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Old 09-02-10, 01:00 AM   #1
LudoGuppy
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Multiple Questions on Gearing

Recently, I purchased a Gios Spazio road bike that I intend to outfit for self-sufficient loaded-touring. Specifically, I live in Taiwan and intend to do a long tour zigzagging along all three of the cross-island highways as preparation for a longer continental tour.

The problem is that, even without additional weight, I find myself using the lowest possible gears to propel myself up some mountain roads. Currently, my chainset is a 48-38-28T and my cassette is a 12-25T with Shimano Tiaga front and rear derailleurs (I hope I'm using the correct words, my bike parts vocabulary and knowledge are both in their nascent stages.). Hence, it's become clear to me that I need to lower the gearing on my bike.

What I am worried about are compatibility issues, so here are my questions:

1) I would like to purchase a Shimano XT M770 11-34T cassette for the back, which means I will definitely need a new rear derailleur to accommodate the larger cassette. I intend to buy a Shimano XT rear derailleur for this purpose. Am I overlooking anything in the relationship between the rear and the front parts of the "chain system" (?) or is that all I would need to buy?

2) Hopefully the larger cassette will do the trick, but if not, then I intend to change out the chainset to one with a smaller chainwheel with 22 teeth, something like a Shimano XT. Are there compatibility issues with installing mountain bike oriented chainsets onto road bikes? I anticipated having to buy a new front derailleur, but the guy at the shop (in Chinese) tried to explain that the chainset and derailleur would be incompatible with my roadbike shifters. He said I would need to change out the shifters and probably the handlebars, because roadbike shifters need less extension to initiate a shift than mountain bike shifters. (At least that's what I think he said...)

Thanks for any help. We've got until Chinese New Year to get this settled.
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Old 09-02-10, 10:20 AM   #2
sstorkel
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Quote:
Originally Posted by LudoGuppy View Post
1) I would like to purchase a Shimano XT M770 11-34T cassette for the back, which means I will definitely need a new rear derailleur to accommodate the larger cassette. I intend to buy a Shimano XT rear derailleur for this purpose. Am I overlooking anything in the relationship between the rear and the front parts of the "chain system" (?) or is that all I would need to buy?
You'll need the cassette, rear derailleur and possibly a new chain and cables. Increasing the size of the largest cog in the cassette by nine teeth will mean that you need a longer chain. You can probably reuse the existing shift cable, but might need a longer cable.

In addition, you'll want to buy the correct version of the XT rear derailleur. Know the difference between Low Normal, High Normal, and Shadow before you make your purchase...

Quote:
2) Hopefully the larger cassette will do the trick, but if not, then I intend to change out the chainset to one with a smaller chainwheel with 22 teeth, something like a Shimano XT. Are there compatibility issues with installing mountain bike oriented chainsets onto road bikes? I anticipated having to buy a new front derailleur, but the guy at the shop (in Chinese) tried to explain that the chainset and derailleur would be incompatible with my roadbike shifters.
There can be compatibility issues when installing mountain bike parts on road bikes. If you try to use a mountain bike crank with a road bike front derailleur, you need to worry about "chainline". Mountain bike cranks typically place the chain further from the bike frame than road bike parts. As a result, you road bike front derailleur may not extend far enough to shift onto the largest chain ring. I ran into this problem when installing a 48/38/26 trekking crank on my Ultegra-equipped bike. Worked around the problem by removing one of the bottom bracket spacers.

The other potential problem is the shape of the front derailleur cage. Usually, the derailleur cage is curved to work with chain rings in a specific range of sizes. A road bike front derailleur, for example, might be designed to work with chain rings from 34-teeth to 53-teeth in size. If you go outside this recommended range, you may find that shift quality starts to suffer.

Finally, Shimano mountain bike front derailleurs are not compatible with Shimano road bike shifters. Don't know about SRAM, but I would assume they have the same problem.
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Old 09-02-10, 10:24 AM   #3
chasm54
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No, I don't think you're overlooking anything except that you will also need a longer chain to accommodate the bigger sprockets at the back (assuming your chain is the right length at the moment).

My own view is that swapping the rear cassette will do the trick. My biggest gear on my touring bike is 28/32 and with that I can climb a 20% gradient with a 50lb load without too much trouble. With a 28/34, and a bit of practice, you will be able to get up all but the most extreme of inclines. Bear in mind that 28/34 is a massive drop from the 28/25 you've been using. You really shouldn't need to change the chainset.
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Old 09-02-10, 08:58 PM   #4
badamsjr
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I agree with the above, but would add that your 28t small ring is probably a 74mm BoltCenterDiameter. You could replace it with a 26t or even a 24t small ring, should you find the 28/34 is not low enough for you. This would preclude the need for a new chainset and front derailleur. The new chain you will need for 28/34 should have no rpoblem with 26/34 or 24/34.
I ran into the incompatibility of MTB FD and road shift/brake set. I ended up going to bar end shifters to make it work the way I wanted it to.
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