Gear Issues or Rider Issues?

[andyfraser]

Hi,

I've been having a little difficulty setting up the Sora front derailleur on my new Felt F95. I can get it so 34x25 is ok but 34x11 will rub. When I tension the cable 34x11 is ok but 34x25 will rub. 50x11 is ok and I haven't bothered with 50x25 feeling that that shouldn't work anyway.

So the question is, for a compact chainset am I supposed to have access to all rear sprockets on the small chainring or should I be shifting up before I reach the smallest rear sprocket? The front derailleur shifts perfectly fine otherwise.

On my Kona I have full access to all 9 sprockets on the middle (38) chainring but the cage is a different shape. I wouldn't dream of going to the extremes on the 30 or 48 chainrings so I may have picked up a bad habit of staying on one chainring here.

 

[lejogger]

Cross chaining should be avoided as much as possible, and you should be avoiding the extremes such as 50 x 25 and 34 x 11. Both will probably cause the chain to rub with a Sora front mech and compact chainset combination if it's set up properly.

 

[andyfraser]

Thanks for that. I was starting to think that way. I need to learn better habits.

 

[Nigelnaturist]

what @lejogger says, is there not a trim function on the left shifter, there was on my Sora 3400 triple, I can get all my rear gears in whatever front ring I am on without rub, 50/38/26 and a 12-27, though the lower 3 or 4 gears, i.e. 1-4 on the 50 ring there is chain noise, but this is because it is at extremes and the chain itself when at an angle will make a noise very similar to chain rub, also on different rings the combinations will make different noise, the current set up ran quieter on the 30/40/52 i had before I changed it this week in the same combinations. The reason is that the 52/40/30 chainset should have had a 116mm BB but was on 115mm so the chain line wasn't as extreme, the new set up is on a correct 113mm BB so the chain on the front outer is further away than it was

 

[andyfraser]

How would I know if there was a trim function?

 

[lejogger]

How would I know if there was a trim function?

A trim function is essentially giving the shifter half a click to move the front mech slightly... which can prevent chain rub at times. It still wouldn't help with your problem when on the very outer limits I wouldn't have thought.
The same ratios as these extreme gearings will be available to you on the alternative chain ring, it just requires a more frequent change of front gear which is probably the main disadvantage of the compact chainset.

 

[Nigelnaturist]

How would I know if there was a trim function?

Do you know what modal the shifters are, but usually certainly on triples if you part press the lever either up or down it will move the cage only so much, the same again will move the cage to the next ring.
I dont know if this will help
http://si.shimano.com/php/download.php?file=pdf/um/UM-6VX0A-001-00-ENG.pdf

 

[andyfraser]

The shifters are Sora STI. I'll have to check this out further. Although @lejogger is right, the ratios will still be available.

 

[cyberknight]

How would I know if there was a trim function?

If its a double shifter then when you shift up you can normally shift up a a click more to push the mech further out.
My front mech is set so i can get on an 8 speed cassette 2-5 with no rub then an extra click for 6-8 that also works from about 4-8 with no rub .So if i am on a hill but dont need the small ring but maybe need to run number 3 at the back which is 21 back and a 50 front and i had been running a big gear on the flat with the front mech as far out as it would go i can do a small/shallow movement of the left shifter in the down direction till i feel a click and the mech will move in enough to prevent rubbing without changing down to the small chainring.

 

[cyberknight]

The shifters are Sora STI. I'll have to check this out further. Although @lejogger is right, the ratios will still be available.

You can normaly get a similar ratio on a small chainring
heres a claculator for a 50 /34 12-25 cassette
50..........34
109.5..74.5
101.1..68.7
87.6..59.6
77.3..52.6
69.2..47.0
62.6..42.5
57.1..38.8
52.6..35.7
so a 50x25 =56.2 inches per rev , 34x 15 gives 59.6 , 34x17 gives 52.6 so very close and a better chain line.
I personally run a 12-25 with a 50/36 as i find it gives me better ratios and less of a drop when i have to change down to the small ring so i am not spinning my legs like a nutter till i slow down enough for the gear .

 

[andyfraser]

I know of a couple of ratio calculators online. I should work out what's where and learn to use my gears a bit better I think. Thanks for that info. That's been very helpful.

 

[andyfraser]

I've just had a look at my front shifter and I do indeed have a trim option. I'm going to reset my gears (the rear cable has stretched a bit anyway so needs doing) and see how I go with the trim.

Thanks so much to everyone for pointing this out.

 

[andyfraser]

Gears now reset (now takes me less than 10 minutes to do front and back) and everything is good using the trim. Many thanks again.