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Say the current setup uses a zero offset seatpost, and a 100mm stem. If the seatpost were replaced another seatpost that has a 20mm offset, should offset be subtracted from the stem length?

My current situation is for a road/cross frame, dunno how important that is to the topic vs mountain/etc frames.

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You can generally slide the saddle forwards/backwards about 15mm on the rails, so the change in seatpost may not change your saddle position much. And both should be selected to fit your body and riding style -- no formula. –  Daniel R Hicks Apr 15 '12 at 1:47
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2 Answers 2

up vote 3 down vote accepted

Saddle setback is a fixed position for each rider.

It is necessary to place the saddle in the position which places your knee in alignment with the pedal spindle, when at the correct saddle height.

Arbitrarily changing the saddle setback can interfere with a proper fit.

You should set your saddle height, then fore/aft position to align your knee correctly, then choose a stem length that makes the saddle position reasonably comfortable.

If the 20mm setback post is necessary to align your knee, then, yes, you may need to subtract that extra distance from your stem. But that will leave you with a very short stem in this case, which will affect how the bike handles.

I would expect that if that change is necessary, you are on the wrong frame size.

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The seatpost setback has nothing to do with the stem. Set the seat to the same distance behind the bottom bracket that you had on the other seatpost. Problem is, it may not be possible, which would mean your position won't be right now. (Unless you are deliberately changing your bike fit.)

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