Why Is This Happening?!Phantom Shift?!
 

So I'm huffing on an uphill road, slowly making my way. I have my friction shifter set to a comfortable gear, and WHAM! The sum***** says, "Oh hey, you doing ok? Nope!" And the chain snaps into a lower gear. If this has happened to yall, then you know how stupid one could feel when this happens right when your in the groove of things. I figure one of many things is going to happen; I'm going to chip a tooth on my cassette, I'm gonna break my chain OTR, or I'll go ass over handlebars at some point (preferred). Can some kind soul tell me why this phantom shift occurs? And how it is remedied without consulting a scrapyard exorcist?

The most common cause is a shift lever which has been slipping ever so slightly with each pedal stroke. Since the cable runs along the side of the frame (and down the center of each tube) as the frame flexes with each pedal stroke the cable gets a small tug then release, over and over again. There is friction within the cable's path which "stores" this slight cable movement as it builds up. At some point the amount of cable slack that has built up is played out all at once. If enough it will allow the der to move enough to shift. Yes this means that the lever is actually moving. The more flexible the frame, the greater the der's return spring tension is, the greater the cable path friction is, the less the lever's friction is then more this will happen at moments that don't seem to correspond with anything.

Modern index shift levers have what essentially is a lever with near infinite friction WRT the holding the shift lever in place. So this is rarely seen on a indexed system which is also well adjusted. Back in the day the more flexible AL frames (of traditional diameter tubes) saw more of this then the stiffer steel frames of that era.

There is an easy test for this. Disconnect the shift cable from the rear der and secure it off to the side where it won't snag the wheel or chain. Get a piece of "cable" only a few inches long with a end head on it. This could be a spoke, a gear or brake inner cable. Thread it through the rear der's cable stop (or adjuster) and anchor bolt. Manually shift into a cog you can ride on the up hills then tighten the anchor bolt on this short length of "cable". Play with the clamping location or the cable adjusting barrel to fine tune the der's being centered under the cog so the chain runs smoothly. What you have done is to essentially make a single speed that no longer has the full cable and shift lever holding the der in place. Now go for a ride up some hills. If my theory is correct you'll have no auto shifting under any conditions. Why? Because the cable route is now only within the der which sees no flex from pedaling.

I have used this test to show customers why their bikes are the reason they have auto shifting, not that some one didn't adjust the gears right (like me). After doing the test return the bike to the lever controlled der function (reattach the cable) and repeat the hill riding to confirm that the auto shifting returns.

Things that effect auto shifting is first the lever's friction. Enough friction and the cable will not move, of course you won't be able to shift either. The der's return spring is what's pulling the cable as the cable goes through the tug/relax cycle. So a der with a strong parallelogram spring has a greater pull. As the cable route's friction get greater from rust, grit, roughened BB guide or kinks the cable will hang up at some point along the route. Then some shock/force comes along (a bump or strong pedal stroke) and overcomes this friction and the cable releases or moves a bit. So a cable route with minimal friction will actually reduce auto shifting. This is why index shifting is so cable friction dependent. Lastly is the rider's component. Some riders use their strength in a jerky manor, others have a smoother spin. Guess which one sees more auto shifting all else being equal. Andy.

Some older Campagnolo shifters had a bad reputation for "ghost shifting" as the tension bolts would gradually back off until the friction was inadequate to hold the cable in place. I had a set of Campy C-Record friction downtube shifters on an older bike for a while and they did this unless I tightened the tension bolt every couple of days. Replacing them with a set of Sun Tour Cyclone levers solved the problem.

This is the issue that Simplex "retrofriction" levers and SunTour "Power" levers were designed to address. In the Simplex lever, a spring in the lever serves to counteract the derailleur's return spring so shifting against the derailleur's spring takes much less effort. The SunTour levers use a ratchet wheel in a similar manner. Either design is a superior replacement to plain friction levers.

I completely agree. have since 1973 when I got my first set of ST Power shifters, attached to my Fuji Finest. Continued agreeing through the 1980s with their bar end levers. Andy

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