Charles Wahl

I bought and cleaned/refurbished a 2nd-year Stumpjumper that had been only slightly used; 3 x 5 drivetrain, using stock equipment according to catalogs. Did not change derailleur, crankset or freewheel. Sized chain equivalent to what was on the bike (114 links) and checked this with the method suggested by SheldonBrown.com, Park Tool, etc.: wrap chain around large cog and large chainring without threading through derailleurs, and where they meet, add one full link (1"). The "meeting" was just a bit loose, but I couldn't wrap chain tighter and get the next link to meet.

Problem is: with chain on smallest chainring and largest sprocket (lowest gear), the top jockey wheel has a bit of interference with the large sprocket.

When on middle chainring, derailleur looks like this


Derailleur & chain when on largest sprocket and largest chainring -- could I get away with removing a half or whole link to shorten chain and increase rotation of the jockey wheel cage? Note how far axle is toward front of dropout.


Derailleur and chain when on largest sprocket and smallest chainring -- hard to see, but upper jockey wheel and sprocket seem to be enmeshed with each other, and somewhat noisy

Rear view, chain on largest sprocket and smallest chainring

Front view, chain on largest sprocket and smallest chainring


I've tried adjusting the B-screw, but it doesn't seem to make much difference. I have the rear axle in approximately the same position in the (horizontal, short) dropouts as it was when I got the bike, rather close to the dropout opening, though it could go a tiny bit more forward. As noted above, I've considered removing a link of the chain (or half a link, with a "half link") but wonder if it's too tight as it stands. Should I try screwing the B-screw all the way in, to tilt the derailleur parallelogram further downward? Moving the wheel forward to increase offset between axle/sprocket and derailleur?

Any recommendations will be appreciated (short of "get rid of that MounTech derailleur" -- I'm aware that it might come to that, but it's in perfectly good shape).
Thanks,
--
Charles Wahl

smd4

Did you try putting the chain on the smallest cog/smallest chainring, and seeing if you could add a link?

Bill Kapaun

Size of largest cog?
How much chain are you wrapping?

Charles Wahl

I don't see what adding a link would do to help. That would rotate the jockey wheel clockwise, and (by my reckoning) further into the largest cog. As I said above, I think that either the jockey cage needs to be rotated less at the big sprocket-small chainring (lowest gear) extreme, or the axle needs to be farther (forward) from the mounting point of the derailleur, to keep the jockey wheel from fouling the big sprocket.

14-30 freewheel, and 46 big ring, 24 small ring; so 16 + 22 = 38 teeth wrap total.

smd4

Chain looks too tight to me, thereby pulling the derailleur forward and up into the cogs.

Bill Kapaun

Thinking about it, with the type of drop outs you have, there probably isn't a "ledge" for the B screw to push against?

If you have the GTL version, it should barely have enough wrap capacity.
https://velobase.com/ViewComponent.a...93e71&Enum=108
Else, not-
https://www.disraeligears.co.uk/site...leur_4900.html

Put it on BIG:BIG and see how much excess chain you have if any.

Reynolds 531 

You should never be on "Large-Large" in any case
It's called "cross-chaining".

https://outdooruae.com/articles/cros...dos-and-donts/

Charles Wahl

Chain seems a bit too loose to me. Starting from the big-big combination, where the chain is tightest (3rd photo), the jockey wheels both rotate clockwise as the derailleur wraps chain. Staying on the largest cog, and shifting to the smallest chainwheel, the upper jockey wheel has now rotated to a position where it's starting to interfere with that sprocket. If I shift to the next-to-largest sprocket, the derailleur moves outward, and the jockey wheel has more clearance at that sprocket. Thus, with a longer chain, that interference just happens sooner, because the jockey wheel cage starts at a more rotated position.

I just rotated the whole derailleur a bit more clockwise, by screwing in the B-screw more (dropping the jockey wheels slightly). I think that has helped.

Charles Wahl

I actually don't have quite that much; certainly not enough to remove a whole 1" link.
Re chain wrap: I am dealing with the GTL version, longer steel cage, not the 4900 but the 5500 (next one at DisraeliGears.co.uk). That's got 34 tooth cog, 38 teeth wrap max -- I guess I'm livin' on the bleeding edge of 1984 technology.
Re B-screw ledge on dropout: yes, it has one, and it's apparently functioning.

Reynolds 531 Of course, I know that one shouldn't cross-chain. But I believe it should be possible to do so, even if it's noisy (the beast complains) and inefficient.

Thanks, everyone. I think I've gotten it to "good enough" status.

Bill Kapaun

Longer B screw?

jccaclimber

It might require a change in chain length, but would letting the axle sit toward the back of the dropouts buy you more clearance?