Brake judder after a cheap pad and disc job
A 2021 compact SUV with 46,000 km arrives with steering-wheel shake when braking from 80–100 km/h, worse when the brakes are hot. Front pads and discs were replaced 5,000 km ago at a tyre shop and nothing was measured. Your readings: disc thickness 24.9 mm at twelve points (new 26.0 mm, minimum 23.0 mm), disc thickness variation 0.042 mm (limit 0.015 mm), lateral runout 0.11 mm measured on the car with a magnetic base (limit 0.05 mm), hub face runout 0.02 mm, wheel bearing end play 0.01 mm, caliper guide pins free, pad thickness 9 mm and even. Tyres are at 3 mm, pressures correct, no ABS codes stored. The owner has a 400 km highway trip in three days and asks you to just machine the discs because it is the cheapest option.
1. Interpret the measurements and decide between machining and replacement.
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The judder is caused by thickness variation, not by warping in the everyday sense. A DTV of 0.042 mm is nearly three times the 0.015 mm limit; as the disc turns, the varying thickness pumps the piston and generates the pulsation felt at the wheel. Runout of 0.11 mm is also over double the 0.05 mm limit, but the hub face is only 0.02 mm, so the error is at the disc-to-hub interface or in the disc itself - typically rust or debris left on the hub face and uneven lug nut torque from a rattle gun, which is consistent with the previous fast fit. Machining can correct both faults, but the discs have already lost 1.1 mm of their 3.0 mm wear allowance; a 0.3 mm skim would leave 24.6 mm, only 1.6 mm above minimum, with reduced thermal mass on a vehicle that will be braking from highway speed loaded. With unknown prior heat history, replacement of both discs and pads as a pair is the defensible choice, and the owner should be told why the cheaper option carries a comeback risk.
2. Describe the fitting procedure you would use so the fault does not return, including the checks and the numbers.
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Clean the hub face with a rotary abrasive disc until bare, bright metal, then check it again with a dial indicator; anything above about 0.03 mm means the hub itself needs attention. Fit new discs with the locating screw, measure runout on the car at the outer face and index the disc one stud position at a time if it exceeds 0.05 mm - often one position brings it inside spec, which proves the disc and hub high spots met. Torque the wheels in a star pattern with a calibrated wrench to the manufacturer figure, never with a rattle gun, because uneven clamping is what distorts hot discs. Fit new pads, clean and lubricate the guide pins with the correct high-temperature grease, and check the caliper piston retracts squarely. Then bed the pads in with six to eight moderate stops from 60 km/h, cooling between them, and road test for pulsation and for an even pedal. Record thickness, DTV and runout on the job card.
3. The customer refuses new discs on cost grounds. How would you protect the vehicle and yourself?
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Set out the options and their limits in writing. If machining is chosen, skim only within the manufacturer's allowance, finish with a non-directional pattern, then re-measure DTV and runout on the car and record both numbers; if the result cannot be brought under 0.015 mm DTV and 0.05 mm runout, stop and recommend replacement. Note on the job card that the discs are already 1.1 mm worn and that thermal performance is reduced, and that the 400 km trip should be driven normally with no heavy towing or repeated high-speed stops. Fitting new pads against discs with existing DTV is not acceptable, so pads must be replaced or the discs corrected. If the customer still declines new discs, have them sign the advisory and book a recheck after 500 km. Never release a car whose measured brake judder you have not corrected or clearly documented, because braking performance and liability both sit with the workshop.