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C15 - Frame & Suspension




The frame on the C15 consists of two main members. The front engine cradle and the rear seat  support are bolted together, the top joint having a steel bush fitted through the members,  and the lower at the swinging arm pivot where it is retained by bolts. The rear seat section also provides attachment of the rear suspension and mudguard. The C15 and B40 frames are similar have different diameter top tubes, the B40 is bigger.

The easy way to tell them apart is by the front mounting lug for the petrol tank The tank front sits on 2 rubbers, which are mounted on a pressed steel bracket. On C15 this bracket loops up and over the frame tube, saddle fashion, and is welded to it. On B40 this bracket is a straight channel shaped pressing which is bolted to two small lugs welded to either side of the top tube.

Click on the pictures view detail



Swinging Arm

The swinging arm suspension is mounted onto the rear of the frame by a large steel king-pin. This has grease nipples in the bolts at each end to lubricate the brass bushes within the swinging arm unit. The brass bushes fitted to the rear suspension swinging arm are unlikely to need replacement for some considerable time. When it is found necessary to renew them, first remove the suspension units by detaching the top pivot bolts and the bottom retaining nuts. Remove the rear wheel and chain guard. Then undo the swinging arm spindle nuts and carefully tap out the spindle using a suitable drift. Lift the rear fork until it is clear of the side plates, when it can then be turned and pulled away from the rear. The bushes should be removed with a suitable drift and the new bushes will need to be reamed out to size after fitting into the swinging arm.




Front Suspension Units

The front forks consist of a steel stanchion, this slides within the lower fork tube which contains the damper oil. The bearing surfaces consist of two brass bushes, one fixed to the bottom end of the stanchion, and the other at the top end of the lower fork tube, through which the stanchion slides. Fixed to the bottom of the  lower fork tube is the damper restrictor rod which passes through the stop nut that is screwed into the bottom of the stanchion.  The restrictor rod is tapered at the bottom end to give progressively harder damping as the suspension begins to bottom out. At the top of the lower fork tube there is a screwed collar, which is chrome plated and contains an oil seal. The cover tube pushes into the top of this collar and on later models also contains an oil seal. On reassembly it is recommended that grease is smeared around the bear metal of the fork stanchion to prevent corrosion while in use.

Click on the pictures view detail


Rear Suspension

The rear suspension units offer little opportunity for maintenance. You can remove the springs, but the damper units are sealed. The C15 models were fitted with a spring of 124 lb/inch, which is retained by chromed collets fitted at its base. A service tool was made by BSA to facilitate removal. As the above tool is unlikely to be available, I have designed a similar tool which will do the job. Approximately half the tube on one side has been removed (B) to facilitate the extraction of the chrome collets. This is assembled as shown and when the nut is screwed down sufficiently the spring is compressed releasing the collets. The collets can then be extracted through the apertures in the tool and the spring comes away when the tool is removed. Reassemble is in the reverse order. 



Additional Rear Shock Information

BSA C15 Shocks up to 1962 were 10.9" Long; from 1963 they then went to 11.9"

Click on the pictures view detail

Difference between C15 & B40 Frames

C15 and B40 frames have different diameter top tubes, the B40 being bigger. It is not easy to spot by comparison without careful measurement. An easier way to tell distinguish them is by the front mounting bracket for the petrol tank The front of the tank sits on 2 rubbers which are mounted on a pressed steel bracket. On C15 this bracket loops over the frame tube, saddle fashion, and is welded in place. On B40 this bracket is a straight channel shaped pressing which is bolted to two small lugs on either side of the top tube using spacing tubes.


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