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  1. #1
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    New question here. Advice for upgrades: 2012 GT Sensor 3.0 9r

    This is my first post here, as I'm new to MTB'g. I have picked up a used 2012 GT Sensor 3.0 9r, all stock. Overall, I really like the bike. I grew up on dirt bikes (now 45 years old), so I really wanted full suspension. I mainly ride singletrack, rocky stuff (Austin TX area). I'm "fluffy" at 255 lbs.

    I know I need to upgrade the suspension, but my more immediate concern is what I believe to be a bent right side crank. When I really get on it, the crank hits the swing-arm. I want to go ahead and replace with a new crankset and BB. There seem to be 8 million options and terminology that I just don't get yet. So, anyone willing to give me a few recommendations of the exact crankset/bb that I need? I'd like to upgrade the front derailleur while I'm at it. Back too maybe. I hate the squeaky brakes too!

    Open to any other recommendations/suggestions on what parts to change out on this bike.

    Many thanks in advance.
    Paul

  2. #2
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    I would install a XT crankset.

  3. #3
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    Quote Originally Posted by cbj2012 View Post
    I would install a XT crankset.
    Thanks CBJ. There are numerous XT cranksets, and I guess that's my problem, I don't know how to identify which one I need, which one's fit. My current is a 3 ring. Is it ok to go to a 2 ring? Do I get a 165mm, 170mm, 175mm or 180mm? Can I vary off of stock on sprocket counts? Do I get a sealed BB or threaded?

    I realize I sound like an idiot. I can tear down a dirt bike and split the cases, but know zero when it comes to bicycles and the real specifics like replacing a crank, bb, derailleur, etc. Feel free to PM me any direct links to specific cranksets if you wish. I don't mind doing the research, which I've done, I just can't seem to figure out some of the things mentioned above.

  4. #4
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    Number of rings depends on what you like. If you are a big guy get 3 rings.

    Crank length depends on your how you fit the bike. Most likely 175mm is fine.

    Threaded bb just buy the matching XT level BB.

  5. #5
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    Before replacing the crankset, make sure they the culpret first. Make sure the left crank is tightened onto the bottom bracket (screw in the center of the crank arm), that the frame isn't flexing because it cracked somewhere, and that the rear triangle's bearings aren't so bad that it's causing the rear triangle to flex on you. If the pedal is at a slightly odd angle when at top center of bottom center, congragulations, you have a bent crankarm.

    Your current bike uses a threaded square taper bottom bracket. This is old technology, heavy, but very durable and strong. You can replace it with a new square taper crankset for about $30 (search "vuelta MTB crankset" or "Shimano square taper crankset" on amazon) or you can get a whole new crankset. The Jenson online retailerhas a deore external bearing bottom bracket/crankset for $50. Either way, you'll need to get tools to replace them or get your LBS to do it. The bigger question for you is if you want to learn to do the work yourself or pay to have the LBS do it.

    Shimano Deore M610 Crankset > Components > Drivetrain, Brakes and Pedals > Cranksets | Jenson USA

    The 170 vs 175 big is the length of the crank arms. If you're tall, get the 175mm. If you're short, get the 170mm.

    Also, changing out the crankset may require you to adjust your front derailure. Easy to do and only requires a couple allen wrenches, patience, and the desire to learn. Normally front derailures don't go bad, they just get knocked around and out of alignment. I wouldn't replace it, but fixing it will be part of the patience bit I mentioned.

  6. #6
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    I really really appreciate the info! This gives me a greater understanding, and yes, I want to do all the work myself to learn how to do it. I have checked for frame cracks, none found. I can literally push/pull the crank into the swingarm, there's that much flex. I'm going to do the BB and the crankset, then patiently learn how to adjust derailleurs

  7. #7
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    Good job!

    Ordered the crankset with bb, bb tool and few other things. Looking forward to diving in. Many thanks again for the advice and specifics. I'll keep ya posted on how I progress...

    Paul

  8. #8
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    Good job!

    update as promised... installed the new crankset and bb, and a few other things, and adjusted the front derailleur. had to buy a crank puller and another wrench to tighten the outer bearings, but now i have those tools plus the other bottom bracket socket thing. couldn't have done this without your help. much appreciated!

    Advice for upgrades: 2012 GT Sensor 3.0 9r-newcrankset.jpg

  9. #9
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    looks good. I think the front derailleur might be a little high. but if it works and shifts clean, go for it.

    Looks like the old bottom bracket might have been bad, you had loose chainrings, or the drive side crankarm was seriously bent. Shouldn't have chainring gouges on your frame.

  10. #10
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    Thanks for the comments. I will double check the height, although it is shifting pretty smoothly at this point. I think it was a combination of bad bb and bent crank. You could tell more easily when off the bike how the crank was bent. The flex is now gone, and the chainring isn't close to the swingarm, nor is the crank, like it was before. The old sealed BB was obviously bad. You could feel the bearings roughness when spinning it in your hand.

  11. #11
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    Well, still have the GT Sensor 3.0, and it's still going strong; however, I want to upgrade the front brakes. Anyone have a suggestion of a kit to replace on this bike?

  12. #12
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    bikewagon has shimano hydraulic brakes for cheap. If you're good with your levers, shimano makes some mechanical disc brakes for something like $25 a caliper. They use the larger brake pad size similar to an avid BB7. If you are OK spending more money, get an Avid bb7 set. I got one years ago for about $75 off amazon. Still going strong.
    "a hundred travel books isn't worth one real trip"

  13. #13
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    Thanks Watts! I'm off to shop........

  14. #14
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    Bought the Avid BB7, rotor and new pads. No noise yet after break in and they are stronger than the hydraulic ones that were on there. Thanks again for your help! I'll probably be back for more questions........

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