Sanspistons

This one's for the experienced members (of which I am NOT!!!).

I'm about to redo the brakes on my '85S with GSL rear disks; I have a few spare disks hanging around and had some problems turning them correctly.

-So far every rear disk I've turned is warped to Hell and back. Normal for this axle?

-Every disk required machining down to the minimum thickness to correct even if it was almost unworn otherwise; also normal?

-Am I setting these up correctly? I've been using a solid hub-centric setup with a locating cone (like it was a front disk). Should I be using a spring/double cone like a drum instead? Nobody I ask seems to know any better than me...

The set on the car now are the new ones I bought when I installed the new axle. I'm kind of afraid to chuck them up and grind away if I'm not setting them up correctly. My boss is sure this kind of rotor is made to be machined on the car but I don't remember that setup being readily available in '83...

As an aside I recieved some advice from a board member (I don't remember his name, now...) to use Porterfield front pads to cure the extreme touchiness of my fronts in wet weather. I e-mailed Porterfield for their advice and received nada for an answer. Anybody have an opinion on Porterfield pads?

Sorry for all the questions but I don't get alot of time to be on the 'net so I have to make good use of it when I can....

Sanspistons for less warpage

lovintha7

I'd just use the cone setup like the front rotor. Are the rear rotors on a 85 solid or vented? If they're solid, don't waste your time, they're near impossible to turn without chatter, unless you have a $10k lathe.

Check your ebrake cable, and make sure it's not sticking. Hot rotors and e-brake pressure will warp them in a heartbeat. If you have to brake hard, try to stay off the brakes after you stop, that will warp them also. Let the rotors cool, and try to avoid water with hot brakes.

Pele

iTrader: (28)

Can you post pics of all the little adapter pieces that came with your lathe?

Every time I've done discs that don't have hubs built in (IE You don't have to dismantle the wheel bearings to get the rotor off.) Ive set it up as follows:

Lathe spindle nut --- Spacers --- Cone --- Brake rotor --- little cone that fits inside the center of the rotor --- spring --- Cone --- Spacers if required --- End of brake lathe spindle where it's attached to the lathe.

For the cones, I use the largest ones I can that fit inside the hat of the rotor.

For the little cone that fits inside the center of the rotor, I use one sized so that the rotor doesn't just sit on the edge of it, but one where it'll sit right on it, somewhere in the middle.

Hope that helps.

Felgar

Do you get uneven pad wear at all? My worst warping problems have been the result of the calipers siezing up... Similar to lovintha's point. If the pads aren't wearing evenly then you probably have caliper issues.

Other than that I haven't had rear disc issues at all really. Lovintha's other advice is good too.

10,000

here is the trick on non vented rotors, only cut one side at a time. try it and you'll thank me.

Pele

iTrader: (28)

I've cut solid rotors at the shop before. I usually cut both sides, but it's my practice to cut all rotors in a staged process...

Cut em 0.0010" deep on both sides at a high cut speed (Usually the spindle speed is fixed, but the device that moves the blades along the rotor or drum has two speeds.)

Repeat till it's flat and there is no place on the rotor that the blades have not touched.

Cut them once more at 0.0005" deep on a slow cut speed.

Hold sand paper against the rotor as it still spins on the lathe. Move it in and out to remove any spiraling the blades may have left.

Always use a rubber belt around the edge of the rotor to dampen any vibrations. Some brake lathes also include a little spring loaded clamp that you put on the rotor above the blades to dampen vibrations.

I've never had someone come back for pedal pulsation or noisy brakes... Although it did take me an hour and a half to do a brake job that everyone else did in 30 mins... (No rotor turning or caliper slide lube... Old pads off, New pads on, Back it out.)

For the record, I'd LOVE to have a brake lathe at home... Just a little 120v bench top model like we had at the tire shop. (Actually we had two, one usually set up for drums, one for discs.) Anyone know where to get one... Maybe used... For a couple hundred bucks?

I don't trust the auto parts clerks to turn my rotors.

Sanspistons

Got them turned just fine, thanks to the advice of the board members. Pele, I used only the spring-loaded inner cone since I couldn't find an outer cone small enough to clear but no problem. Rotors not even warped bad; one fast cut and one slow did the trick. This machine has a REALLY slow cut and I took less than a .001" so it didn't chatter; didn't have a band small enough to fit it, but it rang like a bell near the end...

Sadly, even though the rotors are now flat the right rear axle is still warped (dammit, I'm tired of swapping this axle looking for a straight one! -the third time!). Got to find yet another axle. Or maybe a looooong rod to straighten it with........!!!!

Thanks again for the advice; the first place I turn to for RX (in more ways than one!)

Sanspistons; the few, the proud, the grateful!

hornbm

At my work we use a procut on car brake lathe, MUCH MUCH better than the ammco because it takes hub runout into account.

As for the sandpaper thing. You may have not had any comebacks, but thats not to say there isnt a better way to do it. The best way to do it IMHO is to use a sandling block, and just hold it there. The idea is to get the rotor as smooth as possible. Smoother = more surface contact area.

Then be sure to wash the rotor with SOAPY WATER. Just using brake clean will not get all the rotor dust off. If you dont believe me clean a freshly cut rotor off with brake clean. Once you cant clean it any more. Now try some soapy water and you will get more metal dust off.

Just for the record I havent had any brake comebacks either

Rx7carl

Good reading guys. Im totally clueless on the subject so thanks for educating me a little.

Pele, try ebay for the brake lathe. I see alot of shop equiptment on there. Also look around your town, there might be a place thats deals in used equiptment.

Pele

iTrader: (28)

I was using what I had. We had no block of wood. Hell, we didn't even have a shop vac at that shop...

I'd also go out and break in the brakes for the customer before they picked up their car... (Couple soft, slow stops, then progressively harder stops and from higher speeds.)

I've never washed the rotors off before... Why would you do that?

hornbm

Why? Brake rotor dust has a good chance of causing squeeling concerns. Heres a hood way to check. Take a freshley turned rotor, and put on a latex glove. Run your finger across the face of the rotor. You finger will be completely silver. All of that does not help any stopping, and the only thing it can do is transmit to noise when the conditions are right.

Hell I even learned to wash rotors way back in high school.

I also wear in the brakes before I give the car back to the customer. Its wierd though, some pads can feel like **** for days and some only feel like **** for 5 minutes.