Standard to Compact

[e-rider]

Sorry mate it makes no difference what tyre size is for comparative purpose but for 700x25c according to Sheldon's calculator 34x26 and 39x30 are 34.5 and 34.3 gear inches respectively. So ok 34x26 is in fact marginally higher than 39x30, but how do you explain dropping 5 up front (39 to 34) is so closely matched by 4 at the back (26 to 30), when you say 3 at the front worthing 1 at the back is worth remembering?

If the rule is true I would expect dropping 5 up front is worth 5/3 or less than 2 at the back, but even with 2 i.e. 39x28 is 36.7 inches, it is not even close.

The rule isn't right.

If one thinks about the maths I think one can work out that the rule can only be right or close when the chain rings have around 3 times as many teeth as the rear sprockets being considered, e.g. 39x13 is IDENTICAL to 36x12. Q.E.D.

39x13 is identical to 36x12

That is what I mean. Surely 42x14 and 45x15 are also all the same?

 

[RecordAceFromNew]

39x13 is identical to 36x12

That is what I mean. Surely 42x14 and 45x15 are also all the same?

As I said that 3:1 rule only works when the chainrings are also 3 times larger than the sprockets. Given the OP was talking about 39ish up front and 26ish at the back, which is nowhere near 3:1, the "rule" doesn't even come close.

3:1 is clearly a useless rule. If one wants a rule, the rule is X number of teeth reduction up front would be roughly the same as 1 tooth increase at the back where X is the number of teeth of the chainring being considered divided by that of the sprocket being considered at the back. For what the OP was considering, X is 39/26 or 1.5.

 

[Bigbud]

Lots to think about thanks guys, this will also have a bearing on my new steed too