Monday, 6 February 2012

Frank's Links

So, to bike building, starting at the back.

My Square Four, like many others is fitted with the famous Anstey Link rear suspension, designed by Frank Anstey to overcome the chain-tension problems inherent in the plunger design, whilst avoiding the complex frame re-engineering required when providing a true swinging fork. Mr. Anstey joined Ariel from Triumph, as Ariel's Chief Designer, in 1937. Here's an extract from Patent 498544, filed by Ariel Motors & Mr. F Anstey on 16th December 1937:

Patent Number 498,554. Spring frames for motorcycles. ARIEL MOTORS, Ltd., and ANSTEY, F. Dec. 16, 1937, No. 34840. [Class 136 (iii)] A spring mounting for the rear wheel of a motor-cycle comprises in combination, ...on each side of the wheel, a lever j arranged horizontally or nearly so at the rear end of the frame a, one end of the lever receiving one end of the wheel spindle m, as by means of a slot k, a fulcrum piece g having a pivotal connection, i with an intermediate part of the lever j means for carrying the fulcrum piece g on the frame so as to allow it to move in a vertical or approximately vertical direction, and means for attaching the other end of the lever j to the frame so as to allow this end to move in an approximately horizontal direction, as by means of a vertical link n or a horizontal guide, the movements of the parts being controlled by one or more springs.; The fulcrum piece g is slidably mounted on a guide pin f secured in a bracket c formed on or attached to the rear end of the frame a, the sliding movement being controlled by a main spring h and a rebound spring or rubber buffer, with or without a vibration damper, such as an hydraulic damper combined with the guide pin mounting. The lever j is formed with a forked part passing around the bracket c, the pivot pin i extending across the forked part. Thesprings are enclosed in telescopic tubes t, u. The two levers j on opposite sides of the wheel may be interconnected by links &c. and controlled by a single spring or set of springs

The  design uses all the 'normal' elements of a plunger suspension, but instead of suspending the wheel from sliding member it incorporates a cast steel horseshoe, which carries the wheel and pivots on the sliding member. This horseshoe is retained by a link, pivoting above the lower frame members and forward of the plunger allowing the horseshoe to travel through a short arc so that the drive chain tension was kept more constant during compression and rebound. Of course, this is not a perfect solution since the centre of the horseshoe's rotation is still a long way from the gearbox sprocket centre, but it is a lot better than a conventional plunger.

Of course, there are lots of pivots, sliding members & bushes to wear out...

Here are the bits I have acquired:

You can identify most of these from the sketch above. The long screw poking out of the horseshoe (and painted that WD green colour) is the chain adjuster, which replaced the snail cams. You will also see that my '51 bike has volute-type rebound springs in place of the previous coils.

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