Pre-ride inspection of a used 125 cc before a 40 km ride home
You buy a used 2019 125 cc commuter with 28 000 km on the clock and plan to ride it 40 km home. In the seller's driveway you check it over: the drive chain has about 45 mm of slack at its midpoint and a noticeable tight spot; the rear tyre shows 1,6 mm of tread at the centre and 1,1 mm near one edge, with a plug repair in the tread; the front brake lever feels spongy and pulls almost back to the grip; the engine oil on the dipstick sits on the lower mark and is black; the tyre pressures read 1,4 bar front and 1,8 bar rear against a placard of 2,0 bar front and 2,2 bar rear solo. The seller says the roadworthy certificate was done "last year sometime". The route home is 40 km, mostly a 100 km/h rural road.
1. Which of these findings stop you riding the bike home today, and which are merely things to fix soon? Justify each.
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Ride-stoppers: the front brake and the rear tyre. The chain is a strong second.
- Spongy front brake lever pulling to the grip - do not ride. A spongy lever means air in the hydraulic system, old fluid that has absorbed water, or a leak. The front brake does most of the stopping on a motorcycle; a lever with no firm bite can go to the bar in a real emergency stop. This must be bled and inspected first.
- Rear tyre with a plug repair and 1,1 mm of tread near the edge - do not ride. The legal minimum tread depth in South Africa is 1 mm, so 1,1 mm at the shoulder is right on the limit and effectively worn out. A plug repair in a motorcycle tyre is at best a temporary measure and can fail under load or heat at 100 km/h.
- Chain with 45 mm slack and a tight spot - do not ride as-is. Typical slack is roughly 20–30 mm depending on the model. A tight spot means the chain is kinked or unevenly stretched; under load it hammers the countershaft bearing and can snap.
- Oil on the lower mark, black - fix before riding, but not a hard stop if it is topped up. Low oil on a 40 km ride at highway speed is how a small engine gets destroyed.
- Pressures 1,4/1,8 vs 2,0/2,2 - inflate before leaving. Under-inflation makes the bike slow to turn, overheats the tyre and accelerates wear. It is a two-minute fix, not a reason to cancel.
2. Explain what a tight spot in a chain means and how it must be adjusted, including what happens if you simply tighten it to remove all the slack.
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A tight spot means the chain has stretched unevenly or a link has seized, so tension varies as the wheel turns.
How to set tension correctly:
- Rotate the rear wheel and find the point where the chain is tightest - that is the point where tension must be measured, not the loose point.
- Adjust to the manufacturer's spec (commonly around 20–30 mm of up-and-down movement at the midpoint of the lower run) measured at that tightest point.
- Check the rear wheel is still aligned using the swingarm marks or a straight edge, and re-check the axle nut torque.
Why you never just wind out all the slack: if you set the chain tight at the loose point, the tight spot becomes over-tensioned. That loads the gearbox output shaft bearing, can damage the seal, and causes the rear suspension to bind because the chain has no slack to accommodate swingarm movement. On a long ride it can snap the chain - which on a motorcycle often locks the rear wheel or whips through the crankcase.
For this bike: a chain with a pronounced tight spot at 28 000 km is near the end of its life. Adjust it to get home if the front brake and tyre are sorted, but replace the chain and sprockets as a set rather than nursing it.
3. List the checks you would still do after fixing these items, before joining the 100 km/h rural road.
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A short, systematic pre-ride check - tyres, controls, lights, fluids, chain, paperwork.
Tyres and wheels
- Pressures set to the placard (2,0 front / 2,2 rear solo here) when the tyres are cold.
- Tread and sidewalls: no cuts, bulges, exposed cord or foreign objects.
- Spokes tight on a spoked wheel; wheel bearings have no play when you rock the wheel.
Controls
- Front brake: firm, does not creep to the grip when held.
- Rear brake: pedal has free play but bites.
- Throttle snaps back to closed on its own from any steering angle.
- Clutch operates smoothly; lever and cable not frayed.
- Steering turns freely lock to lock with no notchiness, and the headstock has no play.
Lights and electrics
- Headlight on main and dip, tail light, brake light on both front and rear controls, indicators, horn.
Fluids and mechanicals
- Engine oil at the correct level; no leaks under the engine or at the forks.
- Chain tensioned and lubricated; sprocket teeth not hooked.
- Sufficient fuel for 40 km plus reserve.
Rider and paperwork
- Helmet fastened, gloves, jacket, closed shoes, eye protection.
- Valid licence disc and licence card, and a roadworthy certificate you can actually rely on - "last year sometime" is not good enough; verify the date before you ride.