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  1. #1
    Sofa King We Todd Did
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    'Rule of Four' in practice

    At the mo, I have a 46Tx18T gear combo. It's a magic gear. Now, given that dark art called the 'Rule Of Four', if I want to use a 16T cog in the rear, what's that mean to the chainring size I need to go to? 48T chainring? 44T chainring?

    I never fully grasped the concept of the rule, so I figure here's my chance to dive right in.

  2. #2
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    48t

    or
    46/18
    48/16
    50/14
    44/20
    42/22

    Quote Originally Posted by SpinWheelz
    At the mo, I have a 46Tx18T gear combo. It's a magic gear. Now, given that dark art called the 'Rule Of Four', if I want to use a 16T cog in the rear, what's that mean to the chainring size I need to go to? 48T chainring? 44T chainring?

    I never fully grasped the concept of the rule, so I figure here's my chance to dive right in.

  3. #3
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    Good job! Yep

    Them there all add up to 64 teeth tooth count! Add/subtract two teeths from one end, you gotsta subtract/add two to the udder.



    Quote Originally Posted by steelbike
    or
    46/18
    48/16
    50/14
    44/20
    42/22

  4. #4
    Sofa King We Todd Did
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    Cheers, fellas. If you'll excuse me, I'm off to set things on motion to completely wreck my knees now...

  5. #5
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    Bonking ... not feelin' well Hey yeah!!

    Des, what the 'ell are you doing running that bigga gear anyways??? The heck with your knees, you're gonna have one a them cardiac episodes or sumthin'.

    I'm an idiot for running 38/18 up here in hilly Preskit!



    Quote Originally Posted by SpinWheelz
    Cheers, fellas. If you'll excuse me, I'm off to set things on motion to completely wreck my knees now...

  6. #6
    Sofa King We Todd Did
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    What a nosey bugger!

    This gearing isn't for off-road use, mind you. At the mo, I've got a very nice setup with the 46x18 gear combo on my road-going MTB. I can handle most of the flats and mild ascends with ease with that gearing. But I know I can go faster and I know that I can try and build up a bit more strength if I tweak the gearing a bit, hence the tougher gearing. Granted, that's a helluva tweak, but what the hell, let's see where this goes.

  7. #7
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    The "rule of 4" isn't 100% foolproof when running "magic gears" on bikes with vertical dropouts. For example, 36/20, 34/18, and 32/16 all run really well on my bike, which has vert drops. That is just as the rule of 4 would predict. But the "rule of 4" also says I could do 36/16 and 32/20, without any chainlength change. Well, there is soem change, I know because 36/16 is really snug (doable with a "stretched" chain), and 32/20 is very loose and would require a tensioner for any real riding.
    If you really want to get into just which ones will REALLY work (and why) you need to check out fixed inovatons / fixmeup. http://www.peak.org/~fixin/personal/...fmu1_1_big.php
    Fixemeup indicates that 44x16 will work exactly as 46x18 would (and is a good bit higher ratio) but that 48x16 requires a signifigantly shorter chainstay- IE, it would be very loose, and likely require a tensioner. There also a lot of gears lower than 46x18 that would work for yah, but not to many ones that are higher, at least not with practical chainring sizes.

  8. #8
    Recovering couch patato
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    It's both the total teeth and the difference between the 2 that need to match. Rule of 4, or with halflink, 2, applies to the total, the difference can't be overcome, chainlength differences are tiny but significant.

  9. #9
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    what's the rule of four?

  10. #10
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    Doesn't work this way

    If you are running 36/20, how could you run 36/16 without a change in chainlength? You have an extra couple of links hanging around. Remember that the total amount of teeth has to be equal. Using the rule of 4 you actually get 36/20, 34/22, and 32/24 not 36/20, 34/18, and 32/16.

    "]The "rule of 4" isn't 100% foolproof when running "magic gears" on bikes with vertical dropouts. For example, 36/20, 34/18, and 32/16 all run really well on my bike, which has vert drops. That is just as the rule of 4 would predict. But the "rule of 4" also says I could do 36/16 and 32/20, without any chainlength change. "

  11. #11
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    Your answer is:

    Quote Originally Posted by SpinWheelz
    At the mo, I have a 46Tx18T gear combo. It's a magic gear. Now, given that dark art called the 'Rule Of Four', if I want to use a 16T cog in the rear, what's that mean to the chainring size I need to go to? 48T chainring? 44T chainring?

    I never fully grasped the concept of the rule, so I figure here's my chance to dive right in.
    If 46x18 is magic then 44x16 is magic. You'll take out a link but that's not important.

    (rules work unless they confuse people, so I use Excel)
    Attached Images Attached Images

  12. #12
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    Quote Originally Posted by steelbike
    If you are running 36/20, how could you run 36/16 without a change in chainlength?
    I meant "chainstay length"- In other words, no need for horizontal droopouts.

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