snailracer
Über Member
When you double-shift down, the number of teeth the chain engages suddenly reduces, because you are shifting to a smaller cog and smaller chainring at the same time. This suddenly introduces slack in the chain, that must be taken up by the derailleur and the rider's pedalling. The derailleur (and chain) has mass and limited spring pressure, and the rider's legs cannot "catch up " instantly, so cannot take up the chain slack instantly. The resulting short period of slack in the chain is when it may de-chain. How often you actually de-chain then depends on lots of other factors (spring tension, derailleur design, gear tooth design, chainstay length, etc.)I asked the qn on the forum about 18 months ago and was told that double shifting increased the probability of de-chaining. No explanation was forthcoming.
My own experience refutes the assertion but nonetheless I try to avoid doubleshifting. If the episode is unplanned then probably a lot of power is going through the system and so double shifting may not be the best option.
Double-shifting up is less likely to de-chain because the chain does not go slack - tension increases, and this increase is much more gradual because the gear cable action is slower changing up, compared to when changing down.