Front rotor minimum thickness?

[JeepSnake]

Hey gang,

I've got the front brakes apart to put in new pads and try to cure a front end shudder that occurs during braking. It looks like a brake rotor turning is in order.

Alan, Maurice, and I all expected that the rotors would have minimum tolerances cast or stamped into them. No dice. I've searched Wilwood's site and it looks like I have rotor PN#160-5839; they list a thickness as new, but no tolerances. I've just sent them an email, and hope to have a reply Monday.

But, in the meantime, does anyone here know the thickness tolerance for turning the standard front rotors?

Also, I've been getting an occasional "pop" in the right front of the car. It's in the suspension, but I don't see any readily evident problems. It has occurred a couple of times under very hard braking and a couple of times while backing up with the steering turned full left. Nothing looks out of place on first inspection, though. Anyone experienced something like this and have an idea of where to look?

[pgermond]

Zach,

I had some strange noises in my front-end that turned out to be something easily remedied - although your issue sounds a bit different.
http://www.uniquecobra.com/forums/showthread.php?t=7120&highlight=safety+wire

[JeepSnake]

I'll definitely check those bolts, Phil, as there's no safety wiring in place on them (yet).

One of the possibilities I had in mind was that the brake pads might be "loose" in the caliper - i.e. have enough clearance to shift forward or backward ever so slightly during braking. I've had that happen on another vehicle; not a safety issue as they're still constrained, but the noise can scare you to death until you figure out what's going on and correct it. These pads are tight, but a loose caliper bracket could yield the same result - only it WOULD BE a dangerous situation.

All bolts on the hub, hat, calipers, etc. have been tight so far, but I haven't checked the caliper brackets. Sounds like a task for this evening (after I get the water heater back up and running at the house!!!).

[rdorman]

Check with Wilwood to be sure....... as I recall, the don't recommend 'turning' them. If they where to be resurfaced at all, it would be on a mill. Rule of thumb is 90% of its new thickness. I check run out and if it is less than and .009" (the closer to zero the better!), the rotor shows no signs of damage (cracking etc) and the thickness is still good (do the math) then I will just dress them with 120 grit on a sanding disc allowing the rotor to rotate creating a nice circular pattern. That is just when I am changing compounds (like for the weekend track guys). With that said, if the pads are worn out, I will normally replace the rotors, rebuild the calipers and replace the mounting bolts to the top hat (they get hot!).

[JeepSnake]

Well, I got exactly the answer I expected from Wilwood...

"These rotors are racing rotors and do not have a minimum thickness as they are shipped to you already machined to the minimum thickness. We do not recommend cutting the rotors under any circumstances. If a rotor is warped, it needs to be replaced. "

I'm going to get in touch with our local machinist tomorrow to see about checking runout.

The thickness is a-okay, and the original machining marks are still barely visible in the rotors. I'd just hit them with the sanding disc (done it many times myself!), but that still doesn't address the steering wheel shake I'm getting under braking...

Anything besides a warped rotor that would cause that?

(FYI - caliper brackets are all tight as can be.)

[rdorman]

You can check the runout on the car... it is the best place to do it. All you need is a dial indicator and stand. If the axial and radial run out is out of spec, index the rotors and hats to get it to minimum. If that does not bring it into spec, determine which piece is the cause (which you should already know from your axial and radial measurements of each piece) and replace it. As far as shaking under braking, the two primary causes (related to the brakes themselves) are uneven transfer film layer caused by improper bedding of the brakes and un even heating of the rotors which causes the rotor to 'warp' when hot.

[rdorman]

I also forgot to directly mention thickness variation.

Assuming that you have not indexed rotors before... it is a simple task, but a bit time consuming.

Pull the rotors off and completely disasemble them. Clean everything (including bearings) very thoroughly. Mount the hub (leave the grease seal out at this point) and the top hat back on the spindle adjusted for zero end play. Measure radial, dial indicator on the outside circumfrence of the top hat face, and then measure axial runout, dial indicator on the outside edge of the top hat.

Make a little drawing and label each position writing down the results. Label the position of the top hat so you don't loose track of what measurement is what position. Repeat this rotating the top hat one bolt position from the start until you have measured every possible combination. Once you have measured all of them, bolt it in the position that give you the least axial and radial runout.

Now mount the disc and perform the same operation.

What you are looking for is radial run out of the rotor near the OD to be less then .006, axial less then .01 (much less important) and thickness variation of the rotor to be less then .001. These are maximums... .003 or less radial is best.

Once the you are done, time to set the bearing correctly. On you perfectly clean bearing, hubs and spindles (you did replace or carefully inspect the bearings did you not?) with NO grease seal in place, back the nut off slightly until there is some play in the hub in relationship to teh spindle. Now while spinning the rotor tighten the nut slowly until a slight drag is fealt. Repeat this several times to get the feel for it. This should be zero play. Check it with a dial indicator mounted to the rotor (you did get a magnetic stand didn't you?) to the nose of the spindle while prying out evenly on the rotor. If it is zero, good. Now back the nut off until the first position to be able to insert the cotter pin. Insert a pin and back the nut off as far as it will go. Measure the play again. It should be between .001 and .005... no more, no less. If you can not align the cotter pin and keep it in spec then the 'washer' below the castle nut will need to be replaced or shimmed. You can calculate how much by the threads per inch and how much you are off. These are special washers and shims... use only those designed for the application. Mark that location on the castle nut in relationship to the washer. Pull it all down.

Now locktight red all the rotor bolts and torque to spec. Safety wire them all. Put in the grease seal and pack your bearing and put it all back on tightening the castle nut to the previously marked location. Cotter pin and grease cap and there you go.

[ralphscott]

Zach I indexed mine when I put the big brakes on the front. As you do this you will be surprised at the variation as you rotate the rotor to different positions on the top hat. Once you determine the best position after doing all possibles re mount at best position and do a final recheck. Don't forget to safety wire. Remember I am a belt and suspenders kinda guy. Great directions from Rick!

Ralph

[will butterworth]

I remember you doing that, did the big rotors give you better braking--slide front tires? will-alabama

[ralphscott]

Much better braking Will. I even have installed a remote brake bias adjuster. No the front tires don't slide neither do the rears, but the positive feel it is like having power brakes!


Hope all is well,

Ralph

[rdorman]

Update from Wilwood... they indicate .007 or less to be the tolerance for runout.

Thanks for the kind words on the procedure. I have done them that way for years. The only thing I have changed is the way I set the end play on tapered roller bearings. I adopted the procedure above after reading a Timken tech bulliten. I use to do them packed and with the grease seal in place. Not any more and have yet to have a bearing failure since (had a few before though).

[JeepSnake]

Rick, do you have the torque values for the hat to hub fasteners and the rotor to hat fasteners? I'm hopefully going to start reassembly this evening.

FYI - got the right rotor down to 0.002 runout, left rotor down to 0.004. Thickness on rotors was consistently only about 0.004 less than thickness as new. I went over them with the old sanding disc and got a good swirl pattern on them. Bearings are as new.

On inspecting & rebuilding calipers, it appears that instead of the right caliper not releasing, the LEFT caliper had a couple of sticky pistons that may not have been applying fully. Calipers cleaned up and went back together just peachy keen. Don't know what brand of brake pads were on the car, but they were NOT Wilwoods. They had plenty of friction material left, but the backing plates were garbage. They were heavily rusted, and had what appeared to be latex house paint peeling off of them!?!?

Now, the only thing ahead that I look on with dread: bleeding the brakes. I've tried all the methods, all the short cuts, everything, and it's still a task I despise. If for no other reason, there's always the chance of getting some fluid on the paint!

Thanks!!!

[eliminator]

Use the standard bolt torque for the type of fastner you are using , (size fastner, grade, material and thread type), these charts are listed in a lot of places.

[JeepSnake]

OK - that's what I planned to do unless I heard differently.

Had intended to have this done by this point, but I've gotten caught up in working out in the yard the last several days. The weather's just been too warm not to - I even cut the grass and staged an all-out offensive against the fire ants yesterday. They're going crazy now that we're finally getting a little rain.

Thanks again!

[ralphscott]

Zach I've had two nests in TN that I treated with a couple of gallons of gasoline. Took out any visible signs of them. But who knows what will pop up this spring. What are you using?

Hope y'all weathered the storms and did not get damage. We did not get any tornadoes but we got the wind and rain. We are in the valley between the Cumberland and Smokey mtns. which has really helped us out over the years.

Ralph