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Gearing and chain tension

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Gearing and chain tension

Old 05-14-15, 12:44 PM
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peugeot84
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Gearing and chain tension

Hi new to bike building so need advice please. Project is a 1980's Peugeot 531 P frame and sourcing appropriate components.
Got 48 T / 36 T chainrings and 6 speed freewheel 12 T - 17 T, my rear mech is a simplex SX610 which seems to be working ok. The chain tension is correct when on the big chainring but when on the small ring it is way too slack. Is the mech not working properly or is it not compatible with the gearing I have ? Any advice appreciated.
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Old 05-14-15, 01:06 PM
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How to Calculate Chain Length - Bicycle Tutor Video
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Old 05-14-15, 03:20 PM
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Easy to figure out. The rear derailleur has a limit to how far forward and how far rearward it can swing. With the chain on the smallest cog (rear) and smallest chainring (front) the derailleur's spring should bring take all of the slack out of the chain before the derailleur reaches its rearward limit. If the chain has slack and the derailleur has not reached its rearward limit you have a mechanical problem with the derailleur, possibly a broken spring or it's incorrectly assembled. If the derailleur is firmly against the rearward limit but the chain is still slack, either the derailleur cage is too short to take up all of the slack or the chain is too long. Regardless it is imperative that the chain be long enough to be shifted into the big cog - big ring combination to avoid serious damage.
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