Wednesday, May 18, 2011

tweaks and alterations

Today I worked on the problem of the chain cross which looked to be developing on the ride I took the bike on round Pitch Hill in Surrey. Chain cross is where the angle of the chain in the extreme ends of the gears is too wide. This puts stress on the cogs and chain and increases the wear and so eventually the frequency at which parts need to be replaced. Another issue was the failure to retain my lowest to gear cogs. At the start of the ride both these gears were readily accessible, but as the ride went on (lots of time in low gear climbing hills) I began to progressively lose the gears. For me this was terrible news as on a 42 tooth chainring at the front you are really going to need those bottom to cogs at the back.

Gear selection problems first. Investigating the cable housing I discovered that I'd actually cut a couple of sections too short (only just), but enough to induce some slack in the cable tension as it is pulled up by the shifters. The correct response to this problem, is I am sure, to cut new cable housing. However, I have taken a short cut and used some superglue to hold the cable housing to the end caps. This might not be a great solution and I could find that it all comes apart on the next hard ride, but I really couldn't face unlacing all the cable to do it properly. Eventually I will probably will have to do it, but lets hope its not until the whole cable is due to be replaced.

Chain cross is described in detail by Sheldon Brown. it is enough to say that on the ride the grinding noise was a good enough indication. When I rode out later, there also seemed to be a problem of chainrub on the retention device, which let me tell you makes it awfully hard to turn the cranks round. Going back to basics I measured the distance from the centre line of the bike to the chain ring, only to find it was outside the tolerance for the chain retention device. Not by much, just a couple of mm, but that can sometimes be enough. So it was a simple matter to remove the chain retention device but keep the bashguard. As it is unlikely that I will be doing any major downhill in the near future I probably won't miss it. There probably is some nifty way of fitting it which will allow me to add it back later when I come across it.

It all leads me to reflect on the original bike, which had a chain retention device. This used a BB-01 bottom bracket and not the BB-04 which I had to use. Clearly, the BB-01 must have been narrower and allowed the configuration. So one of the issues of retro-building is coping with evolving componentry. Some evolution is obviously better in terms of performance, but others are driven by market forces to develop customer sales. A bicycle is a fairly simple bit of equipment, real evolutionary developments can be probably be counted on the fingers of one hand. The changes though can have a profound effect on the modular nature of a bicycle. That is to say putting parts A and C on one frame and then changing them for parts B and D. The again who would want to put a rabbit's tail on a T-Rex?

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