Originally Posted by
pfaustus
"Certain cranks like older Stronglight and SR Apex have particular problems with even 8s chain though, but this is easily corrected with minor modification to the small chainring, something I have been doing from time to time for over 20 years since it is always worth the effort to be able to use narrower, better modern chain."
I've still have several of those cranks. What is the mod to make them work better?
Those particular cranks from the mid-1970's and before would sometimes have "chain-skating" problems even with the old, 8mm-wide conventional chains, so can be even more prone to slipping (skating) while shifting down to the smaller ring if a narrower chain is used.
Initially, I try to correct the problem by first dislodging the chain so it hangs from the bottom bracket shell, allowing the cranks to be turned freely.
Then I take a long file, stand it up on one end on the ground so it contacts the smaller ring's teeth at an angle.
I then have to position my hand/arm so the cranks can turn all the way around without hitting my wrist or whatever, and rotate the cranks so the small ring's teeth are moving across the file's surface in the cutting direction.
I end up with a bevel to the tip of the small ring's teeth, which re-locates the tips of the teeth slightly toward the big ring, reducing the effective chainring spacing.
Where the beveling turns out not to be sufficient to prevent "chain skating" after a downshift to the small ring, I will then remove the small ring and bend each tooth slightly toward the big ring side, using a small adjustable wrench. I use caution to keep the bending to about a half-millimeter, which is barely visible.
Some of the SR Apex and Stronglight cranks, particularly those with half-step gearing, can have a particularly strong tendency for the chain to "skate" or slip atop the small ring's teeth, so these are the cranksets that tend to need the most attention when using a narrower 8s chain.
I will add that some 8s chains are wider than others, and that some have more outer plate flaring to better snag the chainring's teeth and resume transmission of power. The Shimano 7-8s chains are better in these regards, but are a bit wide for use on any Suntour 7s or Ultra-6 freewheels, so best to avoid the combination of these freewheels with the older Stronglight and SR Apex cranksets!
I use the narrower 9s chains on a lot of old bikes, and even they work fine on most cranksets, with at most a bit of the filing/beveling that I described.