Modification To Stop Drive Belt Slipping

As I explained in My C5 Story, I have long had problems with the toothed drive belt on my C5 slipping, causing a loud "clicking" sound when power is applied to the motor. Previous attempts to cure this slipping involved packing the motor bracket with plastic shims, and making the motor bracket very tight. However, these "solutions" only proved partly successful, and never fully cured the problem.

I first became aware of an official Sinclair Vehicles modification for this problem when I loaned some C5 material from Perran Newman. Among this was some information about a metal bracket, which had been designed in 1985 to cure a problem found with some C5s - the slipping drive belt. This modification had been fitted to many of the C5s still in stock in mid-to-late 1985.

The problem is caused by the design of the motor mounting bracket. To save on plastic, the ledge that supports the bottom of the gearbox housing is very narrow. This enables the gearbox housing to slip behind the ledge, and instead of the motor being held parallel to the axle, it can now be pulled towards the axle under the tension of the drive belt. This means that the belt is not running square to the drive gears, and the belt tends to slip, damaging both the gears and the drive belt in time.

There are two ways you can fix this, both of which are shown below. The official modification is the metal bracket. An easier option, which seems just as effective but is less elegant, is to use a Nylon cable tie wrapped around the motor mounting bracket. See details below.

Bracket Method

The bracket is shown in the photos below. It simply pushes the motor support back away from the belt, and stops the gearbox housing from dropping behind the ledge. I obtained the dimensions of the bracket, and got a metalworking friend of mine to make one. As soon as we fitted the bracket, I noticed an immediate improvement in pulling performance - the C5 was quite happy to perform a standing start, although this is not to be recommended as it places a huge strain on the motor and gearbox!

Below are two drawings: The first is a technical drawing showing the design of the bracket - all dimensions are in mm. Armed with this drawing any competent metalworker should be able to make a bracket. The second drawing is the official fitting instructions from Sinclair Vehicles.

Thanks to Perran Newman for the original information, Paul for making the bracket, and Dennis for the technical drawing.

Nylon Cable Tie Method

You need a heavy duty, long Nylon cable tie. This has got to be long enough to pass all the way around the motor mounting bracket as shown in the photo, and be tightened to pull the top support on the bracket underneath the gearbox housing. If this is unclear, look at the bracket photos above - you are trying to do the same thing with the cable tie.