My wifes cannondale road warrior keeps throwing the chain when shifting to the small chainring it has thumb shifters it only happens when she ride I cannot duplicate the problem on a stand or on the road.would a chain deflector be good to use in this case

Perhaps you could adjust the lower limit screw to move the derailler cage more to the right side of the bike. Probably adjust it until you can JUST downshift onto your smallest chain ring. And make sure the derailler is aligned properly relatively to the chain rings.

Ok I covered all that she is very hesitant with the shifters which may be the problem like I said it only happens when she rides so I don't think its an adjustment since nobody else has a problem with the bike.that' s why I was thinking of a chai deflector

Third Eye Chain Watchers cost less than $10 and, installed properly, prevent the problems you are describing. In fact you can set the inner derailleur stop to give a very firm downshift to the granny without worrying that the chain will come off.

Almost any LBS has them and they are easy to install.

Thanks I think I will try one as it is now its a pain to keep puting her chain back on also she hesitites to use the granny on hills

Is the FD mech positioned correctly. If it is too high then under stress, it will throw the chain to far inboard due to the profile of the FD.
What is the min/max chainring size?
What style of FD?
What is the gap between FD and max chainring?

Agree. That's what I would do too.

I'm going to double check der height but since this problem only happens to one rider I doubt that's it but its worth a look thanks to all for the advice

if shes only using the granny on hills - are you sure shes not shifting under load ? i can force my chain off the bottom ring by flicking the gear down while climbing without backing off the pressure - but ive set my mech to give a nice positive downshift, so i only have to back off for a quarter revolution...

You're correct that shifting before it's absolutely necessary and relieving pedal pressure works better and reduces chain spilling problems. However, a lot of riders, particularly inexperienced ones, don't shift until they are forced to and anything that helps them is worthwhile.

You know you may have it she doesn't like the thumb shift at all so I think she may wait too long before shifting causing the chain to jump now if I can work with her without getting a divorce the problem will be solved or maybe stay married and get a chain watcher. Thanks for all the input

Put her on the stand and watch how she uses the gears.

My wife had the same problem; turns out she would crank extra hard while shifting (she remembered the sales guy telling her to NEVER stop pedaling while shifting when she bought her first bike) and the extra force was binding the derailleur.

I had her pedal steadily while I shifted for her and she picked up "the feel" really fast.

As to the divorce part, ask Dr. Phil...

Ok this sounds reasonable it has to be something in her shifting style practice on a trainer mey be just the thing thanks