Brake Judder After Pad Replacement
A 200 cc motorcycle returns two days after a front brake pad replacement with the complaint of a pulsing lever and vibration through the bars when braking from about 60 km/h. The technician who did the original job reports he cleaned the caliper, fitted new pads, and pushed the pistons back with a screwdriver. The wheel spins freely on the stand with no obvious drag, but a dial gauge on the disc shows 0.18 mm of lateral run-out against a limit of 0.10 mm. The disc shows a faint blue tint over part of its swept area. The front tyre pressure is 1.6 bar against a recommended 2.0 bar, and the steering head bearing has noticeable free play when the front brake is held and the bars are rocked. The customer insists the fault was caused by the new pads and wants them replaced free of charge.
1. Separate the findings into those that explain the pulsation and those that are incidental. Explain the mechanism by which each relevant finding produces the symptom.
Read the worked answer
Findings that explain the pulsation:
- Disc run-out of 0.18 mm vs a 0.10 mm limit. As the wheel rotates, the high spot of the disc pushes the pads and pistons back further than normal, then the low spot lets them return. Lever travel and braking force therefore oscillate once per wheel revolution - exactly the frequency a rider feels as a pulsing lever at road speed. This is the dominant cause.
- Blue tint on part of the swept area. Localised overheating indicates the pads were dragging on a high spot, or that the disc has a thickness variation or a hot spot from uneven pad contact. Either way it confirms the disc is not running true and is being heated unevenly.
- Steering head free play. Play allows the fork legs to move under braking load, superimposing a shake on the pulsation and amplifying what the rider feels through the bars. It is a contributor to the vibration but not to the lever pulse.
Incidental or secondary:
- Low tyre pressure (1.6 bar). Degrades steering feel and can add wobble, but cannot produce a once-per-revolution lever pulse.
- Pushing pistons back with a screwdriver. Poor practice that risks damaging the piston face and seal, and may have forced contaminated fluid back into the master cylinder, but the free-spinning wheel shows no current drag.
The new pads are not the cause; they merely transmit an existing disc fault that the rider now notices because fresh, thicker pads sit closer to the disc.
2. The customer demands free replacement pads. Write the explanation you would give, and set out the repair sequence with the checks that must follow.
Read the worked answer
Explanation to the customer: the pads are a friction material that follows the shape of the disc; they cannot create a pulse in the lever by themselves. Measurement shows the disc is running out by 0.18 mm where the limit is 0.10 mm, and the blue heat marks show it has been heating unevenly. The new pads are doing their job but are now transmitting that disc fault more noticeably because they are thicker and sit closer. Replacing pads again would reproduce the same symptom within a few days. I would offer to show the customer the dial gauge reading on the bike.
Repair sequence:
- Remove the front wheel and check disc mounting bolts for correct torque and seating; re-check run-out with the disc properly seated.
- If run-out remains out of limit, replace the disc rather than attempting to true it.
- Replace the pads as a set only if they are contaminated or unevenly worn - inspect both sides.
- Clean the caliper, check piston and slide operation, and flush the fluid if it was contaminated by the earlier push-back.
- Adjust the steering head bearing to remove free play, then re-check for notchiness.
- Set tyre pressure to 2.0 bar.
- Road test at increasing speeds, confirm no pulse, and document before-and-after run-out figures.