CrowSeph

Allignment , rotation , limit screw, cable tension seems all fine.... but this sucker won't shift down to the smallest cog.
video ---> https://www.youtube.com/watch?v=1vxiXrayqg0




note to myself: next mtb must have a microshift or another brand for the drivetrain. I hate everything branded shimano on this bike

hokiefyd 

Is that the front mech that came with the bike? I can't see a lot in the video, but it looks like very tight chainline (almost like a road bike) and you have a Deore front mech -- it may simply be designed to not travel that far inward (mountain bikes usually have a higher chainline measurement). I see you can force the chain down to the small ring by hand, but the mech is obviously supposed to get it there itself...if it's the right mech for the application. It appears you're using a mountain mech where you might should be using a road mech (Sora, etc.). Just a guess based on the limited info.

hokiefyd 

As a follow-up -- to confirm that, loosen the cable adjustment entirely (so the cable does not operate the mech) and back out the lower limit screw so that the mech is bottoming out "on itself". In other words, no external inputs like cable or limit screw. Work it by hand and "shift" to a larger ring and then let the mech snap back home while pedaling. Does the chain move to the smallest ring then? If so, then you probably DID have a cable adjustment or limit screw problem. If it still doesn't, then I think my earlier advise may apply -- it may just be the wrong mech for the application.

cxwrench

A lot of those top swing derailleurs will bottom on themselves before they can mover far enough to shift the chain to the small ring, especially on bikes w/ aluminum frames/larger diameter seat tubes. I've seen them spec'd on new bikes that absolutely will not shift to the small ring. Nothing you can do except replace it with one that works...I'd always go bottom swing...I HATE top swing derailleurs.

CrowSeph

The derailleur is a piece of original equipment, the crank is not since i had to replace both from. Deore (m530?) to the alivio one (it was the almost similar and kinda cheap).

alcjphil

If you changed the crank and didn't make sure that the new one didn't need a different bottom bracket spindle length, look there

CrowSeph

Do you have any idea how much longer the bb must be?

hokiefyd 

Thanks for that -- I didn't think of a replacement crank. It's possible that the Alivio crank requires a longer BB spindle and installing it on a shorter-than-optimal spindle puts the small ring too far inboard. This is usually noted with the specs of the crankset (the correct BB spindle length). If it's a square taper BB, you do have some options if you cannot find the spindle length specs for the crankset -- estimate how far more inboard you need the mech to move. It's probably not much -- maybe a few millimeters. That's how far you need the crankset to move outboard (because the mech is already bottomed out). Multiply that (2mm for example) by 2 and that's the additional spindle length you need (because the spindle is generally centered in the BB (although offset ones do exist). So if your current BB is a 113mm and you need an additional 2mm, then look for a 117mm (or similar) BB. It would be better to err too long rather than too short -- too short keeps you in the same scenario you have now.

CrowSeph

Thanks for that. An alternative is to move from 3x9 to 2x9.
Actually i'am using a custom 40-11 cassette with 22-32-38 chainrings at the front.
Do you think i can keep the 32-38 or i should use a more appropriate gear with a bit lower teets?

HillRider

Your current 22x40 low gear is lower than even loaded tourists typically use and the 32x40 gear is about what they use. If you don't really require the extremely low gear that 22T chainring provides then using your crank as a 32x38 double should be fine.

CrowSeph

My current bb is 68*113
How much longer bb should i use?

masi61

.

There is nothing inherently wrong with Shimano. Setting up that triple front derailleur to work with your (different) crank might be a simple matter of doing the set up procedure for the front derailleur (for a mountain bike triple) from the beginning. That orange set-up block that comes with a new front derailleur is helpful or you can use a 5 mm allen key to space the cage pivots out ever so slightly when you first snug up the cable with the shifter in the granny ring. So long as your low limit screw is set correctly, your shifter should shift into the small chainring OK.

CrowSeph

Tried everything, trust me 😅

Iride01 

Can you put the old drive crank and chain rings back on? Then you can measure how far from the center of the seat tube to the center of the middle chain ring. Then if you do the same measure with the new crank on the old BB, you'll have a decent idea.

But...... Square tapers are ticky things. Different standards and even among the same standard, it doesn't take much difference in the tolerances to make the crank arm and spider sit further out or in. Probably one of the many reasons why they moved away from square tapers, though some things are still made with them.

If these aren't square tapers, then something else is up. The newer standards of ISIS and Octalink by design put the chainrings more in a standard position so all that was necessary was to know the bb shell width.

CrowSeph

Seems i spotted the problem. i compared the two cranks and seems the old is more external that the new. Basically by rusted math i should solve the problem by fitting a bottom bracket with the total lenght of 120-121 instead of the 113.

hokiefyd 

That sounds reasonable. That's a total difference of about 8mm and, assuming your current BB and a replacement BB have centered (vs. offset) spindles, that should position your chainrings outboard about 4mm. That sounds like a plan.

CrowSeph

Last question....... "68" means english threads?

hokiefyd 

68 very likely refers to a 68mm bottom bracket shell...the shell itself is 68mm wide. This does not tell you the type of threads. Most modern bikes use English/British threads but there are other standards out there. Sheldon Brown's page on this has some good information.

If your bike currently has a cartridge type bottom bracket (like a Shimano BB-UN26 or similar), then it probably has English/British threads, but it would be best to check to be sure first.