Tuesday 5 January 2021

Wheels, Gearbox and Chain Alignment

With the purchase of the 1931 AJS SB8 project I had a frame for the bike in which the engine would fit but I needed to convince myself sooner rather than later that I could also squeeze in a gearbox and a rear wheel with which I could get chain alignment.

I had 3 candidate gearboxes: the AJS/Sturmey Archer gearbox that had come with the SB8 project bike, a Norton/Sturmey Archer “Dolls Head” gearbox and a Sturmey Archer EIV gearbox that I had actually acquired for another project. The first step was to narrow down these choices.

The EIV gearbox was dismissed immediately because it simply wouldn’t fit into the available space.

That left either the Norton/Sturmey Archer Dolls Head gearbox, shown below

 Or the AJS/Sturmey Archer that had come with the SB8 project

The Dolls Head would have been a technically superior gearbox; it has a positive stop incorporated into the “head” part of the Dolls Head, it is 4 speed, it is a very strong gearbox and this one happens to be close ratio. Some years ago I was running one of these on a Levis sprinter.

And apart from having to face off the dogs on one of the gears it never ever gave a problem and was a joy to use.

However…. although the Dolls Head appeared to squeeze in the available gap, it didn’t really fit, at least not with any chain adjustment and this gearbox therefore also had to be excluded.

And so, by default, the AJS/Sturmey Archer ‘box that had come with the project bike was the only one that would physically fit and allow longitudinal movement. Decision made! Although I would only have 3 gears and would need an external positive stop (or hand change) this is nevertheless a strong ‘box and looks much like the gearbox on the original bike. As I had previously stripped this gearbox to make some thread repairs I knew that the case and the internals were in excellent condition – all gears, shafts etc were “as new” and I had already replaced the bearings.

The next step was to sort out chain alignment. I was not too worried about the alignment of the primary chain because I had yet to make the crankshaft, the engine sprocket and, indeed, the complete clutch and so I had freedom of design on this part of the transmission. I had decided to use the Royal Enfield rear hub that was used on many vintage bikes – Norton, Scott, Broughs etc because it contains an excellent shock absorber and obviates the need to put a shock absorber elsewhere in the transmission. My experience of this rear hub on 3 vintage Nortons has been very good; one can feel the “thump” of a firing stroke being absorbed by the cush drive within the hub on these single cylinder bikes, especially under low engine speed and high load conditions.  This link provides an excellent set of pictures of all the main components. Luckily, there is an engineering company (JHB Engineering in Rye – no website) that makes all the parts for these hubs in the UK and Scottparts make the rear sprocket. A good buddy of mine had a spare rear brake plate and brake shoes and so all the constituent components for the rear hub were there.

The next step was to juggle the gearbox around in the frame by putting it on pieces of wood of varying heights to work out where the chain line would go so that the offset for building the rear wheel could be calculated.

These details, together with all the components for the rear hub and the original front hub were then sent to my local wheel builder. The front wheel has a reasonable size 7” diameter brake - the same as fitted to my 1930 cammy AJS and which is a decent brake and the Royal Enfield rear hub has a 7 ½” brake which has excellent stopping power. It is important to paint the hubs before building the wheels as this is not easy when all the spokes are in place.

Steve, at Wheelwise, has built a lot of wheels for bikes that I have restored in the past. I just checked on his website and on this page I counted 8 bikes for which he has built my wheels: a Matchless G80C, a Velocette Thruxton, a Velocette KTS, 2x Model 18 Nortons, the AJS K7, the AJcette and an Ariel V-Twin. I wanted the wheels for this bike to have the same “skinny” appearance of the original  bike and so new 21” rims were laced onto the hubs with black spokes and nickled nipples.

A few pieces of aluminium were welded together to support the gearbox in place and some aluminium strips were set up as temporary engine plates.


And, together with new tyres and tubes, this allowed the engine, gearbox and wheels to be assembled into the frame for the first time to resemble a V-Twin motorcycle.

 And check that the final drive chain had ended up in the correct place

The chain alignment was fine but the chain was rather close to the intermediate frame tube

And I therefore decided to gain a bit of extra clearance by using a 5/8” x ¼” chain rather than 5/8” x 3/8”. The thinner chain would be quite strong enough – the Levis sprinter had used the same chain and the 500cc JAP engine running on a 14:1 compression ratio using methanol would have been rated at around 45 bhp @6000 rpm. I would estimate this V-Twin to be rated at less than 40 bhp and so power transmission with the lighter chain should not be a problem. The rear sprocket was thinned accordingly to accept the ¼” width chain and a new final drive gearbox sprocket was made.

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