Need mental and ratio help! Please!

Discussion in 'Motorized Bicycle General Discussion' started by conradcliff, Dec 5, 2008.

  1. conradcliff

    conradcliff New Member

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    :eek: Ok, so here I go again. I hope you guys aren't getting sick of me yet. I think, yes think, I have things pretty much figured out. But my big issue now is the ratio calculations. At first I thought I had it figured out but then I thought to myself: "Hmm, you know...the diameter of a #40 sprocket with the same tooth count is going to be nearly twice that of a #25 sprocket. I bet that's going to completely screw me up." so now I'm back to trying to figure out these ratios. I think I have a method for the calculations that should work but it looks like it's a bit off.

    What I figure is that instead of going by tooth count I can go by outside diameter instead for the ratios.

    Now, before I go any further, let me try and explain my setup so that you can have an idea about what I'm planning on. Picture this: My engine is rack mounted, it has a 6 tooth #25 sprocket being driven off of its left side (note: to my knowledge no one makes a #40 sprocket for these bell housings). The chain from the 6 tooth sprocket on the engine runs just forward and slightly down from that to the left side of a jackshaft to a 28 tooth #25 sprocket. On the right side of that jackshaft is a 10 tooth #40 sprocket. A chain runs from this sprocket down and slightly forward to a 44 tooth #40 sprocket. Attached to the same freewheel as the 44 tooth sprocket is a 38 tooth #40 sprocket. This runs back to a 14 tooth gear which is attached to a 26" OD wheel. Basically just picture the SBP setup with a rear mounted 4 stroke engine.

    Here are two different versions of basically the same calculations. One uses the tooth counts and one has the outside diameters. (Note: the tooth counts for the #25 sprockets found on the engine and the left side [input side] of the jackshaft have been reduced by about half to mimic the approximate size of the same OD #40 sprockets. A 6 tooth #25 sprocket is approximately equal in outside diameter to a 3 tooth #40 sprocket and a 28 tooth #25 sprocket is approximately equal to a 13.28 tooth #40 sprocket [even though 13.28 tooth sprockets don't actually exist])

    [​IMG]

    As you can see the final calculations are off by a moderate margin.

    What I'm aiming for is an approximate speed calculation of 40MPH when I'm shifted into my final gear, which is 14 teeth.

    One other difference here is that I had originally shot for 40MPH at 6000RPMs taking into consideration the HP an torque curves of the GXH50 and pocket bike engines. Now that I'm probably going to be utilizing the Titan engine I fear that the max HP and torque range are going to lie within a different RPM range, which would necessitate different calculations entirely..oh well.

    So what I really need to know is what size #25 sprocket I need for the input side (left side) of my jackshaft in order to reach 40mph at 6000RPMs...or whatever RPM range you think the Titan has the most power at. And I understand that the Titan may not be able to get me to 40mph..that's ok, I still would like to keep that as the target speed for the given(or optimal if you know it's different for the Titan) RPM range.

    Here is a link to the sprocket catalog I got my measurements from.

    Here is a link to the ratio calculator I made with the help from the guys at maxtorque. (If you find any problems with it let me know because I just sort of figured it out without any prior knowledge about ratio calculations)

    So, I guess that's about that..thanks again for putting up with me and for all the help you guys have provided thus far..hopefully I can get this figured out without having to buy 10 different sprockets to get the right one. :grin5:

    P.s. Although I appreciate the concern..let's try to keep the discussion about "40mph being too fast" to a minimum if we can.. :D
     
  2. Ghost0

    Ghost0 New Member

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    There should be no difference in your ratio calculation do to the different sprocket types. This would only come into effect if you were trying to use a 6t #25 to a 40t #40. Then you have a problem. But a 40t/6t for a #25 chain is 6.6 to 1 and a 40t/6t for a #40 chain is still 6.6 to 1 so don't let the different sprocket/chain types mess with you.
     
  3. conradcliff

    conradcliff New Member

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    Thanks for the reply..that would all be well and good if I didn't have two different sprocket types on the same jackshaft. So the ratio calculation for the jackshaft is where the trouble is and that's why I need to figure out what the size of the input sprocket needs to be.

    Thanks again though. (^)
     
  4. Ghost0

    Ghost0 New Member

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    It still doesn't make a difference, you still have a #25 driving a #25, then you have a #40 driving a #40. Trust me on this.
     
  5. Creative Engineering

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    Ghosto is right!

    It's easy to get confused.

    Just look at each part of the drive separately, (as it's own entity), and you'll find that it isn't so complicated.

    Ditch the software and work it out, (methodically), with a calculator.

    Jim
     
  6. Creative Engineering

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    Better yet,

    Since you are in Tampa, bring it to the shop and i'll help you with it...No charge if it's on a weekend!

    Jim
     
  7. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Conradcliff, the reason for your second calculator's error is because you only have THREE teeth on the clutch sprocket.^5

    FWIW, follow the general consensus here, but research where the Titan engine's max torque and hp range is. Then gear for that.

    In quarter-mile racing, they recommend that you overgear slightly at the top end. However, since you are sustaining this top end speed, ya don't want to overgear.
     
  8. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Change the engine sprocket to 6t and the first input jackshaft sprocket to 24t. You'll arrive at a negligible fraction off of the first calculator.

    You will arrive at 6.87:1 on first calculator and 6.85 on second calculator, which is WAYYY too high of a gear for 6,000rpm/7,000rpm engine range.

    The last sprocket combo from crankset to rear drive sprocket is NOT a gear reduction.

    For 40mph at 6,000rpm, gear ratio should be 11.6:1.

    For 40mph at 7,000rpm, gear ratio should be 13.55:1.

    more info later.
     
    #8 5-7HEAVEN, Dec 5, 2008
    Last edited: Dec 5, 2008
  9. Saddletramp1200

    Saddletramp1200 Custom MB Buiilder

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    I am most likely wrong, but I may see what is getting at. I have seen a sprocket with 25 teeth that was 4 inches IN DIA. I have seen a sprocket with 25 teeth 4 FEET tall. Is this what your saying?
     
  10. Creative Engineering

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    You're not wrong at all!

    It makes no difference what size chain is used...as far as ratio is concerned.

    Jim
     
  11. conradcliff

    conradcliff New Member

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    Hmm, now that you say it again it's got me thinking harder about it...I guess the reason why is because no matter what, it is always going to be a 1:1 across the jackshaft. It isn't a matter of ratios between gears on the jackshaft but only between gears attached by chain. I think that means my calculator is way off then. I'm going to have to go re-engineer it and post the numbers and see if it looks right to you guys.

    Thanks for not giving up on me despite my arrogance. (^)

    Thanks for the advice Jim...my problem is I had piggy backed off of someone else's calculator and changed it around without really knowing what I was doing. Maybe you could give the numbers I'll post in a bit a quick looking over? :D ....

    ...and yeah! That sounds fantastic! I'd love to just stop by and check the shop out. I'll send you a PM..

    Yeah, I had done that because I was trying to mimic the size of a 6 tooth #25 sprocket using measurements from a #40 sprocket and how large the OD was in reference to tooth count. Turns out I didn't need to do that at all...


    Ok, so here are the new numbers...do they look right to you guys?

    [​IMG]

    Thanks again for all your help! If these numbers are right that means I wont have to be returning any sprockets! dance1
     
  12. conradcliff

    conradcliff New Member

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    How do the new numbers look to you?

    Yeah, that was pretty much my reasoning process..but as it turns out it was a flawed one...bf.
     
  13. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    A gear ratio of 11.41:1 would spin an engine at 6,000rpm to reach 40mph.

    You would need 6t/32t sprockets(5.33333:1), then

    8t/44t sprockets(5.5:1), then

    36t/14t final drive(.389:1).^5

    5.33333 X 5.5 = 29.33333

    29.333333 x .389 = 11.41

    I might be wrong, so someone else can doublecheck.^5
     
    #13 5-7HEAVEN, Dec 5, 2008
    Last edited: Dec 5, 2008
  14. conradcliff

    conradcliff New Member

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    I seem to be getting pretty much the same overall ratio with with the gears that I posted...am I doing something wrong? Or would the current configuration that I have not give me a final ratio of 11.41:1? I couldn't get a smaller sprocket than a 10 tooth that's why I had to go with the larger 40 tooth for the jackshaft input.
     
  15. conradcliff

    conradcliff New Member

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    Here's a good question..at least I think it is..SBP's kit only uses a 17 tooth sprocket on the input side of their kit and say:

    Why are their ratios so different? It seems like something is amiss.

    Edit: Ohhh, I think I read somewhere that the china engines have an internal gear reduction..is that right?
     
    #15 conradcliff, Dec 5, 2008
    Last edited: Dec 6, 2008
  16. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Your present calculation gives you 6.87:1., and you need 11.41:1.:ride:

    An easier resolution is to calculate gear ratio with the next-to-last(larger) sprocket on the rear hub.

    Tell me what it is, and we can do more easily attainable calculations.^5
     
  17. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    When he mentions 58t sprocket and 5.8:1, you must multiply this by 4.15 internal gear reduction to calculate to 24.07:1.

    Multiply 2.7 by 4.15) internal gear reduction to get 11.21:1 top speed gearing.(^)

    The ratios are so different because he is using all the gears in the cassette.

    Can you see that the SPB kit has 11.21:1 final gear?
     
    #17 5-7HEAVEN, Dec 6, 2008
    Last edited: Dec 6, 2008
  18. conradcliff

    conradcliff New Member

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    Following your procedure above I get:

    6t/40t sprockets(6.67:1), then

    10t/44t sprockets(4.4:1), then

    36t/14t final drive(.389:1).

    6.67 X 4.4 = 29.33333

    29.333333 x .389 = 11.41

    I trust that I'm doing something wrong as I haven't had any experience with this but I just can't see where..could you point out my error?

    Thank you so much for all the help you've provided.

    Edit:
    Yeah, I figured that the change in ratio was due to the cassette I just didn't think about the internal gear reduction at first which was throwing me off.
     
    #18 conradcliff, Dec 6, 2008
    Last edited: Dec 6, 2008
  19. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Conradcliff, you did good.(^)

    The 6t/40t, 10t/44t and 36t/14t is EXACTLY what you need.

    Now ya just have to locate the 40t sprocket to accept a #25 chain.:ride:
     
  20. conradcliff

    conradcliff New Member

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    Oh, awesome! I thought I was going crazy again there for a second..yeah, I already ordered the 40 tooth sprocket 2 nights ago. This was my original setup that I had configured with my ratio calculator that I thought was wrong. It turns out that the round about way I had done things ended up working out after all (^)

    Also, it's the same configuration in the latest revision of my ratio calculator that I posted last..

    Thanks again for all your help! :D
     

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