Getting front derailleur to shift up smoothly

danhoro

Regular
I've recently acquired a Planet X RT-58 alloy with Sram Apex groupset. From the start I've found that it's not very smooth when shifting up to the bigger chainring. Sometimes it catches straight away, but around 50-60% of the time it takes what feels like 1/8 (maybe a bit more) of a turn of the crankshaft before the chain properly catches on the bigger ring. In the meantime, I get an unpleasant scraping noise as the chain is pushed up against the ring. Once it shifts up I don't have any issues with the chain rubbing the derailleur cage.

I've tried tightening the cable tension, which doesn't seem to have improved things. I assume it won't be an issue with the limit screws, so I haven't played with those. It looks like the derailleur is an appropriate height above the large chainring - about 2mm at the smallest distance.

I had a couple of theories. Maybe there is some flex in the bottom bracket meaning that it's harder to shift up when pedalling down on the right pedal. Also, the chainring has a number of smaller teeth, so maybe it doesn't properly catch until one of those rotates around. I don't know how likely either of these are?

I did wonder if it was just an idiosyncrasy of the mechanism, but I found this video of someone setting up the same derailleur and they get it running much more smoothly without a problem:

View: https://www.youtube.com/watch?v=HVI8q6GLLFw


Any advice much appreciated!
 
Location
Loch side.
I've recently acquired a Planet X RT-58 alloy with Sram Apex groupset. From the start I've found that it's not very smooth when shifting up to the bigger chainring. Sometimes it catches straight away, but around 50-60% of the time it takes what feels like 1/8 (maybe a bit more) of a turn of the crankshaft before the chain properly catches on the bigger ring. In the meantime, I get an unpleasant scraping noise as the chain is pushed up against the ring. Once it shifts up I don't have any issues with the chain rubbing the derailleur cage.

I've tried tightening the cable tension, which doesn't seem to have improved things. I assume it won't be an issue with the limit screws, so I haven't played with those. It looks like the derailleur is an appropriate height above the large chainring - about 2mm at the smallest distance.

I had a couple of theories. Maybe there is some flex in the bottom bracket meaning that it's harder to shift up when pedalling down on the right pedal. Also, the chainring has a number of smaller teeth, so maybe it doesn't properly catch until one of those rotates around. I don't know how likely either of these are?

I did wonder if it was just an idiosyncrasy of the mechanism, but I found this video of someone setting up the same derailleur and they get it running much more smoothly without a problem:

View: https://www.youtube.com/watch?v=HVI8q6GLLFw


Any advice much appreciated!
It sounds perfectly normal to me. The chain doesn't shift exactly the moment you move the lever. The chainring only has about 4 places where the shift happens so 1/8th of a revolution is good. You can see the shifting ramps by looking at the left side of the chainring and you'll either see round pins or ramps machined into the metal. It only shifts there. Secondly, you have to de-power when you shift. We call this feathering. It is a technique that most of us do without realizing but it is crucial for a smooth shift. If you pedal too hard whilst shifting, the chain can bend to the new position and scrapes like hell until through some miracle it hooks and pulls over.
 
OP
danhoro

danhoro

Regular
Thanks, that's reassuring then. After thinking about it again, I'd say it would often be up to a quarter turn of the crankshaft, but presumably that would still be okay if there are only four shifting points. It still seems to me like that video shows it shifting quicker than mine does consistently, but I guess it might be a case of being over-sensitive to anything that might be a problem on my new shiny piece of kit!
 
OP
danhoro

danhoro

Regular
Okay, it looks like I spoke too soon! Over the past few days I've been trying to keep an eye on it and observing exactly what's going on. Quite often it definitely takes more than a quarter turn to engage the big chainring. Sometimes between 3/4 to a full turn. I've tried the feathering technique, to the extent of actually stopping pedalling altogether, shifting, and then pedalling again lightly. It seems like it might help with the shifting a bit, but certainly doesn't solve the issue. I really feel like the shifting is much slower and less consistent than the seven year-old Sora groupset on my last bike. I can't help feeling that something is awry with the way it's set up. Any ideas?
 
Location
Loch side.
OK, lets have a look. Are you in a position to remove the chain? If so, do it, shift to the two extremes and post photos of the FD directly above the small and large chainring as in the position you would have shifted had there been a chain on.
 

TheJDog

dingo's kidneys
Feathering doesn't mean stopping turning the pedals, it means turning the pedals with little or no power going through them. The shifts in the video above are very smooth because he's just turning the pedals with his hand. Can you lift your back wheel and try shifting up and down a few times turning the pedals constantly with your hand? That is probably reasonably smooth. By stopping altogether, shifting, then starting again you'll probably get the worst shift ever. Your set up might be fine, but you aren't doing it right. On the other hand, one of your pins might be damaged, and it's not catching the chain. Or something else entirely.
 
OP
danhoro

danhoro

Regular
I still have the same problem when feathering in the way you describe unfortunately. I had just tried the stopping pedalling thing as a bit of an experiment, and have tried all sorts of varied degrees of force on the pedals during shifts as well. I had a play around with holding the back wheel up and shifting the other day, and still had the problem of it sometimes taking well over a quarter turn of the pedals before it engages.

Will try to take a few photos this evening.
 

oldroadman

Veteran
Location
Ubique
Check the front changer cage is correctly aligned. Check the chain for wear, Check the chainrings for wear. Correct any faults found, and see if improvement happens. First thought is alignment of the cage.
 
Location
Loch side.
Check the front changer cage is correctly aligned. Check the chain for wear, Check the chainrings for wear. Correct any faults found, and see if improvement happens. First thought is alignment of the cage.

A worn chain, chainrings and cassette does not influence shifting at all. Alignment, yes.
 

migrantwing

Veteran
Excuse my jumping in on this thread, but are the pins and ramps on a Shimano chainring perpendicular or parallel to the drive side pedal?
 
OP
danhoro

danhoro

Regular
Okay, so I couldn't find a chain tool so wasn't able to take the chain off unfortunately. I took a few pictures last night, although I don't know if they're any help, with the chain being on and also bad light + camera flash. Anyway, to took photos of the derailleur from the top whilst on the little ring, on the big ring, and also a side view to show the clearance between the cage and the big ring. It's hard to see from the photos, but there didn't seem to me to be a problem with alignment.

http://imgur.com/nAzbkfz
http://imgur.com/VNE5GLQ
http://imgur.com/eoCOU7w
 
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