gear ratio

7 Articles

Bicycle Tows 15,000 Pounds

January 4, 2026 by 13 Comments

An old joke in physics is that of the “spherical cow”, poking fun at some of the assumptions physicists make when tackling a new problem. Making the problem simple like this can help make its fundamentals easier to understand, but when applying these assumptions to real-world problems these assumptions are quickly challenged. Which is what happened when [Seth] from Berm Peak attempted to tow a huge trailer with a bicycle — while in theory the bike just needs a big enough gear ratio he quickly found other problems with this setup that had to be solved.

[Seth] decided on a tandem bike for this build. Not only does the second rider add power, but the longer wheelbase makes it less likely that the tongue weight of the trailer will lift the front wheel off the ground. It was modified with a Class 3 trailer hitch, as well as a battery to activate the electric trailer brakes in case of an emergency. But after hooking the trailer up the first time the problems started cropping up. At such a high gear ratio the bike is very slow and hard to keep on a straight line. Some large, custom training wheels were added between the riders to keep it stable, but even then the huge weight still caused problems with the chain and even damaged the bike’s freehub at one point.

Eventually, though, [Berm Peak] was able to flat tow a Ford F-150 Lightning pulling a trailer a few yards up a hill, at least demonstrating this proof of concept. It might be the absolute most a bicycle can tow without help from an electric motor, although real-world applications for something like this are likely a bit limited. He’s been doing some other bicycle-based projects with more utility lately, including a few where he brings abandoned rental e-bikes back to life by removing proprietary components.

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Automatic Transmission For Manual Transportation

May 15, 2025 by 31 Comments

The drivetrain of most modern bicycles has remained relatively unchanged for nearly a century. There have been marginal upgrades here and there like electronic shifting but you’ll still mostly see a chain with a derailleur or two. [Matthew] is taking a swing at a major upgrade to this system by replacing the front derailleur with a torque converter, essentially adding an automatic transmission to his bicycle.

Most of us will come across a torque converter in passenger vehicles with automatic transmissions, but these use fluid coupling. [Matthew] has come up with a clever design that uses mechanical coupling instead using a ratchet and pawl mechanism. There are two gear ratios here, a 1:1 ratio like a normal bicycle crank and a 1.5:1 ratio that is automatically engaged if enough torque is applied to the pedals. This means that if a cyclist encounters a hill, the gear automatically shifts down to an easier gear and then will shift back once the strenuous section is finished.

[Matthew] machined all the parts for this build from scratch, and the heavy-duty solid metal parts are both impressive but also show why drivetrains like this haven’t caught on in the larger bicycling world since they’re so heavy. There have been some upgrades in internally geared hubs lately though, which do have a number advantages over traditional chain and derailleur-based bikes with the notable downside of high cost, and there have been some other interesting developments as well like this folding mechanical drivetrain and this all-electric one.

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Behold A Geared, Continuously Variable Transmission

December 14, 2024 by 72 Comments

When it comes to transmissions, a geared continuously-variable transmission (CVT) is a bit of a holy grail. CVTs allow smooth on-the-fly adjustment of gear ratios to maintain a target speed or power requirement, but sacrifice transmission efficiency in the process. Geared transmissions are more efficient, but shift gear ratios only in discrete steps. A geared CVT would hit all the bases, but most CVTs are belt drives. What would a geared one even look like? No need to wonder, you can see one for yourself. Don’t miss the two videos embedded below the page break.

The outer ring is the input, the inner ring is the output, and the three little gears with dots take turns transferring power.

The design is called the RatioZero and it’s reminiscent of a planetary gearbox, but with some changes. Here’s how the most visible part works: the outer ring is the input and the inner ring is the output. The three small gears inside the inner ring work a bit like relay runners in that each one takes a turn transferring power before “handing off” to the next. The end result is a smooth, stepless adjustment of gear ratios with the best of both worlds. Toothed gears maximize transmission efficiency while the continuously-variable gear ratio allows maximizing engine efficiency.

There are plenty of animations of how the system works but we think the clearest demonstration comes from [driving 4 answers] with a video of a prototype, which is embedded below. It’s a great video, and the demo begins at 8:54 if you want to skip straight to that part.

One may think of motors and gearboxes are a solved problem since they have been around for so long, but the opportunities to improve are ongoing and numerous. Even EV motors have a lot of room for improvement, chief among them being breaking up with rare earth elements while maintaining performance and efficiency.

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Lathe Outfitted With Electronic Gearbox

September 19, 2024 by 45 Comments

Running a metal lathe is not for the faint of heart. Without proper knowledge and preparation, these machines can quickly cause injury or destroy expensive stock, tools, or parts. The other major problem even for those with knowledge and preparedness is that some of their more niche capabilities, like cutting threads with a lead screw, can be tedious and complicated thanks to the change gear system found on some lathes. While these are useful tools for getting things done, [Not An Engineer] decided that there was a better way and got to work building an electronic gearbox to automate the task of the traditional mechanical change gear setup in this video.

What makes change gears so tricky is that they usually come as a set of many gears of different ratios, forcing the lathe operator to figure out the exact combination of gears needed to couple the spindle of the lathe to the feed screw at the precise ratio needed for cutting a specific thread pattern. It is possible to do this task but can be quite a headache. [Not An Engineer] first turned to an Arduino Nano to receive input from a rotary encoder connected to the shaft of the lathe and then instruct a motor to turn the feed screw at a set ratio.

The first major problem was that the Arduino was not nearly fast enough to catch every signal from the encoder, leading to a considerable amount of drift in the output of the motor. That was solved by upgrading to a Teensy 4.1 with a 600 MHz clock speed. There was still one other major hurdle to cross; the problem of controlling the motor smoothly when an odd ratio is selected. [Not An Engineer] used this algorithm to inspire some code, and with that and some custom hardware to attach everything to the lathe he has a working set of electronic change gears that never need to be changed again. And, if you don’t have a lathe at all but are looking to get started with one, you can always build your own from easily-sourced parts.

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Gear Up Your Gear Knowledge With Gears

February 19, 2020 by 16 Comments

Gears are fairly straightforward way to couple rotational motion, and the physics topics required to understand them are encountered in an entry level physics classroom, not a university degree. But to really dig down to the root of how gears transfer motion may be somewhat more complex than it seems. [Bartosz Ciechanowski] put together an astonishingly good interactive teaching tool on gears, covering the fundamentals of motion up through multi-stage gear trains.

Illustrating the distance traveled at different points on the disc

The post starts at the beginning – not “how to calculate a gear ratio” – but how does rotational motion work at all. The illustrations help give the reader an intuitive sense for how the rate of rotation is measured and what that measurement actually represents in the real world. From there [Bartosz] builds up to describing how two discs touching edge to edge transfer motion and the relationship of their size on that process. After explaining torque he has the fundamentals in place to describe why gears have teeth, and why they work at all.

Well written explanatory copy aside, the real joy in this post is the interactivity. Each concept is illustrated, and each illustration is interactive. Images are accompanied by a slider which lets you adjust what’s shown, either changing the speed of a rotating gear or advancing the motion of two teeth interlocking. We found that being able to move through time this way really helped form an intuitive understanding of the concepts being discussed. This feels like the dream of interactive multimedia textbooks come to life.

Lose The Beer Belly By Brewing Beer

June 27, 2011 by 19 Comments

If you’re milling your own grains for that next batch of beer you might be able to melt all of those extra calories away while you’re at it. [Eucherboy1] repurposed an unused exercise bike to power his grain mill. The propane tank is serving as a weight to hold the base of the mill in place; it’ll be used later when boiling the wort. A belt transfers power from the bike to a wheel replacing the hand crank on the mill. Check out the video after the break to see [Euchreboy1] working up a sweat. We think there’s much room for improving the gear ratio of the setup. Or he can just man up and push through the pain.

We’ve gotten used to seeing ways to power a bicycle, like using wood-fired steam, or even by incorporating a chainsaw. But the hacks that use a bike as a power source are a bit less common. Our local hackerspace made a bicycle blender a while back. Got any projects of your own that are bike powered? Send them our way!

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