1991 Trek 950 magic gear- Mtbr.com
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  1. #1
    Sofa King We Todd Did
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    1991 Trek 950 magic gear

    Trying to build up the bike mentioned in the title as a singlespeed ride. Would love to avoid using a tensioner if at all possible. I plan to use this mostly on the roads and light trails. Also planning on using the 46T chainring up front that came with the bike. Therefore, if anyone has any experience with this frame, any suggestions on a cog size in the rear? I could experiment with the cogs from the 7-speed cassette that came with the bike, but thought that if someone could help me 'cut to the chase', that would rock.

    If the 46T chainring offers up an unreasonable gear ratio for a magic gear, I wouldn't mind giving the 36T chainring a try.

    Any thoughts/advice?

  2. #2
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    You can use SS convert to figure out a magic length... however, it doesn't figure in a half-link option.

    Quote Originally Posted by SpinWheelz
    Trying to build up the bike mentioned in the title as a singlespeed ride. Would love to avoid using a tensioner if at all possible. I plan to use this mostly on the roads and light trails. Also planning on using the 46T chainring up front that came with the bike. Therefore, if anyone has any experience with this frame, any suggestions on a cog size in the rear? I could experiment with the cogs from the 7-speed cassette that came with the bike, but thought that if someone could help me 'cut to the chase', that would rock.

    If the 46T chainring offers up an unreasonable gear ratio for a magic gear, I wouldn't mind giving the 36T chainring a try.

    Any thoughts/advice?

  3. #3
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    just finished building up a 930 singletrack. chainline 'n tension is sweet using a 34 -20 combo. sorta an old man's gearing, but it works nice, and she's so light.

    try the divides by 2 rule - it has worked pretty well on both my treks..

  4. #4
    Sofa King We Todd Did
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    Sweet. That 34/20 gearing is a fair bit lower than what I was hoping to achieve. Could I get a quick refresher course on this 'divide by two' rule? I vaguely remember reading something about it, but can't be sure.

  5. #5
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    If 34-20 works, 30-16 will as well, if I understand well. 38-20 is longer and a bit tighter chain.
    Klok - XC - Skate - Ski

  6. #6
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    Is this that 'rule of four' stuff? I'm looking at the gear ratios you're suggesting Cloxxki, and I'm noticing the pattern of adding or moving four teeth on one end. Maybe I should go google this.

  7. #7
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    i have heard of both "rules" but found this from experience for the trek frames that i run - usually 19" or 20", i found that a 34 front and either 19 or 20 rear will work nicely depending upon the "meat" - the diameter of the chainring.

  8. #8
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    Quote Originally Posted by SpinWheelz
    Is this that 'rule of four' stuff? I'm looking at the gear ratios you're suggesting Cloxxki, and I'm noticing the pattern of adding or moving four teeth on one end. Maybe I should go google this.
    Rule of four: It takes 2 links to completely cover 4 teeth, but the chain only wraps around half the teeth of a cog or chainring (approximately). The result is it takes only 1 link to adapt a chain to 4 more teeth - in any combination 4+0, 1+3, 2+2. To be exact you have to account for a chain covering more than half a chainring and less than half a cog.

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