Catastrophe! Training for our upcoming ride, I had put two gallons of water on the front rack as if low riders, and had four gallons on the back end. We were 3.3 miles from home, coasting a gentle downhill to a stop sign. I down shifted the front end to granny and downshifted the back end to first and WHAM!!! Chainsucked the rear end.
Squeeze drops her load (28 pounds of water) and cycles home to get the VW Bulli for vehicle recovery.
Had to take a chain tool and "cut" the wheel out of the rear end mess. Wheel is at the bike shop. No spokes broken. Don't know if the freewheel survived. At this stage, I don't know if the derailleur caused and/or was destroyed by the event. I just know that my BD just gutted itself a few weeks before its first grand tour ride.
I am prepping the "grocery getter" for a 185 mile tour. Now, I'm scared to take the BD 185 miles. Will it do this again?
Do we know a risk and frequency for chainsucking the rear end of a BD?
I need some input as to whether or not to trust and take the BD or to use my (38,000 mile/1984) grand touring bike.

I hate to mention this but it is a possibility, a real one and a dirty trick at that, but someone that knows what they are doing with bikes and has something against you or has a nasty sense of humor or is just plain twisted. Long story short, if you reach down to someone's rear derailer on their bike and bends the bottom of the derailer inward towards the wheel while it is one of the higher gears. When the unsuspecting rider shifts into the lowest granny gear on the rear shifter while moving the bottom end of the derailer grabs one of the spokes in the wheel and the wheel and the derailer tear themselves mutually apart.

I've had it done to me more then once and finally caught the scum pulling the nasty stunt in the act and put an end to the problem. Someone might have done the same to you. Was your bike ever out of your sight in an area where someone could have done this to you?

'chainsuck' refers to the specific phoenomena of a chain failing to disengage smoothly from the front chainrings, thus getting pulled upwards (sucked) and wedged between chainstay and chainrings.
Is this what happened?
or was this actually rear derailer issue? 'chainsucked the rear end' being a misnomer....

chainsuck is comonly caused by worn chainrings -the teeth wear into sharkfin shapes that snag the chain

if it was the rear derailer; could be a couple causes
limit screws not set right, derailer hangar misalignment

Were you using a spoke protector?

Ok, maybe I used the wrong word. My BD came to me as a used vehicle and does not have a disk to keep the chain out of the spokes. The chain did suck, wrap, lodge around the rear axle. I had to use a chain tool to break the chain to get the wheel off so that the LBS could use their tool to take off the freewheel. The inches of chain thus freed were destroyed in the wrapping and cannot be re-used.
I think that what happened was that I managed to downshift to granny on the front and first on the back at the same time causing a ripple in the chain. I just don't normally shift left and right at the same time.
I need a protective disk on the front and an anti-suck device on the front. LBS says that I would have to lose first gear to put one on.

...again your terminology is unclear...

but it sounds like your chain overshifted off the largest cog and wedged between the spokes and freewheel.
typically this occurs if the rear derailer limit screws are not set properly
or if the rear derailer has been misaligned via impact (not necesarially from a crash, can be as minor as someone at the bike racks next to you knocking it by accident)

I'd check those two things: derailer alignment and limit screws
If your LBS is good, they already checked that out when they did your repairs; and you should be fine now.

+1 on rear derailleur adjustment / limit screws.

Someone doing what turbo described above, on purpose, is just nasty. You need to know a bit about bikes to even pull that kind of stunt.

Also replace any spoke that shows the slightest mark from the chain. Even a small scratch causes stress concentration, and that spoke will break soon... Maybe not the next 20 miles, but likely within the next 200.

Don't shift like that. I haven't owned a bike that would like that. Get your bike tuned up and yes, you can trust it.

I agree with the above posters. Avoid shifting both derailleurs at the same time. A properly adjusted rear derailleur will not be able to push the chain further than the gear closest to the spokes. Fine to ride after adjusted.

Late to the party but I totally disagree that it is bad to shift both ends at the same time. In fact the rear, going to a bigger cog, tightens the chain. Shifting the front, which gives up chain length by going to a smaller ring, provides chain slack easier than the derailleur spring and will provide a better shift.

I almost never shift the front without shifting the back at the same time; On my bikes a chainring shift is about the same as two or three cog shifts, thus a shift on the front and rear gives a smaller gear ratio change and is typically what I want.

I do agree with the posters above who suggest that the derailleur/hanger was bent or the low limit screw was improperly adjusted and that spokes need to be replaced after being damaged by the chain.

PS. There are two ways to shift the front and back at the same time: Shifting for big changes (like the OP was doing) requires less chain from the derailleur spring. Partially canceling a ring change by also shifting a cog or two requires more derailleur spring but has the advantages of a smaller gear change and less soft pedaling than for two separate shifts.

When coasting downhill, always shift to a larger chainring. That is an old habit I learned in the start of MTB in the 80's. Chainsuck was a big deal in the early MTB days, and the bigger chainring, 'wraps up' more chain and moves it away from the rear tire. The chain flapping around would hit the knobby tire, and get sucked between the chainstay and tire and that would start a catastrophic lockup of rear wheel.

So get out of the granny gear, unless you are using it. AND only with the two biggest, maybe three cogs in the back. Then a properly adjusted derailleurs, and lack of loose chain will keep you safe.
Bob E