As promised, the Mercian that I ride with my son; about 1978 vintage. The only twin lateral frame that Mercian ever made (so they say). Unusual in that it does not have an eccentric front BB and it was originally set up for straight through drive - tension in the driver chain is maintained with a 56T TA chainwheel - when the chain is worn it falls out. It is an improvement on the original tensioner that was very Heath Robinson and excessively noisy. I have kept this arrangement and upgraded it to a triple - with a quad rear chainset on the back. It requires the longest front BB spindle I have been able to find with a load of spacers. When that wears out I shall have to think about what to do. Left hand drive kiddy cranks also take quite some sorting out too as you have to swap the pedals on their axles - they are not symmetrical. The wheels are 650B with 32x584mm Wolber tyres - a lovely ride as well as quite swift when Alexander and I get going. It took me 16 years to get my hands on a pair of 40H 650B NOS rims which I imported from the States earlier this Autumn - well worth it as I was then able to fit the drum brake on the back which works far better than the useless rear Suntour SE brake that is fitted at present. My son thoroughly enjoys riding it with me(he has an independent freewheel). Next year he'll graduate to my double marathon frame as a true stoker - no freewheel then!
Mick F wrote:What an interesting drive-train! Any chance of a close-up shot?
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As requested.
Details:
a Stronglight 49D chainset at front with a 38T driver, connected via the 56T TA 'ghost ring' as driver chain tensioner (lighter than an eccentric) to a 38T rear driver chainwheel. This is mounted outboard on the outer bolt circle of the 48T chainwheel with a 40T middle and 26T inner fixed onto the inner bolt circle (IIRC). The RH crank was broken (an old TA cyclotouriste) so that has been sawn down to form a mounting boss (avoids a spare crank whirling around on the RHS). The Kiddy crank drive has a 26T driver connected via a 1/8" chain onto the largest singlespeed freewheel I could find at the time. I think that was 17T. That is mounted on a freewhhel threaded boss that mounts onto the square taper of the bottom bracket spindle. This is the downside; it means that the kiddycranks are overgeared, but at least my lad can rest when he wants to. Obviously, with the crankset reversed, the pedals have to be as well; ie pedals swapped from axle to axle; another remotely possible downside - the pedals could unscrew on account of (what Sheldon Brown calls) precession and Mr. Juden called the contra-rotating load principle...I think. however, my son is a 5 year old (today in point of fact) and not strong enough to make this a reality. the rest of the drive train is traditional to a 6 speed freewheel with a changer on Suntour command shifters - it does index - eventually, so I prefer to use them in friction mode. It took some sorting out but I have done this a few times with three tandems and two previous daughters so I am used to the tinkering that fitting kiddy cranks occasions
You can use the ghost ring to tenion the chain on a fixie - the trick is figuring out how to thread the chain stay through it, once you've got that sorted you are on to a winner!
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