A toothed (timing) belt carries the drive from the gearbox's output pulley to the large drive gear on the rear axle. The motor drives the left-hand rear wheel only. The belt is tensioned by rotating the motor in its clamp: the motor's end mouldings are deliberately eccentric, so turning the motor moves the pulleys apart.

The pedal drive is separate. It runs through the chain and freewheel.

The Owner's Handbook (p. 25) describes refitting a belt that has slipped off:

1. Remove the boot (see Boot clip).
2. Release the motor mounting bolt.
3. Rotate the eccentrically mounted motor to bring the toothed pulleys as close together as possible. Slide the motor about 40 mm clear of the drive wheel.
4. Place the belt over the drive pulley. Then, while sliding the motor back, feed the belt onto the motor pulley, and push the motor fully home to the left.
5. Rotate the motor until the belt is taut, with 1.5 mm deflection.
6. Tighten the clamp bolt to 8 Nm. Check alignment and that nothing binds, then refit the boot.

The Sinclair output-gear service note adds that tension is normally right when the motor brushes are roughly horizontal, with the red lead at the rear. Make sure the gearbox cover rests on the support web.

Fitting a new belt means removing the left rear wheel. The handbook advises calling a service engineer if the belt has come off completely, rather than just slipped.

• Inspect the belt for cuts and frayed edges, and check its tension. The handbook says to do this at every routine check; Hoover's 6-monthly service included it.
• Both extremes cause damage:
  • Too tight: bearing wear and friction, and the motor tilts, loading one edge of the belt.
  • Too loose: the belt wears and slips.

The most common C5 drivetrain complaint. Under load, especially when starting off, the belt jumps teeth with a loud rapid clicking.

• Cause. The plastic ledge (a vertical tongue) that should hold the gearbox end of the motor parallel to the axle is narrow. Under belt tension and high torque, the gearbox housing slips behind it, so the motor twists towards the axle. The belt then runs out of square, slips, and wears both the belt and the pulley teeth.
• Factory fix (1985). Sinclair Vehicles issued a galvanised steel motor bracket that pushes the motor support back and stops the housing dropping behind the ledge. It was fitted to many C5s still in stock in mid-to-late 1985. See Motor support.
• Owner fix. A cable tie around the motor bracket holds the tongue in place. It's cheap and effective in many cases, though not all.
• Other causes to check:
  • worn teeth on the gearbox output pulley or the drive gear;
  • a worn belt;
  • the gearbox having come off its lip on the motor end;
  • on high-voltage conversions, internal motor damage.
• Prevention: always pedal away first, then use the motor.

The word “narrow” on the MATCH-AURORA list suggests a wider belt may also have existed by then.

• Any HTD 400-5M-15 belt (400 mm, 5 mm pitch, 15 mm wide) fits. These are a standard industrial size, widely sold for a few pounds.
• 18 mm wide belts (HTD 400-5M-18) are used by some owners to reduce slip. Check clearance before fitting.
• Some owners pair a wider belt with a metal drive gear. Chas's 72-tooth version needs a matching longer belt.

Gearbox · Drive gear · Motor · Rear axle · Motor support · Chain

• Sinclair C5 Owner's Handbook (1985), pp. 23, 25, 30–32
• Sinclair Vehicles, Notes on Replacement of Output Gear (Part No 101036)
• The Unofficial Sinclair C5 Service Manual, “Notes on C5 Motor/Gearbox Assembly” and “Known Problems”
• Hoover C5 Service Engineer Training Notes (1984): 6-monthly service schedule
• Roy Woodward, “Drive Mod: Modification To Stop Drive Belt Slipping”, sinclairc5.com (c. 2003; Internet Archive)
• ESG parts and price list, December 1991; MATCH parts price list, January 1991; MATCH-AURORA order form
• c5owners.com forum (closed to new posts, still readable):
  • Parts: motor brushes, drive belts etc.: Fezza_rs, Jul 2013
  • Noise from motor, sounds like machine gun: Dan, Mar 2014
  • Help wanted: power: Ray1, Oct 2014
  • C5 mint condition: Karl, Dec 2014
  • Metal gearbox insert (68 vs 72-tooth drive gear): SINCLAIR C5 CHAS, 2013
  • C5 for sale (18 mm belt fitted): C5mick, Feb 2015