Wattage meter for Cycling

Ok this is a tough one folks: want to build something like this, which is a watt meter measurment for cycling any ideas on this? If it can be developed at low cost, it could be a great deal. It would be like the ANT+Sport enabled type devices currently be offered at 500 to 2,000 dollars for the PowerTap Hub or GPS enable Garmin type units for competitive cycling

Thanks OUCH

I can't say that I remember the name of the company that makes these things, but when I was in school, there was a guy doing a senior design project for a company who makes floor polishers/burnishers, etc. The idea was that you put this device, a special strain gauge, on the shaft. Knowing the mechanical properties of the shaft and the rpm of the shaft, you could determine from the deflection in the strain gauge and the rpm, the torque being applied to that shaft. Problem is, I can't remember who makes the strain gauges.

I suppose you could use force sensors on your pedals and then measure the rpm of the pedals. Knowing the length of the pedals, how much force is applied, and RPM, you could calculate the torque. The only thing is, there is, it may be difficult to determine the tangential force only. Maybe you could put a strain gauge on the pedal lever arm itself...

Forgive my long-winded thinking. Just trying to sort through some ideas. Hope it gives you some inspiration.

I haven't seen any response from anyone here. Hopefully, my answer was somewhat helpful. It just looks as though the thread died after my answer.

This would be a fun project. The electrical portion (other than sensor design) is quite
easy. Unless there is some creative way to accurately infer watts without a custom machined hub I doubt this would be less money than the Power-Tap.

Does anyone know how the Power-Tap measures power?

I probably would use BT instead of ANT. You could do a Droid app that links data
received with the GPS coordinates. Make a handlebar holder for the Droid and display
all the stats as an overlay to the map.

(* jcl *)


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Cycling power meters are not for the faint of heart. They are a serious electrical engineering/mechanical engineering/computer science challenge. There's a reason these power meters are expensive. While I suspect that the net parts cost of the power meters out there are a lot less than they are selling for, the prices need to pay for some pretty serious engineering development costs. I ended up buying an iBike. It wasn't cheap. But, it was ultimately worth the price I paid. I suspect that a homebrew bike power computer could be made. But, one would probably need to dedicate serious time to do it.

If you marked a spot on the hub and a corresponding spot on the rim - would they go out of phase as torque increases?

I done a project like this. Use a alternator connected to the wheel to generate a dc voltage and a voltage divider to scale the voltage,. Use a sparkfun current sensor to measure the DC current. power = current x voltage.

Use a alternator connected to the wheel to generate a dc voltage

[Disclaimer: I'm no mechie] The strain gauge method would be capable of distinguishing when the power was being generated by the rider, but this method would still generate voltage when the bike was free-wheeling.
I think.

There might be some mechanical way to measure chain tension with an adapted derraileur idler-pulley unit? Spring it to twist the chain out of straight (in the top section of the chain).

The greater the tension in the chain the more the unit is turned - thus providing a measure of tension.

Disadvantages - mechanical losses, not linear.
Advantages - can connect wires to it as its not rotating!

In fact a single idler wheel held up in a fixed position but with a force sensor (load cell) in the mounting might be
simpler, more linear.

Crack open a digital scale, rip out the strain gauges, superglue them to the pedal, remember to use Wheatstone Bridge Circuit to get mesurements right and a good amplifier circuit

Happy pedaling :stuck_out_tongue:

But if you want to buy some

[edit]They cost from $45 in package of 10, so $4.50 a pop[/edit]

I've actually been working on this conceptually for several years (since hearing about the iBike) and initially started to try to implement one on an MSP430 before jumping to Arduino around last Christmas.

Curious if anyone else has made any progress. I've made a very accurate cycling computer and am working on interfacing additional sensors now.

I've also simulated the setup in MATLAB to determine what some of the sensor tolerances would need to be to be useful for training. This is the part that hurts... the wind speed (dynamic pressure) sensor needs to be extremely sensitive and I've had a hard time finding a good cheap part to do this... my first attempt only got me to about 1 mph or wind resolution although I think I can get to .1mph or resolution with oversampling. Also, for grade effects you need a tilt sensor accurate to at least 1/1000 of a g.

There are of course other ways to go... I also had thought about the instrumenting my pedals or shoes but the above mentioned type of strain gauges won't work (wrong plane) and pressure transducers are notoriously inaccurate (although never thought about using whatever is inside a scale! Anyone done this?) Also, a small but measurable amount of pedaling torque is produced by the upgoing leg... not sure how you'd measure that with a pressure trandsducer.

I'd also thought about using strain gauges on the chain stays. By measuring the forces in all the rear triangle structural members and doing some vector math you can back out the tension in the drive chain. Once you have chain tension you multiply it by chain speed to get power.

Alternately you could go the Polar route and measure the vibration of the chain which also correlates directly with tension. (I actually did this using a guitar pick-up as a sensor feeding and FFTed it...worked great on tuning forks an inch or so away but I just got "noise" near a vibrating chain... maybe I should re-look into this). You could also use a laser diode to measure the vibrating chain.. but you'd have to be able to track the chain as it moves across the different gears.

In summary - there are several ways to skin this cat. You can go the iBike route and measure the everything except the power and then back out power, or you can try to get at power directly by measuring force/tension in the chain, pedals, cranks, crank shaft of hub. The later two may be the hardest as you'd need to get a strain gauge into a rotating bearing. Now that I think about it, strain gauges on the cranks measuring their flexing might be the easiest way to go if you can figure out how to get the signal off each crank (arduino mini's plus bluetooth?) Anyone have any expertise on the art of affixing strain gauges to structures? Do you need to glue them down or something?

I'd like to document what I've done so far...can anyone recommend a good package for documenting projects on a blog or website?

Feel free to PM me...

Bear with me... this will take some "explaining" without being able to do a drawing and point to it.

It is an idea not mentioned above.

Before I start, let me say that I like the "drive an alternator with the bike, measure power produced by it" answer... better than my answer! Could someone please address the "but it would count power produced when bike free-wheeling" problem? I suspect it isn't really a problem, but can't quite say why.

So... my alternative answer....

It all started with an exercise bike I sometimes ride. It has an adjustable brake, to make riding it easy or hard.

The brake doesn't work exactly like this, but essentially it does, and what I am going to describe can, I think, be adapted to provide information on power.

In the first place, the "tire" on the bike isn't like a road bike's tire. It is turned by the pedals, but the "tire" surface, that which would touch the road in an ordinary bike, is metal, and flat. Flat and at right angles to the main dimension of the wheel. And about two inches wide. So the wheel is more like a train wheel without the flange, if you see what I mean.

Now, the brake:

The brake consists of a rugged strap. One end is fixed to the bike's frame, then the strap wraps around the wheel, and the other end connects to a spring which connects to a different point on the bike's frame. You can change the tension in the spring. The tighter the strap is held against the wheel, the more friction, the harder it is to pedal.

For my idea to work, you would need the spring to be at the "uphill" end of the "U" of the strap around the wheel. The harder you pedalled, the more the spring would be extended. Only slightly more, but the difference would be there.

If you measured the DIFFERENCE in the pull on the frame by the spring between when the wheel were at rest and when the wheel was turning, you would have a measure of the force the cyclist was exerting to overcome the friction, wouldn't you? And you would count the turns of the wheel, throw in the circumferance of the wheel and you have a distance "travelled" Force x Distance = work??

If you didn't want to use a strain gauge, you could, I think, with some "amplificaton" by means of cable and pulleys, measure the slight change in the spring's extension, and calculate the force from that?

As I said... I think the alternator is the way to go!

Take a look at this....

If you use a trainer style brake load then measuring rpm and time will give you an equivalent work - energy. Measure the torque load once and use it to calc the energy.

Correct me if I am wrong... but the problem with using alternator/generator or brake solutions is that the measurement sensor then adds resistance that the rider must overcome on top of whatever effort he is exerting to maintain his speed. This isn't desirable... the other methods sense the power output affecting the bike/rider system.