pbass

I've been riding a 1x for so long, I kinda forgot how triple chainring setups behave. Just got my son a new entry-level MTB, with a triple. Granted it's brand new and might need some adjusting (though it came from my fave LBS and they do great work--they built it up/tuned it up before we picked it up). But here's what's happening - while in the middle front ring, if you downshift to the lowest cog in the back it really wants to jump to the small ring in front, and usually does. I can see the chainline is "crosschained" but it doesn't look that extreme. I seem to recall being able to camp out in the middle front ring on a triple and use the entire range in back on bikes I've had before, but not on this bike. There's a barrel adjuster for the front derailleur at the shift lever, so I've started turning that clockwise in teeny increments but it hasn't helped.
Is this something that can be adjusted?

70sSanO

You should be able to camp out in the middle, even with 9 speed cassette.

It could be the chainline is way off, but people have been know to crosschain big-big on a triple and not lose the chain off the ring, so that is a stretch. I'd take it back to the shop and let them sort it out.

John

pbass

Thanks. Yeah, I've been sitting here making micro adjustments to the cage screws and not getting any improvement.

dedhed

The High & Low limit screws are just that - "LIMIT" screws that are only stops to limit the FD movement on the two ends of travel and do NOTHING anywhere else in the travel.

TakingMyTime

Guessing.... chain too long? Does the take-up spring on the RD keep the chain tight? Maybe just too much slack and it makes the jump?

Steve B.

Could be the size of the bike has shorter chain stays, thus the chain angle middle/small is having the chain catch the front derailer, causing the chain to move to the small ring. You could back out the upper limit screw, trouble with that is it’s a fine point where a shift to big ring will take the chain off onto the crank. As this gear isn’t needed, you should maybe plot it out on a gear chart and teach him when to shift to the big ring.

Bill Kapaun

I would expect it to want to climb the big ring?
Is the chain making heavy contact with the rear of the FDER when this occurs? (FDER rotated too much so that the "tail" is in too far)
That's the only thing I can think of is that it is using that as a "pivot" point to "lever" the chain over to the small ring.

Digger Goreman

Have you tried angling the front derailleur?

pbass

Thanks for all the replies. Took it in - turns out the bottom bracket isn't the correct size. Crazy. They're going to order a new one.

fietsbob

Sort out a ratio grid (a little Math) so you know what the gear sequence is,, Hint there are Overlaps

ideal : smallest on crank biggest on wheel. 3 shifts and next shift the from granny to the middle

as you get to the smaller cogs on the wheel the next change is to the big chainring..

chainline middle of the cassette aligned to middle chainring..

the screws are stroke limits they only keep the chain from being pushed too far in either direction.. .