29 Jan 2016

29th January 2016 Shimergo rethink.

Friday 29th 41F, 5C, heavy overcast, very windy and wet. A wild day is in prospect with gusts to 20m/s or over 45mph with rain. A low is passing north of Denmark after crossing the North Sea from the UK. 

For those of you following my Shimergo experiments I have come up with a new idea. A shall trap 16mm, 5/8" rubber tap washers, behind steel washers, flat against the pull ratio changing lever. This will reduce the offset of the cable arrival and exit. It should also avoid the damage the present clamps and sharp, cable bends are doing. The new "pulleys" will be mounted one above the other with the cable following a new route. The rubber washers and increased degree of cable wrap will provide extra grip without needing the present clamps. They will also be much kinder to the gear cable.

I had been completely blind to the possibility that the cable could enter and exit between the pulleys. This had narrowed the available range of pull ratios by forcing too great a distance between cable exit and entrance. By using CSK head screws, instead of the present very deep, hex socket head screws and cable clamps, the "pulleys" should become much thinner. The cable skewing should thus be much reduced.

By bringing the cable entry and exit between the pulleys their separation can be increased. Providing a much greater difference in potential pull ratios by means of the lever effect. I need an increase of 1:1.38 to match the 11 speed Ergo levers to the Shimano 11 speed MTB  rear derailleur. The rough digital drawing at the top of the page shows what I have in mind. With the weather being so awful today I shall have plenty of time to play in the shed.

The gear indexing hasn't been too bad [at all] over much of the cassette. Though there were some problems with the second largest rear sprocket. There was some indecisiveness which suggested that the present pull ratio did not quite match. The cumulative effect caught up at the far inward movement of the rear MTB derailleur travel. I tried cable tension adjustment but this did not have the desired effect. The cable bends were far too extreme and causing some slack where the cable could not follow a straight line.

The images show the first trial with the inner, parallel tangents of the rubber washers at 64mm and 88mm for ~1:1.375. I double checked the previous set-up, with the twin cable clamps and that was a little over 1:1.4. The fixings shown were all I had in the correct size so I will look out for some countersunk head screws and matching Nyloc nuts. Stainless steel fixings would be better if I can find them to avoid rust.

It is blowing a gale, with continuous sheets of rain falling, so outside photography is rather difficult.

Here is the ratio changing lever in the rearward position for top gear. [Smallest sprocket.] Notice how I have lowered the lever pivot clamp to make the pulleys lie equally above and below the chainstay. This was necessary due to the wider spacing of the new pulleys.

Winding the cable in a figure-of-8 around the pulleys needed some dexterity working between the chainstays. It helps to have the rear derailleur in top gear before commencing the cable threading. Thankfully the cable does not slip on the rubber pulleys and the washers make excellent cable retainers. This helps to avoid cable derailing on rapid changes over many gears which the Ergo Chorus lever can provide. The cable might become loose if the rear derailleur did not keep up. I could slice the rubber washers to half their width to reduce cable offset.

I looked again at the potential of running the cable under the bottom bracket but the angle is still too steeply inclined upwards to meet the top pulley. To use this route would not only upset the cable ratio changes but would introduce variability between each end of the cable pull.

So I am stuck with a short length of outer trapped in a clamp on the down tube as my cable guide. What I really need is an over the BB guide tube or channel. Though I'm not sure these are made any more. They used to be commonly available in the past when cable stops were usually fitted on top of the chainstay.

Ideally I would also move the lever pivot backwards. This would improve the symmetry of the lever movement over its full travel. At the moment it moves between vertical, when fully forwards in bottom gear. Then moves a long way backwards in top gear. Which makes the smallest sprocket indexed gear changes more insensitive due to the constantly changing difference in active radius at the pulley.

The present lever pivot is a bolt passing through a modified front changer clamp. Since making the changes I have been experimenting with the indexing and getting rather variable results. So I really must hang the lever in a more neutral [backward] position when the chain is running on the center of the cassette.

I am also struggling with the rear changer's "B-link" being too long. Which forces the cable entry too close to the trike's axle reinforcing loops. This may well be increasing friction at the bend. Wolf Tooth in the US do several straight, replacement links. Which I will have to examine in case these might help the Shimergo cause. They claim their Roadlink can make the long cage, Ultegra RD can run a huge sprocket! [42t] My immediate worry would be the larger gap between the pulleys and the smaller sprockets. I don't see how one can simply lower the rear derailleur without adding other problems elsewhere. Perhaps the derailleur body can automatically fold closer to the smaller sprockets?  

My emails have suddenly packed up again. Even web mail won't show new mails. I will have to wait until Monday for tech support. The gales grew worse all afternoon. Rest day.

Click on any image for an enlargement.


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