Everytime I come across with this question on a bike forum my thoughts automatically go back to the 2003 Tour de France Prologue.
On that day, the scottish David Millar lost the opportunity to win the initial time-trial for less than a second due to his chain that came off the front ring.
News footage from the French TV shows how brave is Millar placing the chain back with his fingers upon arrival. A delicate operation while you’re riding over 30 mph.
This information points at the missing front derailleur as the reason of the problem.
It’s hard to believe that a bike mechanic involved in such a race like the Tour de France, world class riders and equipment (not the case with Decathlon bikes though :roll:), decided to remove the front mech, because Millar was committed to use a single chainring setup, without fitting a chain stopper on its place.
I can only blame ignorance to be mother of imprudence. Front derailleurs allow to switch from chainwheels, and what is as important: stop the chain from jumping off. On bumpy stone paved roads (pavée) and a NOT enough rear mech to lock up the chain tension while it is working crossed along the cogs we can only predict the chain is going to fall off sooner than later. Even on flat surfaces it will slip off.
Whether if you want to convert your MTB to singlespeed or 1x type transmission, bear in mind you will need a device to prevent the chain from coming off. It can be a double bashguard, a chainkeeper/chainguide/chain stopper or a combination of both.
Many people that are simplifying old vintage road bikes should do the same for fixie/single speed purposes.
All these advises (and devices) are taken into account considering we are talking about pure vertical dropout frames with no other methods to take up the slack in the chain, such us: eccentric BB/hubs, sliding dropouts, and so on, representing the vast majority of frames.
There are only a couple of exceptions that freaks will proudly point out. SRAM XX1 single chainring gruppo and high chain tensioners on singlespeed bikes where the chain works straight! (Bromptons for example).
Even though, I’ve experienced chains coming off in my bike that uses a perfect aligned chain on a Rohloff hub and the extremely high tension provided by the DH tensioner. XX1 failures are even more common, and have been reported frequently despite engineers taking the piss with the deep profile teeth design of the XX1 chainring encouraging not to use a chainguide. I would like to recall that Hermida lost the podium in the last Olympics because of the bloody stupid tendency on bikers to remove the front derailleur and the “cojonazos” of his team fitting new-unproven parts they’d just received.
If you’re switching to single speed or whatever 1x setup is in your mind, fit a chainguard. You’re not likely to lose a gold medal or the yellow jersey but you teeth instead. Remember how bad the Cofidis bicycle mechanic should have felt that night after the first stage of the Tour de France: “somebody shoot me”.