Brake guys, talk to me about turning rotors and runout

Steve, I remember reading here somewhere that it's rare for these rotors to need this. You might want to throw a gauge on it first to see if really needs it or if something else isn't awry. I will look for the thread and report back.

Found it:

http://www.stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths

original thread:

do search: brake+rotor+turning then go to warping rotors

 

That's correct. Turning on a lathe is a tad simple and can result in a tiny spiral on the discs that could put unexpected forces on the pads; although I never had issues with that. Perhaps the fineness of the ground has influence.
That stoptech article is indeed great and led me to change the things they mentioned; I bought better pads than before, bedded them properly and changed my driving style to not sit on the pedal. I never had an issue with juddery brakes again.

Not sure what you mean by an issue with runout? As long as the shop does not remove too much material and the minimal thickness of the discs is not exceeded, just put them back on and re-bed the brakes.

 

So after reading this what do you think about Blanchard grinding?

http://www.f-body.com/forum/showthread.php?t=9387

 

My personal experience from repairing hundreds of road cars over "several" decades is this; Metal brake rotors sometimes develop lateral runout, whether this from invisible deposition of pad material or actual deformation of the metal I cannot definitively say but the net effect is the same and I call it warping.
Remachineing by turning on a lathe will bring them true again but the thickness is then uneven as more material has been removed from the high spots. The overall thickness will be constant but the relation between the braking surface and the cooling vanes is altered.
Removing material in any case diminishes their ability to withstand heat and doing it unevenly as described here makes them even more prone to further "warping". If your rotors have "warped" from hard use and not a corrected malfunction I'd expect them to do so even faster after "turning" with similar use.
As far as the spiral grooves left by turning go this is essentially a non-issue if the proper feed rate is used when turning. A quick scuff with an orbital sander will remedy the effect in any case.

 

FIrst things first I would see what is causing the vibration. It may be the rotor or it may be the caliper.. Plus once you have the rotors off you'd be nuts not to replace the pads at this stage. The lathe will tell you if she is warped from heat. Plus when you take material off the rotor will be able to retain less heat and it would probably return. I'm not big on cutting rotors.. As long as they are straight, when changing brakes I take a scotch bright pad and scuff them up.. then place new pads and I am done.. If the rotors are warped or have a crease line from worn pads (metal to metal) I would turn them maybe once if I knew for sure that they were never turned.. If the rotors were cheap I would just replace. Usually when I check my tire pressure I check brake pads for wear just to keep track..

R

 

If the rotors are warped enough to need to worry about such runout differences, I'd replace them!
Not sure about shipping, but for these prices I would not mess about:
http://www.superformance.co.uk/a-308/brakes.htm

 

So it seems the take home here is to make a good diagnosis first as to where the problem really lies because removing material is not absolutely a good idea. Would you all agree with this conservative view? If so what are the causes of a vibration in braking?
1) warped rotor
2) DTV
3)?

Quote from EBC website:
Many drivers confuse rotor warping, or warped rotors, with a condition called DTV (disc thickness variation). DTV occurs when a vehicle brakes are serviced and the brake rotor has runout in excess of 0.1mm or 0.004". This runout causes the pads to contact the rotor when the vehicle is being driven "off brake" and after 3-4000 miles, a thin spot develops on the rotor. The rotor runout itself does not actually cause a vibration. It is the effect of the thickness variation of the brake disc or brake rotor under braking that causes pulsation. Even small thickness variations such as 0.01mm will exhibit itself as pulsation. A warped rotor is a very rare occurrence and in 30 years in business, we have seen no genuine cases of warped brake rotors.

 

Although the repair process is fairly obvious, the better question is why? Also, why just one side? As there is huge liability involved in a repair this type of repair, I'd want to know precisely what caused this occur, as brake rotor-related vibration on the 308 are pretty unusual.

Hypothetically, I'd consider looking at:
-What is the hub run-out?
-Do we still have the "coming on 30 years old" rubber flex lines, among other things...
-What happens if we give someone the rotors versus the rotor/hub assemblies to machine

This number is not Ferrari-specific. Total hub and rotor run-out should not exceed 0.004"

If this were my car or a client car, I'd:
-Disassemble the front hubs and do a proper wheel bearing service
-Replace the likely original rubber flex hoses
-Install new OEM rotors
-Install either Hawk or Porterfield R4S-compound brake pads
-A proper flush and bleed of the brake fluid
-Inspect the rest of the front end for possible collateral damage.

Just sayin'....

 

I don't. On a flywheel or cylinder head, sure...Total overkill and potentially dangerous on a brake rotor.

 

I ordered the Porterfields ("BEST : PORTERFIELD AP04S "R4-S") in prep for doing the same but I think I read somewhere after I ordered them that they should have been ordered with something else otherwise "they will just be paperweights". I can't find the thread but you might ask around to find out. I ordered the one's on the 308 Xref list and am keeping fingers crossed that I will be okay.

Does anyone want to jump in here and assure this as being correct or admonish for getting the wrong part?

http://www.topbrakes.com/carSeriesDetails.php/6787/Street+Brake+Pads/PORTERFIELD/Porterfield+R4-S/1985+FERRARI+308GTB%2C+GTS+4+VALVE

 

Yes according to the Porterfield website. That's what I ordered also. Is there some sort of shim or clip that can be ordered with these also?

 

The shims should be on the existing pads.

R

 

Thank you. I did read that there were some other little parts then. Whew, I thought the pads would be paperweights.

 

A dial indicator is a very inexspensive item these days. I'm just sayin diagnose first and buy last. Lost

 

If the flex lines are still the original ones they're way overdue. In fact, they are a likely cause of the problems to begin with, next to the calipers themselves.
On my cars the hoses get tossed at 10 years. Since you're probably going all-in with new rotors, pads and flex lines, service the calipers too (new rubbers for these are cheap and they are bound to be gummed or even rusty under the boots).

Last, seriously consider rebuilding or changing the master cylinder as well. I once obtained a car that had gotten a complete brake service, only to have NO brakes for a few pumps a few months later. This with absolutely zero warning, it very nearly got me into a serious accident. It turned out the brakes were serviced but the MC was not. A niece of mine had the same a few years later in a similar event with an unrelated vehicle that ended a little less fortunate. In both cases the master was the problem, and in both cases it was relatively short after a brake service that involved bleeding the system, possibly because the master travels in a different range than normal during a brake service. No external leaks, it just did not build pressure for a few times and then mysteriously 'healed' itself (waiting for another moment to strike). MC was in both cases approx. 12 years old.

 

this is 100% spot on. warped rotors will quickly warp again after turning. might be time to replace.