Laser spoke power meter

Concept, Road 12 167

If you regularly visit cycling sites on the web, it is hard to miss articles about the benefits of training with a power meter. When the first SRM power meter was developed in the late 80s, it was definitely a training tool strictly for the pros. These days though, there are many more options available and it seems like the majority of cyclists I see on group rides are using some system to monitor their power. Of the power systems on the market today, the Look Keo and Garmin Vector pedal based systems are probably the easiest to install and use on multiple bikes (though you do have to switch both the pedals and the transmitter pods attached to the crank ark) . With retail prices in the thousands of dollars though, those systems are not reaching as broad a target market as they could at a significantly lower price point

Engineer Gennady Lubarsky aims to provide accurate power data to more cyclists with his Laser Spoke power meter, which according to the website is based on a “laser and position sensitive detector mounted on the hub, and an optical right-angle prism attached to the rim.” When torque is applied to the drivetrain, the hub-based sensor monitors the deflection of the wheel based on the beam projected back from the prism on the rim. Lubarsky claims that his relatively inexpensive optical system “delivers a level of accuracy similar to that obtained with most sophisticated systems available on market.”

Currently, the project is on IndieGoGo, where you can see a 3 minute video that explains the concept. The current prototype relies on an SD card to transfer data to a computer after the ride, but plans for communication with ANT+ devices (including bike computers and smart phones) are in the works. It definitely sounds like an idea that has potential, so I am looking forward to seeing how the project progresses.

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12 Comments

  1. Andy May 24, 2012 at 1:53 pm -  Reply

    I’m not following how that measures power. It seems to measure how hard you mash on the pedals instead. If I stand up and mash every stroke, the speed will constantly be jumping up and down a bit, applying the same function as torque as the rim responds with the slight delay. But if instead you ride at 30mph but with the smoothest stroke, the deflection would be nearly zero, and it will assume that you aren’t outputting much power. Neat concept though, and they even made a prototype, so bonus points for that.

  2. Mzungu May 24, 2012 at 2:52 pm -  Reply

    Andy, I think it’s measuring the rim’s circumferential deflection and not the out of dish deflection as you would expect from hammering side to side. so, I think it may be somewhat insulated from that. Still, I am not sure I like this wheel base design either, I would of prefer to measure the power output from a point much closer to the rider(like the pedals/BB bracket) than further away like the wheels. The further you are at taking the measurements, I think it adds to more uncertainty of the measurement….. like accounting power lost from the flex of the wheel, or the flex of the frame,, ect.

  3. Willem May 24, 2012 at 2:54 pm -  Reply

    ….But if instead you ride at 30mph but with the smoothest stroke, the deflection would be nearly zero….

    not zero: power required for 30mph = torque * rpm = Constant * deflection * rpm.

    if you mash the pedals, the torque may vary, but on average still is equal to above and > 0.

  4. Mzungu May 24, 2012 at 2:57 pm -  Reply

    oh, yeah. not sure if I like an un-balance wheel this may create either. had globs of those anti-flat sealant on my bikes once, and the unbalance feel of the wheel on the bike stand just drive me nuts. not that I can feel it on my mountain bike on a trail, but some thing about it don’t feel right. Measuring the strain on the pedal/Crank/BB to derive power output is still the way to go.

  5. Stu Cox May 24, 2012 at 3:25 pm -  Reply

    Should have gone to Kickstarter!

  6. Mohsen May 24, 2012 at 5:07 pm -  Reply

    I think that instead of asking riders to adjust stiffness of the wheels for calibration, it is better to calibrate the device by applying a known force(e.g. 10kgF) to the pedal and measuring the deflection of the rim to that force and calibrate the device based on the proportion of sample force/deflection.

  7. Bubba Nicholson May 25, 2012 at 8:22 am -  Reply

    A similar laser-based rather than a magnet-based speedometer would increase appeal, albeit slightly–perhaps as slightly as the speed improvement.

  8. Mike May 25, 2012 at 12:29 pm -  Reply

    I can’t imagine this would ever be accurate without constant calibration. The device depends on even spoke tension and a consistent amount of torsional deflection from the tire. You’re also going to get goofy thermal expansion effects from both ambient temperature changes and heating the rim from braking. Dish deflection in a sprint will also give bad readings, and you’re also going to see weird readings every time you go over a bump. More power to the designers if they can engineer all of this out, but I’m not optimistic.

    • James Thomas May 25, 2012 at 1:10 pm -  Reply

      They definitely do have some issues to deal with. Something as simple as dirt or grime on the prisms could possibly throw it off as well. It is interesting though, so I am looking forward to following the project as it develops.

    • art May 25, 2012 at 2:49 pm -  Reply

      Stiffness of the wheel in any direction is not dependent on spoke tension unless they’re so loose they’re going slack. Even if you expand the rim thermally, you still have the same torque-rotation relationship between the hub and the rim. Elastic deflection of the tire doesn’t matter either, since it’s in series with the rim. You will get negative power under braking, but presumably this thing has a cadence sensor and knows when you’re not pedaling.

      Dish deflection is linearly independent of power, so unless you’re bending the wheel so far that the laser misses the prism it’s not an issue.

      There will be a bit of a sneeze if the wheel hits a bump when the prism is at the bottom. Adding a beam splitter and a second prism at 180deg would take care of this and the aforementioned balancing issues.

      The only real problem I see with this is dirt fouling.

  9. Gennady June 1, 2012 at 7:26 am -  Reply

    We have some great news to share! I’m happy to announce that Laser Spoke ltd has applied for an Invest Northern Ireland grant (£50K) to run research and development on this project.

    We have reviewed carefully our budget taking into account this serious government support and success of our IndieGoGo campaign. We are able now to introduce new prices for our perks for the period of this campaign.

    We want to invite even more people to our project. Please share this great news with your friends, cycling clubs and welcome.

  10. Ron George July 1, 2012 at 11:50 pm -  Reply

    The interesting thing with these power meters is the development time. I’m always entertained by new concepts, but until a reliable, mass producable product comes to market that has the accuracy required of a powermeter, I’ll simply be….entertained.

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