need for speed

Discussion in 'Motorized Bicycle General Discussion' started by Crysisfreak7, Jun 4, 2014.

  1. Crysisfreak7

    Crysisfreak7 New Member

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    i am using the sprocket that comes with the chinese 2 stroke. and i can only reach 32 mph in a straight road. i am going to also be carrying replacement parts, 1 gallon os gasoline, headlights breaklights turn signals rear view mirrors and a bike rack, and i want to make atleast 40. the best speed would be 45. what sprocket should i use, links would be appreciated.
     
  2. crassius

    crassius Well-Known Member

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    sprocket choice depends mostly on rider weight and how hilly your riding area is

    smaller sprocket goes faster - larger pulls better uphill or with heavy rider

    top speed is difficult to assess until 300 miles of run-in are done and carb tuning is then completed
     
  3. Davezilla

    Davezilla New Member

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    I agree... you'll need to run in your engine really good and it'll be faster after 300 miles or so... Then you can think about expermenting with sprocket sizes depending on your weight and how many hills are in your area... a 36 tooth will make the bike faster, but it'll also slow down it's acceleration and climbing ability considerably. The 44 tooth will give you good climb and acceleration with a moderate top speed, but it's still capable of pushing you above 40mph with sime mild engine upgrades like a good pipe, the proper carb tune, better compression by using a Puch Hi Hi head or Fred Head, and some mild porting, You can also buy a ported jug from several of the vendors in here like Arrow Racing, Fred at CRmachine.com, Jake's, and a few others, don't fall for those so called high performance jugs sold on ebay for under $40 because they're the same thing you already got, get one from a reputable builder like those mentioned and you'll be happy.
    You will need to break in your engine all over again if you buy a new ported jug but you can remove yours after break in and port it yourself if you got the ability to, or buy a ported one and break in the engine again.

    With these upgrades you should be able to hit around 40mph with the 44 tooth sprocket and still have excellent acceleration, or even faster with a smaller sprocket with decent acceleration... There's a lot of hype going on with these engine upgrades so beware of what you buy and go more for the stuff that counts like compression, pipe, and porting as even a stock NT carb and stock CDI will still get you up to the speeds you're looking for
     
  4. mapbike

    mapbike Active Member

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    DITTO.......

    I'll also add that I have run a 41T sprocket that still gives good low rpm power but adds 3-4 mph to top speed.

    Also Jake's offers a 38T for use with the nice hub adapter he sells, I've been concidering giving one a try myself since im plenty happy with a 40-42 mph cruise speed and I figure ill still have that easy with a bit more power for the hills.

    Great advice Dave.
     
    #4 mapbike, Jun 4, 2014
    Last edited: Jun 4, 2014
  5. Nashville Kat

    Nashville Kat Active Member

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    40-45 is probably not a realistic goal for a China girl-

    Mine, with light narrow 700c wheels and a 34 sprocket might push 35 if I really pressed it- I don't- the motors are notoriously unreliable, while my bikes are like a second car- I want long performance out of them- not break neck speed.

    Granted, I'm only running a stock NT Speed carb- so a change there might yeild me some more screaming RPMS, but I feel like I'm pushing the limits on a bike anyway at that speed- and I raced USCF fourteen years road and track in my crazy days. Plus my bikes are set up with as much alloy and titanium as practical- but also baskets racks and bags to be able to carry stuff.

    I usually cruise out about 25 on my analogue speedo- probably not too far wrong- , but can get that up to 30 to stay with traffic if I want- I ride them slower all the time- at 60 I want to just cruise around the lake on my John Candy pontoon boat and enjoy life staying alive.

    My 50 cc cruiser- set up much the same otherwise is about 5 mph slower all around, with a 39 alloy sprocket- and it's all flat here- a few years back I was getting up near the tops of the steepest mile long climbs in southern Indiana before bogging down on a 36 if that helps- all on standard kit carburetion.

    xct2 We're running with the shadows of the night.....
     

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    #5 Nashville Kat, Jun 4, 2014
    Last edited: Jun 4, 2014
  6. mapbike

    mapbike Active Member

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    NK

    Your right about the most common speeds being in the low to mid 30's and for the average china girl engine that is gonna be where most of them will be, acception being the better balanced 40mm stroke PK-80 types like what dax and Neil are selling as " Gen IV & GenV " engines, I have two of them built and running and each will pull my 215 lbs down the road @ mid 40's and one is hitting upper 40's and has registered an unofficial 51.5 mph lately, but without good port work and some other goodies the average badly balanced china girl will not ever hit those speeds and will most likely find there sweet spot in the low to mid 30's even with a 36T sprocket, I know because I have a couple old bgf engines that run this way, one of them needs a 36T just to have a comfortable cruise speed of 32-34 mph and it will do it all day but beyond that the vibes are crazy bad and would kill that engine quick if it were run that way for long above low 30's.

    Some just dont realize what it takes to get a china girl to do comfortable 40 mph speeds unless it has a good balance and it is built so that it can breathe well and set up on the bike right.....

    Good advice Kat you are correct.

    Map
    .wee.
     
  7. Davezilla

    Davezilla New Member

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    I gotta agree with Map on this since he does have some exceptionally fast bikes without a lot of the hype... just a good bottom end and some decent port work... I think he's even still using the stock NT carb.
    I recently built another engine with the DAX Gen4 bottom end and used my original ported cylinder, cut piston, and Fred head with a CNS carb then I put a good pipe on it and it's plenty fast even with the 44 tooth sprocket. I'm really happy with the performance and smoothness of this engine and can't wait til it's got about 300 or so miles on it. I may go up to a 40 tooth sprocket for a little more on top but I want to see how fast it really is before swapping anything else out. It does accelerate really nicely with the 44 tooth and I'm guessing high 30's from the few full speed runs I was able to get on it so far...

    By the way Kat... You got a really nice looking ride too :)
     
  8. mapbike

    mapbike Active Member

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    By the way Im running a NT speed carb on one of my fastest bikes and an RT dellorto clone on the other which is turning out to be what is the fastest of the two bikes now.

    The NT carb will easily give someone 40's mph if all else is right... its a good simple easy to tune carb that just plain works on these little engines.

    Map
     
  9. mapbike

    mapbike Active Member

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    Oopz didnt realize you had posted before my last post Dave... yep an NT Speed on my best all around cruiser bike and a RT Dellorto Clone on the old 63 Western Flyer which has 24" tires and is running a 30T sprocket I bought from SBP its made for the shift kit but I drilled it for a rag joint set up and it works great, I've done several sprockets up this way, just use and old kit sprocket for a template and drill the holes and bam... a new sprocket for about $7 -$10 depending on which. You get or where you get the victim sprocket.

    Map
    .wee.
     
  10. Crysisfreak7

    Crysisfreak7 New Member

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    so even though my 80cc engine hits 32-36 on flat roads with the stock sprocket (44 or 48?) i still cant hit 40-45 with a 40 or 36t sprocket?
     
    #10 Crysisfreak7, Jun 4, 2014
    Last edited: Jun 4, 2014
  11. mapbike

    mapbike Active Member

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    If you can get 36mph with a 44T then probably 40-42 mph is easily doable with a 36T, whithout some upgrades to the I ternals like maybe a better quality wrist pin bearing ou may not want to do those speeds for long distance, but if vibes are not to bad and the carb is tuned well and the exhaust flows good enough 40+mph top speeds sounds like it would happen for you with the smaller 36T sprocket.

    Map
     
  12. crassius

    crassius Well-Known Member

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    depending on your weight, at some point the next smaller sprocket may just bog down and go slower than the last

    you are the heaviest part of this system - skip breakfast for an extra 2mph
     
  13. CTripps

    CTripps Active Member

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    On my beast I have a 36T sprocket, and the 'Speed NT' carb (red rectangular cover).. With the carriers loaded, on a flat ground run I can get it close to 70 kph.. (43.5 mph) However I start to get a speed wobble just over 65 kph.. weight all together is about 250 lbs (165 lbs of me and 85 lbs for the beast). However, hill climbing does suffer. My knees have to be pumping about as fast as the piston helping it up to the house from the river-side paths.

    On stock 44T sprockets (with stock carb), most of my builds have very easily (and happily) hit 55-60 kph (35-37) on the first ride, and can get me uphill pretty smoothly.

    On the wife's with a 48T/stock carb, top speed is around 50 kph or so (she's not one for pushing it to the limits) but it'll climb hills with a trailer full of groceries hooked on with no problem.
     
  14. mapbike

    mapbike Active Member

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    Very true, gear optimal gearing depends on several factors, and the engines ability to pull the gearing is crucuial, I put a 28T sprocket on one of my bikes with 26" wheels and it was a performance killer, it is much faster with the current 34T than it was with the 28T and it was gutless on the hills around here with the smaller sprocket also, my engine on that bike cruises at mid 40's mph and is has good port work and several other upgrades and it pulls me at 215lbs good up to 46mph, but that engine doesn't have the grunt for a smaller sprocket than what I have on it now, maybe 2 teeth smaller would still work so so, but I think the 34T is about as small as I should go probably.

    Yes weight and build profile of engine is a very important factor in determining which gearing will work best, also terrain that will be road makes a huge difference also, big hills require big sprockets period unless someone weighs about 90lbs or something...lol!

    Map
     

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