Chain coming off front chain ring

[Willd]

Last time out no issues, today came off 3 time in 30 miles. Nothing has happened to it between rides. Happens when I change to the smallest cog at the rear (11) on the largest (50) chain ring at the front. The front chain ring looks true & measuring it is a consistent distance from the frame, no play in the bottom bracket, rear cassette looks fine - no issues at the cassette end, chain comes off at the top by the front derailleur (no contact with front derailleur). Looking forwards from the cassette it all seems lined up ok.

Of course I can't replicate it turning the bike upside down and going up and down the gears. Seems to happening under light load i.e. when changing to the 11 cog going down hill. Under load (peddling) on the flat seems fine. Loose/worn chain?

 

[I like Skol]

Freehub might be getting sticky and is trying to drive the chain from the rear wheel would be my guess.

 

[Big John]

If you've got a chain wear guage then it's a five second job to check if it's worn. If you haven't then check it with a 12 inch ruler. YouTube will have a vid how to do that. If it's worn and needs changing then chances are the cassette will need changing too. Give the chain a good inspection and make sure the links look ok and aren't tight anywhere. It's an odd one for sure. I've heard of chains coming off when you change up or down on the chainrings but not when you drop into the small sprocket at the back. When you do solve it it'd be nice to know what caused it. Good luck.

 

[cougie uk]

Try taking it up your block a bit before going to the big ring. I think your set up doesn't like shifts from small to big when you're in the 11.

 

[si_c]

I'd have guessed the same as @I like Skol - if the cassette is driving the chain forward it will lift it slightly meaning it will drop off at the front.

If it is the freehub you'd be better getting it sorted sooner. Depending on the hub make you might be able to give it a service.

 

[slowmotion]

Freehub might be getting sticky and is trying to drive the chain from the rear wheel would be my guess.

My chain is tending to drop off the chainring when I tee it up backwards about 120 degrees or less in anticipation of riding off after a brief stop. I suspected a freehub problem. The last time I had the same trouble, it made more sense to buy a new wheel than try and find a replacement hub because the hub itself wasn't serviceable and the wheel rims were pretty worn anyway.

 

[Willd]

Thank you all for the useful suggestions. Freehub appears to be ok, chain is a bit worn, but not overly so. However, it was rather crudded up and stiff, I've been using dry lube and it's been rather dry and dusty lately. So I gave it a good clean and put some chain oil on it. I think one or two links were bunching up a bit when it was slack.
I only tested it for about 6 miles, as it was tad warm at midday today and it seemed fine.