Originally posted by serottalt
Should I avoid shifting when the chain is crossed over that far??
Definitely! Here's an approximate gear-inch table for your setup:
Wheel Diameter: 24.50 inches
Gears: 39/53 12-13-14-15-16-17-19-21-23
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SP CRxFW GI diff
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1> 39x23 41.54 12.43
2> 39x21 45.50 15.05
3> 39x19 50.29 18.59
4> 39x17 56.21 0.79
5> 53x23 56.46 10.25
6> 39x16 59.72 6.64
7> 53x21 61.83 5.86
8> 39x15 63.70 14.29
9> 39x14 68.25 0.29
10> 53x19 68.34 16.20
11> 39x13 73.50 9.06
12> 53x17 76.38 10.19
13> 39x12 79.62 4.81
14> 53x16 81.16 17.00
15> 53x15 86.57 19.43
16> 53x14 92.75 22.41
17> 53x13 99.88 26.15
18> 53x12 108.21 0.00
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SP CRxFW GI diff
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You can get pretty close to the same ratios on your 39, if you just practice and get used to shifting over sooner! In addition to throwing the chain less, you'll get smoother shifts and less wear and tear on equipment.