I built up my first 1X9 with a SLX crank using the factory chain ring and a medium cage X7 derailleur. On this bike, I couldn't keep the chain on, any downhill or rough technical section would drop the chain. I added a bashwich and it's been golden since.
Shortly after, I built a 26er and used a cheap single speed crank with bash guard. Naturally, this crank has a single speed chainring. I've been running that bike with no chain control device at all and I've never dropped a chain.
I just converted my Kona Unit single speed to geared with the single speed crank and a medium cage X7 derailleur. Took it out for it's first run and had no problems with it either. Went down the same fast rough downhills that would drop the chain several times on the other bike and no chain drop issues. And it's a fully rigid bike so it's worse case.
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