breadbin

I was going up a really steep bit of hill today, tried to anyway but I stood up out of the saddle and was pressing my entire weight down and the chain skipped. Just for a second but enough to knock me off guard. I'm fairly heavy so I'm not surprised but is it to do with the weight or do I need to adjust the derailleur? It works perfectly on the flat, no noise or skipping at all and changes gears no bother.

There is a sign at the bottom of the hill that says 15%!

DiabloScott

1. Derailleur misalignment
2. Worn chain
3. Worn cog on the cassette.
4. Bike flex
5. Superhuman strength.
6. Some combination of two or more of the above.

corrado33

Good on you for climbing that hill.

In addition to the suggestions from Scott above, it could also be.

1. Wrong chain for bike (chain falls between gear.)
2. Worn chainrings.
3. Cross chaining.

DaveSSS 

Unless you changed to a new chain recently, it's probably not chain skip. Chain skip most often occurs when a new chain is installed on an old cassette. Chain skip means that the chain stays on the intended cog, but rides up over the teeth.

For an old chain to skip with a used cassette, it must have a large amount of elongation, in the 1.5-2% range.

The other things mentioned are not really chain skip, if the chain moves to another cog.

Worn chainrings cause chain suck, not chain skip.

Trakhak

Could also be pilot error---riding mostly or exclusively with the chain on the smallest cog on the freewheel or cassette. Although the OP probably wasn't doing that while climbing a 15% grade.

FBinNY 

Not enough info, including details of the age and condition of the drivetrain, gear selected (though my money is on low), but isolated skipping isn't all that rare.

What happens is that for some reason, the RD doesn't feed the chain onto the sprocket perfectly and it climbs and rides on the tops of the tooth. That puts it at a larger diameter, so the next roller is out of phase, and 2-3 links ride the top until a roller is in phase enough to drop in as it's supposed. You now have a short section representing the loss of 1/2" (1 link) on the top, and when it comes around to 12 o'clock you get that 1/2" back in a hard pop.

If you can't imagine it, think about the last time you buttoned your shirt and missed one. The skip is like that extra bit of shirt on one side vs. the other.

jacksbike

Could be a stiff link in the chain. Or the freewheel teeth have worn and you need to replace both the chain and freewheel at the same time.

maddog34

amazing... all of these "mechanics" replying, but NO MENTION of the possibility of a BAD RATCHET in the freewheel or freehub....

if everything else looks good/serviceable, then it was probably the ratchet in the freehub or freewheel.... YES, they FAIL. Excessive slop in their bearings is a good way to inspect them... and i've found one that had managed to back it's retaining bolt out, too! most of the walmart freehubs are not removable, and the entire hub will need to be replaced... suggest upgrading to a better hub, in that case....

i even replaced an Ultegra hub recently, that would not engage every so often... the skipping of Walmart hubs is fairly common, so if it's a low end bike, that is almost always the cause of "chain skip", if everything else is ok.

ALSO.... i took in a couple NEW Walmart/Bell chains from a scrapper last winter... they were "WORN OUT", right from the package!... i tossed them back into his next load of scrap... but bent them up so no one else would have to deal with them...

Bad ratchet springs, severe wear after RUSTING, or old (stiff) grease, are the usual causes of ratchet failure.

DaveSSS 

Maybe because a bad ratchet mechanism is just that, not chain skip?

breadbin

thanks for the replies, it's not a low end drive train by any stretch. it's Ultegra mostly. Very low mileage even though it's on the bike a couple of years. All the parts were put on new at the same time too.

Now that I think about it, it sounded more like I just powered through the ratchet somehow. It was just like a 'bang' and I felt my feet go forward a bit. Sorry, it's hard to explain. I don't think I should have attempted it! The front of the bike was coming up off the ground and I was very close to toppling over! There was a girl coming down the other way though - women! I'll get it out for a spin tomorrow and see how it is. There was never any sounds coming from it, really buttery smooth always.

oh yeah, the chain was on the smallest ring on the front but probably the biggest one on the back!

TheMountBike

The best recommendation for fixing a skipping chain is to add tension to the rear derailleur. When your bike chain starts to skip, you could have problems switching gears and navigating your two-wheel chariot, and the most common culprit is a loose cable. If the chain skips under pressure, it is most likely worn; if you have replaced it recently, you are waiting longer and have worn your rear cassette gears. They don't mesh with new chains. If the chain skips every third or fourth pedal revolution, you probably have a tight or bent link. My gears won't shift smoothly from one to the other, making a lot of noise. That's why my mountain bike chain skips.

Schweinhund

This ^
The pin in the roller is worn to the point of misalignment when under pressure.