I have been scratch building a road bike using a Trek compact frame,and mostly Shimano 105 components. Mostly I build older vintege bikes, so its not often I touch 10 speed gears.

Normally I measure new chain length using big to big +2 links, but I had read that for 10 speeds you should use small to small and thread the chain through the derailleur, so I used that method.

However, when I cam to set up the SI, I could not get it to work. The derailleur did not seem to be indexing and would not reach the two largest cogs. I adjusted the angle in case the derailleur was hitting the cogs, I double checked parts numbers in case I had a non 10 speed, changed the outer cables, as I had been a bit tight - no change.

The only change I had made was to measure the chain. So I split the chain, re-measured my normal way and there was a 4 link difference (too long). I rechecked the new way, and I appeared tom have that correct. I had images to check against as well as instructions.

As soon as I shortened the chain, everything worked, but I cannot get my head around how this impacted on the SI. Nothing else was changed.

Can anyone provide an explanation.