Feel occasional slip through the pedals when accelerating
 

Sometimes, when I push hard on a pedal I feel it give a little (maybe 1/2 an inch) when it should not. It only ever does this once; I can't make it slip more by pushing again. If I pedal another 30 seconds or so it might do it again, it might not. I haven't been able to make it slip by pushing really hard on both pedals (while not moving), but because it doesn't do it every time even when I'm accelerating hard I don't actually know if this rules out some possibilities. I had thought at first that it only happened after using the foot brake, but now I've noticed it happening after I stop with the front brake too.

I've been noticing this for about two weeks now. I ride about 1hr a day, to work and back.

The bike has no derailleur. The rear hub is a sram T3 with a coaster brake.

If I could make this happen while looking at the bike it would probably be easy to see what's going wrong, but I've not been able to.

Any ideas? Is it dangerous to ride as is?

I would look at the adjustment of your hub gears. Maybe your adjustment is right on the edge and frame flex is causing a little more or less cable pull that is putting the hub between gear selections.

When this hub is under any force it usually cannot switch gears; if I flip the shifter to a higher gear while pedaling with almost any power the cable moves easily (which one would expect; it's loosening) but it will delay the shift until I back off. Flipping to a lower gear is impossible; the cable just won't move.

So I don't think it's frame flex, but I will check the shifter adjustment tomorrow morning and see how the commute goes. Thanks for the suggestion.

I checked the cable adjustment this morning; it seemed right. But to be sure I loosened it all the way and did the adjustment again. Then on the way to work I noticed slippage once or twice. So I don't think that's the issue.

could be you freehub is about to crap out and one of the pawls inside is slipping or giving way

Do internal gear hubs have freehubs? This hub is a six month old sram T3.

I have a similar problem with a late 80s Campy cassette and new chain. It seems the slip only occurs when I'm in the three lowest gears. During normal spinning it's fine, but if I'm accelerating or up out of the saddle, it'll happen repeatedly. The cassette doesn't appear to be worn out. The clunk-clunks and loss of acceleration is driving me crazy... I'm at a loss about what to do.

Check to see if you have a stiff chain link. The stiff link stays bent at a sharp angle as it goes around the RD and rear cog (smaller cogs are worse) and then straightens suddenly once it gets around the cog and starts heading for the cranks. This makes the pedals jump forward just a bit all of the sudden before the link straightens and goes back to normal. And it's only noticeable to you when you're pushing hard - the other times you're not mashing hard enough to notice the sudden change in chain tension. Because it's just probably one stiff link in the whole chain, the chain has to make it all the way around again before you can possibly recreate the conditions and chances are you're not pushing hard then.

You can find the stiff link by bending each joint in sequence and finding the one that doesn't bend so much, or watch the rear gears while you slowly work your chain around - the stiff link will stick out from the cogs or RD jockey wheels and not conform to the circumference like it's supposed to. Bend it back and forth both in line with the chain (make a Z with the stiff link in one direction, then the other) and also perpendicular to the chain. Some lube down between the plates on the chain moves things along. You're trying to force the plates on the outside away from each other just a tiny bit to correct for squishing them together when you put the chain on. Things are done when the stiff link isn't stiff anymore.

EDIT - Don't need a RD for this to happen - the small cog on the internal hub will still cause things to happen. It's not dangerous on it's own, but will cause the chain to stretch faster if you leave it and can make your feet slip off the pedals if you're not using clips. Plus, it's annoying and not hard to fix.

It doesn't have to, chain stretch and sprocket wear can cause chain skipping long before it's visible to the eye. Trust the symptoms, not the inspection.

Just to be clear: 3 smallest sprockets or 3 biggest sprockets?

The smallest sprockets tends to wear faster, and stretching the chain with them. If that's where you do most of your riding it'd be a nice explanation for why you have chain skip on the biggest sprockets that hasn't seen that much wear. OTOH due to the smaller no of teeth engaged chain skip is still more likely to occur on the smaller sprockets.

You can hope to find a stiff link, but the usual cure is a new chain and a new cassette.

+1 on the frozen chain link.
I had the same problem when I installed a new chain.

I'll look for a stiff link after work, but one difference between what I notice and what Ablaut has is that for me there's no "clunk-clunk"; it's totally silent. If there were noise I could try and see where it was coming from and I would be less worried.

Cbr I think supcom has the right thought on cables. 6 month old hub and been doing this about 2 weeks sounds like cable stretch, it has the gears on the edge of clutch to change. The internal hub does not have as much tension on the cable so takes a little longer to break in the cables.

A worn casette with a new chain will skip like mad when youre mashing, particularly idf you're doing it on one of the more worn cogs. Here is Sheldon Brown's take on it (scroll down to where it says "A new chain on a worn sprocket").

Check the original post again; there is no cassette, he has an internally geared hub.

He was referring to my post. Yes, the smallest 3 sprockets.

I was trusting the symptoms, just trying to be unreasonably optimistic and hoping there could be a different problem... I don't want to go through the hassle of finding another 8sp cassette. Checked the chain, no stiff links. So if it IS a worn cassette, well... crap. This might be a dumb question, but is there any chance chain stretch might sort of wear into the cassette and slip wouldn't happen as often?

Cbr, sorry if I'm hijacking your thread with my own problem.

I also checked my chain and found no stiff links. I also noticed that this happens in all three gears, so it can't be slipping down to a lower gear. Looking at the hub, though, I noticed it's making a strange high pitched noise that sounds like loose bearings or something. I took a video of it, mostly for sound purposes:

http://www.youtube.com/watch?v=bCfO1fax3bw

I'm not talking about the normal clicking that happens when it coasts but the weird speed-proportional scraping/sliding sound.

No worries.

Jeff

It DID turn out to be my chain! I didn't see any stiff links, but just to be safe I OCDedly went through the entire chain with the bending technique. Well, something worked so I'm happy.

Good luck, cbr!