Bicycle gearboxes have been around for almost a  century, but it's only recently that they have  

truly proven themselves as a worthy alternative  to derailleur gears. Gearboxes trade a small  

amount of drive efficiency and weight, in order to  achieve a significant reduction in maintenance and  

component wear, as well as a higher performance  in poor riding conditions. Additionally,  

you can pair gearboxes with belt drive, which  eliminates chain cleaning and lubrication, and  

allows you to pedal over 30,000 kilometres before  replacing any parts. These days you can find  

gearbox bikes that will cross continents, complete  your daily commute, or slay mountain trails.

Right now, you're probably thinking gearboxes  are pretty cool. But have you ever thought  

about building your own? At home with old  cassette cogs, and chains, and plexiglass?  

Well, "champx", a user on Instructables  did just this a while ago. And today,  

I'll be showing you how this homemade bicycle  gearbox works, and how it was constructed.

The first step was to put the idea on paper.  The inspiration for the design was based on  

a lawnmower gearbox, which uses a similar gear  selector, but differs a little when it comes to  

the use of spur gears, rather than chain and  sprocket gears. Champx first ran the numbers  

to determine the gear ratios and drivetrain  parts that were required to make the gearbox  

work. Once the sprocket sizes were found,  the concept was turned into a set of plans,  

which were drawn to scale. From an idea floating  in his head to having a full set of plans, it  

took champx just two weeks. If you're scratching  your head and trying to understand how it works,  

it's time to listen up! Looking down at the  gearbox from above, we can see the crank arms  

as well as the bottom bracket axle. This image  of a crankset from above should give you a  

sense of orientation. The force from the pedals  travels inside the gearbox to a gear selector,  

which slides along the axle and allows you to  engage one of three different gear ratios. The  

first gear provides the lowest drive gear ratio,  and the third gear the highest. Your pedal power  

now travels via one of three chains to the next  set of sprockets on a separate axle, which then  

turns a sprocket on the opposite end. All four  of these cogs are welded to the axle, which means  

that all four chains inside this gearbox spin when  you pedal. The chain on the right-hand side then  

connects to one last sprocket inside the gearbox.  This sprocket is attached to an external sprocket  

meaning they both spin at exactly the same rate.  Importantly, these sprockets are decoupled from  

the axle, allowing the external sprocket to spin  faster or slower than your cranks. The external  

sprocket then goes on to drive the rear wheel.  Hopefully, you now have a good understanding of  

what's going on inside this gearbox. Let's  now take a look at how it was constructed.  

The first thing that champx made was the bottom  bracket axle and gear selector. Special rings with  

internal notches were machined with a mortiser,  and attached the cassette cogs to the axle.  

The notches inside the rings allow the gear  selector arm to engage the desired gear.  

The axle was later hollowed out so that the  spring-loaded gear selector would fit inside.  

To prevent corrosion damaging the  gear selector it was made from brass,  

hence the gorgeous gold colour. With the hardest  bit out of the way, champx made a test rig  

to try out his concept. The secondary axle was  assembled with the four necessary cassette cogs,  

which are all welded to the axle. The four chains  were then hooked up and the rig was ready to test.  

If you watch the red stripy tape on the right-hand  side of the axle, you can see it spinning at  

different speeds when each gear is selected.  Here's the lowest gear. Here's the middle gear.  

And here's the highest gear. Okay, so the  concept clearly works. Champx now needed to  

make the box part of the gearbox. He chose  to use stack together sheets of plexiglass  

so that you could see the sprockets spinning  inside. A friend machined the bearing locations  

into the outer sheets, but otherwise, champx used  a jig saw and hole saw to shape the plexiglass,  

which is no easy feat. With the gearbox coming  together nicely, the next step was to make it  

look pretty. Champx worked his way from coarse to  fine-grit sandpaper to give it that glassy look.  

The box wasn't quite wide enough, so one more  six-millimetre sheet of orange plexiglass was  

added. This turned out to be the cherry on the top  and ties in really nicely with the orange chains.  

The remaining shifting components now needed to be  added to the gearbox. Inside you can see a small  

red rotating hand that pushes the gear selector to  the left and right. This connects to a rod outside  

the gearbox, which is lifted up and down using a  spring-loaded cable. Unfortunately, the original  

spring mechanism didn't provide enough cable  tension, so champx built a heavy-duty version  

that works well. With the gearbox now complete,  it was time to build a bicycle frame to house it.  

Given the novelty of this project, it made  sense to build the gearbox into a fun bike.  

Champx welded a frame that he calls the "swing  monster". This cruiser-style bike uses headset  

bearings at both the fork and seatpost, so  that it can steer from the front or rear wheel.

I say mission accomplished on this project! The  bike looks amazing, and would be a fun challenge  

to ride too. As a bike nerd, tinkerer, gearbox  aficionado, and lover of cool projects - I had to  

share this work! I hope it inspires some of you  to build something that's normally out of your  

comfort zone. If you know of some more out of the  box DIY bicycle projects, I'd love to hear about  

them in the comments. And if you'd like to learn  more about the two most popular bicycle gearboxes,  

check out my video comparing the 14-speed  Rohoff hub with the 18-speed Pinion gearbox.