43mph 48cc stock internals +- 1500 miles later

Discussion in 'Motorized Bicycle General Discussion' started by Ahmedy, Nov 8, 2015.

  1. Ahmedy

    Ahmedy New Member

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    Hi there. Just wanted to post this up to find out if anyone has achieved this before. I'm running a 48cc bottom end and a so called 66cc head. I'm also running a standard carburetor with stock jets and venturies . My total milage thus far is over 1500 miles and I've tampered with a few exhauste systems as well. Recently I built (for the 1st time ever) a home made exhaust for it and I achieved 43mph. Aslo I'm running a standard CDi. I'd like to hear from one and all .
     
  2. Ahmedy

    Ahmedy New Member

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    Forgot to add that I'm running a 36 tooth rear sprocket. Thus I also achieved a calculated +- 9300rpm
     
  3. mogollonmonster

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    I'm at about the same, but my bike is basically stock. Read my review for more info.
     
  4. Ahmedy

    Ahmedy New Member

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    Thanks I'll give it a read
     
  5. Ahmedy

    Ahmedy New Member

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    Chromemoly rings . I'll remember that , thanks Excessive. . . I don't know what I'm going to do about the engine block itself. It has cuts in it that (assumingly) the piston made . I can feel them with my fingers by wiping the cylinder walls . Its on the side where there isn't any ports. But I guess I'll worry about it when it finally gives up .
     
  6. Agreen

    Agreen Member

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    I rock a 48cc on a gt2-a frame. I weigh in at 200 lbs on a 6'2 frame. I clocked 43 as well on a flat surface. So far I have: (in order)
    -tore down engine upon arrival to clean out metal shavings
    -smoothed and matched the transfer ports
    -Lapped cylinder head and replaced head gasket with a very thin layer of copper rtv (no leaks)
    -homemade poopoo pipe (3/4" conduit w/ stock muffler at rear)
    -re-jetted stock NT carb


    With just that little bit of work, I consistently run 40+ mph. Very smooth and lots of power band. And that's on a 44t sprocket with the factory tensioner. If set up properly, it all works well. The important things to remember is that if it creates drag, it is slowing the engine down. Too much chain slack causes drag. Too little slack causes drag. Flat tires, misadjusted wheels, loose or tight bearings, lack of grease in the bearings, too much grease in the bearings, etc. All that stuff will make you lose speed.

    I will say, though, that it's not about speed. Sure I can get there when I want, but I'm not going to keep my bike at that speed for more than a minute or two. That's a quick way to destroy that little chunk of melted down soda cans between your knees.
     
  7. Ahmedy

    Ahmedy New Member

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    Agreen that's really impressive, wow. I'm just thumb sucking an estimate . You're motor at that speed is running at roughly 11300rpm. That's through the roof lol .
     
  8. Greg58

    Greg58 Well-Known Member

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    The only time either of my 48cc bikes did 43 mph they were in the back of my truck, my bike with the 44 tooth sprocket on a good day will do 31.
     
  9. Ahmedy

    Ahmedy New Member

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    I'm currently using a 36 tooth sprocket at the back and standard infront. 26" bicycle wheels and tyers. Reason I caught that speed was mostly due to the free flowing exhaust . Running roughly 9300rpm at 43mph . Also on a reasonable downhill.
     
  10. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

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    Estimates and 'reasonable down hills' make the point completely moot.
     
  11. mogollonmonster

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    Ditto, AND my bike is a "60/80" cc, AND I'm 50lbs lighter, AND I agree with Joe. The numbers do not compute. Estimates are worthless, spend $10-$15 on a speedometer.

    Technically you need one to be legal. Try booking through a school zone at 25, past a cop, on a pedal bike. You'll get a ticket. Promise you, EVERYTHING must obey the speed LIMIT. Get a speedo, $15 to save you a $100+++ ticket.

    Gods forbid a traffic incident involving a pedestrian and a motorbike, let alone a speeding one. Bad publicity for all of us. DO NOT SCREW WITH THE MOTORBIKE MOJO!
     
  12. Ahmedy

    Ahmedy New Member

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    My speedo is on par with my GPS . On the flats i can cruise at a comfy 25 with my old pipe (not standard pipe) . The estimates I worked out was on the engine revolutions. Roughly 8 revs per 3.28 foot (1m). I might be wrong on my calculations . For eg. 10 000 meters divided by 60 mins multiplied by the number of revs (8) . note that I got 8 revs using a 26" wheel and 36T sprocket. Thus at my maximum (downhill) of 43mph (70kmh) works out to 9333rpm. I weigh in at 176pounds (80kilos) and my bike weighs 61pounds(28kilos).
     
    #12 Ahmedy, Nov 12, 2015
    Last edited: Nov 12, 2015
  13. cannonball2

    cannonball2 Active Member

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  14. Agreen

    Agreen Member

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    It's not rocket surgery guys. Many people think that the bigger engine means bigger power, but it's not going to be the case if you get one that's so imbalanced that it shakes itself to pieces. Ive found that the 48s are just more balanced from the beginning, and will rev higher than a 66 any day. Add that to a properly set up bike (which most just don't do, let's be honest here) and it's a recipe for success.
     
  15. thxcuz

    thxcuz Member

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    Can you still get the 48cc kits? I never see them on ebay and the dealers in the margins over there seem to have been out of stock for several years
     
  16. Ahmedy

    Ahmedy New Member

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    I got my engine kit from a guy here in south africa . www.ecotrax.biz/ . There's the link to his site if you'd like .
     
  17. langolier

    langolier Member

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    Facts:
    1) If going downhill or have wind at your back the argument is moot. I can hit 60 mph if the wind is strong enough or the hill is steep enough with the engine not running.
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    2) Numbers don’t lie. With a given ratio, Speed (mph) and engine RPM are directly related and cannot be disputed (unless wheel is spinning (not making full contact to ground) or clutch is slipping).
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    3) Resistance is what will keep you from achieving your intended goal of RPM or MPH (again both directly related).
    Resistance = Going uphill, Against wind, Weight, Drag (wheel bearings, fat versus skinny tires, drive chains and sprockets …. etc.). And with only about 2.5 hp ? and ? torque It’s our biggest enemy.
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    4) Bicycle wheel diameters: An important calculation Uto determine. The size of your wheel will determine how many times it needs to go around to achieve a distance of 1 mile.
    Calculation example: For a 20” dia tire. It would be 20 x Pi (3.141593) / 12 = 5.2360 ft or …….. 1 revolution of a 20” tire travels 5.2360 ft. Since there is 5280 ft in 1 mile if you calculate 5280 / 5.2360 you get 1008.4034 or …………..
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    ………….. a 20” tire has to turn 1008.4034 times to travel 1 mile. Shortcut # used later (1008.4034 / 60 = 16.8067)
    ………….. a 24” tire has to turn 840.3361 times to travel 1 mile. Shortcut # used later (840.3361 / 60 = 14.0056)
    ………….. a 26” tire has to turn 775.6949 times to travel 1 mile. Shortcut # used later (775.6949 / 60 = 12.9282)
    ………….. a 27” tire has to turn 746.9655 times to travel 1 mile. Shortcut # used later (746.9655 / 60 = 12.4494)
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    Ok back to ratios : Using a standard 2cycle China girl the internal gears are 20 (crankshaft) to 80 (clutch) and motor exit sprocket has 10 teeth, Rear wheel sprocket varies from about 32 to 48 teeth.
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    For this example (my bike) I have a 26” tire with a 40 teeth rear wheel sprocket and with my big but of 220 lbs I can achieve a top speed of about 31 mph (checked with a GPS with +\- error of about .3 mph).
    Since most of us know (assume) how fast we are traveling in mph let’s calculate to figure what RPM our motor is turning at that speed.
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    20 (crankshaft gear) x 80 (clutch gear) x 10 (motor output sprocket) x 40 (rear wheel sprocket) = .0625 (1 revolution of the motor = .0625 revolution of the rear wheel)
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    Since the wheel is traveling in MPH ”miles per hour” and the motor is in RPM’s are per “minute” we need to adjust our wheel speed to revolutions per minute to do that we simply divide by 60 (60 minutes in 1 hour)
    Which is why I created the “shortcut wheel numbers above. OK back to our calculation …
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    20/80 x 10/40 = .0625 (revolution of the rear wheel per 1 revolution of the motor)
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    12.9282 (adjusted tire rev per min in 1 mile) / .0625 = 206.8512 (# of revs motor has to turn in 1 min per mile an hour)
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    206.8512 x 31 MPH (your actual speed) = 6412.4 (actual motor RPM @ 31 MPH)
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    So back to Ahmedy’s example: China girl standard engine with 26” wheel and a 36 tooth rear wheel sprocket:
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    20/80 x 10/36 = .0694
    12.9282 / 0.0694 = 186.1661
    186.1661 x 43(mph) = 8008.141 (motor rpm)
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    You can go smaller with the wheel sprocket but if you go too small you may run out of the horsepower or torque to achieve the motor rpm’s & mph (again directly related) you want.
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    Bottom line is: If ya wanna go faster either reduce the resistance or increase the horse power or torque.
     
    #17 langolier, Dec 7, 2015
    Last edited: Jan 21, 2016
  18. Ahmedy

    Ahmedy New Member

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    Its just as well I started this post otherwise I'd never have known how to properly calculate those revolutions. Thanks for that langolier. I counted or rounded of those piston revs as I measured it off incorrectly. . . One things for sure I get the same speed as you on a level road . . . I guess getting a new bicycle might help a bit aswell. Noticed the china bicycle's spokes are turning loose , and the wheels are sitting in different angles /\ when looked at from the front or back of the bike . Another big thing I want to do is do a compression ratio . Reason for that is the cylinder wall has a scratch running from top to bottom. Other than it messing the compression ratio I'm sure it has an effect on the smoothness of how the engine should work. . . Once again guys I'm no pro at this , so point out if yawl see something wrong.
     
  19. Ahmedy

    Ahmedy New Member

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    Anothe few things I'd like to ask is what compression ratio's are you guys running ? Altitude ? And are you guys running stock exhausts and carburetors . Oil to fuel ratio's . I'm running 30ml to 1 liter of fuel.
     
  20. langolier

    langolier Member

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    No problem Ahmedy we are all learning in here, I've just been seeing some "fuzzy math" around lately and thought I could maybe narrow it down some.
     

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