5 Dec 2015

5th December 2015 Shimshramergo II: The hideous [virtual] reality!

The image shows how two simple levers fixed together and pivoted near the front gear changer can alter the pull ratio of the cable to match Ergo road levers to an MTB rear changer. The radius arm lengths shown are nominal and can be altered provided they maintain the correct length ratio of 1: 1.38 at the cable clamps at their tips. Note how the tips of the radius arms move on a tangent to the original cable runs. This is important to minimize any complications like variations in pull-per-click, badly skewed pulls on the cable stops, adjusters and bottom bracket cable guide. It is vital that the rear arm [shown in green] clears the bottom bracket when pulled fully forwards in bottom gear. 

An alternative approach would be a single radius arm. Though this would require the downtube cable is redirected over the bottom bracket to meet the single arm at the correct point and angle to maintain the necessary length ratios. Otherwise a single radius arm would have to be made ridiculously long to provide the correct ratio change with an under-the-bottom-bracket cable run. 

Down tube clamps, with rear changer cable guides [or even pulleys] have been available for many decades and could easily provide the necessary cable detour to the upper clamp over the bottom bracket. The higher the radius arm is pivoted the easier it becomes to obtain optimum pulls, more accurate pull ratios and flatter curves at the cable clamps. The closer the radius arm is pivoted to the seat tube the better the tip follows the normal chainstay cable run. The cable need not even be divided at the radius arm and could easily rely on friction on a labyrinthine path of pegs instead of cable clamping bolts before continuing on to the rear changer. It's all "swings and roundabouts" with "alternative" Shimshramergo designs. ;)

And now, a dose of mock-up reality. I fixed a front changer clamp as high as possible on the 35mm seat tube. Then dangled a long hex key in the changer fixing hole to simulate the radius arm. You can think of the ball end as the lower cable clamping bolt. The tape on the chainstay is 36mm wide to show the full arc required by the rear changer cable movement. 

The 94mm mm dimension is the radius from the pivot down to the first cable clamping bolt.  This is where the cable from the gear lever is anchored at the correct ratio of 1:1.38 relative to the lower cable clamping bolt.

The hex key proved that the radius arm would need to be cranked outward at the bottom, over the chainstay, to avoid the front changer cage when riding on the inner ring. Otherwise there is nothing else to get in the way.

I have [rather crudely] drawn in a downtube pulley to suggest how the cable running beside the downtube could be turned to pass over the bottom bracket. This is to ensure the cable reaches the upper pinch bolt as close as possible to 90 degrees. This angle is important to minimize variations in cable pull between sprockets as the rear changer moves across the cassette. There is very little room for error on an 11 speed index gear system.

There would be room for the radius arm to actuate between the chainstays if it was cranked slightly. This option has cosmetic potential for a stealth pull ratio conversion on a trike but would be in direct line with the rear tyre, road spray or the mudguard on a bike. Fitting the radius arm under the front changer is probably optimum for those daft enough contemplate copying my idea for themselves. Those using triples, which just clear the chainstay, would need to be wary of contact between the lever and the inner chainring or chain.

Click on any image for an enlargement.

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