Bicycle Mechanics - What does the indexing on front and rear?
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10-20-11, 06:59 PM
Hey guys I am new to road bikes and was wondering what does the actuall indexing for the front and rear derailleur? Is it the shifters or the derailleur? I ask because when I originally set up my shifting, when I released the cables the deraillur's both move thru the full stroke with no stops. Seems like the shifters would be the controling factor.
The shifters. The derailleurs are just a mechanical device that move the chain inward and outward depending on the cable pull.
10-20-11, 07:12 PM
Thats what I was thinking. One more question, my bike has sora 8 speed/triple and I came across 105 front and rear derailluer's for a triple, and 9 speed rear. If I set the limit stops on the deraillur's would they work? Thanks
10-20-11, 07:27 PM
The derailleurs will work as long as they are the same brand, both Shimano, which sora and 105 are. The rear should be no problem as long as the cage is the same length. If the cage is shorter on the 105, you may not have the same gear range, but it may still be enough. If the cage is longer than what you currently have you may need a longer chain.
10-20-11, 07:30 PM
Awsome thanks for the info!
10-20-11, 08:57 PM
If the cage is longer than what you currently have you may need a longer chain.
Nope. Cage length on a rear derailleur has no effect on required chain length. It does effect how much chain it will "wrap up" and, therefore, how wide a gearing range it will tolerate before going slack in small-small. Notice a rear derailleur at full stretch (i.e. in the big chainring-big cog combination) is fully in a straight line with the chain and it's length has no bearing on the situation.
Remember, the safest way to size a chain is to wrap it around the largest chainring and the largest cog without going through the rear derailleur and then add 1". The rd cage length is not a factor.
10-20-11, 09:28 PM
Although it's the shifters that create the indexing by pulling or releasing a specific amount of cable for each shift, that amount is not the same for all shifter brands, even if the cassette spacing is the same, like Shimano and SRAM. The difference is due to the RD's actuation ratio. Shimano 10 shifters only pull and average of 2.3mm of cable per shift and the pulls are not uniform. SRAM shifters pull 3mm per shift and the pulls are identical. Campy shifters are similar to Shimano, in that they use nonuniform pulls, but the average pull is 2.8mm to shift a slightly larger 4.12mm spacing, compared to Shimano's 3.95mm.
FWIW, a Campy 10 shifter pulls 2.5mm five times, 3mm twice and 3.5mm twice. If you have the equipment to make more accurate measurements, you might detect even more small differences in the pulls.
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