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The Simple Mini Chopper Drive-train - Sprockets, Chains, and Centrifugal Clutches

The simplest drive-train, and also the most common on mini choppers, involves only a few parts - an engine to provide the power, a centrifugal clutch to disengage the engine from the load and to transfer power to the rear wheel via the sprocket, and a rear sprocket. The rear sprocket is connected to the axle and the two sprockets are connected using a chain. This is the same simple setup I used on my old school mini chopper project.

The downside to such an arrangement is that you are fixed into one gear ratio for both low and higher speed operation. So, it is very important to choose the best combination of these components to suite your goals. 

Imagine a car that contains a 4 speed transmission, but can only operate in one gear. Which gear would you choose? If you choose first gear, the car might take off the line with the tires squealing, but you won’t get past 10 mph before the engine RPMs are maxed out. If you chose fourth gear, the car would barley be able to accelerate (if at all), but would be capable of very high speeds You would likely choose one of the middle gears, which would allow some amount of acceleration but still allow for some amount of top speed. This scenario is exactly what you are faced with when choosing a clutch/sprocket combination.

The glory of such a simple set of components is that there is a direct relationship between these components and the ultimate top speed of the bike. The engine turns at some specified speed (RPMs) and the difference in sizes between the front and rear sprockets determines if the rear wheel turns faster or slower than the engine. A typical installation has a rear sprocket that is much larger than the front sprocket (gear reduction), making the rear wheel spin much slower than the engine. For instance, if the front sprocket is 2" diameter and the rear sprocket is 10" diameter, then the rear sprocket will turn at only 20% of the engine speed (S1/S2), but with more power. If the rear tire is 14" in diameter, then it travels 43.98" with each full turn (Pi*D). If the engine is turning 2000 revolutions each minute (RPM), at a 5:1 gear reduction (as described above), then the rear wheel is turning only 400 revs per minute (2000*.20). This means that in one minute, the rear wheel is traveling 17,593" or 1,055,576" each hour (400*47.81*60). Now divide that by the number of inches in a foot (12) and then by the number of feet in a mile (5,280) and you get 16.65 MPH. If you change the RPM to 3500, the speed increases to over 29 MPH.

That's a lot of math  to calculate the speed each time you want to change sprocket sizes or RPMs or the rear tire size. So, I created this online calculator to help determine what top speed you can expect from your mini chopper, given the size of  the sprocket on your centrifugal clutch (drive sprocket), the size of the rear sprocket (driven sprocket), the diameter of your rear wheel, and the operating RPM of your engine. By putting this information into the calculator below, you can quickly see what to expect when you make changes to any of these variables. Just remember that when you change sprocket or wheel size to gain more top speed, you will sacrifice torque and will experience reduced off-the-line acceleration.  If this is taken to an extreme, the bike may not even move and will result in a quickly ruined clutch.  So, if your calculations say your top speed is going to be 70 mph, you need to take a second look at your ratios because your gearing is probably way to high and the bike would eat a clutch before it ever moved an inch!

 The following calculator is just a guide. You will have to make sure that the combo you choose matches your engine torque and power. To start out, you may want to use the component specifications below and  keep the RPM in the operating range for your engine.  Many builders recommend gear ratios of about 6:1. This may need to be adjusted up or down slightly, depending on your rear tire diameter. Again, these are just guides and you will need to work out the details for your final setup. Just remember, always Be Safe - never gear for speeds that are unacceptable to your components and frame. Think about what your brakes capable of? Are all of your bearings capable of high RPMs? Will your rigid frame handle the engine and road vibration and shock? 

Most centrifugal clutches for #35 chain have 10-12 teeth, rear sprockets are 60, 66, 72, and 80 teeth
Briggs  5.5 HP Powerbuilt engine maximum torque is at about 3,000 RPM 
Briggs 6 HP Vanguard engine maximum torque is at about 3,000 RPM 
Other Briggs engines are probably similar, but you should check your owner's manual or get a free copy online at www.briggsandstratton.com

You can purchase various sprockets and centrifugal clutches at Northern Hydraulics

This was designed to work with Internet Explorer, so I apologize if it doesn't work with some browsers.

Driven sprocket - Rear (# teeth or inches)-----
Drive Sprocket - Front  (# teeth or inches)----
RPM  --------------------------------------------
Diameter of rear wheel - including tire (inches)

Expected Speed (MPH) BE SAFE!!!

Use This Only as a Guide - You Should Always Check Your Own Math and Calculations 


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