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Thread: gearing

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    gearing

    So I was assuming that I would need a smaller gear, but it appears I may not. According to Sheldon Browns gear calculator, the Downtube combo of a 48t chainring and 32 max cassette is 2.2 vs my roadbikes 39x30 of 2.6. Does this make sence that the differences in wheels will cause the gearing to be the same??? So it apears that the 48 tooth chainring on a 20 incher acts much like a 39 tooth on a full size bike.
    So with the gear ratios very similar will they climb the same up the hill on my commute? (I would just ride the bike up the hill but I haven't bought it yet, but I'm much closer as my car is now posted for sale!!)
    Scott

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    Professional Fuss-Budget Bacciagalupe's Avatar
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    Personally, I don't like using the gain ratio thing.

    DT in gear inches: 30"
    road bike: 34"

    It does make sense that smaller wheels will alter the gearing in this fashion. My guess is that the DT will feel like it has about one extra lower gear than the road bike. However, other factors like frame stiffness or suspension would suck up energy. Also, if the DT is significantly heavier (15 lbs or more) than the road bike, you might notice it on a climb.

    So it would make sense if they performed about the same on the hills.

  3. #3
    Senior Member Speedo's Avatar
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    Quote Originally Posted by ncscott
    So I was assuming that I would need a smaller gear, but it appears I may not. According to Sheldon Browns gear calculator, the Downtube combo of a 48t chainring and 32 max cassette is 2.2 vs my roadbikes 39x30 of 2.6. Does this make sence that the differences in wheels will cause the gearing to be the same??? So it apears that the 48 tooth chainring on a 20 incher acts much like a 39 tooth on a full size bike.
    So with the gear ratios very similar will they climb the same up the hill on my commute? (I would just ride the bike up the hill but I haven't bought it yet, but I'm much closer as my car is now posted for sale!!)
    Scott
    Actually it's more like a 48 tooth chainring on a bike with 20 is more like a 35 tooth chainring on a bike with 700 C wheels.

    It may be easiest to think of this in terms of "gear development". Gear development is the distance your bike will move for one turn of the crank. The magic formula is

    GD=Pi*WheelDiameter*Chainring/CassetteCog

    So for the Downtube, the lowest gear development is 94.25 inches.
    For your road bike the lowest gear development is 110.27 inches.

    So the lowest Downtube gear should be a bit easier to push then the low gear on your road bike.

    Speedo

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    Senior Member Speedo's Avatar
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    Quote Originally Posted by Bacciagalupe
    Personally, I don't like using the gain ratio thing.

    DT in gear inches: 30"
    road bike: 34"
    I actually prefer the gear inches specification. But I didn't want to have to explain why anyone would think of specifying gears that way.

    Speedo

  5. #5
    Gone, but not forgotten Sheldon Brown's Avatar
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    Quote Originally Posted by ncscott
    So I was assuming that I would need a smaller gear, but it appears I may not. According to Sheldon Browns gear calculator, the Downtube combo of a 48t chainring and 32 max cassette is 2.2 vs my roadbikes 39x30 of 2.6. Does this make sence that the differences in wheels will cause the gearing to be the same??? So it apears that the 48 tooth chainring on a 20 incher acts much like a 39 tooth on a full size bike.
    So with the gear ratios very similar will they climb the same up the hill on my commute? (I would just ride the bike up the hill but I haven't bought it yet, but I'm much closer as my car is now posted for sale!!)
    Scott
    Actually, 2.6 is significantly higher than 2.2, about 18% higher according to my trusty slide rule. There may be some round-off error too.

    Not sure what wheel size values you're using. I used 700 x 28 for the "road" bike and 20 x 1.75 for the folder. You might have used the "20 inch nominal" value which is significantly larger.

    Running those calcs in with the chainring size multiplied by 10 to get more significant digits, I get 26.0 for the road 39/30, vs. 20.9" for the 48/32 with 20 x 1.75 wheels. That's slightly over a 24% difference. This is a Big difference!

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