Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 46 Post(s)
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Neither right nor wrong, but more wrong than right. It depends on the size of the sprockets.
For comparison purposes it's simple division. The front chainring divided by the rear sprocket is the gear ratio, so if you had a 28T granny and a 28t rear sprockets, adding or subtracting 1 tooth form either is very similar. At the opposite end if you had 52/13 or 4:1, 48/12 is the same 4:1 ratio.
So as you see, you can't blindly apply some simple rule of thumb. Get a hand calculator and compare ratios. If you want a quick rule of thumb for the effect a 1 tooth change divide the change by the number of teeth there originally, ie. adding 1 tooth to a 25t sprocket is 1/25 or a 4% change, while adding 1 to to a 50t chainring is a 2% change.
BTW- if you haven't noticed yet, shrinking a chainring is the same effect as enlarging a rear sprocket, so keep the direction of change in mind.
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