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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.
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.
….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.
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.
Should have gone to Kickstarter!
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.
A similar laser-based rather than a magnet-based speedometer would increase appeal, albeit slightly–perhaps as slightly as the speed improvement.
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.
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.
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.
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.
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.