single gear question

miatakid97 · May 15, 2011

I live in Asheville, NC and its pretty hilly and im tryin to figure out the right single gear set up for my bike.

79cc 2.5hp HF motor
165lb "me" + gas + bike and motor and book bag = around 225 pounds
good bit of hills nothing really flat but medium grade hills
sprocket on motor has 12 teeth.
rim is 26 inches

Im not looking for the fastest bike gear ratio but i want to maintain a good speed up hills and not get bogged down.

Thanks in advance for you guys help.

2door · May 15, 2011

the 41 or 44T sprocket (standard in most kits) is generally a good all around gear for most bikes/riders. A smaller sprocket will hurt your hill climbing ability and a larger one will reduce your top end speed.
Tom

MotorBicycleRacing · May 15, 2011

2door said:
the 41 or 44T sprocket (standard in most kits) is generally a good all around gear for most bikes/riders. A smaller sprocket will hurt your hill climbing ability and a larger one will reduce your top end speed.
Tom

Tom, you gonna confuse miatakid97
Your advice would be correct for a Chinese 2 stroke motor.
This question would get better answers in the 4 stroke forum.

He has a FOUR STROKE 79cc 2.5hp HF motor
miatakid97
You need 2 gear reductions on a 3600 to 5000 rpm 4 stroke.
You can make your own or buy a gearbox by EZ Motorbike or
AGK makes one too for the 79 cc harbor Freight.
www.AffordableGoKarts.com in Santa Barbara who is also an EZ
Motorbike dealer.

You can not put a gear on the motor shaft and another sprocket on
the rear wheel as the rear sprocket would be huge to get enough reduction.
What are you using for a clutch?

miatakid97 · May 15, 2011

wheres a good place to look for one and buy one?

MotorBicycleRacing · May 15, 2011

miatakid97 said:
wheres a good place to look for one and buy one?

I realize that you didn't read my post yet.
2 door gave you incorrect advice. He thought you had a 2 stroke motor
with 4.1 to 1 internal gearing. Ignore what he said as he obviously
didn't read what you said. lol

The 4 stroke 79 cc HF has no gear reduction built in.

What clutch are you using is the first question
Read the 4 stroke forum to find other 79cc HF motor builds.

miatakid97 · May 15, 2011

BiMoPed said:
I realize that you didn't read my post yet.
2 door gave you incorrect advice. He thought you had a 2 stroke motor
with 4.1 to 1 internal gearing. Ignore what he said as he obviously
didn't read what you said. lol

The 4 stroke 79 cc HF has no gear reduction built in.

What clutch are you using is the first question
Read the 4 stroke forum to find other 79cc HF motor builds.

haha sorry for that we must of been posting at the same time. I got a clutch from nothern tools for it and its a 12t. It is a 4 stroke motor too. I kno a lil bit about ratios but finding a good one for hills is a lil bit different than flats and 2 stroke motors.

MotorBicycleRacing · May 15, 2011

miatakid97 said:
haha sorry for that we must of been posting at the same time. I got a clutch from nothern tools for it and its a 12t. It is a 4 stroke motor too. I kno a lil bit about ratios but finding a good one for hills is a lil bit different than flats and 2 stroke motors.

You need something like this

affordablegokarts: Jackshaft Assembly for 79cc HF Engine

This jackshaft assembly is a great way to get the gear reduction you need. Use with our Max Torque clutch. Jackshaft has a 22t input sprocket and a 9t output sprocket. We use this with a 56t sprocket on the rear wheel for total reduction of 12.44:1

miatakid97 · May 15, 2011

Im not tryin to get that toruey. 12.44:1 to much torque?

MotorBicycleRacing · May 15, 2011

miatakid97 said:
Im not tryin to get that toruey. 12.44:1 to much torque

Download the file gearratio.exe from
Index of /ratiocalc
The program allows for up to three ratios in series. If you have fewer just leave the unused ratios set to 1:1.

The program will also calculate speed based on engine RPM, drive ratio,and wheel diameter. Be sure to measure the outside diameter of your wheel for accurate results.

then you can figure out your own "torque" requirements.....

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single gear question

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