I decided that I wanted to use my Phillip's Twenty (one of them) to pull my utility trailer which would require me to fit a larger rear cog to reduce the gearing which would make the bike pretty impractical for commuting and utilitarian riding.
So I decided to build a dual drive.
I had already built up a dual drive on another SA three speed and figured that I could do it on my Twenty to give me another 6 speed with a good gear range and no overlap.
A Twenty differs from a conventional 3 speed in that they have fairly short dropouts, that were never made for a derailer, that also have a pronounced upward slope to the rear so I needed to modify the derailer hanger to overcome this.
Mounting the hanger in the dropout conventionally did not allow enough room for the axle, anti rotation washer and nuts to sit far enough back. My solution was to set the hanger as far back in the dropout as possible, mark where the retaining screw goes on the dropout plate and drill a small hole for the retaining screw.
Because of the angle of the dropout the stop on the hanger had to be ground down a little to allow the derailer to swivel forward and sit properly to allow for maximum wrap on the chain to prevent skipping. I had experimented with a few derailers with no success and found that an NOS Suntour Honour was perfect since they allow for greater forward adjustment than the Shimano derailers I was trying.
Because my Twenty is also equipped with a rack for paniers I had to grind a small half circle at the back of the hanger to line up with the existing hole in the dropout plate (which was half covered by the hangar) so I could reattach the rack.
The chain ring on a Phillip's Twenty is 3/32...
I removed the 1/8 SA cog and spacers and replaced this with two Shimano 3 speed cogs (3/32) that are dished, when placed back to back they allow good spacing for the 3/32 chain I used and take up the space that was occupied by the SA cog and spacers... there is just a little wiggle that I have yet to address although it does not seem to be causing any operational issues.
The shifting duties are being handled by a simple Shimano friction thumbie mounted on the left side of the handlebar although I do have some nicer thumbies like a pair of Huret half ratchets and some equally nice Suntour half ratchets.
So now I have a Phillip's Twenty with a dual drive and 6 speed range of 35.8, 40.3, 47.7, 53.7, 63.6, and 71.6 gear inches with no overlap.
The first ride was perfect as the shifting between the 16 and 18 tooth cogs is buttery smooth and for cruising home I just left the SA in third and moved between the high and low cogs which is a gentler 12% gearing change rather than the 25% change one gets in an SA hub when you change from 2nd to third.
The bike (Grace)
I can't wait to hook up the trailer but I have to modify my hitch assembly a little to accommodate that damn Twenty.
I hope you enjoyed the little twist.