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Old 09-07-08, 06:54 AM   #1
hankbrandenburg
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LHT Drivetrain advice please

Hello Touring Collective,

I'm currently in the process of building a LHT with the intent of some progressively longer tours over the next two years; I hope to ride trans Am in 2010. I have no experience at building a bike from scratch myself and am working with my LBS to get this new machine fully functional. My questions relate to drivetrain; specifically what range of gearing is even feasible to pursue and how to do so. Here are some pertinent specs on the bike as it now stands.
  • 62 cm LHT frame
  • Shimano Ultegra STI shifters
  • Shimano XT Deore Rear Derailleur
  • Shimano XT Deore Cranks
  • Three chain rings - 48/36/24
  • SRAM 9-Speed Cassette 11-34
  • SRAM PC-991 Chain

We attempted to use a Shimano XT front derailleur but apparently it was unable to shift across the whole range of rings in the front. Right now the bike is equipped with a Shimano SORA in the front, which does access all three rings. To make this work the smallest ring had to be reversed from its normal installation, and apparently the cranks are 5 mm out of symmetry because of this.

It is clear that this configuration will not support all 9 cassette positions with all three chainrings. If the bike is on the smallest ring, only the 4 largest cogs on the cassette are usable before the chain loses all tension. With the largest ring and cog selected, the real derailleur appears to be over extended.

My question(s):

1 - Is it even feasible to pursue this wide-range gearing configuration?
2 - If the answer to 1 is yes, can you provide any recommendations on components to accomplish this?
3 - Is the lack of symmetry between the cranks anything to be concerned about?

As always, thanks for any advice or guidance you can provide.

Last edited by hankbrandenburg; 09-07-08 at 07:11 AM.
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Old 09-07-08, 07:45 AM   #2
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There are a couple of issues here:
  • Chain wrap -- (48-24)+(34-11)=47 teeth. The XT will officially wraps up to 45 teeth, so you should be pretty close, but may want to narrow things a bit.
    • most folks only use the bottom 4 or so cogs with the granny, so that's OK
    • you should never be using large<->large, but as long as you can shift into it, you're OK
  • Alignment -- the Front Derailleur should be able to shift to all three rings if they are correctly installed, and the Bottom Bracket is the correct length
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Old 09-07-08, 07:58 AM   #3
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If the bike mechanic can't figure out what's up with your FD without doing wacky things I would seriously consider going to a different shop. The RD as described sounds normal. This should be old hat for the mechanic you're working with.
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Old 09-07-08, 09:14 AM   #4
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consider changing out the bottom bracket. it is possible the wrong spindle length is not allowing for certain gear combinations. i run a 48-36-24 with a 11-32 and have almost all possible combinations (except for 48/32) available to me in part because i made sure that the correct bottom bracket was used.
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Old 09-07-08, 12:26 PM   #5
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Originally Posted by ernok1923 View Post
consider changing out the bottom bracket. it is possible the wrong spindle length is not allowing for certain gear combinations. i run a 48-36-24 with a 11-32 and have almost all possible combinations (except for 48/32) available to me in part because i made sure that the correct bottom bracket was used.
Thanks for the tip ernok - what BB are you running?
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Old 09-07-08, 07:17 PM   #6
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Try fitting spacers under the driveside BB BEFORE you go to the trouble of getting a new one. And honestly, go a different bike shop. The XT F. derailer had little chance of working with the Ultegra brifters. You need a top shelf wrench to get this set up.

At this point in the game, I'm betting a good mechanic could most likely fix your bike WITHOUT spending more money.

You're on the right path, so you can relax, hankbrandenburg.
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Old 09-08-08, 12:04 PM   #7
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I would wonder about the BB too. My LHT has the same gearing you describe, using I think XT rear derailer, Tiagra front. Oh yeah, friction shifting on the front, though, unlike you.
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Old 09-08-08, 12:27 PM   #8
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Quote:
Originally Posted by GeorgeBaby View Post
There are a couple of issues here:
  • Chain wrap -- (48-24)+(34-11)=47 teeth. The XT will officially wraps up to 45 teeth, so you should be pretty close, but may want to narrow things a bit.
    • most folks only use the bottom 4 or so cogs with the granny, so that's OK
    • you should never be using large<->large, but as long as you can shift into it, you're OK
  • Alignment -- the Front Derailleur should be able to shift to all three rings if they are correctly installed, and the Bottom Bracket is the correct length
I'm running a 46/34/22 (RaceFace Turbine) with an 11-34 and an XT rear without issues. Same wrap as above. 105 shifters with a Tiagra front. No problems.

I have another bike running an XT octalink with a Tiagra shifter/Ultegra front derailer without any special cabling nor with any issues on shifting.

There shouldn't be any problem with this set up, hankbrandenburg
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Old 09-08-08, 05:57 PM   #9
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The stock LHT complete has a 48/36/26 (Sugino) paired with an 11-34, and it works fine for all combos- has Tiagra FD and XT RD, SRAM PC971 chain, 110 mm BB
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Old 09-08-08, 06:22 PM   #10
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I have Tiagra brifters, FD, and RD and I am running 11-34T cassette in the back and a 52-42-24T on the front. It pushes the limits but it does work if you dial it up right. You just have to avoid the stupid extremes (52->34 or 24->11) and be a little gentle with the front shifting but it works wonderfully. There are tricks like putting washers in to make the FD a bit wider that you can maybe try to help eliminate a little rubbing if needed.

I think switching to a friction shifter for the FD would make this setup work in all situations and it's the route I may go eventually.
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Old 09-08-08, 09:04 PM   #11
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Quote:
Originally Posted by hankbrandenburg View Post
Thanks for the tip ernok - what BB are you running?
113 mm
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