So this has happened to me three or four times in the last couple months and it is kinda scary. It happened yesterday when I was shifting from the middle to largest chainring, not under heavy load. Pedals just locked up suddenly and wouldn't move. It is scary because my legs still have momentum and each time I've been lucky to stay on the bike by just reacting quick enough to not fight it and coast. I'll click the shifter down and jiggle the pedals and it will work again, I've never had to get off the bike and physically untangle the chain (I assume the chain is somehow tangled and locked up, but since I've always been able to get it back without stopping the bike, I've never seen what this problem looks like). But to be clear, when I say the pedal lock up, I mean TOTALLY lock up without warning.

An internet search led me to the term 'chain suck', which seemed to sound very similar to what I am experiencing. However, none of the causes the websites listed really fit with me. It said that a majority of cases are new chains on worn out chainrings, or really muddy conditions on mountain bikes.

Well, this is a road bike, not a mountain bike, there is no mud. And the whole drivetrain is brand new, just a couple months old. SRAM PC1050 10sp chain, 105 cranks, ultegra cassette. The chain is well lubricated and mostly clean (when this happened yesterday, for example, it had only been about 150 miles since I last cleaned the chain, and I don't live in a particularly dirty place).

So, what are some other reasons that this is happening that might be plausible? I would really like to have this fixed because one of these days my pedals are going to lock up and I'm going to crash.

Thanks!

A likely cause is that the chain is a link or two too short.

When you're shifting to the big chainring, where is the chain in back? If you're going to big/big and it's doing that, your chain might just not be long enough. At this point, you should also check to make sure the rear derailleur hanger is still straight.

I usually shift to the large chain ring when I am about four or five cogs up.

In that case, you might have to just stop the next time and take a look.

You said you use a 10-speed chain but you didn't specify if the rest of the drivetrain is also for 10-speed. A 10-speed chain is a bit narrower than a 9-speed one, so if you have a 9-speed drivetrain the chain might be getting stuck between the chainrings. If so, try using a 9-speed chain.

Sorry, should have specified. Yes, it is all 10sp.

I've seen this happen when the front derailleur was set a bit too low. Also check the high limit screw setting. If the derailleur is too low and swinging too far out it may cause the chain to bind between the inside rail and the big ring.

Check to see that the chain is long enough to allow shifting to the big chainring and biggest cog, even though you should not use this combination it might accidentally happen.

Check for a tight link in the chain, this can cause serious shifting problems.

Al