Attention! Little math help here!

Discussion in 'Motorized Bicycle General Discussion' started by mogollonmonster, Nov 11, 2015.

  1. mogollonmonster

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    Good news is, I've got a pocket full of cash to spend on my bike. Dax has sprockets, and I'm gearing down for the hills. Question is, which sprocket? I think I may order two sizes. It's almost 200 miles to Phoenix, down the Mogollon Rim, which is basically a mountain range.

    They've got 54, 56, 60, and 72. I've got a 44t on Tanglebones currently.

    I'm running about 30, if I rack it out, 25ish cruising. What speeds y'all reckon those will give me?

    Or, even better, can someone give me a little math lesson?
     
  2. crassius

    crassius Well-Known Member

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    if your rpms stay the same, you'll get 30 on the 44, 30 times 44/54 on the 54, 30 times 44/56 on the 56, and so forth, however, it gets easier for the motor to turn on the bigger sprockets, so your rpms available may go up a bit
     
  3. mogollonmonster

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    Clear as mud to me bud.
     
  4. bluegoatwoods

    bluegoatwoods New Member

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    Okay.....well......I'm a bit rusty on that stuff. But I ought to be able to get you someplace.

    Let's set your 25 mph cruising speed as our benchmark.

    You're doing 25 at X engine rpm --it doesn't matter exactly what the value of X is --with a 44 tooth sprocket. I've already forgotten what your sprocket choices are. But did you mention a 56 tooth? Let's go with that.

    To figure out how fast you'd go at X engine rpm you'd need to figure 25 x 44 divided by 56......equals.......mumble, mumble......lemme find a paper and pen.........just a minute........I'll be right back......

    okay...25 x 44 divided by 56 equals................Infinity???!!!!

    Oh, mannnn!!!......I'd better get a calculator........

    25 x 44 divided by 56 equals.....19.6 mph at X rpm.

    Maybe I made that more complicated than it needs to be. When calculating results of different sprocket sizes just take the ratio of the sprocket you've got against the sprocket you're considering. In this case the ratio of 44/55 which equals point 8. So with a 56 tooth sprocket your speed at X rpm will be point 8 times your speed with a 44 tooth sprocket.

    Or you could calculate your torque at X rpm using the inverse. 56/44. Which equals 1 point 27. So at X rpm, on a 56 tooth sprocket you'd have 1 point 27 times the 'pulling power' that you'd have on a 44 tooth.

    Unless that Mogollon Rim is very, very steep you probably don't need a sprocket any larger than 56.
     
  5. mogollonmonster

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    Blue, you ROCK! That made perfect sense to me. And we do have some pretty serious grades. Yet I'm more concerned with the long ones, some places miles at 6%

    I think I'll get a 56t and a 60t. The sixty for curiosity, and a possible future build.
     
  6. bluegoatwoods

    bluegoatwoods New Member

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    "Blue, you ROCK! That made perfect sense to me." ---mogollonmonster

    That made sense? Man, I'm still confused.

    But anyway......if your climbs are particularly long, then I guess you might want to go for as much torque as you can get. So that you're not 'winding out' that engine for miles at a time.

    At a guess, I'd say that both a 56 and a 60 is a good idea. It's hard to imagine you needing anything larger, since I think you mentioned once that you're not a particularly heavy guy.

    In any case I think I'd recommend gearing that has your engine right in its mid-rpm range on those long uphills. That's where you're going to want that engine to be in its comfort zone. And it's likely that either the 56 or the 60 will fit that.
     
  7. mogollonmonster

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    No I'm not very heavy, 160-165. Perhaps the 56 will do, there's really only one way to find out...

    Either way MY gears are turning. If my trip to the Pheonix/Glendale area goes well, and the bike does well, I'll plan something big. Big like cross country big. Or at least a full tour of Arizona. I've lived here all my life, but I've never seen Montazuma's castle. The canyon of course, but there's a lot more of Arizona than that. Of course that's after I get a back-up engine put together, and possibly rebuild this one.
     
  8. Tony01

    Tony01 Active Member

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    I made a small XL spreadsheet table for you, right next to my sprocket size calculator. Beware the size of a bigger sprocket. You'll see that the 44t is about 7" diameter, while a 60t is almost 10" diameter. Make sure you have the space/chain clearance before proceeding!

    As said previously you will probably pick up some RPMs, but the table on the left assumes the same RPM as the one you got 30mph with.

    [​IMG]
     
  9. mogollonmonster

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    Damn I love this forum! Thanks Beginner! Looks like I did the math right, at least. Those are the same speeds I came up with, still saving the chart though. I'll order the 56t, if it's not enough, I'll get the 60. Double thanks for the clearance info, I don't think I'll have any issues, but at least now I know what to expect.

    Motorbicycling Dot Com Rocks!
     
  10. leo

    leo Member

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    i'm running a 56t with 29er rims now, top speed on level ground is only around 20mph. but she climbs hills like a champ.
    i don't think there would be a noticeable difference between a 56t and a 60t
     

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