4 Dec 2015

4th December 2015 Shimshramergo: All the pretty toys!

I need a lower bottom gear but am unwilling to compromise with a triple chainwheel. So it's double or nothing and a major headache to achieve a sufficiently low gear. My present bottom gear is 33t front x 32t rear for 29". A 33/36 would give me 26". Which may not sound like much difference until you realise that the 10th largest gear of the new, 11 speed MTB cassette has  32 teeth. So an extra  crawler gear lies beyond that. Which means that I can maintain my cadence on even steeper hills or pedal with less effort on the same hills. As an officially recognized old fart I ride a trike which is often laden with shopping. So you carbon weight weenies can stop sniggering at the back! I need that granny gear as much any other old fart. [Male or female.]

The [non]shiny new Shimano XT11 M8000GS MTB rear changer has a rather low lateral cage movement per lever click of 1.1. To balance this the MTB handlebar lever has a rather long throw per click of 3.6mm. The final results are the same as any other perfectly matched indexing system. The chain is moved safely across the full width of a 39mm 11 speed cassette with precise indexing in perfect order. 

Now throw a very large spanner into the works and demand that the 11 speed Campagnolo Ergo "brifter" lever does the indexing. Being a bit of an idiot, though cosmetically smart, the MTB rear changer won't know what to do with the wimpy 2.6mm per click of cable movement. It will scramble the first few changes up the cogs and then go completely haywire as it rapidly runs out of total cable movement. 

Something has to be done to increase the Ergo's 2.6mm cable movements [per click up and down] to match the [officially approved] MTB levers. The difference between 2.6 and 3.6 is 1.38. So a 1:1.38 ratio increase in cable movement anywhere in the cable run between lever and changer will fool the M8000 into thinking it has an MTB lever yanking on the other end of its control cable. Earlier posts have covered the choice between a bellcrank or double pulleys to achieve the change in ratio. After struggling to make my DIY pulleys pivot on a non-existent space on the bottom bracket shell I have settled back on a bellcrank system for the moment.

The advantage of derailleur gears is that it is all about cable pull without any push. The rear changer spring tries to maintain the same tension whichever gear is selected. So anything which intervenes in the cable system is always under tension. This simplifies bellcrank design down to resisting only tension between the pivot and the the cable anchor points. Two radius arms, fixed together, but with different lengths matching the 1:1.38 ratio, will provide our increased cable pull per click. Indexing with the the Campag Ergo lever should be identical with an MTB shifter. As per the images in the next post. There should be no extra lever resistance when changing gear because the pull tension is so much lower on the rear changer. All thanks to the very long actuating lever.   

This image shows the back of the Shimano XT MTB rear changer [as usually seen through the spokes on a bike.] I have marked the important measurement between the cable stop and the clamp on the actuating arm in the foreground. To achieve a change in cable pull ratio demands that the cable moves 36mm to cover a 39mm total sprocket width on the cassette over 10 clicks using a 1.1 shift ratio. The Campag Ergo lever manages only 26mm total cable movement.  [11 speed uses only ten clicks each way otherwise it would be a 12 speed. The chain is already resting on the largest or smallest sprocket so either of these sprockets whether you start counting from gear no 1 or gear 11.]  The M8000 is sent out with the stop screws fully in which limits it to 26mm total movement. I backed off the stop screws with a tiny hex key to find it can cover a much greater width of cogs.

The top face of the M8000 changer is the one you usually admire as you glance down desperately hoping to find another lower gear is available.

Complex, but superbly styled to appeal to the [middle of the track] MTB'er without bottomless pockets. Other MTB changers can run up to nearly 4 times the price. This one still does 11 speeds but without some of the bling and weight saving measures of the top end stuff.  The cable stop is at the top in the right foreground and the cable clamp below that in the image. It all looks very compact, when folded up like this, but can stretch to 11 speeds and a giant, 42 tooth largest sprocket. It even has a clutch to limit chain suck and floppiness over the rough stuff.

This image shows the shiny new Sram PG1170  11 speed 11-36 cassette disguised as a 10 speed 12-36t. I was about to email the supplier when I discovered the 11 tooth 11th gear sitting on the back of the packaging clip under the locking ring.

Just in case there is any doubt about the size of this thing I have arrowed the marking on the 36 tooth bottom gear. The largest sprocket measures 146.5 mm in diameter across the tooth tips. That's 5 3/4" in old money!

The cassette feels surprisingly light considering all the metal involved. It must be all that air filling in all those empty spaces. I presume this level of complexity is laser cut these days rather than CNC milled. Or even water cut?

The obverse shows the 11tooth sprocket tucked under the locking ring and held by a retaining clip all mounted on a dummy plastic free-hub for carriage and display. Sram really know how to make a cassette go faster with all those pretty perforations! You could clean the whole thing with a blast of soapy water. Or collect an even larger load of muck than a solid object! Still, it's a truly remarkable engineering design for what could otherwise be a very heavy and clunky object. 

I believe the Sram 11 speed MTB cassette has the same overall dimensions as a Shimano offering. i.e. 39mm wide with a 3.9mm sprocket pitch being the most vital statistics. 

BTW: Lever cable pull per click = sprocket pitch divided by rear changer ratio. So, 3.9 / 1.1 = 3.54mm. [Listed as 3.6mm in the tech websites below.]

Now all I have to do is design and make a suitable bellcrank to carry out the 1:1.38 increase in cable pull using the simplest possible mechanical conversion.


Click on any image for an enlargement.

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