Derailleur Question

[stevej37]

Riding bike this am and my bike chain jumped off in high gear when I hit a pothole.
The other day it did the same twice when in the higher gears and heavy bumps.

I feel I need to tighten the whole works up somewhat. Where is a tightening screw or bolt located? It doesn't need much.
I don't see any way to do it by moving the axle back in the drop-outs.

[look 171]

Steve, isn't that and no need to move axle back. Large gear meaning the largest cog, the one closest to the spokes?

[stevej37]

^^^ No...the next to the smallest gear...which is 7
I hardly ever use 8

The bike is in 2 in the pic....I just parked it in the garage

[look 171]

Does it jump into the next larger or smaller gear? I think the cable stretched.

[stevej37]

It always stays in the same gear but the chain comes off the front sprocket. It's easy to get it back on, but a greasy mess on the hands.

The bike is about a year old with 3200 miles on it.

[look 171]

At the crank? Two chain rings? It jumps off the small one?

[stevej37]

The bike is an 8 speed...so a single sprocket on the front.

When it bounces off, it always ends up with the front end of the chain hanging from the crank.
The rear of the chain remains on the gear that it was in.

I feel that if the chain was shortened/tightened up just a wee bit....it wouldn't happen.

[LEAKYSEALS951]

Is that chain a newer chain?
The derailleur doesn't seemed very tensioned and you are in the next to what should be the "most" stretched derailleur position.

Put the bike in what you call "8" and see if the chain is floppy b/c not enough derailleur tension.

[stevej37]

I should also state that it shifts up and down perfectly as it should.

[stevej37]

I have a bike stand...I'll have to use that...which means tomorrow.
It's buried in the tool shed.

But I do know that when it is the higher gears the chain there seems to be less tension on it.
(which I thought was somewhat normal)

[Flat Six]

Used to be a way (on the old, old Shimano derailleurs) to adjust the spring tension. IIRC, it was done at the pivot (just above/11 o'clock from the 'A' in Altus). Of course this was ~40 years ago; design -- and my recall -- might be just a little off.

Regardless, if that's the original chain it's likely stretched more than you'd like; if it's original I'd just replace it.

[look 171]

get us a pic of the bike from the side showing the chain. Put that on the smallest cog in the back. I like to see the tension of the rear mech like Leak said.

Going out for a ride now before the sun sets for a couple hours. get back to you

[stevej37]

Original chain....3090 miles...I just checked.

There is a small set screw behind the pivot....I didn't dare turn it before asking.

[LEAKYSEALS951]

Reason I ask- if it's a new chain, it might be a link or two too long.
edit- nevermind=you beat me to it

There is something called a "b" screw- it's in your pic, which can move the derailleur back, but if the chain is too long to begin with, might be better to remove a 2 links to get the tension.

What you want to see is
How stretched is the derailleur in the "8" position?
How stretched is the derailleur in the "1" position?

Post some pics if you get a chance.

plenty of YT videos on adjusting the derailleur b screw, but want to see the chain at the extremes.
Worst case scenario with B adjuster- turn it to pull the derailleur back, remember the # of turns you turned it, and turn it back if it doesn't work. Adjusting the b screw can affect shifting responsiveness because it essentially pulls the derailleur away from the cogs sometimes.

Seeing how stretched the derailleur is in the one and eight positions, and how loose the chain is in those positions (floppy, loose etc..) is a good place to start.

Also- longshot, make sure you don't have a bent tooth on the front chainring. A single bent tooth can throw the chain if tension is low.

[stevej37]

Just started raining here...I managed to get it into 8 by holding the rear end up, shifting with another hand and turning the crank all at the same time.

The pic shows the bright screw that the pivot rests on. (I'm tempted to back it out a quarter turn to allow the pivot to rest further back.?)

[LEAKYSEALS951]

I'm gonna throw up this video- didn't even really watch it, but it looks good
Should talk about what the b screw does and does not do.



Still want to see the chain tension. I've never had a chain stretch so much the derailleur can't keep it tight- not in 3000 miles. It might have been on the long end to start with.

Weird.

Dang, wish I was there. This is the kind of thing, if Look or one of us was there, it'd be fixed easy peasy. It's tough sometimes diagnosing over the interweb.

[stevej37]

It never happens on normal pedaling...it's only happened when hitting extreme potholes or bumps.
And I'm usually out of the saddle when it does it.

[LEAKYSEALS951]

Other easy thing to do while you are in that gear is slowly pedal the chain backwards- see if there is a stiff or bent link. Sometimes it won't feed correctly through the derailleur if you backpedal.

The above pic looks good adjustment in/out. The derailleur is right under the cog. Thats why it shifts fine.

[herr_oberst]

So, Steve, if I understand you right, the chain is bouncing off the big chainring, hanging up in the crankarm, (outside the sprocket, so to speak) when you are in big ring front and 7 on the rear cassette; does that sound like I'm understanding the issue?

edit; and it's not going into 8?

post edit. you don't have 8 on the cassette. And that chain is really dry.

[LEAKYSEALS951]

While youve got it in the 8 gear- take a pic from the side of the bike. It looks like the top of the chain is dangling somewhat.

Same with the bike in the 1 gear.