The NuFixie Challenge: Can you build a fixed-effort bicycle?

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Following on my recent posts on the hackable Monkeylectric LED spoke lights and the Altoids tins of bicycle hacks, I’d like to share a new idea. I don’t have the parts, tools, or know-how to build this myself, but perhaps you do, so I’ll share:

The idea is to build an automatic continuous transmission for a bicycle, by wiring up a controller for a NuVinci CVP designed to maintain a constant level of effort from the rider.

In the past year or two, I’ve spotted articles and reviews discussing an interesting new technology for bicycles: the NuVinci CVP from Fallbrook Technologies. The “CV” of the NuVinci CVP reflect the fact that the gear ratio is continously variable: While most bicyclists are familiar with the clicking feeling of switching between discrete gears, the NuVinci CVP offers an alternative in which a continuous spectrum of gear ratios are available. The demo video gives a nice overview. While a few high-end bicycles now incorporate the NuVinci into configured bike, you can also purchase just a hub or purchase just a pre-built wheel. Most user reviews so far have been quite positive about the technology, ratio speed range (350%), and test rides, but have been less positive about the added weight (~8lbs), reduced efficiency, and price (~$400). (See the reviews at bikehugger, veloblog, and bikecommuters).

…build a controller that continually adjusts the NuVinci’s gear ratio to maintain peak efficiency.

So let’s assume for the moment that the NuVinci CVP lives up to its billing. I’m suggesting that rather than constantly adjusting the NuVinci’s “CruiseControl” twist-shifter to maximize your efficiency, we build a controller that continually adjusts the NuVinci’s gear ratio to maintain peak efficiency. If I’m not mistaken, the effect should feel something like riding a stationary exercise bike. Or am I mixing up effort, power, and cadence? I did a bit of looking and came across one off-the-shelf power output sensor (the Polar Power Output Sensor Kit), and there are a whole slew of relatively cheap cadence sensors available.

Automatic controls for gear-shifting has made a recent resurgence, thanks to the three-speed Shimano Coasting group. Bicycles built on the Coasting components (or a similar system) automatically switch gears based on the bike’s speed (?? correct me if I’m wrong on this.) The goal of the Coasting system has been to enable simple bicycle designs for non-riders, by removing those pesky hand controls for shifting and braking. Clearly I have a different goal in mind than Coasting: elegance, not simplicity.

A bicycle that automatically and continuously maintains the preferred gear ratio (or cadence?) for the rider. Too good to be true? A simple afternoon hack? Please share your thoughts on the NuFixie Challenge…

Update: Be sure to check out Ron’s detailed assessment of the NuFixie Challenge.



dan goldwater wrote on May 29, 2008
you don't even need a CV transmission to have auto-shifting, without CV it just picks the closest of available choices. either way i think there have been a number of auto-transmissions developed for bikes, of course they dont seem to have taken off much so i have never seen or tried one. google for auto-shift bike and you will find stuff like: the former one uses a Shimano setup, i dont know if shimano still makes it though. the latter one seems to have abysmal user reviews although i dont think that is inherent in the technology, merely that implementation. i remember in the past having seen more high-end auto-shifting prototypes announced here and there.

ari wrote on May 30, 2008
I hadn't seen the LandRider before, and do like that it has more than 3 gears. As far as I can tell, both systems shift based on wheel speed, and I was thinking more along the lines of cadence or power output. Perhaps the effect in the end is similar enough to be unnoticeable. My guess is that while a gear-based auto-shifter would be constantly clicking through gears, a NuVinci-based auto-shifter would feel much more smooth. Between a cadence-, wattage-, and velocity-based controller for adjusting the gear ratio, which would we expect to feel the most consistent (in terms of effort required) across a variety of riding conditions?

Ari Bader-Natal: NuFixie Challenge | BikeHacks wrote on June 2, 2008
[...] RSS feed. You can also subscribe to BikeHacks by email. Thanks for visiting!Ari is looking for some help. He has an idea for a new drive train and is interested in reader [...]

Ron wrote on June 4, 2008
Hi Ari. I wanted to respond to your idea. Very interesting. Too long to type here though. Visit my blog post for my reply Dan : CVT and the existing auto transmissions you mentioned are not the same.

Chris Lyon wrote on June 4, 2008
A couple of thoughts: Each of us has a power output sensor: our legs. The feedback loop can be a simple as a cadance sensor with a +/- range. Set it for 100, and as the load increases, your legs slow, and the shifter downshifts - and tother way around. Therefore, we need a power supply, programmed chip, an input device, and a servo mechanism

Ron wrote on June 5, 2008
Hey, I hope you're checking the comments section on my blog. More ideas and links are coming in. This is a fun project, making things simple first would be the way to go for a prototype. The aim of the project, and I hope yours too, is not to increase speed or decrease weight, but to improve COMFORT in bikes, primarily through an all automatic "intelligent" shifting utilizing a Nuvinci hub, and programming. That said, Nuvinci is not for race bikes, it clearly has its niche and I hope the company recognizes too that its not competing in that market.

ari wrote on June 5, 2008
Ron, thanks again for your analysis! For those of you who haven't take a look at Ron's blog post, I highly recommend checking it out: A number of thoughts, suggestions, and links have been raised in comments, and I haven't gotten a chance to read through all of them yet. The ones that stuck out for me were: I'll reiterate that I don't have the knowledge or materials to build the NuFixie myself, but I'm certainly enjoying the discussion. Perhaps an enterprising engineering student or two can take up this challenge for a future class project?

Efried wrote on June 7, 2008
Well I rode the NuVinci. and I find it too heavy- would love to have a leaner solution- possibly without a chain

Crankzone » Dwell, the Trailcart, and a challenge wrote on July 5, 2008
[...] and Bikes blog has a question/challenge for all you designers and engineers who might be reading: Can you build a fixed-effort bicycle? The idea, which he is calling "NuFixie", is to build up a bike that uses the NuVinci CVP hub, [...]

David wrote on September 9, 2008
You would want to change gears based on speed, not torque. Otherwise it would change to higher gears based on the amount of pressure you applied to the pedals, not on the speed. You would have to inversely control the gear ratio according to pressure, the greater the pressure the lower the gear. Why do that when the end result is that you want to control speed. Cadence is the same thing as speed. Your steps are cyclic, and are translated into rotation of the wheel, same thing.

shannon wrote on December 29, 2008
looks like Fallbrook have at least part of the answer to your quest... It's a cool idea I think that a constant cadence should be the optimal measure of 'effort' and this new electric bike from iZip sort of covers the idea too click on the EVO express bit: