17 Dec 2015

17 December 2015 Being pulled both ways.

Thursday 17th 48-50F, 9-10C, very heavy overcast, very mild, misty, drab, grey and miserable with occasional, light misty rain and a steady breeze. Visibility was about 50 yards and then everything rapidly disappeared with greater distance. The mist cleared and returned slowly and was a bit patchy and thicker with greater altitude. Even if it was only a few meters higher than the average. At least there was no need for gloves today. I walked my usual forest loop trying to avoid puddles and mud. If it gets any wetter I shall need waders!

Spent the dark and rainy afternoon playing with pull-ratio conversion, cable pulley ideas. I eventually made an eccentrically placed, middle pulley to act as a gentle ramp for the cable to climb between the smallest and largest pulley. It ended up midway between the two in size as well. This pulley will provide a tangential flat for the cable to lead on and off without needing cable clamps, bends or kinks. Making the groove narrow really increased the friction compared with the wider V-grooves of the original pulleys. This suggests that deepening and narrowing the other two pulley grooves would have similar benefits. The middle pulley has very little cable wrap so will not add much friction by itself. I have yet to file [or mill] flats on the rims to allow the cable to cross to the next pulley along. It's quite a hefty lump of metal now! I'm really  not sure the pulleys need to be so thick at 5mm each for 15mm total. The gear cable is only 1mm in diameter.  Not easy to photograph the triple pulley combination with flash. Far too dark not to use it. No ride today.

Friday 18th 48F, 9C, modest winds, very heavy overcast. The promised potential showers did not materialize. Spent the morning looking at improving the gear change arm. I modified the front changer clamp which I use for the arm pivot. I have bored it oversize for a brass bush which will be squarer to the clamp than the original [skewed] hole. Using a 10mm end mill I produced a flat on the back of the steeply sloping area. I have also filed away the curved area where the front changer would normally have fitted. The 35mm pulley clamp will provide support for a short length of cable outer instead of the previous brake noodle. This will guide the downtube cable over the top of the bottom bracket to meet the ratio changing arm as near as possible to a right angle. Hopefully with less friction than before.

Indexing relies heavily on low cable friction to avoid variation in cable pull between gear changes. Particularly when changing to higher gears.[i.e.Smaller cogs.] Which relies entirely on the tension of the rear changer spring to pull the cable back through the entire cable run. Changing down, to larger cogs, relies only on the finger strength of the rider to pull on the lever. Though even here the ratchet pawl will trip and release the cable if the resistance is too high. Apart from any friction in the lever itself there is the length of outer following the handlebars. Then the downtube adjusters take their toll along with whatever is used to bend the cable around the bottom bracket. Finally there is the chainstay stop and the loop to the rear changer.

Modern gear cable outers are lined with a low friction hose and usually have longitudinal reinforcement instead of the usual steel spiral. This is to achieve tight control over outer length so that it does not affect the indexing through unwanted backlash. All inner cable movements are subject to the total length remaining as fixed as possible to achieve absolutely fixed steps in cable movement per gear change. The more gears available the smaller the allowable variation before the chain rattles on the next sprocket or overshoots it.

The image shows how the angle of the cable changes depending on its route. The route over the BB [green] is better in one sense but the pull is from off to one side due to the seat tube intervening. The lower route, [red] under the BB, climbs at a steep angle but approaches the arm straight on. Neither is ideal since non/perpendicular cable pulls will produce variations per click over a complete stroke of the arm. Oblique angles of cable pull will load the arm and its pivot with unwanted forces. Flexing the arm sideways, with an oblique pull, may easily cause variations of pull per click.

A small pulley behind the BB could help to straighten the lower route back to a nicely horizontal pull but might add friction. While  moving the arm on its pivot further back will reduce both cable approach angles at the expense of further complication. A much longer arm could be pivoted from a clamp fitted above the front changer. The slope of the seat tube ensures the arm moves steadily backwards with increased pivot height. The downside is a more "untidy" appearance unless the clamp can be made much less visually intrusive. 

The extra weight of a longer arm is too slight to be worthy of further consideration. One obvious advantage of the longer arm is that the spacing between the cable clamps is much wider for the same ratio change. So that the clamp bolts will safely clear the chainstay. The arm could then be made to hang much closer to the chainstay if required. Which would further reduce the obliquity of the cable pull. The image [left] was from an earlier trial to see how a longer arm would look and behave. The sagitta of the arc is also shallower with a longer arm. Further reducing pull errors across the entire stroke. A black pivot clamp might look better since it would match the front changer clamp. All this could be avoided with pulleys but those too have their own disadvantages. I have still to seriously examine the idea of a pair of pulleys to avoid kinking the cable sharply around the present clamps on the arm. The shorter arm does not lend itself to such pulleys. The pulleys would have to be made woefully small to avoid overlapping.

One can easily understand why the manufacturers demand the user stick to one group of their own products to optimize indexing. Those who, like me, decide to change that rule don't have a leg to stand on if they start complaining about the resulting gear change. It is incumbent upon Shimergo fans to make certain that we carefully optimize whatever combination we think will work. We must pay particular attention to the geometry of lever pull ratios, lateral changer movement and sprocket pitch. Hoping for the best, while breaking these most basic rules, will lead to bitter disappointment or even broken equipment! Set up is even more critical to avoid a rear changer in the spokes or the chain jammed fast outside the top gear sprocket. Even a single missed gear can be life threatening if one is sprinting to avoid a fast moving vehicle! Let's be careful out there!

Just a short, hilly ride in the middle of the afternoon returning after dark. It was quite windy by then. My loudly whistling tinnitus has lasted all day today without a break despite an oiling of my left ear. Only 10 miles.

Click on any image for en enlargement.

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