Sprockets

Discussion in 'Motorized Bicycle General Discussion' started by georgeknight, Oct 31, 2008.

  1. georgeknight

    georgeknight New Member

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    Hello all,
    Just have a question. I had a 44 tooth sprocket that ran 32mph, but I wanted to cruise without being wide open at 30 mph. So it was suggested that I buy a 36 tooth. I did, now it won't run above 21mph. The engine is not in any bind but it won't get no where near wide open throttle. Any advice? Thanks George
     
  2. HoughMade

    HoughMade New Member

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    40 tooth perhaps?
     
  3. stude13

    stude13 New Member

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    ill buy the 36. please pm me
     
  4. Creative Engineering

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    georgeknight,

    There is not even the slightest chance that a tooth reduction from 44 to 36 on your rear sprocket is causing this problem.

    When it comes to mechanical things, coincidence can be frustrating...but coincidence this is.

    Look further, grasshopper!

    Jim
     
  5. Large Filipino

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    What engine? Remember that low displacement makes for an unhappy engine with too tall gearing. The only way you're gonna be happy with that gearing is that you have to help pedal it up to speed.
     
  6. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

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    I'll respectfully disagree...

    If the engine can rev up high enough to get "on the power curve" he would suffer a lower top speed than with the 44T.

    I have ridden several motorcycles that would go faster in 4th than they would in 5th gear.
     
  7. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

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    To the OP- Try a 40T.

    Depending on gross weight, and other factors, there will be a point where higher gearing won't improve anything.
     
  8. Creative Engineering

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    Bikeguy Joe,

    I may be wrong, let's run the numbers. I have not tried a 36T sprocket on the Point Beach bike I have, so the experience of those who have; will of course over-ride my thoughts on this.

    I assume you meant to say "if the engine can't rev up high enough to get "on the power curve" he would suffer a lower top speed than with the 44T. Yes that's true, but looking at the numbers, I can't believe these engines have such a narrow power band!

    The reason some motorcycles go faster in fourth than fifth is wind resistance. At 100+ mph wind resistance is huge.

    Assuming a bicycle with a 26" diameter rear tire is used, the numbers work out as follows.

    Pi X D = circumference, or a circle laid flat.

    One revolution of a 26" Dia. wheel will make the bike travel 81.681 inches or 6.806 feet.

    There are 5,280 feet in a mile. 5,280/6.806 = wheel revs per mile.

    776 wheel revs per mile.

    32 miles per hour will correlate to 24,832 wheel revs per hour.

    24,832/60 = wheel revs per minute @ 32 miles per hour. 414 per minute.

    414 x 4.4 for a 44T sprocket = 1821 jackshaft RPM
    414 x 3.6 for a 36T sprocket = 1490 jackshaft RPM

    Engine RPM through the 4:1 ratio clutch gearing...I'm not at the shop so I can't accurately count the gear teeth. from photos I'm sure 4:1 is close. It looks like 80T on the clutch gear and 20T at the crank from photos.

    44T = 7,284 engine RPM
    36T = 5,960 engine RPM

    A difference of 1,300 rpm well into the powerband of this engine.

    Working back through the numbers 21 MPH and 36T would require an engine RPM of 3,911. Seems like enough RPM for the engine to be making power?

    Georgeknight: Have you tried a fresh NGK plug?

    I don't know, the numbers seem to suggest the problem lies elsewhere.

    Jim
     
  9. Creative Engineering

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    Hello all,

    I forgot to mention that the above assumes a 10T jackshaft sprocket.

    Jim
     
  10. georgeknight

    georgeknight New Member

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    I've tried a new plug, no help. My bike has a 10 tooth front sprocket.
    Weight should not be a problem as I weigh 140. It does seem to try to get to the power band but falls just short. The motor seems to be lacking enough fuel to get going but it runs fine with the 44 tooth. It just turns too many rpm's running 30 mph, but the stock carb may need to be modified somehow. I wish I had a carb that I could adjust the air fuel mixture
     
  11. georgeknight

    georgeknight New Member

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    PS
    The motor is a 80cc China girl.
     
  12. Creative Engineering

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    I checked the tooth count on the primary gearset, 20 crankshaft, 82 clutch.

    ratio 4.1:1

    At 21 MPH with a 36T rear and 10T front your engine should be turning 4,008RPM.

    This should be well eough into the power band for it to keep climbing.

    We actually had a similar problem on ours, but it had nothing to do with a sprocket ratio change.

    We used non-synthetic oil and the factory plug during break-in. Right at the end of the break-in the engine would blubber at upper RPM's.

    We changed to a high grade of synthetic, and an NGK B6S plug. It made a HUGE difference.

    Best Regards,
    Jim
     
  13. Motormac

    Motormac New Member

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    Jim, after break in ,what oil gas ratio are you using now with the synthetic? Mac
     
  14. jburr36

    jburr36 Member

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    Also terrain and wind would made a difference here. Nothing had been mentioned about that yet.
     
  15. georgeknight

    georgeknight New Member

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    Thanks Jim,
    You might have hit the nail on the head. I ran some other gas that a friend had about the time I changed sprockets. I'll get another new plug and try it with fresh gas
     
  16. Creative Engineering

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    As a point of comparison, your engine was turning 4,900RPM at 21 MPH with the 44T sprocket.

    This is only 900RPM difference. This isn't much of a difference for a 2 cycle engine, especially on the high side.

    If we were comparing 900RPM to 1,800RPM it would be a different story, but when the engine is already up there 900RPM should not be causing your bike to slump.

    Jim
     
  17. Creative Engineering

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    You're welcome,

    Let us know how it works out!

    Jim
     
  18. Creative Engineering

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    motormac,

    20:1

    Jim
     
  19. Andyinchville1

    Andyinchville1 Manufacturer/Dealer

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    Hmmmmm...Interesting situation...."80" CC engine, 140 LB rider weight and an altitude of only 1280 FT above sea level should leave plenty of power to go faster than 21 MPH with a 36T rear sprocket....

    I weigh 157 lbs , run a Dax 70 and am only RPM limited when I hit 34 MPH using the 36T rear sprocket....Something is definitely wrong...

    Have you tried to "read" your spark plug to make sure the mix is correct?
    (I am suspecting you may be running rich....)

    Assuming nothing was done other than just changing out the sprocket, maybe the Air / Fuel ratio is off and when you were running the 44T but there was enough OOMPH to swing it to a high RPM (engine relatively lightly loaded) but when you "loaded" the engine up by trying to pull a steeper gear, the fact that you weren't making "normal" power would become more pronounced.....

    My other business is lawn maintenance and I have had weed eaters to rev to the moon when lightly loaded BUT when heavily loaded that same weed eater would bog WAY down....

    A properly operating weedeater would rev about the same lightly loaded or more heavily loaded (of course you have to give more throttle).

    The above analogy is similar to your operating the engine in a relatively unloaded condition when using the 44T sprocket VS changing sprockets to a 36T and thus demanding more "power" from the engine at any given engine speed (like cutting thicker / taller grass).

    More often than not in the case of our weed eaters, the exhaust port is plugged with carbon deposits (you'd be surprised how little it takes to adversely affect power ).

    So I guess in summary, I would look into the possibility of running a little to rich and then check the exhaust port / muffler for excessive carbon build up...

    Barring this perhaps your kill switch or some wire was accidentally bumped so that it is partially grounding out some ignition power? BUT if I were a betting man I'd check the other 2 issues first.

    Hope this helps you.

    Good luck and let us know what you find as it may help others in the future...

    Andrew

    PS - Any chance you filled up with "bad" fuel in the meantime?
     
    #19 Andyinchville1, Nov 1, 2008
    Last edited: Nov 1, 2008

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