The used bike the night before a two-up tour
You have bought a used 2019 650 cc twin with 24,000 km on the clock, approved for A2 with a 35 kW restriction. Two weeks before a 300 km trip with a passenger you go over it in the garage. On the left fork leg there is a ring of oil on the stanchion and a drip hanging from the dust seal. The front brake lever comes back further than it did last month and feels spongy; the pads have about 3 mm of friction material left of 4 mm when new. You measure chain slack at 45 mm against a specification of 25–35 mm, and one tight spot needs noticeably more force to move. The chain has several stiff links and the rear sprocket teeth show a slight hook. Tyre pressures are 1.9 front and 2.2 rear against 2.5/2.9 for a loaded bike. Brake fluid has never been changed according to the service stamps. The trip starts tomorrow morning and the passenger is meeting you at 08:00.
1. Which of these faults would stop you from riding tomorrow, and what is the reasoning for each?
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The fork seal and the spongy front brake decide the trip. Oil from a failed fork seal runs down the slider onto the disc and pads; fork oil is a lubricant, so the pads glaze and lose friction exactly when you need it, and the loss is worse with a passenger on board.
A lever that comes back further and feels spongy points to air or water in the hydraulic circuit. Brake fluid absorbs moisture from the air and should be replaced at least every two years; water in the fluid turns to vapour under hard, repeated braking - a mountain descent with two people aboard is the classic case - and the lever can come back to the bar.
The chain at 45 mm with a tight spot and stiff links is at the end of its life and can snap or destroy the sprockets. Tyre pressures are a two-minute fix and are not a reason to cancel, but they must be corrected before loading. Verdict: ride nothing until the fork seal, the brake fluid and the chain are sorted.
2. How is the fork leak connected to the brake problem, and what do you check after the repair?
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Fork oil migrates down the slider by gravity and airflow, lands on the brake disc and is thrown into the caliper, soaking the pads. Contaminated pads cannot be cleaned back to full friction reliably - replace them, and clean the discs with brake cleaner until no film remains.
When the fork is apart, check the chrome slider for stone chips and pitting: a single sharp edge will tear the new seal within a few hundred kilometres. Fit a new seal and dust wiper, use the specified oil volume and grade, and set the air gap correctly, because too much oil on a long-travel fork can hydraulic-lock under compression.
After reassembly, test at walking pace, then at 50 km/h in a safe area, and bed the new pads in with a series of moderate stops rather than one hard stop. Check the fluid level and colour in the reservoir: dark fluid confirms moisture and age and is a reason to flush the whole system, not just top it up.
3. Explain why the chain and the tyre pressures are judged as they are, and what you would actually do.
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Chain slack of 45 mm against a 25–35 mm spec is already outside the limit, and a tight spot means the tension varies as the wheel turns: setting it to spec at the tight spot leaves it far too loose elsewhere, and setting it at the loose point overloads the tight one. Stiff links do not free up once the O-rings have gone and the pins have corroded - lubricant only hides the problem. A hooked sprocket tooth confirms the set is finished. Replace chain and sprockets as a set; a new chain on worn teeth wears out in a fraction of its normal life.
Pressures of 1.9/2.2 are below the loaded recommendation and add flex, heat and vague steering with a passenger. Set them cold to 2.5/2.9 before loading, not at a warm forecourt gauge. Also check the chain free play again after the bike is loaded and the suspension is compressed, because tension increases as the swingarm rises.