|   2x9 (converted from 3x9) setup questions | jeffj
Jan 12, 2004 4:04 PM | | I recently added a 20t granny gear to my 3x9 (now it's a 20-32-44 RaceFace Turbine, compact, square taper) cranks using an 11-34 rear cassette. I'm sure more experienced cyclist already know my dilemma. Shimano recommends no more than a 43 tooth difference between the largest ring and the smallest ring. RaceFace does not make a 9 speed 42t 94mm big ring and I am not willing to give up the single best improvement to the climbing ability of my bike I have ever made (the 20t granny gear helps my 270lb carcass ascend better than ever without having to stop). The chain is stretched out straight at 44/34 (3rd ring/1st gear) and the long cage XT derailleur cannot take out enough slack to prevent the chain from rubbing on itself and the cage when at 20/11 (1st ring/9th gear).
I have decided that I can live without the third (big) ring more so than the other two and am willing to 86 it in favor of a rock ring along with a 2x9 setup (20/32).
I decided to try it by removing three links from my chain and going for a ride without using the large front ring and did not terribly miss it.
1) How should I adjust the front derailleur (LX bottom swing) so that I cannot shift past the middle ring?
2) Should I lower the derailleur (closer to the BB) on the seat tube since it doesn't need to clear the outer ring?
3) Is there anything else I should be aware of when setting this up?
TIA |
| re: 2x9 (converted from 3x9) setup answers | club
Jan 13, 2004 8:03 AM | | that 43t is total capacity, no? not just small/big ring difference.
instead of calculating total capacity based on small/large cog plus small/big ring, calculate it based on the Useful Gears, the gears you will actually need and use. No earthly reason to ever run in the 20/11 or even the 20t with the smaller half of the rear cluster. Ditto the big/big, no reason to ever use that gear. Both of the above are actually bad for the rings and chain, they're called Cross Chain Gears because they force the chain to work at the most extreme angle from front to rear.
With 3x9, you could go one of two ways: shorten the chain so it doesn't hang limp in the little/little, and then be very careful to Never Ever shift into the big/big. A reason to go this way would be if, when in the granny ring and large cog, the upper pulley rubs the chain up against the big cog even though you've already screwed the B tension screw all the way in.
The other way to go is set the chain length so you can just get into the big/big combo without damaging the derailleur, and then just don't even shift to the small/small or its close cousins, so what if the chain hangs limp in gears you never use?
If you switch to a double crank, you might want a "big" ring larger than 32t to give you more high gears. With 20 and 32t rings you'll have too many duplicate or close duplicate gears and will be wasting your reduced number of gears with overlapping ones.
If you really need a 20/34t low, fine, I have that on my backcountry bike for altitude. But if you don't you might also go to a larger granny ring so you can cruise along in the granny more often. A popular double setup uses like a 28 or 29t inner with a 42 or 44 outer, with the 34t low in back.
to figure out the actual gearing and avoid duplicates, use the gear inch formula: #front teeth divided by #rear teeth multiplied by wheel diameter (26 unless you got a 29er). Plot out all the gears and you'll see what you want/need to have a good range with aminumum of duplicates. Welcome to the weird wonderful world of gear freaking, Frank Berto would be proud. (do a goggle search or read back issues of Bicycling from the '80s if you never heard of the King of Gear Freaks). |
| Thanks for a thoughtful reply... | jeffj
Jan 13, 2004 10:26 AM | | It's appreciated.I do really need the 20t in fron. It's the single best thing I've ever done for my bike and myself. I'm a big guy at 270lbs and climbing has always been tough. I don't ride as consistently as I should to either drop lbs or get fit enough to not "need" the 20t granny. I am able to keep pedaling in situations where I would have needed to stop several times. The 20/34 combination helps me to not get too winded, exceed max heart rate or have legs quit or all of the above. It's that much better for me than the 22t I've been using for years. I shift up out of low more often, but it's those times when I need to climb hard for more than a relative few cranks that used to get to me. I think being able to continue pedaling will get me in better shape quicker than starting and stopping and I'm not nearly as winded when I reach the top of a climb. If I am climbing on typical trails here in SoCal, I am usually in lower gears (no middle or big ring climbs for me). Rolling, flat or downhill is the only times I am out of the small ring and rolling or flat trails are few and far between around here.
One more question I have is whether or not I should drop the derailleur closer to the bottom bracket so that it is really close to the top of the middle ring (like it was on the big ring before)? |
| yes. 2mm gap between cage bottom and largest chainring nm | club
Jan 13, 2004 10:58 AM | | |
| You have created a monster . . . . . >8P | jeffj
Jan 14, 2004 8:43 AM | | No explanation necessary... |
| You have created a monster . . . . . >8P Part II | jeffj
Jan 14, 2004 8:44 AM | | |
| yup | club
Jan 14, 2004 7:00 PM | | other than the low gears being on top (I always start those charts with the small cog and work down) that's pretty much a large version of little itty bitty squares of adhesive label with lil numbers on it that I was sticking on my road bike stems back about 35 or so years ago until I memorized all the road bike gear inches, course that was with 5 and 6 sp. freewheels so there weren't so many numbers to memorize. as a reminder not to use the little/little and big/big combos, I'd put Xs there instead of numbers.
I'm always surprised how few riders have a clue about gear inches and how knowing them can help you figger out stuff, like comparing BMX/road/MTB/Bigwheel/Alex Moulton 16" wheels, etc. |
| You have created a monster . . . . . >8P | jeffj
Jan 14, 2004 8:43 AM | | No explanation necessary... |
| |
