I recently replaced my chain and cassette on my Cannondale M800 with Shimano FC-M737 42/32/22 and noticed that if I back-peddle with the chain on the outer 42T chainring and the two low cassette cogs, the chain will derail. The original cassette and chain didn't do it. The front and rear derailleur are proper adjusted. The chain doesn't touch the front derailleur chain guide when this condition occurs. So, the condition appears to be a function of the chain and cassette only. Is this an indication that I need to try a different chain?

The new cassette and chain are: Shimano HG51 8-Speed 11-28 & KMC X8.99 (uses 7.3mm master link).

The original cassette and chain were: Shimano LX 8-Speed 11-28 & Sedis chain.

I'm thinking of trying a SRAM PC-870 or PC-850 chain, because they have good reviews and I think they take a 7.1mm master link. I'm thinking the KMC X8.99 chain might be a bit too wide and catches the next higher gear when back-peddling in max cross-chain condition.

No, it's an indication to stop doing that.

+1, Just pedal forward or Coast.

if you use the cogs and chainrings that are straight in a line alignment with each other , Middle to middle,

& the derailleur is not aligned so it is pulling to one side or the other , then you may have a better chance to spin the pedals Backwards.

Why are you pedaling backwards?

I don't actually back-peddle while riding. I was checking my derailleur adjustments on a bike stand after changing the cassette and chain. I noticed that when I rotated the crank counterclockwise the chain would derail in low gear. As I indicted, the previous cassette and chain didn't derail in this gear condition. I think it's just a condition of this combination of hardware. I'm going to try a SRAM PG-850 11-28 cassette and SRAM PC-870 chain and see if it still derails in this gear setting.

I don't get the victim-shaming going on here. There's plenty of normal times you back pedal a little, especially on a MTB when you approach an obstacle.

This is near max tension in the RD, and cross chain, and when you backpedal you increase the tension further. I'd be looking at RD hanger alignment and condition of the RD. If the RD can flop around when the tension changes, that could do it. Where does it derail, does it fall outside the front ring?

Who's shaming the OP? Shimano's own documentation says not to use the extreme cross-chained combo with a triple, and there is never a legitimate scenario where the hardware needs to support it, forward or backward.

Many victims are self-created.

Thanks for the things to check.

I've got the Park Tool derailleur alignment tool on order. It should arrive any day. I was hesitant to post before I checked everything. I just ordered the SRAM cassette and chain. I installed the KMC chain with the same number of links as the original chain.

I've noticed that when I rotate the crank counterclockwise very slowly, it will still derail.

When the SRAM items arrive, I'll first try the SRAM chain with the Shimano cassette to see if it still derails.

When I approach my garage which is uphill in this max cross-chain condition and then dismount and back-up the bike to park in the garage, the crank rotates counterclockwise as I roll the bike backwards. That's when I first noticed the derailing and tried to adjust the derailleurs to resolve this condition.

Why is he Max Cross Chaining..?

I'm aware of the recommendation to avoid the max cross chain condition, but I'm in this condition, because I usually stay on the 42T front chainring and shift the rear as needed based on effort. With my original cassette and chain, I never noticed any noise or issues when I was in max cross chain. I didn't think it was much of an issue with my MTB 42T-28T gear setting. I think it is an issue on my road bike with Shimano 105 53T front chainring and 11-speed cassette.

When pedaling forward the RD guide wheel keeps the chain on the selected cog. When you back-pedal while cross chained, there is nothing preventing the chain from derailing and it will naturally want to straighten itself out. When you pedal forward again it will get back on track. The shift gates which ease shifting normally will also do the same when back-pedaling.

Perhaps your new chain, having more lateral stiffness, will exacerbate this behavior.