I have a GT Zum 8 speed. It has a Shimano HG-31, 8-speed cassette. I like the bike and have about 300mi on it. I took it to my LBS to get it tuned up and noted that when peddling backward to get the peddles in position at a stop, the chain sometimes jumps off the rear sprocket. The owner made an adjustment and I no longer have that problem. Now the chain will jump off the front sprocket when I am in first. I took the bike back in yesterday and got an explanation of what cross chaining is and how a single front sprocket 8 speed may be prone to it. He said he could install some sort of guide, but it would cause the chain to rub and make noise. This seems to mostly happen when i am down shifting coming up to a traffic light or when I am in 1st spinning around the neighborhood warming up for a ride. It doesn't really happen when I am going slow.

Is this an issue of my technique shifting into 1st when it is not appropriate or something mechanically wrong with the shifting mechanism? I am not a luddite, but am only familiar with the concepts of the mechanics of a bicycle. Thanks in advance for any help or advice.

Sounds like the lower limit on the front derailleur is set too far inboard. Cross chaining shouldn't necessarily cause your chain to drop. Noisy, perhaps. Whether it's 8-speed or 9 or 10, it should affect the front rings.

What sort of 'guide' needs to be installed? Seriously? BTW, I cross chain all the time, and yes, it's not the smoothest or quietest, but it's fine for a brief period of time.

8 speed with correct chainline and properly adjusted shouldn't have any issues crosschained, even pedalling backwards.

There is no front derailleur. It has a single sprocket in the front.

Should I be able to properly adjust this or should I leave it to the LBS?

Sounds like this bike is 1x8, so LBS must be talking about a chain guide.

That is correct. Thank you for clearing this up.

If you held a ruler against the front ring and extended to the rear cogs, which cogs would it line up with? Should line up somewhere in the middle.

Anyway, also possible your derailleur, the derailleur hanger, wheel or frame is bent/out of alignment.

+1) Best chainline with a single chain ring is lined up with the space between the 4th & 5th cog in back .. of 8 , an even number..

its also where the middle chainring of a triple is best..

A standard chainring will act as you describe. A single-chainring system should use a specially-toothed chainring, which has taller teeth to prevent derailling. It sounds like you've got the wrong chainring on your bike.

That makes a lot of sense. If it still happens, I will contact my LBS and see if there is a taller toothed chain ring. I will be going back there next week to pick up my wife's new ride.

For a bike with a single chainring, Nothing should make the chain fall off. Chain too long?

I have had this issue when I had A.) a loose driveside crank and B.) a bad BB so the spindle could slide.

As mentioned by others, bent derailleur hanger ... or even the chain not threaded through the derailleur properly.

There is no "cross-chaining" on a 1x ( a bike with a single chainring.) If the bike shop guy told you there was, he is ... mistaken, charitably. Avoid him at all costs.

The teeth in a single chainring are not taller, they are alternately wider and narrower to better fit the chain links. I have one on my 1x10 setup and have never dropped a chain.

With service like you have described, I'd be looking for a better bike shop.

Not in the sense of small/small or big/big, of course. But in biggest or smallest cog the chain would be at a steeper angle, so max "crosschain" for 1x.