JACKSHAFT KITS FOR MOTORIZED BIKES
transfer More Torque to your rear wheel
BICYCLE ENGINE JACKSHAFT KIT
If you are a motorized bicycle enthusiast, you will have heard of jackshaft kits. If you have been thinking about a jackshaft kit motorized bike, then this article is for you.
With more than 25 years experience fitting bicycle jackshaft, I have encountered a few common issues that can be easily avoided. There are a few popular misconceptions, however, these kits can be exceptionally useful for many gas bike builds.
While shifter kits enable gear selection through the bikes own rear cluster, there are other benefits (and a few drawbacks) that are not so obvious. Some applications can have a huge impact on the complexity of a motorized bicycle project.
Let’s explore some of the key benefits of shifter kits and why they can be a great option for your bicycle.
WHAT IS A JACKSHAFT ?
A jackshaft kit improves the power delivery, and pace of bicycle engines, by transferring power through the existing gears.
A mororized bicycle engine kit typically provides a fixed ratio sprocket and chain which is independent of the gear cluster of the bicycle. The drive sprocket of the engine spins a chain which is connected to a 44 tooth sprocket mounted on the left side of the rear wheel.
Jackshaft kits replace the standard chain and sprocket provided in the bicycle engine kit. The system connects the engine to the gear cluster on the rear wheel of the bike.
The rider is then able to maximize torque to suit the terrain. Low ratios can be selected to assist the power plant pull up hills. High ratios can be used to improve cruising on level roads.
2 SPEED JACKSHAFT
A two-speed motorized bicycle jackshaft kit can be of benefit to bikes with no inherent gears.
A 2 speed jackshaft provides two different output velocities from the engine drive sprocket. The first provides a low ratio to increase the torque output from the engine. This helps starting (where recoil start is not fitted), and acceleration from a standstill.
The second gear is high ratio to increase the bike’s top speed while reducing engine RPM.
As with other shifter kits, the 2 speed jackshaft is mounted between the engine and the rear wheel.
2 speed jackshafts are useful to gas bikes as they allow better performance, and greater versatility across a range of riding conditions.
JACKSHAFT SETUP
Jackshaft systems use two additional chains, a shaft, and a sequence of sprockets, to connect with the pedal crank:
From the drive sprocket of the engine, a small chain connects to a small sprocket fixed to the shaft.
The shaft is mounted to brackets on both ends . The brackets are fixed to the rear of the motor.
The shaft then transfers power from the left side of the bike, turning a small sprocket on the right side of the shaft.
The right side sprocket drives a second chain attached to a larger sprocket paired to the inside of the pedal crank.
The pedal crank is connected to the inner sprocket via a ratchet bearing. The ratchet bearing allows the pedals to spin freely when the motor is driving. When the rider exerts force, the teeth ratchet gear engage and the bike can be peddled normally.
Although the engine does most of the work, the rider still needs to pedal from time to time:
- to start the engine,
- when additional input is required (steep hills for instance), or
- where the engine develops a fault or runs out of fuel.
WHY FIT A JACKSHAFT TO A MOTORIZED BIKE ?
Motorized Bike Jackshaft Kit Top Speed
The biggest reason that riders purchase motorized bike jackshaft kits is to improve top speed and acceleration.
While the most common reason to fit kits is to make the bike fly, practical application does not always live up to the hype.
Frame Selection
The chain and sprocket in standard bicycle motor conversion kit, is designed for use with hard tail bicycles. Although it is possible to convert a full suspension bike using standard parts from a bicycle motor kit, it can be challenging.
Similarly, trikes are complex to motorize using the standard components provided in bicycle motor kits. In particular, it is difficult to fit and align the chain and sprocket to the rear axle of a trike.
Trikes. can, however, be readily motorized using a jackshaft kit. Shifter kits do not need a sprocket to be welded to the axle as they drive through the trike’s pedal crank.
Jackshaft kits provide greater flexibility in the type, and size, of bicycle frames that can be easily fitted with an engine. They remove many complexities that enable builders to fit motors to trikes and rear suspension bikes more easily.
A shifter kit provides more frame choices which, in turn, allows you to choose a motor that best suits your needs.
Power Delivery
Changing the gear ratio between the engine and the rear wheel maintains engine revs in the optimal power zone. Jackshaft kits are, therefore, more efficient in transferring the power of your motor to the road.
Gear selection obtains the best from the engine. You can climb hills more easily, carry heavier loads, reach higher velocities, and improve acceleration.
Reduce Fuel Consumption
Greater engine efficiency results in improved fuel economy.
Reduced engine wear
With a shift kit, you can select a high gear for long journeys. This can reduce the revs on the engine, and help reduce wear.
Quieter Operation
Bicycle jackshafts also reduce the pitch of your engine by allowing the motor to run at lower RPMs for a given run.
DRAWBACKS OF SHIFTER KITS
While jackshaft kits have many advantages, there are also some downsides to consider:
I have fitted jackshaft kits to my own powered bicycle projects. After the first few days, I rarely change gear.
I select a low gear only to bump start the motor. Once running, I choose the gear that best suits my cruising pace. Although gear selection was available, I only use the gear that I know works best for the engine. I rarely take advantage of the shifter setup.
Fitting jackshaft kits adds an extra level of complexity to the bike build.
A high level of DIY expertise can often be required to fit the components. It may be necessary to obtain a wider shaft for 4 stroke motors or fat tire bikes. Brackets and fittings often need to be modified. This is especially true where frame space is tight.
Installation is certainly more challenging than fitting a standard gas bike kit. There are numerous parts to assemble and it is rare to find any kits that are accompanied by instructions.
The installer has to figure out how to piece the parts together. Assembly is not as easy as you would imagine.
Some bike frames are not suitable
Jackshaft kits are fitted behind the power plant using brackets fixed to the engine casing. The shaft and brackets require space that may not be available on some bike frames.
The rear engine mount is designed to attach to the saddle stay. The shaft must then fit between the saddle stay and the rear wheel. Although it is not common, some bikes simply have insufficient space to fit a shifter kit.
Fitting
Correct alignment is crucial for the system to function properly. If the components are not fitted correctly, parts may jam, or chains may jump off their sprockets.
Jackshafts are designed to fit 2-stroke engines onto regular, old style, bicycle frames. Fat tire bikes have a wider frame profile which may require a longer shaft.
Similarly, 4-strokes are wider than 2-stroke engines, and require systems designed specifically for them.
It is worth discussing the details of your project with a mechanic before making your purchase.
Increased maintenance
The added complexity of the shaft system requires more maintenance than the default sprocket and chain arrangement provided in most conversion kits.
Additional maintenance includes:
- adjusting the chain tension on 3 separate chains,
- regular lubrication of the chains, sprockets and bearings
- ensuring that all bolts and brackets are properly tightened, and sprockets aligned.
Cost
The additional components and complexity can be expensive.
Although it is possible to obtain kits for around $50, some brands can retail for over $150.
Reliability
The system requires three chains to be routed through multiple sprockets and bearings.
The added complexity introduces more wear and tear than with a standard sprocket and chain. The additional parts mean that there is a higher expectation that faults will develop.
In particular, the most fragile component in the arrangement is the ratchet bearing.
The ratchet bearings used in the kits are designed for bicycle gear clusters.
On a bicycle, the ratchet gear is designed to transfer pedal force through the rear sprocket. Selecting low gear transfers the full force of the pedal, leveraged through a sprocket with around a 2” radius.
On a jackshaft system, pedal force is transferred through the large front sprocket which is more than double the radius of the largest sprocket in the cluster. The ratchet bearing can be subject to loads of 10x greater than intended.
Although replacing the bearing is straightforward, failure can leave the rider stranded.
When the bearing fails, the pedals spin freely. This means that the rider cannot pedal. If the engine is shut off, you cannot bump-start the engine.
Quality of components
The quality of the components in a jackshaft kit can impact reliability. Cheaper or lower-quality components are usually more prone to failure. Choose a kit from a reputable manufacturer that uses the best quality ratchet bearing.
Overall, the reliability of a system depends on the quality of the components, installation and maintenance practices, and the riding conditions.
CONCLUSION
Jackshaft kit motorized bike’s offer several benefits. They can improve acceleration, cruise velocity, load carrying ability, hill climbing, and reliability.
Jackshaft’s offer greater flexibility when choosing a frame, and boost fuel consumption.
The major drawback is that the ratchet bearing is a common point of failure on all kits.
If you’re motorizing your bicycle, a jackshaft kit is a great option to explore.
