Ok just got the 16/42 sprokets in will I need a new (longer) chain or will the stock one work? Or if I just change out the front can I run the stock chain ? Only about 2000 mile on the chain would hate to wait for replacement to come in.
Each tooth is equal to one link. So if you drop the front 1 and raise the back two you will need 1 more link (+2-1=+1), longer than stock to not shorten the wheel base (rear axle position). Half a link shorter in length will be the new axle position - wheel base - with a stock chain. That is about 3/8ths of an inch forward of where you have it now. Or add a link to the stock length and move the axle back 3/8ths inch or so. The axle position is half of the change in link length. I am guessing the link length is 3/4th inch? That is where I get the 3/8ths. It might be longer, more like 1 inch per link?