Front Derailer, cable pull non linear
 

Having an odd issue with a bike I'm rebuilding
old hardrock
biopace triple crank
top swing bottom pull derailer
gripshifter w multiclick/friction/trim

all sprockets are shiftable, however the cable pull is highly non linear
such that in order to get good results the indicator marks on gripshifter must be ignored for 2nd sprocket
ie 1 to 2 requires overtwisting the grip past the 2 indicator mark(nearly to 3 mark)
then 2 to 3 shifts with just a click or two of trim position furthur
so, drastically more cable pull is used up going from 1 to 2 than 2 to 3, yet overall travel is correct - shifter indicator marks for 1st and 3rd line up, but not for 2nd

since the gripshifter is essentially friction, with the indexing positions merely being painted on, it works, but the non linear issue is annoying
any ideas what causes it?

thanks



atm my best theory is the elliptical biopace rings force higher than normal FD positioning, leading to the biased shifting response for higher sprockets....

Is the shift wire routed correctly to the clamping bolt? You have not mentioned the brand of derailleur but many Shimano ones have a somewhat counterintuitive cable routing.

Another possibility is that your cable adjustment is incorrect. I suggest that you follow this procedure Park Tool Co. » ParkTool Blog » Rear Derailler Adjustments (derailleur)

A third possibility is that the shifter and derailleur are a mis-mated pair and will never play right.

I have 2 bikes with Biopace rings and don't need to do anything unusual as regards derailleur adjustment to obtain good performance.

IF the shifter is some cheap off brand as found on Huffy's etal, clicks/numbers are pretty meaningless. About all the numbers do is tell you which way you are shifting. Up or down.

furthur data
a customer's bike came thru shop today for a flat

it also had biopace triple, with a deore top shifter lever in friction mode
it had identical behavior
1-2 takes up more cable pull than 2-3

yes, cable clamp is correct and derailer adjustment is good
shifting works consistently, once shifted the rear cogs are all workable withou need to trim FD in any sprockets
seemingly indicating good responsive FD adjustment

I'am merely puzzled by the skewed cable pull and misaligned indicator mark for 2nd sprocket

more clues-
on the gripshifter I have, indicator marks are evenly spaced, same as with the observed deore top shift lever at work
however, on a different gripshifter i have laying around , the indicator marks have top biased spacing (seemingly correlating with the non linear cable pull Im getting)

suspect compatibility issue between different era components and subtle changes to FD swingarm design over time
undocumented?

ultimately a curiosity than a real problem, I can always remove the painted on indicator marks