I can get 4th low gear even without touching the downtube cable adjuster or having the radius arm working. Which will get me mobile while I search the local bike shops for a 35mm hinged clamp for a DIY downtube pulley. Or even an old fashioned curved cable guide would do. I will take some clamps to show what I need, but in the larger 35mm size.
The image above shows the results of my labours though I seem to have missed some patches of dirt. I don't think the new "dog's wotsits" look too out of place. I'll leave the fine tuning of stops and B-screw until I have the indexing working properly.
Rode to Assens to buy a front changer clamp and was offered an excellent recycled one for small change. I glanced at MTBs on display in one bike shop and every one of them had the XT rear changer! So much for exclusivity. Though I doubt another soul on the planet is daft enough to be trying to use Campag Ergo levers with the XT11. The gears worked fine but with some missing. I can finish the pull-ratio arm later after riding an errand. I came up with another clamping mod which will save me chopping the cable in half. 19 miles with more to come. Plus 7 more miles.
Plan B, this morning, was placing the ratio arm between the chainstays. Not ideal from several points of view but it worked instantly once I threaded the cable through the two clamps and put some tension on it.
I had to lower the arm pivot a little to ensure the cable clamping screws did not hit the chainstay.
The pulley clamp wants to be reversed to place the pulley further out of the way. Though nothing interferes with it, being hidden behind the chainset, it just looks a bit silly and badly exposes the downtube cable run.
Fortunately it changes onto and off the largest sprocket without contact with the frame.
I had problems with chain clearance on the Higgins when I tried to increase the size of the largest rear sprocket to 28 teeth. The sprocket cleared the axle loop but the chain takes a higher route as it climbs on and off the sprocket during gear changes.
Rear view of the arm rather crudely bent around the chainstay. This position wraps itself around the seat tube but it has just occurred to me that the arm pivot can easily go behind the seat tube simply by reversing the clamp. Silly me! I shall change it to the more sensible arrangement after the morning coffee ritual.
It is all in a fluid state as yet with simple nuts and bolts as pivots. I don't even have enough Nyloc lock nuts yet. The arm is a rather crude strip of aluminium alloy but noting more is really required. Cosmetically it leaves something to be desired but I looking for a donor object to improve matters. Something thicker and squarer would probably look well.
I shall see what it needs when I have re-arranged the arm clamp.
There is actually no need for two two clamping screws on the arm as either would do to provide a cable lock. A single screw in the lower position would allow the downtube cable to achieve its natural tension. At the moment the rear cable tension can be higher than the front. Perhaps it needs a small pulley [or two] on the arm instead of a cable clamp to even out the tension?
Click on any image for an enlargement.