Front derailleur won't shift when pedaling with a cyclocross crankset

[vas9999]

I have an interesting problem, which makes me wonder if SRAM Rival/Apex front derailleurs might have a bit of a problem shifting with smaller (46T big ring) cyclocross cranksets. Here is my story:

Recently I've built a brand new bike with the following setup (SRAM Apex):

PG-1050 11-32 cassette
PC-1051 chain
Apex compact 50/34 crankset (same chain rings as Rival/Force)
Apex front and rear derailleurs
SRAM TT500 bar-end shifters

Everything had been adjusted well and shifted up and down perfectly. However, after a couple of days I did notice that the 50/34 crankset was not particularly well suited for this bike (commuter) and my style of riding - the 50T was a bit too high, and 34T was a bit too low. So, I've replaced the crankset with the cyclocross version of Apex - 46/38 (truly love the combo, btw - 46/38 front and 11/32 rear, for general commuting), shortnened the chain by one inch, re-adjusted everything, and went riding.

And then I've noticed the problem shifting between two front rings - at times, it takes seconds and seconds before the shift occurs. If I try to shift while pedaling the chain clunks and rattles until I let all of the pressure off the pedals and just spin freely, then it shifts in a few rotations.
I've already spent more hours tinkering with the FD than it took me to build this bike from the ground up. This is the same derailleur that worked perfectly with 50/34 crankset. Plus - since my front shifter is bar-end friction, things like trim, etc. are out of the question. Here is what I've noticed so far:

1. shifting under power is far worse than shifting while spinning pedals with a hand
2. shifting "up" 38T->46T works reliably while the chain is on the _smaller_ cogs of the rear cassette, but then 46T->38T hardly shifts at all
3. in opposite to #2, shifting "down" 46T->38T works reliably while the chain is on the _larger_ cogs of the rear cassette, but then 38T->46T does not shift

The only way I can get it to shift every time is by relaxing both upper and lower FD limits, but then I start dropping the chain every 5 minutes

I am at a loss. Maybe it has to do t with chain tension. Any ideas/suggestions are very welcome

 

[Peter@vecchios]

Originally Posted by vas9999 .

I have an interesting problem, which makes me wonder if SRAM Rival/Apex front derailleurs might have a bit of a problem shifting with smaller (46T big ring) cyclocross cranksets. Here is my story:

Recently I've built a brand new bike with the following setup (SRAM Apex):

PG-1050 11-32 cassette
PC-1051 chain
Apex compact 50/34 crankset (same chain rings as Rival/Force)
Apex front and rear derailleurs
SRAM TT500 bar-end shifters

Everything had been adjusted well and shifted up and down perfectly. However, after a couple of days I did notice that the 50/34 crankset was not particularly well suited for this bike (commuter) and my style of riding - the 50T was a bit too high, and 34T was a bit too low. So, I've replaced the crankset with the cyclocross version of Apex - 46/38 (truly love the combo, btw - 46/38 front and 11/32 rear, for general commuting), shortnened the chain by one inch, re-adjusted everything, and went riding.

And then I've noticed the problem shifting between two front rings - at times, it takes seconds and seconds before the shift occurs. If I try to shift while pedaling the chain clunks and rattles until I let all of the pressure off the pedals and just spin freely, then it shifts in a few rotations.
I've already spent more hours tinkering with the FD than it took me to build this bike from the ground up. This is the same derailleur that worked perfectly with 50/34 crankset. Plus - since my front shifter is bar-end friction, things like trim, etc. are out of the question. Here is what I've noticed so far:

1. shifting under power is far worse than shifting while spinning pedals with a hand
2. shifting "up" 38T->46T works reliably while the chain is on the _smaller_ cogs of the rear cassette, but then 46T->38T hardly shifts at all
3. in opposite to #2, shifting "down" 46T->38T works reliably while the chain is on the _larger_ cogs of the rear cassette, but then 38T->46T does not shift

The only way I can get it to shift every time is by relaxing both upper and lower FD limits, but then I start dropping the chain every 5 minutes

I am at a loss. Maybe it has to do t with chain tension. Any ideas/suggestions are very welcome

First, no need to shorten the chain. If it was proper with a 50/34, it would have been OK with a 46/38.

Did you also lower the FD when you went to a smaller chainring?

I'm thinking height and alignment. 1mm distance from the FD to the big chainring, height wise and when on the smallest cg and big ring, outer cage aligned with the chain. When in Largest cog in back and small ring, 1mm or less from the chain to the inner FD cage, same for cage to chain when in big ring and small cog.

Does the FD cage 'arc' align with the chainring? Not way tail up because of the really steep seattube?

 

[vas9999]

Thanks, Peter. Yes, I did lower the FD, and it is perfectly aligned - at this point I have it set up so there is about 0.5 mm between the inner FD cage and a chain The FD cage arc seems "normal" - this is cyclocross frameset, and the seattube angle is not that steep

 

[daveryanwyoming]

I don't know the source of your troubles and it seems like you've done all the important alignment steps, especially lowering the front derailleur to be within a mm or so of the big chainring teeth.

In terms of your original question I run a Rival front derailleur and shifters on my cyclocross bike with a 38x46 set of rings and they shift fine even under fairly heavy loads in race situations. I did have to remount the SRAM front derailleur by using the second and higher braze on mounting hole when I went to a smaller big ring to get the vertical clearance down to the mm range but it sounds like you've already done that.

So I don't think it's any kind of SRAM short coming but perhaps something more basic like a kink within the cable housing or some other more common reason that your shifting is slow and inconsistent.

Good luck,
-Dave

 

[vas9999]

I found it. Amazing. The "b tension" adjustment screw on the _rear_ derailleur was way too tight. Two lessons for me:

1. I used to think, naively, that the tighter the chain is, the better it shifts
2. I would not ever fathom that "b tension" screw on the _rear_ derailler would so severely affect shifting at the _front_ - especially with front chainrings not too far part (46/38)

Anyway, the front shifts like a charm now, thanks for everyone's help!
I would really like to learn more about the "b tension", if there are any helpful links you might have, please post them here

Thanks again,

Wesley

 

[Peter@vecchios]

Originally Posted by vas9999 .

I found it. Amazing. The "b tension" adjustment screw on the _rear_ derailleur was way too tight. Two lessons for me:

1. I used to think, naively, that the tighter the chain is, the better it shifts
2. I would not ever fathom that "b tension" screw on the _rear_ derailler would so severely affect shifting at the _front_ - especially with front chainrings not too far part (46/38)

Anyway, the front shifts like a charm now, thanks for everyone's help!
I would really like to learn more about the "b tension", if there are any helpful links you might have, please post them here

Thanks again,

Wesley

If in the lowest gear, biggest cog and the pulley/chain hits the cog, turning this screw increases tension in a spring there, rotating the derailleur body clockwise, moving the pulley away from the cog. BUT you want the top pulley as close as possible for the best shifting.