loimpact

So after a milestone distance ride yesterday (66 hilly miles on my hybrid), I had a chain -drop happen for the 2nd time. (Big chain ring, small cassette gear) This happened a couple months ago in the same place. Here's the situation......

Flying downhill (35-40mph) and I just hold the pedals still. I assume it's any bumps I hit that do it but the first time I had no idea it happened & not only dropped the chain, but my efforts to pedal again lodged it pretty good. Stop, buddy's help, get chain back on & off we go.

Back to LBS, they check everything out, said RD's good, adjust limit to be safe & just used that gear cautiously since.

Yesterday, same situation, did it again but this time I didn't force pedals & was able to use the rear derailer to pull it back up on again & continue on. (Yayyyy!!!)

What else can be done to stop this cuz I'd really like to use that gear to fly!!

TIA

FBinNY 

Normally this might be a limit screw issue, but since it happened while coasting, that pretty much rules the limit out.

However, the very fact that you can drop the chain off the 11t sprocket indicates a problem. The axle spacing should be that there's about 4mm between the outer sprocket's outer face and the dropout. That provides about 1mm running clearance for the chain but is nowhere near enough room for a chain to fall off and jam.

Go back to the shop and have them examine the wheel and determine why the cassette isn't as outboard as possible (just enough for the chain to clear the frame) and either explain or fix it. If they do respace the wheel, they'll also need to redish it to the new center. That's a good thing because it'll reduce the offset, and improve R/L spoke tension balance.

BTW- I'd suspect the skill level of any mechanic faced with the problem, and having the bike in his hands, that didn't notice the excess sprocket clearance, or think to fix it. We're not talking about the normal 1mm or so variation between bikes, we're talking something almost double what's normal.

loimpact

Thanks FB. I'd have replied sooner but am having some image hosting issues. (Damn Skypath) I've measured gear to dropout gap @ .108" (approx 2.74mm) and took some pics at different angles if you have an opinion on what you see......

FBinNY 

It's hard to see, but the gap seems to be much more than the .108 you're measuring. Eyeballing the chain for a reference, the gap would appear to be closer to 8mm than 3mm. In any case, if the gap were LT 7mm or so the 8mm chain couldn't jam itself in there.

Measure again, or if you wish, shift the chain to high, and shoot straight on so we can see the clearance between the chain and dropout (should be 1mm or so) and if it's 4-5mm that's your problem.

As I said earlier, it's normally physically impossible for a chain to jam itself between the sprocket and dropout, though (rarely) chains can get jammed under a seatstay where it meets the dropout.

BTW- can't tell for sure, but there seems to be an inside shoulder on the dropout, which may be a factor. But the dimensions I'm referencing are at the height of the chain (check all the way around), not the axle.

achoo

I don't see how the size of that gap has an effect on the chain falling off the small cog. It shouldn't do that no matter how big that gap is.

It's either bouncing off, or getting pulled off, right?

The chain doesn't look too long, so I don't think it'd be bouncing off because it's loose. Maybe try screwing the B screw in some more to add more tension to the chain?

It's pure speculation, but maybe the derailleur limit isn't set properly - if the same mechanic did it both times it could be wrong the same way. Not likely, but this one's a weird one if the RD is adjusted properly.

I wonder how the shifts down to the 11-tooth cog are done? Are the shifts done while coasting with no pedaling at all?

loimpact

achoo,

That's what I was going to propose. (Man, you guys are quick with the replies. I can't get pics taken & hosted fast enough....LOL)

I definitely only shift under power. I'm very conscious of shifting & know just how to shift for both FD & RD's sakes to keep them happy. I notice a lot of chain slap though whenever I'm pedaling and then suddenly stop. I'd almost be tempted to take a link out of the chain, but might also be accomplished by screwing that B screw in more to tighten up??? (In my ignorance, I'll assume B screw is that one adjacent to the H & L limit screws?)

Here are a few more pics.

1st close-up of me manually dropping the chain for argument's sake. (And, yes to FB, I'd call the "lodge" somewhere at the seat-stay.....((my bike's upside down in the pics)).....and putting tension on the pedals seems to want to help the lodge, thus my ability to keep it from having done so yesterday when I knew I was off the chain)....



Then a little further back so you can see how it all looks in alignment w/ chain off......



And finally with the chain shifted up & pic from straight on so you can see the gap yourself if that helps.......



If it helps, the ABSOLUTE FATTEST measurement of that gap I could get was .210" (approx 5.3mm) and that was at the outer face of the peak of one of the 11t teeth and the deepest width of the stays I could find. (IOW, I think FB's 4mm gap check is probably plenty good considering 3mm-5mm & probably somewhere between)

FBinNY 

Thanks for the better photos. The gap seems to be on the largish side, but not unreasonable. Offhand, it doesn't appear that there's room for the chain to jam there, but it did.

So, let's try something else. Manually move the chain over and see if you can get it between the sprocket and dropout. Also, check to see if there's any cassette play that would allow the cassette to move in and make room. If it's borderline, you might want to have the cassette moved out another millimeter.

Also check that the hanger is tight, and that the chain can't jam between it and the sprocket.

Lastly (or maybe first) the hanger seems to be bent outward, maybe as a result of the chain jamming and pushing it out. Have the shop check that, and reset the outer limit to the most conservative position that still allows the upshift. You can do this yourself. Tighten the limit screw until you cannot shift to high, then back off by degrees until you can smoothly.

Edit

I posted before the last upload.

The fix is to have the cassette moved closer so there's absolutely no possibility of this happening. You want the chain to run as closes as possible to the dropout without touching.

loimpact

I should make some teflon split rings and sell them & make tens of hundreds of dollars!

Seriously, though, while I'd suspect they should have probably made it so that there's no way the chain could dump off the highest gear, I also suspect something as easy as a teflon ring (much like the plastic guard you see on my lowest gear) has been the failsafe for decades! I mean, really how hard is that?

I think I'll try messing with the high limit screw myself, too, first. But I'll also check that hanger & make sure that puppy's plumb.

I also kinda' like on this site (aside from a test-ride version of the same thing which I plan to do w/ FB's suggestion to "sneak up" on the high-limit's....errr.....least amount needed-limit) how this guy even tries to force the issues by pulling outward on the RD to see if the chain will hop off. (smart, imho, but again, I like doubling-down w/ FB's method, too)

Benno's Blog & More: How-To: Adjust rear derailleur limit screws

FBinNY 

Not that many people have your problem, and the better solution is taking a spacer out of the right side of the axle.

But I've been so focused on the gap, that I forgot to mention that (barring a misadjusted limit) the common cause of this is unconscious back pedaling slightly on a bumpy stretch. While the RD guides the chain when pedaling forward, there's no guidance the other way, so it's easy to ping the chain off the sprocket.

If you don't want to modify the axle, consider using body fller on the inside of the dropout at the radius of the sprocket, so the chain has no room to jump off.

loimpact

Believe it or not, I actually thought about that cuz I'm pretty sure that *did* happen the first time I did this a couple months ago. Unconsciously pedaling backwards just enjoying the flight down the hill, then a couple bumps in the road & try to start pedaling again but........whoops.....pedals stuck!

Yesterday (and since then) I didn't back-pedal but that's just consciously holding things still. Though I *think* that just like the "slap" I hear when I'm spinning out and suddenly stop pedaling, I think that same "slap" could be partly to blame.

I'll still be very curious, though, to see how far *I* find the high limit screw to be from what I'd consider "perfect". (Sometimes we assume that paid wrenches know best)

P.S. Thanks, FB, for taking the time to help me here & for so much I see you help/reply around here!

Dirt Road

I wonder if that's why some folks recommend "dead pedaling" when coasting? I figured it had to do with keeping the legs limber and maybe keep blood from pooling due to sudden lack of activity. (Not that we have the kind of descents where one could die of old age during... more like of boredom.)

alcjphil

A possibility that has occurred to me is that the ratchet in the cassette body is catching at times or that it has too much internal friction. Coasting at high speed will cause the freehub to catch and perhaps throw the chain off

Al1943

Which chainring were you on when the chain jumped off? Is it possible that you were cross-chained and that there was slack in the chain while coasting? If cross-chained from small to small there should be no slack in the chain. The rear derailleur spring should still be able to pull the slack out of the chain.

loimpact

It was large ring (48t) and small cassete (11t). Nothing funky going on in the cassette body that I know of. (My cassette is silent as a mouse but my buddy's Specialized is loud as a jake brake).

Again, I'll be curious to see where *I* feel the RD should be. I could probably get the LBS to check it out for free based on their last 'adjustment' but I'm not even so sure they care that much. I'm probably the better judge for myself, at least as far as where I would like to see that high limit be.

bradtx

loimpact, I just measured three bikes and the largest gap between the top cog and the drop out was .126". The other two were .101" and .112". I probably haven't glided as fast as you nor in the same exact gearing.

For a bicycle I wonder if the freehub or freewheel could be the cause. Have you spun up the rear wheel on a stand to watch what's happening when the crank arms are stationary?

Brad

alcjphil

This is a very good suggestion. Get the bike up on a repair stand and spin the rear wheel up pedaling by hand as fast as you can in your highest gear, then let the wheel coast. You could even backpedal a bit and watch what the chain does. It will give you an idea what is happening when you coast downhill. If the freehub has too much internal friction the top run of the chain will go slack. If it sticks, even momentarily you would know what happened
A freehub that makes little noise could easily have sticky pawls