Whacked the rear derailleur off my bike the other day, descending Porcupine in Moab, day 4 of Kokopelli's trail. I really didn't notice it happen, and I wasn't in a particularly rough spot at the time, but...
So it was time to try running single speed. We tried the configuration in the photo, using the upper idler as a chain tensioner -- but it was way too weak. So we shortened the chain to fit the middle ring, middle cog combination. Then, how to avoid breaking the chain as the suspension flexes going over the drops for the next 10 miles? Only time I ever wanted a shock lock-out on this bike!
Answer was to pump the shock to 250, up from 160 or so, set it to ProPedal 3, and try to find the smoothest lines. It worked. The chain would jump to the next-highest cog when I stood up to crank, so I'd stop and manuallly shift it off to drop the tension again, and keep going. It was super fun until we hit the road, then I had to overspin like crazy to hang with the group, they heaping on scorn.
Second pic is just pretty...
Mtbr's 2016 Winter Biking GearReviews and Roundups
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Thread: Single-speed 5-Spot