The Ride Home After a Tyre and Chain Service
A rider collects his 2015 1,000 cc 大型二輪 (38,000 km) from a shop that fitted a new rear tyre and a new chain and sprocket set. The 12 km ride home below 60 km/h felt normal. The next morning, before a planned 300 km day trip, he checks the machine. Chain slack at the tightest point measures 52 mm; the owner's manual specifies 30–40 mm. The rear axle nut carries a paint mark that is now offset by about 20 degrees from the mark on the swingarm. Cold tyre pressures read 230 kPa front (specification 250) and 210 kPa rear (specification 290). Rear brake pedal travel before the pads bite is about 60 mm, against 25 mm before the service, and the wear indicator groove on the rear pads is no longer visible. The front tyre shows 2.8 mm of tread and a 2016 date code. Above 90 km/h there is a light shudder through the handlebars that was not there before.
1. Sort the findings into 'do not ride', 'investigate before riding' and 'monitor', and justify the placement of each.
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Do not ride.
- Offset axle nut mark. The paint mark is the technician's record that the nut was torqued. A 20-degree shift means it has moved, or was never torqued. If the axle migrates, chain tension and wheel alignment change as you ride, and the rear wheel can shift under load.
- Chain slack 52 mm against 30–40 mm. In a chain-and-sprocket change, over-slack usually means a setting error or that the slack was set with the suspension compressed or at a loose spot. It allows jumping and derailment.
- Rear pedal travel 60 mm plus no visible wear indicator. Both point to a rear brake that has not been bled or adjusted after the wheel came out. Do not test it at the first junction.
Investigate.
- Rear tyre 210 kPa against 290 kPa. Roughly 28% low the morning after fitting suggests a bead, valve or puncture problem, not normal creep. Find the leak.
- Shudder above 90 km/h. Wheel balance, bead seating or steering-head adjustment after refitting.
Monitor. Front tyre at 2.8 mm with a 2016 date code: legal, but aged rubber on a 1,000 cc machine is a replacement-due item, not a today item. The day trip should be cancelled, not delayed by an hour.
2. Explain why chain slack is measured at the tightest point, and what the 52 mm reading combined with the offset paint mark tells you about the quality of the service.
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A used chain does not wear evenly. Some links flex more freely, some kink; the run of chain between the sprockets therefore has a tight spot and a loose spot as the wheel turns. Tension is set by the tightest spot, because that is where the chain is already most highly loaded. If you adjust at a loose spot, the tight spot becomes over-tensioned: the countershaft bearing and the output-shaft seal take the load, the chain loses its play and the suspension cannot compress without stretching the chain. If you adjust at the tight spot and the chain then rotates to a slack section, the chain whips.
A reading of 52 mm against a 30–40 mm specification is not a small overshoot on a chain whose tight spot has already been measured. Combined with the offset paint mark, the most probable story is that the axle was not torqued to specification, or that the tension was set before the axle was finally tightened and the adjusters moved afterwards. Either way the geometry the shop left is not the geometry the manual asks for. The correct response is not to guess at a slack figure and ride: it is to re-torque to specification with a wrench and re-check alignment, then re-measure.
3. He decides not to ride the 300 km trip but still wants to get the bike 12 km back to the shop. Set out a defensible plan and the checks that should precede any ride.
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The defensible option is not to ride it at all: ask the shop to collect it, or trailer it. Twelve kilometres is short, but the faults are exactly the kind that fail without warning - an untorqued axle and an unproven rear brake - and a failure at 50 km/h is not made safer by the trip being short.
If he insists on riding, the sequence is: torque the rear axle to the manual figure with a calibrated wrench; set chain slack at the tightest point to the mid-specification figure (about 35 mm) and confirm the adjuster marks are symmetrical; find and fix the rear pressure loss, then inflate to 250/290 kPa cold; bleed and adjust the rear brake until travel is back near 25 mm and the wear indicator is visible or pads are replaced; check that the wheel spins without drag and that both brakes release fully; test the brakes at walking pace in a car park before joining a road. Route on 30 km/h streets, no expressway, daylight, and no passenger. Even then, the honest answer remains: this is the shop's problem to collect.