ShannonM

I'm going to be converting a 1993-ish GT Karakoram to a bad weather / bad roads (in Humboldt County, both of these things are givens) commuter / utility bike.

I'm also a bit of a gear phreak.

The bike was converted to 8 speed, with XT levers and XT v-brakes. The LX crank is 110/74, 46/36/26. This gives stupid gearing with just about every cassette you can buy. After playing around with gear-calculator.com, and considering cassettes you can actually buy, the best setup looks like a 12-14-16-18-21-24-28-32 out back with 48/34/granny up front. A 1.5-step + granny.

The LX front derailleur is spec'd for 22T. I'd like to use a 24T granny, yielding 24T total. Shimano specs are really conservative, and, if this was a friction shift bike, I wouldn't even question it, but are 8-speed XT RapidFire shifters forgiving enough for this to work?

--Shannon

squirtdad

are the XT indexed or friction? no hands on but I would check that if indexed for triple you can get to work on a double.


(HSU hill, stairs, umbrellas)

maddog34

i'd suggest that you step up, be adventurous, and TRY IT OUT FOR YOURSELF

oh, and there are quite a few slightly different "early 90's LX front derailleurs" available..... you may also find that the frame will limit how small you can go on the granny gear... The tail of the derailleur cage might hit the drive side chain stay or be aimed too high by the seat tube angle.... ;-)

once again.. be bold.. TRY IT.

KCT1986

During the '90s there were many different large chainring sizes, 46-50T early and later more 42-44T large rings. The FD cage shape changed to better match the curvature of the large ring. So, early FD's had a larger radius curve (similar to road FD),and later FD's had smaller radius curve.

A FD with a larger radius curve will mean that the tail may be too high to allow a really small granny ring, or at least cause chain rub in the granny/smaller cogs combo.

cyccommute 

As maddog34 said, try it. There’s nothing you can do that would damage the bike or the derailer. If it doesn’t work on the stand, it won’t work on the road. And if it does work on the stand, it will work on the road.

All the kerfuffleage about the derailer hitting the chain stay is mostly moot considering the size of the large chain ring you are proposing to use. It would have to be set higher than your current set up. The small chainring doesn’t dictate the height of the front derailer. I also wouldn’t worry too much about the cage curvature. In practice, it doesn’t matter in my experience. I’m using a road front derailer on a mountain bike crank and have used it for outer rings from 48 teeth to 44 teeth. Never had an issue with front shifting.

By the way, I would argue that your gear set up isn’t half-step. The front to middle chainring difference needs to be much closer to get a half-step…like a 48/44 ratio as seen below.




Your gearing is closer to a “cross-over” (not to be confused with cross chaining). A cross-over is one where you as you downshift, for example, you reach a gear you want and you “cross over” to the middle range. As you can see below, the bottom gearing is a half step where you downshift from high range to middle range to high to middle and so on. In the upper gearing pattern, you downshift from high gear to about the 18 tooth gear (which one isn’t important). The next shift to the 21 tooth cog in the 48 tooth chainring is just about the same as the shift to the 18 tooth cog in the 38 tooth chainring. You can “cross over” to the middle ring and keep downshifting.

You have more duplicated gears with the cross-over but the gear shifting pattern is simpler. You also have a lower middle range if you just want to treat the gearing as three independent 1x systems. The half-step works well in Kansas and Iowa (although those states are hillier than many people think). The cross-over works better where the hills are steeper.

ShannonM

And you would be correct in so arguing. The 45/42/30 x 14-26 6-speed on my road bike is a half-step. 48/34 x 12-32 8-speed is a 1.5-step. Still interleaved, but wider, so that a front shift is 1.5 times as big a jump as a rear shift. 48/45 or 45/42 would be a half-step on the 12-32 cassette, but the LX front derailleur won't work, and the road double derailleur that would work won't index with the XT front shifter.

Hence my question. Indexed front shifting is stupid, and I've avoided it for decades, but on this bike I'm stuck with it, so I want to know if the parts I have will shift the gearing I want to run before I spend any money.

--Shannon

LesterOfPuppets

What's wrong with current gearing?

Personally I'd probably stick with the triple. I also like tighter cassette than that, so would go with 12-25 in back.

As for the indexed front situation, Gripshift Attack front shifter (or similar) is one option. They have a 10-position index for the front so you can trim it.

ShannonM

Nothing's "wrong" with the gearing, I had the exact same crankset on a 1994 Trek back when. It's just that, if I'm gonna redo the drivetrain anyway, I'd prefer a different layout. Like I said, I'm a bit of a gear phreak. And traditional crossovers have always kinda bugged me. I love the half-step + granny on my road bike.

As for the shifters, they're integrated, so changing them out would require new levers as well, at which point I'd just go full friction with thumbshifters. But the whole point of this project is to spend as little as possible. I mean, I'm getting the bike from the co-op I volunteer at, and as many of the parts as possible, too.

--Shannon

HillRider

I know from experience that Shimano's recommended gear range limitations are very conservative. I've had Shimano Ultegra triple cranks that came 52/42/30 and regeared them to 52/42/2. They shifted fine with a 105 8-speed triple FD and STI index front shifting.

cyccommute 

Shifting shouldn’t be a problem. I have indexed front shifting on several bikes with much wider range than you are proposing and they perform flawlessly. I have a road touring bike with a 44/34/20 which has also had a 46 and a 48 outer ring with a 20 toot inner at various times in the past. It works just fine. My mountain bikes also have 44 outers and 20 tooth inners and they have zero issues.

Be willing to experiment and you’ll find all kinds of things that others will tell you “just can’t be done!”

79pmooney

I trust you mean 22 teeth. (I really want to see that 2 tooth chainring. What's the BCD? Now, if we could get one of those 2s in back we could go really fast!)

ShannonM

Ultegra triples were 130/74 bcd, right? In which case, 24t is as small as you can go. (Unless someone, by which I mean TA, made a 23.)

​​​​​​--Shannon