Another chain issue...Help!

[navclbiker]

What is going on here? I understand chain suck (kinda), chain drop (inside and outside)...But what is causing my chain to hover over the small chainring when shifting off the large without seating on the teeth? It really kills your momentum when you hit a hill and start spinning the cranks whilst coming to an embarrassing stop. What wisdom have you? The Park Tool Reference has nothing on this. :crap:

[maunahaole]

how many miles on the chain? is it uber-stretched?

[Louis]

From Sheldon Brown (RIP) here is one reason (which I do not believe is applicable here):

"9-speed" Chainrings
When the move from 8- to 9-speed took place, starting with 1997 Dura-Ace, a very slight change in chainring design occurred. Shimano was concerned about the potential for the chain to "skate" over the teeth of the small chainring.

"Skating" occurs when a narrow chain is used on a crankset intended for a somewhat wider chain. In downshifting, instead of the chain meshing properly with the teeth of the inner chainring, the side-plates of the chain can ride along the tips of the teeth of the small ring. This causes a momentary "freewheeling" forward. If you are foolish enough to downshift the front while standing up, this skating could conceivably cause you to crash.

The difference between "9-speed" cranksets and older models is only in the inner chainring. The teeth on a "9-speed" inner ring are slightly displaced to the right to better accommodate the slightly narrower chain.

Shimano will tell you you need to replace the inner ring when converting an older bike to 9-speed. Their lawyers say that covers them if your clumsy enough to hurt yourself due to "skating" and, besides, they make a nice profit selling the chainrings.

My advice is to not worry about this, and not to routinely replace the inner ring when upgrading to 9-speed. This type of "skating" is very rare, and mostly only happens if you're in top gear front and rear, then decide to downshift the front before downshifting the rear. There's no situation where this is a rational shifting sequence anyway.

[navclbiker]

Oh ...sorry... I knew that would be a question. I have about 1800 miles on this brand new bike.
-Dave

[thwart]

This type of "skating" is very rare, and mostly only happens if you're in top gear front and rear, then decide to downshift the front before downshifting the rear. There's no situation where this is a rational shifting sequence anyway. If that's not the situation, I'd wonder about a chain that's too long, or has a stuck (inflexible) link, or is very dirty (doesn't look like it). What's the RD cage size and cassette that you're using?

Where's Dave? He knows a fair amount about chains...

[navclbiker]

That's definately not the sitch with me. I know not to use the high gear with the small chainring. So, somp'n else is up. I clean/lube the chain fairly regularly. Can a person really wear a chain out in under 2000 miles? The chain is resting on the large ring side of the small ring. I'm no mechanical engineer, but why should this even be possible?

[fierte_poser]

Is the FD not pushing the chain over far enough when shifting to the small ring? What does the FD/chain clearance look like at full low/full high wrt. the shifter?

[thwart]

Man... I'm certainly no expert (actually very close to the exact opposite), but St. Sheldon implies that slack in the chain may play a role in skating, so chain length and RD size and cassette might help come up with an answer here...

BTW, there are no stuck links then?

[pdxmech13]

I have had that happen once on that same crankset. Mainly My fault as I was in the highest gear on the cassette and just shifted the front mech. I would recommend checking the angle on your front mech and maybe if the tail is pointed away from the bike swing it in just a skosh.

[navclbiker]

Man... I'm certainly no expert (actually very close to the exact opposite), but St. Sheldon implies that slack in the chain may play a role in skating, so chain length and RD size and cassette might help come up with an answer here...

BTW, there are no stuck links then?
Yeah...no stuck links. I might give those Ideas a try and then take it back to the shop when I make everything worse. :rolleyes:

[Louis]

NB,

At a minimum you owe it to yourself to check the chain for stretch. Either buy a chain checker or get a good ruler and measure. More from Sheldon:

Measuring Chain Wear

The standard way to measure chain wear is with a ruler or steel tape measure. This can be done without removing the chain from the bicycle. The normal technique is to measure a one-foot length, placing an inch mark of the ruler exactly in the middle of one rivet, then looking at the corresponding rivet 12 complete links away. On a new, unworn chain, this rivet will also line up exactly with an inch mark. With a worn chain, the rivet will be past the inch mark. This gives a direct measurement of the wear to the chain, and an indirect measurement of the wear to the sprockets:


If the rivet is less than 1/16" past the mark, all is well.

If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged.

If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.

If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones.

[stevep]

i thk the throw on the fd is a little bit light.
adjust it to give it as little more throw down. yr shifting too tentatively. let it fly.

notable is that it is a 10spd fd.. and not a left over one from prior iteration.. but that's unlikely.

[Dave]

I agree with others, this is a FD adjustment problem and/or a chainring spacing problem, if you're using a 9 speed crank on a 10 speed bike. It has nothing to do with chain length or wear. Check the angle and height of the FD cage, then try backing out the low limit screw 1/8-1/4 turn. Be sure that the cable tension is not so high that the FD can't move in response to the limit screw adjustment.

I also agree that it's not wise to make the chainring shift from the smaller cogs.

Sheldon's chain wear measuring suggestion is easier if you place the scale on the edge of a pin and look at the amount of pin protruding from the other end. When new, there will be none. Change the chain before 1/2 the pin is visible.

[Kane]

-Measure your chain
-check your cables by pulling on the wire to see if the cable springs back quickly, (you can get dirt in your cable housing even at 1800 miles).
-take your bike to a good bike mechanic, you are quite good with technology (pictures posted on the website), but you don't know much about bikes.
-every time you get your bike repaired, you'll learn something new
-consider buying your bike mechanic a six pack of a nice beer and watch him work (before he drinks the beer)!
-I buy my mechanics a couple of liters of the local micro brew. I bought the bottle and I just pay to have the beer filled up once in a while.

Frankly, I always get great service.

Cheers,

Kane

[navclbiker]

-take your bike to a good bike mechanic, you are quite good with technology (pictures posted on the website), but you don't know much about bikes.

Cheers,

Kane[/QUOTE]

MMMMMM..Duuuuh! That's why I ask questions on this forum...and also toward my good wrenches at the shop. You all may be pleased to know that I will be taking the 6 week Park Tool repair class this April. This will allow me hopefully to ask fewer silly uneducated questions here.
You all are da'Bomb. Thanks
-Dave