Gearing ratio woes on new bike - convert 3x9 to 2x9?

Cj83

Just got a new Giant commuter (Escape 1). However after doing the daily 30 mile commute for a week I now know I cannot live with the gear ratios I can get out of the 3x9 drivetrain (chainset :48,36,something, Cassette: 11,12,13,14,16,18,21,24,28). With the big ring I can use up to the 5th sprocket at a push before a lot of rubbing between front mech and chain. With the middle ring I can get down to the 4th sprocket before the rubbing (and a dodgy looking chainline). The trouble is the gear ratios I like to use (and used on my previous bike) are between these gears.So if you can imagine yourself having a choice of spinning too much or pushing too much of a big gear for a couple of hours a day thats what it seems like for me at the moment - very frustrating!

If i could somehow lose the small chainring (which I will never use anyway) and position the remaining two slightly inwards so I can use up to the 6th sprocket comfortably with the big ring i'd be happy. However is this in anyway possible? Maybe by somehow modifying the BB? The chainset is a Shimano M431 and the BB UN26.

This guide explains how to make the neccessary adjustments to the front mech for a 2x9 set up but not how to modify the chainline with the outer chainring...

any help much appreciated!!

pdw

I'm afraid I don't know the answer to your query, but I would expect to be able to get better adjustment than you describe on the triple. Have you tried tightening the derailleur cable a bit, and also screwing the upper limit stop in a bit?

davis

You're better off fixing what you've got so you can use the gears as intended. Start by looking up how to index your gears.

Cj83

I can use the gears as intended and they are indexed fine but can't get the ratio's i want with those gears. the front derailluer set up seems ok. i think i've found the most hassle free solution though, if i fit a 13-25 cassette i'll be able to get the ratios I want in the big ring and a straightish chainline. cheers for your help.

wombar

You should be able to use every rear gear when in the middle ring, and have very minor rubbing at the worst. On my Ultegra triple I can stay in the middle ring and use every single gear at the back and it's only the biggest gear at the back that I get a tiny amount of rubbing (but it's barely noticeable).

Sounds like your front derailleur needs re-aligning. It's not unusual for them to be misaligned, which results in lots of rubbing. Get them lined up properly and they're just as a good as a double. I've got a double on my road bike and a triple of my CX, both running Ultegra. The triple works just as well as the double.

Triples work absolutely fine, you just have to get the angle right on the FD or they run like rubbish.

The Rookie

+1, the front mech set up right and you should get at least 7 of the 9 gears if not 8 or 9, in which case you'll have the gears you want!

Simon

Cj83

ok thanks for that, i didn't realise you were even supposed to get that many gears for a given chainring, i'll have a play with the front mech. But is this concern that I've got from somewhere about having a relatively high angled chain line (from a running a gear from your small ring to a smaller sprocket and vice versa) unfounded?

desweller

On your middle ring you should be able to use pretty much the entire range of rear sprockets as it's lined up with the middle of the cassette. When you're on the big chainring or the small chainring you're relatively limited as you can only use the upper or lower set of sprockets for the chainline reason you describe.

Blue Meanie

Front Mechs: 3rd hardest thing to set up on a bike.

Thing is, the cage is normally just pressed from mild steel and is often poorly formed; a bit of bendy with the handle of a shifter does wonders.
Check it's parrallel to the chainset (first sign of badly set up mech) and at the right height.
Once you've got that set, you can reset the low stop, adjust the cable and set the high stop. Result: No rub!

I have had to accept that some 'downhill' bikes will always rub, but I wouldn't suffer it on a commuter. Feel free to use diagonal gears, slight loss in mechanical efficiency, but no rub allowed.
I hate catching some 'roadie' who's cycling along with their chain grinding away at the front mech - they never heard of 'trim' on the L/H shifter? Grr...