Brake judder and a left pull: machining, replacement, or pads only?
A 2017 petrol sedan with 62,000 km arrives with steering-wheel judder when braking from about 80 km/h and a pull to the left while braking. A road test confirms the judder is felt through both the pedal and the wheel, and the pull needs constant correction. Measurements at the front axle: disc thickness 24.6 mm right and 24.5 mm left (new 26.0 mm, minimum 22.0 mm); disc thickness variation 0.04 mm right and 0.05 mm left (limit 0.02 mm); lateral runout 0.11 mm left (limit 0.05 mm); pad friction material 6 mm right outer, 7 mm right inner, 6 mm left outer, 2 mm left inner (minimum 2 mm); the left caliper slide pins will not move by hand; brake fluid moisture content is 3.5% (replace above 3%); both front tyres are at 2.1 bar with even wear; rear drums were serviced 8,000 km ago. The customer wants only the pads replaced and asks why you are "selling" discs.
1. Rank the findings by their contribution to the judder and the pull, and explain why fitting new pads alone fixes neither symptom.
Read the worked answer
Judder (pedal and wheel vibration)
- Disc thickness variation 0.05 mm left and 0.04 mm right against a 0.02 mm limit is the primary cause. As the wheel turns, the caliper piston is pushed back and forth twice per revolution; that pressure pulsation is what the pedal and wheel feel. Above roughly 80 km/h it becomes obvious.
- Lateral runout 0.11 mm against a 0.05 mm limit is the second cause and the reason the DTV will come back even if the discs are machined: runout causes the pad to kiss the high spot each revolution and wears a new thickness variation into the disc.
Pull to the left 3. Seized left slide pins are the likely cause. The caliper cannot float, so the left inner pad drags continuously. That explains the 2 mm left inner pad against 6 mm left outer - classic taper - and it also explains the extra heat on the left disc, which accelerates DTV on that side (0.05 vs 0.04 mm). 4. Brake fluid at 3.5% moisture will not cause judder today, but it lowers the boiling point and corrodes the caliper and ABS unit, so it must be part of the job.
Excluded/checked: tyres at 2.1 bar with even wear, and rear drums serviced 8,000 km ago, so the pull is unlikely to be tyre or rear brake related. Confirm with a brake roller or a controlled stop after repair.
Why pads only will not work: new pads pressed against a disc with 2.5 times the allowable thickness variation will simply copy the pulsation, and the dragging left caliper will wear the new inner pad in a few thousand kilometres. The pads are a symptom, not the cause.
2. Decide between machining and replacing the discs, using the numbers, and state the conditions attached to whichever choice you make.
Read the worked answer
The numbers
- Discs measure 24.5-24.6 mm; minimum is 22.0 mm, so 2.5-2.6 mm of wear allowance remains.
- A cleanup cut typically removes 0.10-0.20 mm per side, so machining is technically possible and would leave plenty of thickness above the 22.0 mm limit.
- But: runout is 0.11 mm and the left caliper is seized, and the car has covered 62,000 km.
Decision for this vehicle: replace both front discs as a matched pair. Reasons:
- The seized pins and the runout mean the discs have already been heat-cycled; machining a disc that has been running hot risks warping again.
- A pair of discs is usually close in cost to the machining labour plus a second strip-down if the judder returns.
- The discs must match side to side; machining one and replacing the other leaves a thickness and heat-capacity mismatch.
If you choose machining instead: it is legitimate only when the disc is above the minimum after the cut with a margin (do not finish within 0.5 mm of the minimum), runout is re-measured under 0.05 mm after machining, and the caliper fault is fixed first - otherwise the same DTV is back within a few thousand kilometres.
Regardless of choice, the job must include: new pads on both sides, slide pins cleaned and lubricated with high-temperature brake grease (or caliper replaced if the bore is scored), piston seals checked, brake fluid replaced and bled because moisture is 3.5%, wheel-nut torque to spec with a torque wrench, and a road test confirming no judder and no pull.
3. Write the explanation you would give the customer who says you are "selling" discs, and state what you would record if the customer insists on pads only.
Read the worked answer
Explanation: "Judder is not the pads, it is the disc. I measured 0.05 mm of thickness variation on this disc; the limit is 0.02 mm. That variation is a high and a low spot that pumps the pads in and out twice per wheel revolution - that is the vibration in your pedal. Your left caliper slide pins are seized, which is why one pad is down to 2 mm while the one behind it is at 6 mm. If I fit new pads on these discs, the pads will wear into the same uneven surface and the judder will be back in a few thousand kilometres - you will pay twice. I can machine the discs instead if you prefer; they have 2.5 mm of material above the minimum. What I cannot do is guarantee the judder with pads only."
Show the customer the micrometer readings, the minimum thickness stamped on the disc, the tapered pads side by side, and the moisture tester reading. Give the price of both options - machine or replace - and let the customer choose.
If the customer still insists on pads only:
- Record the measurements and the two quotations offered.
- Record the recommendation (discs and caliper repair) and the customer's instruction, with signature and date.
- Add a line that the braking system is below the manufacturer's specification and no warranty is given on the pads.
- Do not disable or mask anything, and do not mark the job as "brakes serviced" in a way that suggests the fault is corrected.