No matter what brand of components you prefer, when even the top pros can be seen stopping to put their chain back on (remember Andy Schleck a few years ago at LeTour?) it’s clear that “operator error” plays a big and frustrating part in this unfortunate situation. Here’s how you can avoid it.
You might say, “but wait, with the new electronic systems available now, I should be able to shift to any gear at any time, the tiny computer will certainly make things happen perfectly”, but you’d be wrong. Nothing is foolproof as even top pros, riding the most carefully set up and maintained machines in the world, (yes, even those with electronic controls) can make even the best systems malfunction.
The biggest mistake we see, even from seasoned riders, is the classic failure to anticipate the need for a lower gear. Remember that front shifting systems (aside from the ill-fated Browning of the late 80's) despite their pins, ramps and profiled teeth, really just push the chain off the big ring so it can (one hopes) simply fall down onto a smaller one. This worked pretty well when differences amounted to only 10 teeth or so, as in the old standard 42-52 setup.
With 39-53 and 34-50 chainring pairs, the difference is much greater. Differences of 14 or 16 teeth are a big challenge, especially when tension on the chain is high as you struggle up a climb in a too high gear.
Get a gear chart, then count the numbers of teeth on your rear cogset. All of the cogs. Then write down the development when combined with each of your front chainrings. You’ll notice that the 53 combined with your largest rear cog is most likely the same ratio you could enjoy with the 39 chainring and one of the smaller cogs. Using the 39 combination will do two things; First, it will make your drivetrain more efficient as chains prefer to run in a straight line rather than “cross-chained” at the huge angle required by using the largest cog and largest chainring. Greater efficiency is realized too when the spring tension on the rear pulley cage is not as high as when using the “big-big” combination. Of course you want to also avoid the “small-small” though spring tension is not an issue in this case.
Think of driving your car at highway speeds and suddenly jamming the transmission into first gear. You probably avoid this, but many riders do the same thing while cycling. When you ride along in the large chainring and shift only up the rear cogset as the climb steepens, when you eventually need a lower ratio you then are faced with the same situation as abusing your car’s transmission and drivetrain. There’s only ONE shift you can make, asking the highly tensioned chain (both from your pedaling forces and the derailleur spring tension in that “big-big” combo) to fall off the large chainring onto the smaller one. Even electronic systems can drop the chain under these conditions and we’ve seen it happen even to the pros.
Plan your shifts - instead of running the chain up the rear cogset in the large chainring, try shifting down to the small one and moving the chain down a couple of cogs to get a similar ratio. This way you’ll easily be able to shift into a lower gear if needed OR go back to the larger chainring if necessary. Those profiled teeth, pins and ramps do a remarkable job of helping the chain climb up to a LARGER chainring, though backing off on the pedaling pressure still helps.
If you rarely ride on hilly routes, consider ditching the huge difference in chainrings and improving your shifting in all conditions. 14 or 16 tooth differences make for a wide range but chainrings with less difference shift better and let you keep a more constant cadence without so much shifting. Pros at flat races like Paris-Roubaix often set up their bikes with 44-54 chainring combos….they know that 10 tooth difference makes for better shifting.
Need a low ratio but still like the idea of smaller differences between chainrings? Consider a triple chainring setup. While some makers supply triples with a 39-53 pairing and a 28 or 30 tooth inner ring (which really doesn’t improve things) others offer cranksets with closer to 10 tooth differences, like 30-40-50 or 32-42-52. They know this makes for a nice progression and good shifting.
“Shift early. Shift often.” All systems shift UP (to a larger ratio for more speed) better than they shift down, especially up front. Finding out you’re in a lower gear than you need is MUCH better than vice-versa and your drivetrain (and mechanic) will thank you…..and you might never drop your chain again!