by jwasbury

Are you supposed to lube? I would think putting lube there would be a bad idea...it could get on the rotors. Also it would attract a lot of dust/dirt to build up so that eventually they would stop floating properly, no?

thats why I asked, if there was some dry type lube that you knew of that would prevent corrosion yet not effect the rotor surface.
Also make sure to remove the dust shields while you are in there. They prevent cooling air from getting to the rotors and if you do any DE or track events removing these makes a big difference in cooling the rotors.

Do you know if they sell rebuild kits for the 930 calipers (ie seals and what would be a dust boot)? I have not seen it on Pelican.

[jwasbury]

Originally Posted by fredmeister

Do you know if they sell rebuild kits for the 930 calipers (ie seals and what would be a dust boot)? I have not seen it on Pelican.

I don't know. You could call if you can't find it on the website...I'm sure you can order the individual parts.

[onboost]

Originally Posted by fredmeister

Do you know if they sell rebuild kits for the 930 calipers (ie seals and what would be a dust boot)? I have not seen it on Pelican.

Available through the dealer network.. that's where I've bought them in the past.

[fredmeister]

Originally Posted by onboost

Available through the dealer network.. that's where I've bought them in the past.

thanks for the info.

[b5aar]

Reading the thread reminded me of some good technical white paper/articles on the Stoptech website.

No affiliation.

StopTech : Balanced Brake Upgrades

[mooney265]

I got mine from auto atlanta

[jwasbury]

In typical fashion, the job took a few twists and turns but its done and the car is stopping with greater authority than it accelerates.

First off, I went to mount up the VCI rotors and found that they were just a little too thick in the center section where they bolt up to the hats. Just thick enough that they would not float properly. VCI offered to have them machined to correct the issue, but I decided to return them (VCI refunded my $ and were very good to deal with). The guy at VCI also provided me with some very important info about these 917 style floating rotors which I haven't seen anywhere else (wanted to acknowledge this, good service).

I was able to source OEM Zimmermans for $249 a side, which is a very good price.



The important info I learned is this. When you assemble the hats and rotors, the lip side on the ring (see photo below) should be facing up (not against the rotor)



The bevel side of the ring should be face-down (against the rotor)



It almost looks backwards (and mine were assembled lip side down when I pulled them apart). Apparently with the lip side down, the rotors will not float. According to Doug at VCI, this 'feature' allows you to fix the rotors for turning on a lathe, or just if you don't want them to float.

Anyhow, once properly assembled on the hub, the rotors do float noticeably and make a racket when you spin them.



Fresh rotors on the rear



All back together now and the brakes are sweet!

[Helmsy]

I have some replacement Zimmermans on the way and does anyone know the benfit of floating vs fixed (ie having the lip side up or down like Jacob mentioned.)

Does anyone know the torque value for the bolts and does anyone know if you can still buy the lock-tabs and what the part # is?

[sundevil64]

Originally Posted by Helmsy

I have some replacement Zimmermans on the way and does anyone know the benfit of floating vs fixed (ie having the lip side up or down like Jacob mentioned.)

Does anyone know the torque value for the bolts and does anyone know if you can still buy the lock-tabs and what the part # is?

Torque value is 10 Nm or 7.36 ft #'s

[Alex973]

does Zimmerman make gas slotted front floating rotors?
thanks

[Iciclehead]

I have 930 rotors and brakes on my 911S, very similar to your set up. Early on, I had significant problems with a pulsating brake feel, to the point there was significant fore/aft motion felt in the cockpit.

Had the rotors checked for runout, disassembled the whole thing a few times without material improvement. Finally one of the old mechanics at the Porsche dealer felt that I may be getting some pad build up on them as I was (typically) using the brakes lightly in traffic. He suggested serious and very hard braking in a set, enough to get everything good and hot and then let the pads do the cleaning as it were (I had tried different sets - Pagid to the current Mintex).

Well, hallelujah, it worked....so whilst I would not suggest that is your problem, just for guys who have the cross drilled rotors, if you unexpectedly get pulsing - it may be resins bonding rather than a warp in the actual disc.

This was supported by an article published by Stop Tech (not sure if they are around anymore) that gave detailed analysis of causes and remedies for pulsing brakes.

Dennis

[jeos]

is there not some who have an assembly drawing so we can see how the disc with lip must face, because I'm not sure that the lip facing away from the brake disc to be liquid.

[JimmyT]

[aseem]

This is surprising. I had no idea that the ring is supposed to be mounted with the lip away from the rotor. Mine was originally mounted with the lip down against the rotor, and thats the way I have always assemled them too.

Is this worry about your thick center section, and clamping with lip towards the disk and thus making the disk non-floating a real problem? Isn't the forces involved, when the disk heats up and expand, great enough to overcome tha clamping force of the bolts?

I had to replace the original hats with a hat and ring set with T-locks from VCI. Mine were badly corroded (steel bolts in alu hats provide a good ground for corrosion) , and some of the square tabs had fallen off. I went from quiet brakes to very noisy since these hats provide more float. I thought maybe I could grind down the T-locks a little bit to provide less play for the disk.

As for lubing, I used copper spray to try and quiet them down, and reduce friction, but they are just as noisy. Maybe copper paste?

[jwasbury]

Originally Posted by aseem

Isn't the forces involved, when the disk heats up and expand, great enough to overcome tha clamping force of the bolts?

I think that's a great question. I tend to agree that the rotor should probably still be able to expand and contract separate from the hat, despite the clamping force. Since the hat is aluminum, you'd expect it. I suppose it might be more of a "semi floating" with the lip side down, as opposed to "full floating" with the lip side up. Cast iron floating discs that were standard on some of the Ducati motorcycles I used to play with were noisy like these 930 set ups. Regardless, next time I am messing with my 930 rotors, I will be flipping the backing plate over to quiet them down...it's almost unbearably noisy going down the road.