Chain skips under load

[AndrewB]

I could use some pointers as I continue to diagnose this chain issue with my road bike.

When the chain is on the biggest crank and smallest gear on the cassette, and under load, it occasionally slips for a moment and re-catches.

I can't replicate this just by spinning the wheels while inverted or on a bike stand. It only happens when I'm going downhill and over 30mph and pedaling hard. It's not consistent in terms of time (assuming standard cadence) nor cranks (such as, every 2.5 cranks). It may occur between every 5th crank rotation and 10th crank rotation, if at all.

Needless to say, going 30mph is a short lived event on a weekend ride, lasting just several minutes offset by the effort to get up the hill.

I've checked the chain for damage. I've checked the cassette for wear too. It's a 10 speed, so that outer gear only has 11 teeth. The gears change easily, like a hot knife through butter.

[nfmisso]

Hi Andrew;

Some questions that will hopefully lead towards a solution:

How close is your bike to the chain capacity limits of your rear derailleur?
Is the rear derailleur internal mechanism clean and properly lubricated (not too much, not too light)?
Is the chain stretching? 11T is very little wrap, and thus more sensitive to stretch.
Do you have a stiff link on the chain?
Can you calculate the frequency of the problem with respect to the chain?

Good luck. The weather here (in Dongguan) is very HUMID and hot - definitely missing home in SJ.

[AndrewB]

Hi Nigel,

I don't understand your first question about the chain capacity and the rear derailleur. Could you elaborate when you get a chance while on your trip?

Yes, my SRAM Rival derailleur is pretty clean and lightly oiled regularly with a few drops of Tri-Flow.

The chain, a Connex 10S1, is new. I only have less than 200 miles on it. Using my Park CC-2 chain wear detector, the chain is pretty tight - I can barely fit the tool in.

I do not believe I have a stiff link. But that is something I'll check!

I'll have to try the frequency thing when I next do a hill. Probably Saturday if my plans work out right.

I might also try backing out my L screw maybe 1/8 of a turn or less.

Sorry to hear about the humidity there in southern China. The midsection of the US is melting as well. It was in the mid 70's here in San Jose this weekend. I had such a nice ride out Foothill Expwy/Canada on Saturday of 55 miles.


Andrew

[nfmisso]

sorry should have wrote chain wrap capacity http://www.sheldonbrown.com/harris/derailers-rear.html#capacity

Another question: did the issue start with your new chain? Did you replace the cassette at the same time? The old chain may have worn the old cassette.....

[AndrewB]

Ah, capacity - got it. Yes, I'm sure the derailleur has capacity. I don't have clearance in the front derailleur to do something like small crank and small sprocket though without distracting horrific noises! I'll double check this though in addition to checking the chain for a stiff link.

I'm fairly certain the cassette is not over worn. My first chain was replaced in time according to the LBS. After that, I got my Park CC-2 and watched the chain wear until it got to just under .75 - and replaced shortly thereafter with the latest chain.

I don't use the small sprocket a lot to cause wear there as compared to the other gears. I'm usually in the big crank in the mid sprockets and occasionally in the small sprockets or small crank in the big sprockets.

Compact crank, that is.


Andrew

[AndrewB]

Update:

While cleaning and lubing my chain today, it appeared that my rear derailleur wasn't quite lined up with the smallest cog. So as mentioned above, I backed out the limit screw slightly, and I think I got the issue fixed. Until I find a big long downhill that I can pedal on (around 30mph+, and the longer the better), I won't know for sure.

AndrewB