Best rear sprocket size
- Thread starter clay830
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TheNecromancer13
Active Member
Try a 36 if you don't mind pedal assisting on all the hills, otherwise, a 40. Back when I had a 66cc chinagirl I ran a 36 and I could make it up steep hills with pedal assist, but I weigh 155 lbs and have strong legs. If you weigh more than 180 or so, I'd not go below 40 without doing engine mods.
So you dont think a 34 or 32 is a good idea? I weigh 140 and will be going on ALL flat lands.
TheNecromancer13
Active Member
No, I don't think anything smaller than a 36 is good without doing performance upgrades first, even on flat ground. I've ridden a bike with a 34 and although it could go a tiny bit faster than mine with a 36, it took about a mile to accelerate and a lot of pedaling before you could engage the engine. And god forbid you ran into a headwind.
TheNecromancer13
Active Member
A few mph.
Nashville Kat
Well-Known Member
If I were 140 and riding on all flat lands I would probably put a 32 on the bike now- because I'm heavier than that and the unmodified 66 pushes the 66 as fast as a 14 year USCF road racer - now 63- wants to go.
It's a trade-off- you gain speed with a smaller rear gear but you lose pulling power off the start- it helps then - in every way- to build a bike that pedals easily. You can get it up to 10 or 15 easier and let the motor take over- which then can take a better roller up to more speed if you want that. I like to pedal around fairly close to normally- these roll better than most people's normal bikes- and then when i'm outta the parking lot or the park kick in the afterburners. I also really enjoy killing the motor and having a nice cycling like quiet coast and it gives the motor just a little cooling time, before an intersection or at the end of the ride
Given that- as I said above- the best part of smaller sprockets is that- if the bike rolls well- you can always be at less revs. And less revs means less vibrations less noise and less chance the motor or bike frame will be damage- better gas mileage and more miles an engine should e able to go.
You can go with performance mods and get more gas to the cylinder and rev the bike for more speed, but you'll no doubt compromise the reliability and miles the motor can go without further tinkering. Too old to tinker here.
I had a mini-bike when I was a kid- a Rupp with a 3 1/2 hp Briggs and Stratton 4 stroke, I ended up putting a Mac 50 chainsaw motor on- a bit of a mistake- the Mac would go faster but took forever to get there- then I got into competitive cycling- so I have had some tinker skills, but now just want some efficiency and reliability.
The red bike in the links above has a 25 sprocket on the front- and I had only a 17 rear freewheel to complete it for sale- the pedal was almost too light- but not that much by the time the 66 chrome motor and larger tank went on. I built that to mostly get rid of what I had. The alloy cranks have a 42 and 45 set-up matched with a 22 rear- a good gear but lots of smaller derailleur chain.
Notice how on the Red bike below the smaller sprocket keeps the chain further off the chain stays and lets you eliminate the tensioner.
Now I have another cruise frame that may or may not see a spare 50cc I have- I'm putting an alloy 25 sproke on a steel astabula crank , but this time using a 16 on the back- a 14 freewheel might do but they are fat for some inexplicable reason. The combo of small and small on the front and back greatly diminishes the length of the chain for substantial weight reduction- so that a one piece crank is not all that much heavier then than a 3 piece alloy
search gear charts on your computer to compare "inches" of gear
It's a trade-off- you gain speed with a smaller rear gear but you lose pulling power off the start- it helps then - in every way- to build a bike that pedals easily. You can get it up to 10 or 15 easier and let the motor take over- which then can take a better roller up to more speed if you want that. I like to pedal around fairly close to normally- these roll better than most people's normal bikes- and then when i'm outta the parking lot or the park kick in the afterburners. I also really enjoy killing the motor and having a nice cycling like quiet coast and it gives the motor just a little cooling time, before an intersection or at the end of the ride
Given that- as I said above- the best part of smaller sprockets is that- if the bike rolls well- you can always be at less revs. And less revs means less vibrations less noise and less chance the motor or bike frame will be damage- better gas mileage and more miles an engine should e able to go.
You can go with performance mods and get more gas to the cylinder and rev the bike for more speed, but you'll no doubt compromise the reliability and miles the motor can go without further tinkering. Too old to tinker here.
I had a mini-bike when I was a kid- a Rupp with a 3 1/2 hp Briggs and Stratton 4 stroke, I ended up putting a Mac 50 chainsaw motor on- a bit of a mistake- the Mac would go faster but took forever to get there- then I got into competitive cycling- so I have had some tinker skills, but now just want some efficiency and reliability.
The red bike in the links above has a 25 sprocket on the front- and I had only a 17 rear freewheel to complete it for sale- the pedal was almost too light- but not that much by the time the 66 chrome motor and larger tank went on. I built that to mostly get rid of what I had. The alloy cranks have a 42 and 45 set-up matched with a 22 rear- a good gear but lots of smaller derailleur chain.
Notice how on the Red bike below the smaller sprocket keeps the chain further off the chain stays and lets you eliminate the tensioner.
Now I have another cruise frame that may or may not see a spare 50cc I have- I'm putting an alloy 25 sproke on a steel astabula crank , but this time using a 16 on the back- a 14 freewheel might do but they are fat for some inexplicable reason. The combo of small and small on the front and back greatly diminishes the length of the chain for substantial weight reduction- so that a one piece crank is not all that much heavier then than a 3 piece alloy
search gear charts on your computer to compare "inches" of gear
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sbest
Member
Like has been said, below 36t starts to get impractical at low speeds.
You have to pedal up to 10mph just to start the motor but it does run along quiet.
I run a 44t and larger because I am 210 lbs and have huge steep hills the stock motor will not get up. I have that problem solved and I can do 40mph with the single speed. Here is the secret:
There are 4 ways to increase power to the ground:
1) reduce friction, about 5% possible
2) increase displacement, proportional increase but expensive
3) increase average cylinder pressure, 10%-30% typical
4) increase rpm. proportional (with some loses) but decreases reliability
So displacement is probably out because you have what you bought, and you can work on friction (tires, light lubes, teflon, better bearings, wind resistance, chain tension etc). So you are left with increasing the cylinder pressure and rpm, which will multiply each other's effect and can potentially double the effective HP.
Reshape the combustion chamber (read up on squish turbulence) to avoid detonation, widen the intake and exhaust port windows in the cylinder and get a tuned exhaust pipe ("expansion chamber") will raise the pressure and rpm.
When your rpm goes up, you can get more speed out of the same 44t sprocket. So by raising my rpm from 7500 to 10,000 I go from 30mph to 40mph and have the power to haul it. I also have the gearing to pull away from a stop and make it up the hills.
Often when you want something, you have to give up somewhere else.
Not always, if you are reasonable about what you ask for.
Steve
You have to pedal up to 10mph just to start the motor but it does run along quiet.
I run a 44t and larger because I am 210 lbs and have huge steep hills the stock motor will not get up. I have that problem solved and I can do 40mph with the single speed. Here is the secret:
There are 4 ways to increase power to the ground:
1) reduce friction, about 5% possible
2) increase displacement, proportional increase but expensive
3) increase average cylinder pressure, 10%-30% typical
4) increase rpm. proportional (with some loses) but decreases reliability
So displacement is probably out because you have what you bought, and you can work on friction (tires, light lubes, teflon, better bearings, wind resistance, chain tension etc). So you are left with increasing the cylinder pressure and rpm, which will multiply each other's effect and can potentially double the effective HP.
Reshape the combustion chamber (read up on squish turbulence) to avoid detonation, widen the intake and exhaust port windows in the cylinder and get a tuned exhaust pipe ("expansion chamber") will raise the pressure and rpm.
When your rpm goes up, you can get more speed out of the same 44t sprocket. So by raising my rpm from 7500 to 10,000 I go from 30mph to 40mph and have the power to haul it. I also have the gearing to pull away from a stop and make it up the hills.
Often when you want something, you have to give up somewhere else.
Not always, if you are reasonable about what you ask for.
Steve
KipBurbank
New Member
These engines need to rev high to get in the power band. That's why motorcycles/cars have transmissions. A tiny sprocket will lower the torque and you may never reach the power band. Watch YouTube videos and learn. Experimenting is fun. 
sbest
Member
One of the more interesting things I have learned in experimenting with these single speed China Girl engines is to gain high rpm power without destroying low rpm torque. You can have a single speed engine that will pull away strongly from a dead stop and haul steadily up to 10,000rpm at 40mph. This is totally possible by keeping the port timings and volume conservative.
This is going to influence how I port the other 2 strokes in my life.
This is going to influence how I port the other 2 strokes in my life.
I did build a mini bike with a 10 tooth clutch and 40 tooth rear sprocket. It has a predator 212. I also put on 10 inch rims. 16 inches or more with the tire on. I've hit speeds of 50+ no joke. But anyway it does not go up hills at all only small ones when I go off roading. So if I get a 32 tooth on my motor bicycle I feel it would not pull my engine and I would be risking too much such as won't go in winds as I live in the Windy City. I also wouldn't be able to go up any hills. I think I will go with a 36 tooth and shave off a few mph on the 32, but be able to go up a few hills with a 36t. Still not sure
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MotorBicycleRacing
Well-Known Member
If you know your speed ( from phone GPS ) gearing, tire diameter then you can calculate RPM.
You are right that a headwind will knock your speed down.
I like a 40 sprocket for the street.
the rpm of a so called stock china kit varies quite a bit.
You are right that a headwind will knock your speed down.
I like a 40 sprocket for the street.
the rpm of a so called stock china kit varies quite a bit.
TheNecromancer13
Active Member
Not if you want to have even the slightest bit of fun riding it. Trust us, taking a half mile to go from 0-30 is not fun, and neither is having to pedal assist on flat ground just cause the wind is blowing in your face a bit. If you want more power, either modify the engine, or get a bigger engine.
Tyler6357
Well-Known Member
I agree with others, 36 tooth minimum with a stock China girl motor and stock pipe and even that will be kinda sluggish on the low end. Get an expansion pipe, high compression head, port the top end. Make sure you have brakes and a frame that can handle extra speed. Also, you might consider a sprocket adapter if you are using the stock rag joint for the rear sprocket, if you put too much torque on bicycle spokes they will bend or break.
