4 Jan 2016

4th January 2016 Swinging arm design completed.

Monday 4th 23-29F, -5--2C, gales and feeling bitterly cold again. The wind is supposed to drop from gusts averaging 17m/s, 38mph to a "balmy" 15m/s or 33mph. With these winds and the low air temperatures the wind chill factor makes it very unpleasant indeed being out of doors. Just handling any metal in the unheated shed is painful. Heating the uninsulated shed, just so I could work out there, would result in massive condensation and rust on unprotected metal. Besides, it would take ages for tools and metal to reach comfortable handling temperatures. The replacement Campag levers, which I originally ordered on the 15th December from Wiggle, have finally reached Billund sorting center in Jylland. Expected delivery is still listed as "by end of today on the 4th" but it hasn't been marked as "out on the van for delivery" yet. Only as being the hands of the delivery agent.

The trick when it is only 28F in the shed to avoid one's nose dripping on the item being worked on. I botched together a quick arrangement of two [grommet] pulleys on a scrap alloy crossbar. Only to discover I should have been more generous with the spacing to allow the pulleys to clear the central, crossbar fixing screw. Still, it will take only a few minutes work to put something better together.  I also want the pulleys flat on their crossbar to reduce the depth to something which would fit behind a chainset to make it more universal. Having the pulleys pressing on the crossbar would also reduce the risk of their rotating.

The plan after lunch, when my feet have warmed up again, is to to make a longer pulley-supporting crossbar. This will correct a number of interference problems which cropped up with the first trial construction.

Another thing which I hadn't foreseen was the limited range of ratios available from a given arrangement of two pulleys on a pivoted arm. I had fondly imagined that by simply rotating the crossbar I'd achieve a whole range of desirable pull ratio changes. It wasn't to be. Measuring with a vernier caliper, with the narrow jaws sunk into each pulley groove in turn, resulted in some odd ratios. This is the result of two equal sized pulleys, equally spaced from the central pivot. A more asymmetric layout will help to break the lock-step in ratios.

On the left is the new version of the swinging arm with this morning's problem's corrected. The 74mm measurement is simply a suggestion which fits my own trike. The pulleys are now fixed flat and immovably on the adjustable crosspiece. With the 1st pulley bolt now acting as the ratio adjustment pivot for the crosspiece. The Ist pulley is now fixed as to its radius from the swinging arm pivot. [74mm] The 2nd pulley is bolted to the other end of the crosspiece to allow only a little clearance between the two pulleys. [To maximize cable wrap for more friction.]

Rotation of the crosspiece will lift and lower the 2nd pulley relative to the 1st pulley and the swinging arm's own [top] pivot. This movement will set the output pull-per-click ratio to match that desired. Then the 1st pulley bolt can be tightened to lock the 2nd pulley and crosspiece at precisely the chosen, cable pull ratio. The pulleys are merely a convenience to protect the cable from sharp kinks and fraying. While simultaneously providing considerable friction against the cable slipping against the grooved rims.

It might be worth sandwiching a thin, but large diameter, star type, lock washer between the crosspiece and the swinging arm to avoid any chance of the crosspiece ever working loose. Stainless steel screws and SS Nyloc nuts will be used throughout. The swinging arm can probably be trimmed in width for cosmetic purposes.Or neatly chain drilled if you want a "techy" lightweight look. My own swinging arm is just a strip of scrap aluminium sawn from a larger piece. It is very soft but will take a tapped thread if some extra degree of locking is desired.

In my case the bottom of the 1st pulley rim groove [measured at the top of the pulley] is fixed at 74mm from the center of the swinging arm pivot hole. This figure is multiplied by the desired ratio change to match the gear lever to the rear changer. I needed an increase of 1:1.38 to match my Ergo lever's cable pull, per click, to an XT11 MTB rear changer. After  multiplying the Ist pulley [rim] radius of 74mm x 1.38  I then rotate the crosspiece until I can measure 102mm. This measurement is from the swinging arm's top pivot center to the bottom of the 2nd pulley groove on the right. The red line shows the required cable wrap which is deigned to maximize friction of the cable against the rubber pulleys with horizontal input and output cable runs. Any cable slippage would undo the cable pull-ratio setting. If I did discover cable slippage I could easily add a non-damaging cable clamp bolted through a slight elongation of the present arm length. Or I could move the 1st pulley pivot hole upwards slightly to make room for a cable clamp, screw hole on the end of the swinging arm.

The downtube gear cable enters from the right to join the top of the 1st pulley. The cable is then wrapped as shown so that the output from the second pulley runs along below the chainstay from the bottom of the 2nd pulley. Until the cable meets the cable stop for the rear derailleur, outer cable. 

With this simple lever system you can run any rear derailleur with any handlebar lever. Whether the cable pull ratio needs to be increased or decreased on the way makes no difference. The crosspiece is simply rotated until the desired ratio change is achieved on the output end of the cable to match the needs of the RD. It is assumed that the gear lever lets out [and takes in] equal lengths of cable with each click. Some Campag levers alter the pull per click and even have different spacers between different cassette sprockets.

This latest iteration of the swinging arm is probably thin enough overall to go behind a chainset on a bike. With the arm pivot on a clamp just below the FD. There being no room behind the bottom bracket on most bikes a trike has masses of unused room back there. If the 2nd pulley gets in the way of the FD then the the crosspiece pivot could move to the 2nd pulley. With the 1st pulley now fixed on the far left of the crosspiece. There's probably not much more than $1.50 or £1's worth of material and half an hour of [almost] tool-free construction in the entire arm. The Shimano FD clamp, from which the arm is hung, might run to a £enner if you don't have one already. Or can't scrounge a secondhand one from your LBS as I did. 

The "pulleys" are blind rubber grommets designed for sealing a 20mm hole in a thin metal box. Usually where mains electricity is present but not intended for domestic use. The grommets are drilled through their blank centers and a washer [or washers] added to a 5mm center, fixing screw to stiffen the grooved grommet rim against being pulled out of shape by gear cable tension. 

Those with an experimental bent can try making pulleys from different sized grommets to match what is available. Provided you achieve the correct pull ratio the design can be shrunk slightly. The smaller the pulleys the tighter the cable must bend around the pulleys. Perhaps with an increased risk of cable fraying. A shorter swinging arm may introduce cable pull errors at the extreme ends of its arc. Do a full scale drawing and examine how smaller swinging arm radii affect the pull per click on horizontal entry and exit cable runs. Don't forget to allow for the full cable movement at the rear gear changer [RD] when you draw the full arm swing both ways. In my case it is 36mm.  

 If I did turn up some alloy pulley inserts in the lathe, instead of the present washers, I'd need to remember to countersink the screw heads.There is no need to use pan-head screws when stainless steel CSK screws would do a much better job on turned alloy inserts.

My replacement 11 speed Campag Chorus Ultrashift levers have finally arrived. I can now set up the gear system and test for indexing with the XT11 8000 MTB RD. Only 7 miles today into a freezing headwind on the first leg. The tightly knitted balaclava I wore was not very windproof but helped to protect my cheeks. I will wear a GripGrab scull cap underneath next time I need a balaclava on the trike. I must also search for skiing goggles with lighter lenses. My eyes were watering on and off into the headwind despite wearing wrap-around cycling sunglasses. I bought a pair of skiing sunglasses with frame edge seals online but they are so dark they are hopeless in anything but high altitude alpine sunshine on snow! My visit to a skiing shop in the city did not turn up anything with lighter lenses in skiing eye-wear.

Click on any image for an enlargement.


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