Mountain Bike Reviews Forum banner
1 - 20 of 22 Posts

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #1 · (Edited)
So... I've got a chain-rub problem. ( I searched the forum, but never found anything that describes my problem):madman:
I recently upgraded to a new frame and also got a new FD [XT](among others; not affecting)
When i go into my granny-gear, my FD cage will not retract enough to accommodate my setup (28-48 rings on a 68x122 BB) and rubs.
for the obvious Q's: (derailleur fully slack still rubs low ring)
-yes- my 'low' limit screw is backed off fully
-yes-cable tension is slack enough
-yes - the cage is aligned w/ said rings

-The rub occurs when I'm at my lowest in the rear (28t and 1-2 higher)
-A smaller spindle length MAY result in ring-rub/crank contact w/ frame.(see pics)
pic 1=rear gear selection when rub occurs
pic 2=current crank(shimano m480) clearance
pic 3=chain rub & ring clearance
 

Attachments

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #7 · (Edited)
Mods... DELETE THIS POST!!!
sorry guys
talk about not seeing it obviously.:madman:
I feel like an idiot
(atleast i got some 'before' shots)
I will spend my time in the penalty box
-too many sierra nevadas and no. cal's finest harvest:D
THX
 

·
Never trust a fart
Joined
·
4,343 Posts
What kind of frame did you swap over your components too??

You might have to break down and get a new crankset. The longest square taper BB (which fits your M480 crankset) is a 122mm. You might have to go with an external BB bearing type crankset to get the chain line you need.
 

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #10 · (Edited)
frdfandc said:
What kind of frame did you swap over your components too??

You might have to break down and get a new crankset. The longest square taper BB (which fits your M480 crankset) is a 122mm. You might have to go with an external BB bearing type crankset to get the chain line you need.
yeah... i'm realizing that...
i had a nice-sealed BB, now i throw it to the unusable bin :rolleyes:
 

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #11 ·
Update

so, i swapped out my BB w/ 122mm spindle w? a 127 (and shaved a few grams too :p - hollow spindle)
now, my derailleur issue went away.
but i seem to be dropping chains more often because of the increased chainline.
I measured the chainline @ 49 mm (center of ST to middle chain ring).
the all-knowing Sheldon says 50mm being the max for MTB triple, so i'll assume it's technically ok (plus it has to be)

My question now is: Does anyone have any ideas regarding dropped chains and noise due to the more extreme offset?:confused:
I could snap some more pics (and will if requested) but i'm feelin' kinda lazy in my PJ's. Just imagine a fairly extreme left to right chain deflection when looking from the rear (lowest rear gear to middle chainring)

THX
 

·
Never trust a fart
Joined
·
4,343 Posts
I'm guessing dropped chain is chain off little chainring. You just need to play with your limit screw and adjustments on the FD now. Might have to lower the FD a tad bit also.
 

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #13 ·
frdfandc said:
I'm guessing dropped chain is chain off little chainring. You just need to play with your limit screw and adjustments on the FD now. Might have to lower the FD a tad bit also.
well, no...i should've been more descriptive. -say i'm in my middle CR (38) and my lowest cog (28) for a short climb up w/ a dip then climb (rock garden). on the quick dip i just coast and pump over the rumbles and leave the gear 'cause i'll need it in a sec. when i go to crank the next up; my chain is dropped to my lowest CR, i spin my cranks too fast, the F. Der. shifts it (the chain) back to middle, then shifts back to to correct arrangement.
I have only experienced a complete chain-drop on one occasion when i hadn't shifted into a higher gear after cresting a climb and descending some 'steps' while sucking on my water bottle (my hand was too occupied to shift the RD) - That's not so bad since i wasn't cranking into another step-up (still annoying). + the off-camber chain noise bugs me.
Could my cage length be to blame?
I was under the assumption to run the shortest cage your gear combo would allow...
The reason i assume this is because longer cage lengths = greater leverage and greater chain bounce = greater forces on said cage...more force-pulls the cage-which pulls the spring thus slackening the chain...since chain is slack and 'misaligned', it' will naturally gravitate toward the straighter chain-line.
Am I correct??
BTW I am running an XT med-cage.

THX

Edit: sorry for the delayed reply - your responses have been quick w/ accurate diagnosis'. (I imagine a TV show - 'HOUSE' - The MTB [something]'
where a bike problem comes in every week and various symptoms/diagnosis result in more symptoms/diagnosis) -- I'm waiting for it to be 9:50 pm :)
 

·
Never trust a fart
Joined
·
4,343 Posts
Ok, you are going to have to see if your exact FD can accomodate such a huge gear spacing - 20 tooth. If not, you might have to change the small chain ring. If it can then you just need to tweak it some. For the rear you could have a too small of a RD cage. Depending on the tooth difference.

http://www.sheldonbrown.com/derailleur.html
 

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #15 ·
frdfandc said:
Ok, you are going to have to see if your exact FD can accomodate such a huge gear spacing - 20 tooth. If not, you might have to change the small chain ring. If it can then you just need to tweak it some. For the rear you could have a too small of a RD cage. Depending on the tooth difference.

http://www.sheldonbrown.com/derailleur.html
as far as the FD...that is within limits (according to shimano - real world may be different).
and on the sheldon brown page, i used this page for clarification:
http://forums.mtbr.com/showthread.php?t=205890
Being as it is (48-28 to 11-28 - I miss the requirements, but i should be inside of the 'green' in the given combo .
I would never shift into ridiculous combos...ie; 48 to 28 nor 28-11.
a 38-28 seems perfectly fine to me... am i missing something?
 

·
Registered
Joined
·
2,310 Posts
I'm guessing your new hollow bb isn't as rigid as your old one. Plus, there is more leverage with the same weight since it is longer.

A small amount of flex would change the angle of the chain rings enough to cause the problem.
 

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #17 ·
rlouder said:
I'm guessing your new hollow bb isn't as rigid as your old one. Plus, there is more leverage with the same weight since it is longer.

A small amount of flex would change the angle of the chain rings enough to cause the problem.
any suggestions? or just 'deal with it'
i can do that...i'll tune to optimum
 

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #19 ·
nuttin' honey

ol' spindle causes chain drop!!??
really??
hollow?
i'll stay away then
my prob is chain drop on bumps
 

·
~Disc~Golf~
Joined
·
16,496 Posts
Discussion Starter · #20 ·
apologies

i don't mean to come off daft
if i have , i'm sorry
i'm just frustrated. (no excuse)
 
1 - 20 of 22 Posts
Top