front end shudders when braking above 50mph

[floridaboy]

I have a 2001 LS V8 Sport with 47K. Front end shudders hard when braking firmly above 50 mph (feel it in the steering, too). The car stops straight and strong. Has ceramic pads w/50% life, brand new Wagner lifetime front rotors (twice!), and brand new tires. Shudders were there both before and after new tires and rotors. Three shops have checked the front end for other problems, and all three say it's tight. I'm running out of money and patience. Any ideas?

[CGGorman]

Who is torquing the lug nuts? They MUST be torqued equally to 100ft-lbs. Any variation in lug torque can (worst case) cause warped rotors or simply cause vibration.

Pad transfer can also cause problems, but it sounds like you had new rotors and pads installed at the same time, so that shouldn't be a major player.

Got a torque wrench? If not, buy or borrow one with the proper socket to fit your lugnuts (3/4", IIRC) Loosen all your lugnuts, then retighten every other (Mentally number them 1-5 sequentially going clockwise. Tighten 1,3,5,2,4) to 50 and then repeat, but to 100 ft-lbs. Do this on all four wheels.

Now to deal with any possible pad transfer, take the car out to a deserted road and run it up to about 60 and brake firmly to about 10 mph. (not hard enough to engage the ABS, but close) DO NOT STOP. Immediately run the car up to 60 again and repeat. Let the brakes cool off with 10-15 minutes of normal driving and repeat again. Let cool again.

This may help. Let us know.

[floridaboy]

Thanks, Chris, for the suggestions. I watched two of the shops torque the lugs, and they did it in two steps, first partway with the impact wrench, and then tighter with the hand torque wrench in a 1-3-5-2-4 pattern (although I don't know the setting). Just to be on the safe side, I redid the torque per your suggestion this afternoon, with no improvement. As for the pad transfer, I do NOT have new pads, as there was more than 50% life left on the ceramics, and they are EXPENSIVE. But this chatter was happening even on the old rotors, that the pads had several thousand miles "experience" with. Just the same, I took the car out this afternoon, and performed the re-seating drill you described, once again without improvement. I also tugged on the balljoints, tie rods, and steering knuckle, and cannot see or feel any free play whatsoever. Could it be the struts? They're about the only thing left with any significant miles on them...

[el es]

maybe a motor mount is broken.

[CGGorman]

Bummer. Shudder while braking... Don't see it being the motor mounts, but who knows.

Have checked/check yourself the rotor runout while they're installed on the hub. Perhaps the rotors are junk or there is crud/rust on the mounting faces.

[el es]

yeah there could be a knot it one of the rotor or some slight warping both are common items in rotors.

[floridaboy]

I know the motor mount thing is possible on a transverse-mounted FWD car (had it happen on my STS), but didn't think a RWD reacted the same way. Local Lincoln dealer says possibly REAR rotors, even though feedback is through steering, rather than the seat of my pants. I'm gonna have them check BOTH the motor mounts and the rear rotors. I'll let y'all know if either was the problem.

[floridaboy]

PROBLEM SOLVED! It was the REAR rotors. The dealership mechanic says that something about the suspension makes the rears feed back through the steering column!?!? He also says not to drive through puddles when the brakes are hot, 'cause it'll waffle the rotors every time... I live in FLORIDA... You cannot swing a dead cat around here without hitting a puddle! That SUX! Anyway, thanks for the help!

[CGGorman]

Good deal. Glad it's fixed. Lesson learned...don't overlook the rears.

[el es]

Where in Fl do you live? I am in Brandon/Tampa Area.

[floridaboy]

O-do