Wednesday, June 4, 2008

An All-Automatic CVT Based Bicycle

Personally, I'm not much interested in wanting to have an all-auto bike. Maybe its nice to have one in my collection, if such a contraption ever exists, just for the kicks. But the very notion of riderless shifting is sort of repelling to me. But all people aren't like me, so its okay. Its nice to be sensitive to everyone when you're out to design something.

A month or two back, I had written a post called "New Ideas for Cycling Products". It was mostly science fiction, which I admitted. It was fun though.

Well it seems like I missed out on the All-Automatic Bicycle, a bike that thinks and shifts on its own without the rider's mechanical involvement at the handlebars.

This daydreaming of mine is based on a post on Ari's blog, pointed out to readers by James' Bicycle Design. Ari calls it "The NuFixie challenge : Can you build a 'fixed effort' bicycle?"

I want to take a small dab at this. Nothing is too technical or specific. I'm beating around the bush but I hope you see where it is that I'm beating. Drop in comments after you read.

Okay, lets take this step by step, one at a time.


Continuous variable transmission (CVT) is old old old stuff, supposedly originating with the genius Da Vinci himself. The first commercial patent was granted in the late 1800's or so. The CVT concept has never found favor with bikes nor cars but AUDI has had some luck with their multitronic CVT car. Remember that one?

Fallbrook's design is fresh. It is a step less TRACTION or FRICTION based transmission using rolling spheres and some kind of mystery fluid. Take a look.


Basically, what Ari tries to ponder is this, in his own words :

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

Or in other words (if I have interpreted it correctly), take a CVT step less transmission equipped bike, completely eliminate the need for the handlebar shifting operation, and take care of all the upshifting and downshifting based on effort. So the input is effort, the output is a transmission change to appropriate and suitable gearing.

This can be suitable for any type of rider, also to people who can't shift because of a wrist problem, people in cold places who freeze or anyone who has handlebar phobia.


The design goal here is to shift automatically based on EFFORT. But it depends on how to look at effort. What is effort?

Everyone has a different heart rate for a given pedalling load and the perceived exertion is also different, based on what your mindset is, how motivated you are, what your pain tolerance is etc.

Everyone has their own preferred cadence given the same kind of terrain. Furthermore, most people who ride a lot can do so in the same gear even if the terrain gradient changes gradually. Other people can't, won't be able to maintain the previous cadence, and have to shift.

But HR and pedaling cadence are not necessarily objective pointers of effort.

Power output is, and it stays constant over wide variety of conditions.

I think a solution here is to monitor a combination of power input, heart rate and cadence using the sensors available in the cycling market today.

Body temperature and gradient change may also be potential inputs. As we put more effort, our core temperature increases.

As far as gradient is concerned, we all know that when we approach a downhill, we always manually upshift in order to not spin out of gear. Question then is, can you replicate this same behavior in a computer such that a considerable altitude change alters the gearing correspondingly.

If you want to make the system more fancy, go ahead and monitor wind drag and vibration frequency corresponding to a type of terrain. For example, the best and fastest way to move over dirt roads and pave is to get into a big gear and grind at it.


Right off the bat, this is a control issue.

Say this theoretical bicycle had a computer that tells the hub when to shift. I mean, it needs to have a nerve center right? You aren't doing the work of shifting, you want a type of robot, which is the goal.

Roughly, imagine the computer is a rectangular black box that is able to monitor power, HR, cadence, speed and what not. Additionally, a user may be able to change his preferred settings if they want to ride at a particular cadence that day. Whatever...

The complex part of the project is to program this computer. Perhaps you could use a PLC, something small enough in the market today, write a ladder diagram using software and fit all this into a handlebar mounted computer. PLC's are commonly used in CNC machines, AGV's, traffic light control, wherever automatic process control is the name of the game.

The computer will then be coupled electronically to a mechanical shift transducer, in this case, something that will move the idler laterally that in turn will tilt the axis of the spinning spheres in the hub. Digital signals to mechanical motion. Yada yada...


This will be the main headache.

The big issue here is that human physiology varies a lot from person to person. Everyone has their own abilities, their own wattage vs. HR graph,their own lactate thresholds and power profiles and their own preferred cadence for a given situation, which may or may not be always optimum.

If you did write such a program to ready the computer for automatic shifting, could the same system be used for another person, given the above physiological differences?

I don't know the answer to that. Some like to spin fast on a hill, some like the big gears. Some are very efficient at climbing heart rate wise, while others will struggle and possibly die. Some riders wouldn't shift at all were it for a 1 mile commute to the grocery store. Others may ride 25 miles with plenty of climbing every day and plenty of shifting is called for here.

Like I echoed before, such a system will either have to be custom built or the computer must be designed in such a way as to take in a set of inputs based on different rider's abilities.


So the computer operates a mechanical actuation device that in turn rotates the idler.

You could go hydraulic. Okay, no...thats messy. Too much fluid.

Find a compact, geared DC motor that is reversible. Hook a self locking gear mechanism such as a worm gear that mates with a corresponding sprocket at the end of the idler. The selection of the motor mainly revolves around desired RPM, torque, and power output which will need to be calculated. Cost of the motor depends on these specifications.

I mean, there are tons of ways to do this. I just told you one.


Very important with any project.


I don't know the answer to that. Who am I to sit here and say this will work, this won't work? But as with any project, things to keep in mind are :

1. Customer - Who is it, what do they want? Are they racers, recreational cyclists, beach goers, commuters, who??

2. Minimum Specifications and Additional Features - Less moving parts, so much weight, so much dimensions, so much tolerances, durability. Is it too heavy, is it too cumbersome, is it too big that it will induce aero drag?

4. Cost - How much can you make it for? How much will a customer pay for it if you solve their problem?

5. Competitors - What exists already in the market and how will your design address their disadvantages and advantages. How efficient is your design?


This project is fun but difficult. Not impossible. But its complicated and may bring in more problems that that you're trying to solve. One of the problems is there being a lot of components, wires, and moving parts. If you have all this setup and hooked to your bike and then if it suddenly takes a spill, you may be in to cover more costs in damage.

I would not even bother doing this, really. I mean, Shimano's next Dura Ace is an electronic model that just relies on pressing buttons for shifting. We've gotten things down to this level of simplicity. Next year, and the year after that, when these electronic gizmos are mainstream, they might come out with something for hybrids and street bikes. But I don't see any drastic improvements by taking even this 'button' away and making an all-automatic "thinking" bike.

Bicycles are meant to be human powered. Designs are often simpler and cost effective that way.


I haven't tried it. But I like it and would like to get one, perhaps a fully fitted bicycle or just a hub to play around with in my spare time.

A cutout section of NuVinci

I also do not agree with the snobs online who put this down without knowing what it is. Gentlemen, this does not work like those cheap Auto Shift bicycles. There, the primary mode of operation is the moving of the dérailleur through centrifugal force based on pedaling cadence.

Auto shift bicycles fail precisely there - the possibility of shifting at a too low RPM when you don't want it to happen. And for maintenance purposes, what do you do if you want the chain on a particular cog? Sit there and rotate the crank at 90 rpm manually so it can shift??

Not that good design cannot come around all those issues but still...

This is a CVT hub and not a hub based gear cluster. It is meant for step less, noiseless, and smooth transmission giving you an infinite amount of gearing. There is no click-click-click.

I don't care about weight. My main concern is about the moving parts. How reliable is it in high torque, low speed situations such as standing starts and hill climbing? Will it work if I get out of the saddle and stomp on the pedals? Will that kind of friction hold? Will I be wasting my power?

These kind of questions are addressed well in the traction fluid section on Fallbrook's website. The company also got German SRM and an ex-Tour de France rider to test a Nuvinci equipped bike, as shown in this Youtube video. Based on seeing it, it seems amazingly smooth.

And what about the hub itself? If I get out into the elements, can water penetrate the outer compartment and mess with the lubricant? Can the liquid leak? Is it thermodynamically stable? Is it easy to service, maintain etc etc etc, you know where I'm going with this.

I wish that NuVinci would publish more numbers to go with their website. How efficient is this system, say compared to modern chain and derailleur systems?

Chain and derailleur systems have very high efficiencies, typically above 90%. See Chester Kyle's test study here (PDF).

I think somewhere Kyle mentions that for every 1% decrease in efficiency, 12 seconds are lost in a 25 mile TT. Well, the NuVinci may not be for racing at the moment anyway...

NuVinci CVT is very practical for hybrids, city bikes, ladies's bikes, even electric bikes.

But for the hardcore racers who like to go fast and ride ridiculously light bikes, there's no point in sitting and complaining about CVT - if you don't like it, live and let live. Its not for you. Simple.

Now I really have to chill. :)



TD_CBCS said...

I saw the same design on yesterday. That guy only has the drawing though. He calls it NuFixie. I'm not sure if this is the same people or not.

Ari said...

Hi Ron,
Thanks for your analysis and thoughts on my NuFixie idea. I agree with you that it isn't exactly practical, but believe that it is an interesting thought experiment, nonetheless.

While heart rate, power output, and cadence each can indicate effort, but do you think that one would necessarily need all three? What would be the minimal sensor input required to construct a NuFixie?

As for the drive train controller, my thought was that it could probably be located directly next to the NuVinci hub (with a single dial on the controller allowing each rider to specify their preferred effort level.)

Great post! Nice analysis.

Surestick said...

I think the idea of shifting a CVT based on power output is overly complicated.
I feel that auto shifting a CVT could be done by having the shifting set to maintain a specific RPM and then letting the rider have control of the RPM with a manual shifter. If you think about it, most of the time when you are shifting it is to change gears to maintain a specific RPM due to changing wind, hills that can be climbed sitting, fatigue, etc. It's only when standing to sprint/climb or going from road to off road conditions that we really shift to change RPM.

I've though a lot about CVTs, internal gear hubs and bicycle drive trains myself. If anything I don't really see the need for a CVT on a bicycle, one tooth jumps in a cassette give close enough gear ratios unless one is using a huge chainring. For most cyclist I would guess that being able to shift under full power would of greater use, especially for mountain biking.
The armchair engineer in me has always imagined something like a cog split into segments of several teeth sliding in and out on tracks radiating out from the hub allowing the cog to change diameter as being the best candidate for a lightweight, efficient replacement for the dérailleur.

Josh Mitchell said...

LNS - If chain deflection torque meters are "close enough" wouldn't something that counted the linear chain speed be "close enough" as far as cadence is concerned (presuming you entered the size of your chain ring).

LoneStarNot said...

Josh, I'm guessing ... guessing that torque correlates to perception of effort so well that nothing else is needed.

In doing so, I'm eliding LOTS of complexities -- rolling resistance, wind drag, incline, crank angle, etc. -- on the guess that "perception" integrates all those things to get "effort". Thus there seems no point in adding complexity to re-do that job.

While I do expect that the same torque would be perceived as more effort when demanded on an incline, I also expect riders would perceive that as "natural" and appropriate.

I mentioned chain deflection meters because I'm guessing they're cheaper than other torque measurement technologies. Chain speed would likely be more accurate than torque, as a percentage -- but I expect it correlates less well to perceived effort. So I'd guess chain-speed unnecessary in creating a "constant effort" machine.

bike said...

Thanks mate.The video is so cool!

Anonymous said...

Interesting article. I was thinking about taking one of the cvt's from the little pocket bike motorcycles and hooking it up to a bike. I would remove its centrifical clutch and wire up the gearing similar to a grip shift. Has anyone else thought of this? Would the efficiency of the belt drive be so bad as to make it not worthwhile? any comments or suggestions?

Bruce McL said...

This is an interesting thought experiment. For computing power, inclination, and even rough guesstimate of speed, you could use an Android smartphone.

Another computing alternative would be an Arduino.

I agree that measuring crank torque would be just about all of the input you'd need from the cyclist. But I can't think of an easy way to do that. Sensors in the pedals? Then you need wireless to get the info to the computer...