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A rumbling front end...

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DEAR TOM AND RAY:
I HAVE A 1984 DODGE CARAVAN WITH 90,000 MILES. MY PROBLEM IS THAT THE FRONT END RUMBLES WHEN I STEP ON THE BRAKE. THE RUMBLING BEGAN ON A LONG TRIP THAT INVOLVED A LOT OF HIGHWAY DRIVING. AFTER THAT TRIP, I HAD THE ROTORS "MACHINED." THE RUMBLING WENT AWAY UNTIL I TOOK ANOTHER LONG TRIP. NOW, MY MECHANIC SAYS THE ROTORS CAN'T BE MACHINED ANY FURTHER. WHAT CAUSES THE RUMBLING AND WHAT DO I DO NOW?
LINDA

TOM: WHEN YOU SAY IT "RUMBLES," LINDA, I'M GOING TO ASSUME THAT YOU MEAN THE BRAKE PEDAL "PULSES." AND THE CAUSE OF THE PULSING IS WARPED ROTORS. THE ROTORS ARE WHAT THE BRAKE PADS GRAB ONTO WHEN YOU STEP ON THE PEDAL. IF THE ROTORS ARE WARPED, THE PADS ALTERNATELY SKIP AND GRAB AS THE ROTORS TURN--THAT CAUSES THE PEDAL TO MOVE UP AND DOWN RAPIDLY OR "PULSE."

RAY: ROTORS TEND TO WARP BECAUSE OF EXCESSIVE HEAT. IF YOU HAD A STICKING BRAKE CALIPER, FOR EXAMPLE, THAT WOULD CREATE ENOUGH ADDITIONAL FRICTION AND HEAT TO WARP THE ROTOR.

TOM: WHEN ROTORS GET WARPED, THEY CAN EITHER BE REPLACED OR MA?CHINED. MACHINING--THE METAL EQUIVALENT OF SANDING--SHAVES THE ROTORS DOWN UNTIL THEY ARE PERFECTLY FLAT AGAIN. THERE ARE TWO THINGS YOU SHOULD KNOW ABOUT MA?CHINING. FIRST, IT MAKES THE ROTORS THINNER, SO THEY'RE MORE APT TO WARP IN THE FUTURE. SECOND, EVENTUALLY, THE ROTORS GET SO THIN THAT THEY CAN'T BE MACHINED ANYMORE.

RAY: AND THAT'S YOUR SITUATION NOW, LINDA. YOU NEED NEW ROTORS. YOU SHOULD ALSO GET A NEW SET OF BRAKE PADS WHILE YOU'RE AT IT. THE WHOLE THING WILL COST YOU $150-$200. BUT MAKE SURE YOU'RE MECHANIC ALSO CHECKS FOR STICKING CALIPERS. IT MAY HAVE BEEN STICKING CALIPERS THAT RUINED YOUR ROTORS IN THE FIRST PLACE.
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