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  1. #1
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    New question here. Upgrading Shimano Cassette from 26T to 34T?

    I'm wondering if someone could help me out. I have an old Trek 7000 ZX mountain bike that I am dragging out of retirement. The cassette is shot, and I ordered a replacement. But now I'm wondering if the cassette I ordered is going to work.

    The bike came with an 11T to 28T Shimano 7 speed cassette. The derailleurs are old Shimano STX RC derailleurs.

    I ordered a 13T to 34T Shimano 7 speed cassette.

    But I think I read somewhere that I cannot upgrade to a larger gear with my existing derailleur. Does anyone have any experience with this?

    Thanks.

  2. #2
    dru
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    You may have problems, but may not as well. If you can't keep the top jockey wheel away from the cassette when you are in the lower gears with the existing B screw turned all the way in you might want to fit a longer one.

    Drew
    occasional cyclist

  3. #3
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    Thanks Drew,

    I guess I will just have to try it and see. Thanks for the pointer about the B screw.

  4. #4
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    You will find out if your old derailer has enough take up range when you install the new cassette and chain. Also, you will be using a longer chain, or at least not removing as many chain links when you fit it.

    I'm still using a cheap Tourney RD-TX71 with the SGS cage. I used a 48T/36T/26T front crankset with an 11T-34T 7-Spd cogset, and also a 53T/36T/30T crankset with a 13-28T 7-Spd cogset and the RD-TX71 had no problem taking up the slack chain.

    I'm curious about the change from an 11T cog to a 13T cog. Did you really want to reduce your top speed? Granted, you will be able to climb steeper hills with the 34T cog.

    Scott Novak

  5. #5
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    Great info, thanks! I just had the bike shop put a new chain on it and they matched it to the original length. I also asked them for the leftover links, but it seems they gave me the wrong links for the chain.

    So looks like I might have to make another trip down there to have them put a couple more links in my chain, based on the information you just gave me.

    It's a mountain bike, so I could care less about top speed. =) I use it for 85% trail riding, so the extra climbing power will definitely be welcome. I've always felt that 28T was just not large enough for me on some of the climbs I do.

  6. #6
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    This is one of Sheldon Brown's articles on derailers which also includes a section on how to determine the proper chain length.
    Derailer Adjustment

    Unless you are the kind of person that can't tell the difference between a hammer and a screwdriver, I'd recommend that you carry a portable chain pin press in your tool bag. If you break a chain it will at least allow you to put the chain back together, even if it is too short and you can't shift through all of the gears. At least it will get you home. Having a chain pin press with me saved my butt on two occasions that would have been very long walks pushing my bike home. Some multi-tools actually include the chain pin press.

    I use Sheldon Brown's system of Gain Ratios which factors in crank length, tire diameter and sprocket ratios. It gives you a practical way to compare different gearings with different tires and crank lengths.

    Gain Ratios--A New Way to Designate Bicycle Gears

    For instance 165 mm crank arms, 30T chain ring and a 34T rear cog has a gain ratio of 1.71

    This requires about the same force to move the same distance in a given time as 175 mm crank arms, 26T Chain ring and a 28T rear cog, which has a gain ratio of about 1.75.

    With a 26T chain ring, a 28T cog, 165 mm crank arms, and 26" x 2.25" tires, I can pedal steeper hills than my tires can grip.

    I put the formulas into a spread sheet and all I need to do is change tire diameter, or crank length and it spits out the gain ratios for various sprocket combinations. I also set it up so it calculated the percentage of gain ratio increase between sprockets and cogs.

    The Megarange freewheels were a ***** with a 41.7% increase between the 34T and the 24T cogs. I don't like to see any more than a 20% increase and preferably much less. I've heard that some people actually like these, but I haven't got a clue as to why.

    The 13-34 7-Spd cassette (Which I think is the one that you ordered) has a 17.2% increase between the 34T and the 29T cogs which is reasonable.

    But there is no one right combination of sprocket ratios. It's all personal preference

    Scott Novak

  7. #7
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    More great information Scott, thank you. I got the cassette installed last night, but am in a holding pattern because I'm still waiting for my new bottom bracket to arrive before I install the cranks and go through the process of adjusting everything.

    Thanks a ton for the link to the tutorial on derailleur adjustment. I'm definitely going to use that.

  8. #8
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    The late great Sheldon Brown gets the credit for most of the information.

    Since you are changing the bottom bracket it would be a good time to measure your chainline to make sure it's optimum. If your chainline is not within the acceptable range for your derailer it could make your derailer adjustment difficult. Here are a couple more links to read.

    About Bicycle Chainline
    All About Bicycle Chainline

    Chainline on Bicycles with Derailers
    Chainline on Bicycles with Derailers

    I recently discovered that my 1988 vintage Sugino crank arms did not have enough clearance between the crank arm and the outer chainring for a modern derailer to work properly. It was causing me all kinds of grief. I installed a newer set of Shimanio crank arms that had sufficient clearance and then adjusted the bottom bracket for the optimum chainline and Shazaam! The front chainrings now shift better than ever.

    Don't assume that your chainline is correct. It might be fine or it might not be. But it only takes a few minutes to measure it to be sure.

    Scott Novak

  9. #9
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    Quote Originally Posted by Scott Novak View Post
    This is one of Sheldon Brown's articles on derailers which also includes a section on how to determine the proper chain length.
    Derailer Adjustment

    Unless you are the kind of person that can't tell the difference between a hammer and a screwdriver, I'd recommend that you carry a portable chain pin press in your tool bag. If you break a chain it will at least allow you to put the chain back together, even if it is too short and you can't shift through all of the gears. At least it will get you home. Having a chain pin press with me saved my butt on two occasions that would have been very long walks pushing my bike home. Some multi-tools actually include the chain pin press.

    I use Sheldon Brown's system of Gain Ratios which factors in crank length, tire diameter and sprocket ratios. It gives you a practical way to compare different gearings with different tires and crank lengths.

    Gain Ratios--A New Way to Designate Bicycle Gears

    For instance 165 mm crank arms, 30T chain ring and a 34T rear cog has a gain ratio of 1.71

    This requires about the same force to move the same distance in a given time as 175 mm crank arms, 26T Chain ring and a 28T rear cog, which has a gain ratio of about 1.75.

    With a 26T chain ring, a 28T cog, 165 mm crank arms, and 26" x 2.25" tires, I can pedal steeper hills than my tires can grip.

    I put the formulas into a spread sheet and all I need to do is change tire diameter, or crank length and it spits out the gain ratios for various sprocket combinations. I also set it up so it calculated the percentage of gain ratio increase between sprockets and cogs.

    The Megarange freewheels were a ***** with a 41.7% increase between the 34T and the 24T cogs. I don't like to see any more than a 20% increase and preferably much less. I've heard that some people actually like these, but I haven't got a clue as to why.

    The 13-34 7-Spd cassette (Which I think is the one that you ordered) has a 17.2% increase between the 34T and the 29T cogs which is reasonable.

    But there is no one right combination of sprocket ratios. It's all personal preference

    Scott Novak
    I am dealing with almost the same problem and this post is gold! Thanks for all the info Scott

  10. #10
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    Thanks again Scott! An update after I got everything installed. The new cassette dropped right in, and the only adjustment I needed to make on the rear derailleur was to the B screw, as suggested. The derailleur works just fine with the larger rear tooth size.

    But now I'm having another issue related to my front cranks after replacing the bottom bracket, that has me going insane trying to figure out. I'll start another thread, as its a different issue.

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