Idea for a cheap, strain-based power measuring device idea

kmavm

The iBike may be limited, but it's an exciting development for folks like me who are convinced of the benefits of training with power, but underwhelmed by the value proposition of available power meters. They really cost a lot. The PowerTap is somewhat cheaper, but I can think of no finickier bike technology in existence, given how few people actually own them and how many internet posts you see about them not working whenever you look at them sideways. Even if the iBike is a bust technologically, I hope it will make the market more competitive, and bring down the price of some of the higher end units.

As I understand it, the strain gauge based meters have all been pricy because the components into which they're embedded are made of nifty aerospace alloys that are expensive to work with. So, on my ride home today, I had an idea: what about the sole of a cycling shoe? Strain information could be collected there. Perfectly acceptable shoe soles can be constructed out of relatively cheap nylon. Why not embed some strain gauges in the sole of each shoe, and stick a wireless transmitter on the outside of the shoe upper? It might look a little dorky, and be somewhat heavier than a regular shoe. But, if it got me a power measurement system that was much cheaper than the current market leaders, I for one would at least think hard about it.

robkit

i dont think its the housing of the guages thats the problem - for example, a dura ace crank mounted SRM costs $3400 + tax, while the DA chainset that SRM are adapting is about $400. You can pay the latter for a pair of high end shoes.

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frenchyge

If you'd ever worked with trying to read wind speed or air flow accurately under different conditions, then you'd be able to think of a more finicky technology than the PT receiver. Air flow measurements are very dependent on the angle of the flow compared to the receiver, and on turbulence causing obstructions like arms, brake hoods, cables, etc. They're also highly variable, making them difficult to troubleshoot, offset, or even detect.

If another PM offering helps market competition, then that'd be great. Personally, I want to use my PM indoors on my trainer so iBike wouldn't be a viable option for me anyway.

Flatscan

I hope so too (well, I just got a PT Pro, so I'm sort of indifferent), but I don't think that will happen. Each of the current offerings is technically distinct, has its strengths and weaknesses, and (IIRC) has patent protection. A product that directly competes with PT in features would not materially affect SRM pricing. An SRM-like product priced like the PT, on the other hand.... A product needs to have similar accurancy/precision without additional restrictions (e.g. will not work indoors).

I think strain gauges throughout the sole of the shoe would not work. In my interpretation of your idea, the sole would have gauges arranged along its length measuring its deflection. The nylon sole would probably not flex predictably, a shoe is not a lever-arm where you can only apply force at the heel, and torque around the pedal spindle is meaningless, not contributing to input power.

However, borrowing from force-actuated pedals, it may be possible to put gauges under the cleats to measure compression and shear, i.e. input force at the pedals. This would probably increase stack height and might only work acceptably with large road cleats. Cadence can probably be modelled, but a crankarm magnet would eliminate the guesswork. I'm not even going to consider the analysis that would be required to isolate perpendicular (to the crankarm) input force from wasted radial force.

If you're wary of the current PT receiver (recommended mount <3" from a position-stationary, rotating transmitter), what do you think of a transmitter continuously revolving 170mm/6.7" around the receiver?

As I understand it, material is precisely machined under the spider and replaced with the SRM mount. This mount must deform predictably in the presence of torque so that the strain gauge readings map reliably to input torque, thus expensive materials.

Squint

Well, the pedal is the only thing the shoe is pushing against so you could theoretically press anywhere on the sole of the shoe and it would have to be transmitted to the pedal. The problem may be linearity. Pushing at the toe end will flex the sole but pushing on the sole directly over the spindle won't flex the sole much.

The pedal spindle would probably be the best candidate. Pedals would be easy to transfer between bikes and allow you to use any BB/crank/wheelset. They could also tell give you a plot of force applied throughout the pedal stroke for each leg.

Data transmission and batteries might be a problem.


The strain gauges are downstream of the DuraAce crankarm in the SRM spider (pentagon). Inside the SRM is another spider of 4 arms to which the strain gauges are epoxied. As long as those flex in a reasonably linear, predictable manner, what the DuraAce crankarm does is irrelevant.

biker-linz

Don't forget that no-one posts every day to tell you that their PT is working fine. I've had two and they have been almost faultless in all weathers, and Graber's customer service is second to none.

L.

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kmavm

My check for a powerTap Pro has already been cashed, so I hope you're right. I'm not claiming that the powertaps are garbage; just that they have a comparatively higher hassle rate than practically any other piece of bike technology imaginable. Everybody has saddles, chains, cranks, bottom brackets, headsets, hubs, cassettes, etc., and they all work pretty faultlessly baring user error or the extremely rare manufacturing defect. Probably less than 1% of serious cyclists own a PowerTap, yet every message board on earth is full of people saying, "I did XYZ in weather ABC, and now the heart-rate display is in Roman numerals and the speed is in gigacubits/Jovian year!" They then receive the advice that they should dry the unit out, regrease it with marine grease, remount the CPU wrapped in bubble wrap, crash their bike a few times, check their spoke tension, THEN replace the batteries exactly three times, etc.

I'm sure they're fine units. It's just that the technology hasn't really matured to the degree most other bike technologies have.

9606

I have three PT units, an Sl and two Pros. The oldest is about three years. NO problems except for batteries and torn wiring harnesses when I crash.

mises

Add a short wire from pedal/sensor to a battery holder/transmitter mounted with 1 or 2 longer chainring bolts in the space between the arms and have the receiver integrated with the cadence magnet sensor on the chainstay (with wire up to the handlebar unit).

I don't think the price would be any better than SRM once it was all done but using any crank,bb, and wheel, better accuracy than Polar, and pedaling force data too would be worth it.

When do we start production?

MichaelB

The cheapest way would be to attach strain guages to the frame and use a computer to work out what loads are due to the torque at the hub and what is due to bumps and the rider moving around etc... Obviously this type of thing wouldn't be accurate if it was universal but you could probably come up with something.

The shoe idea is last because the angle you press the pedal at varies all the time.

The best method is to measure chain tension but it might get in the way of your foot. Would make a cheap system though.

F1_Fan

No one ever posts to say "My PowerTap worked perfectly today"... so I will... my PowerTap SL worked perfectly today. OK... it sucked until I put a little foam pad under the sensor, CycleOps website is a joke, their documentation is laughable and their software is beyond horrible. So:

dropouts: foam pad
website: I get good info from forums like this
documentation: as above
software: CyclingPeaks

*Now* my PowerTap is perfect.

frenchyge

Don't forget - Lost HR: call for new computer head