5 Dec 2015

5th December 2015 Shimshramergo: Getting my hands dirty.


I started with a strip of 3mm light alloy about 12mm wide [1/2"] and long enough to provide a pivot hole at the top. Plus some room to take up the bends and a space for a bottom pinch bolt. The pivot hole was drilled and dangled from the pivot clamp to check where it needed bending. I had to file a square notch from the front changer clamp which I had donated to the project. This had to clear the Campag Athena front changer clamp to allow the arm pivot clamp to rise as far as possible. The bends would alter the radii from the pivot so it was vital I waited to mark and drill the cable pinch bolts on the final form. A gentle twist was given to the clamp to compensate for the splaying of the chainstay. The arm now runs parallel to the stay without binding at the rearward position. I also checked that the front changer cage did not hit the arm on its fully inward position. A radius to match the bottom bracket shell was filed out of the strip to allow the arm to move fully forwards. It can be improved cosmetically when I have proved that it works as expected. At this stage I have no idea of the bending forces involved so have gone for overkill in the arm's dimensions.

The down tube is 34.9/35mm so I need to find a hinged cable guide clamp or clamp-on pulley in that diameter. It has to be hinged or very flexible to fit over the down tube. I can't just drill a hole for a pulley bolt right through the downtube!  Nothing I have in my collection of bits and pieces is that large and cast seat clamps cannot be forced open to go over a tube with fixed obstructions.

Clamp-on pulleys are readily available to convert pull-down to pull-up front gear cables and are popular with cyclo-cross riders apparently.  Whether they are standard fodder in Danish bike shops is quite another matter. I haven't a clue what to call them in Danish. Spændebånd? That's just the clamp. A pulley is a remskive or trisse. Searching online produces nothing like it using those terms. Readily available in the UK but they want a tenner for postage for an item weighing practically nothing and costing only a couple of squid. The posh one shown here is $30US from Problem Solvers. I'd prefer plain silver to hide against the silvery stainless steel Reynolds 931 tubing. Thought perfect for a cylo-cross bike the pulley shown in the image is also rather outboard for my purpose.

The Athena RD could not cope with 36t much as expected. There just isn't any more adjustment left beyond 32t. Extending the hanger downwards would just move the changer away from the small sprockets. Life and cycling is just not that simple.

Disaster struck when I tried the new XT11 RD on the Trykit. With the 'direct mount' link in place the cable stop casting jams below the rear axle reinforcement loop and other parts against the cassette! I need a completely different hanger from the supplied link to move the RD further forwards, not backwards.

The XT11 M8000 RD in the top gear position with the modified "Direct Mount" link in place. [Stop tab removed.]

I have no need for easy wheel removal on a trike so the 2cm rear offset provided by Shimano's "direct link" is completely irrelevant. I thought I should be able to mount the XT11 RD onto the Trykit hanger but the through bolt is incompatible with a threaded hanger. In fact it wasn't, but there was just not enough room around the cassette to push the bolt through the Trykit hanger to fit the RD directly. More research needed on this problem.

I watched at some YT videos abut the "direct mount" and realised the fixed Trykit hanger was never going to allow me to mount the rear derailleur in the optimal position. Mountain bikes seem to have more rear-set and longer hangers than road bikes [or trikes.] I wish some YT videos showed far more detail in close-up and much less vocal padding/sales hype/bullshit! 

In the end I simple filed the stop tab off the front of the "direct mount" link. Allowing the link to rotate on the brazed on Trykit hanger to the ideal position. Though I certainly do not recommend this modification to anybody else, it gave me enough forward and downward movement for the RD's very long cable stop casting to clear the trike's rear axle loop. I shall probably have to fit a thin, star lock washer or even use liquid thread lock to stop the link working loose.

The gear changer moves very easily with the present [Ergo] 2.6 pull ratio. Exactly as expected, it ran out of available movement just over half way across the block.  My ratio-busting, radius arm should take care of that.

View from below of the XT11 RD in place on the trike showing the [very] tight fit. I have arrowed the close clearance of the RD's cable stop. The XT11 can now rotate on the direct mount link exactly as intended under the control of the B-screw. The RD is curled up in top gear in a cage position it will never reach when a chain is fitted. BTW:That is not rust on the frame joints but is lacquer applied to stop the naked silver solder from discolouring. It just saves having to re-polish the joints with ScotchBrite fiber at intervals. 

Fortunately the 11-36T cassette fitted the trike axle from below provided there was no RD in place. 36t is probably as large as one can go on my Trykit without the chain rubbing on the rear axle reinforcement tubing. Which is extremely fortunate after all the effort [and expense] gone into it so far. Though the cassette and chain need replacement at intervals anyway as they steadily wear. It is also very fortunate that 11speed derailleur prices and cassettes with such large sprockets have become fairly reasonable. Or I would never have tried this experiment.

Bicycles/Maintenance and Repair/Gear-changing Dimensions - Wikibooks, open books for an open world 


Click on any image for an enlargement.

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