Gearing Ratios

Discussion in 'Frame Mounted Engines' started by SlicerDicer, Jun 13, 2007.

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  1. SlicerDicer

    SlicerDicer Guest

    Ok here is my boggle.

    Direct off engine is 12 tooth sprocket no gearing reduction.
    Sprocket on bicycle for the shifter setup I want... 36 tooth thats for the equal to 1st gear.

    That places the motor at 20mph in 1st gear with the clutch engaged. The clutch engages at 2300rpm.

    At full engine speed thats 3600rpm placing the bike at 30mph.... This is unacceptable to me for this kinda speed haha... Consider there will be 7 speeds on this bike that I will be able to use with the motor it seems insane for that kinda speed in the 1st gear.

    So my question I pose to you. How can I knock that down so I can use the gears in a realistic sense. I would really like to hit a top speed of only 40mph with the smallest sprocket on the cassete being around 18

    As of right now that places it at 60mph!!! thats OMG fast.

    so what would I need to do to shave 20mph off?

    I am not a mechanic I am a computer nerd so if I explained this stupid I am sorry. :sad: My calculations may be off too!

    I am going to be putting a low rpm quiet engine on the bike :)

  2. gone_fishin

    gone_fishin Guest

    all i can envision is an intermediary reduction the engine to a (guessing) 30 tooth sandwiched to a 10 tooth going back to the rear gears.
  3. SlicerDicer

    SlicerDicer Guest

    Augi how much torque would that create by reducing it like that? I think I could pull that off does anybody know how to calculate that?

    3600rpm -> 12 tooth drive with the ratio 30 to 10 then 36 tooth in back? As well as maybe the speeds at 2300rpm?

    I am going to be using nearly the identical engine to custom_tecumseh :) Just a newer model.
  4. gone_fishin

    gone_fishin Guest

    i would imagine the drivetrain would be nothing BUT torque that would eventually convert to speed as you climb up the gears. even in high gear, i'll bet it's a responsive suckah :shock:

    (sorry can't help with the calculations)
  5. SlicerDicer

    SlicerDicer Guest

    Augi thats my ultimate plan man!

    I want TORQUE! for pulling power :razz:

    Its going to be a 135cc tecumseh with 3.5hp I am not too worried about the fuzz around here busting me they all seem to think its the coolest thing they ever seen. Waving and smiling :)

    The top end speed is not really my ultimate goal. I want to make sure I can at least do 25-30 however. Once I can be sure that it will hit that easy then I can mount a windshield I will tear off the pedals put on footpegs and leter rip :)

    I found a steel mounting bracket for the tecumseh engine that is for go kart but I can have that welded onto the bike frame so it should add more structural support to the frame.

    But I really want to be able to reduce it down so I can hit no more than 35-40mph thats my concern... Thou shalt not tempt thine speed! :lol: :lol:
  6. gone_fishin

    gone_fishin Guest

    you know what's going on here, don't ya? we're talking each other into some evil builds :evil: :lol: :lol:
  7. SlicerDicer

    SlicerDicer Guest

    its your fault augi... you posted the Wisconsin now I am being obsessive :)

    And the engine is only 100$ so you cannot really argue with that price.
  8. Yeah, thats why I'm trying to go custom like you Slicer. I got my engine for $75 with a centrifugal clutch included.

    Good to see someone else going with a big block engine! :D :D
  9. SlicerDicer

    SlicerDicer Guest

    not only that tecumseh but the engines I have a feeling are much higher quality than "super happy time"
  10. gone_fishin

    gone_fishin Guest

    got that right! i'm totally pumped even considering the possibility of building the beast :lol:

    so what's the topic? oh yeah, gear ratios :lol: :lol:
  11. OldPete

    OldPete Guest

    I look at it from the tire's perspective.
    Convert mph into fpm vs the tire's circumference to rpm.
    1mph = 1.46667fps
    So if the engine peaks at 7k rpm, just figure from there.

    Silly but it works for me.
  12. SlicerDicer

    SlicerDicer Guest


    12 tooth to 30 tooth to 10 tooth to 36 tooth on a 26 wheel? I do not understand how the 30/10 tooth would be calculated...

    Would that make it so the rear tooth was equal to 36x3 being a 108 tooth? as in 30/10 reduces it by a factor of 3? I do not understand gearing very well :)
  13. OldPete

    OldPete Guest



    2.5x3.6=9.0 to 1 reduction.

    Most 26" tires are 2 meters around or 6.5'.

    An 18 to 1 reduction will offer about 28mph w/a 26" tire with the engine at 7k rpm. just a guess-ta-ment
  14. SlicerDicer

    SlicerDicer Guest

    the RPM for full clutch engagement is 2300rpm and the top speed of the motor is 3600rpm.

    So that would mean about 15mph at max 3600rpm and about 10 at engagement?
  15. gone_fishin

    gone_fishin Guest

    ok, so now you "adjust" by changing that intermediary sandwich. again, i'm no help with the actual math :???:
  16. SlicerDicer

    SlicerDicer Guest

    that math is beyond my head too..
  17. SlicerDicer

    SlicerDicer Guest

    Ok now that the gearing ratio has been basically answered. Here is the dangerous part...

    How to reinforce the rear wheel? LocoWelder got any ideas how to reinforce a wheel to be hell for stout?

    I am afraid of ripping all the spokes off in one shot when I get metric tons of torque on that wheel..
  18. OldPete

    OldPete Guest

    What size and type of wheel you looking for?
  19. SlicerDicer

    SlicerDicer Guest

    26 inch wheel for mountain bike with 7 speed cog.. the cog will be welded if needed.

    Is there anyway to reinforce a normal wheel to handle 3.5hp and not shred?
  20. OldPete

    OldPete Guest

    Golden Eagle has rear wheels that are laced 4 cross with 12g spokes for a very good price. Shipping is $20 though. They come in silver or black, packable bearings or sealed bearing type(disc brake). The disc brake type hub can be used on a V brake anyway(I think the rim's side wall is finished for V brakes? Better ask if you go that way.). Could even mount a drive sproket where the disc rotor mounts for that matter.

    Google 4 cross lacing and road speed gear charts, there are lots of hits for both.