Metrigear pedal spindle-based weenie-compatible power meter

Discuss light weight issues concerning road bikes & parts.
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velomanct
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by velomanct

tranzformer wrote:
The Stig wrote:I would think that measuring power at the pedals is like measuring HP at the crankshaft in an engine...I think that the best way to measure power accurately are at the wheels or wheel horsepower as they say, minus the rotational weight/friction of the drivetrain....


Anyone have an idea how much power is lost through the drivetrain? I know a rough estimate for a FWD/RWD car is 10-12% and an AWD car is ~20-25% from the crankshaft horsepower compared to the wheel horsepower.



How about power loss during a sprint? It has to be considerably more than 3% due to vastly increased forces on the drivetrain. This interests me, I wonder how much higher SRM reads compared to powertap for sprint efforts? Does anyone have any data on this?

Even at 3%, that's 60watts for a top track sprinter, or ~0.8w/kg on the power profiling table.


I am quite interested in the pedal powermeter. How often will it record data? Powertap's 1.26 second interval isn't enough for accurate analysis of sprinting.

If Metrigear can offer it for less than $1000, then I might consider it. I already love my Speedplay Zeros and switching from bike to bike would be superb.

by Weenie


fast700c
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by fast700c

PT's sample rate is fine. Max watts number is fairly useless except for boosting ones ego and as you point out can purely be dependant on when a sample is taken in the pedal stroke of a hard effort. Thats why the 5 second number is used for power profiling. After all, it's the sustained power that's going to win you a sprint.

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djconnel
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by djconnel

The SRM reads lower max power than the Powertap because the SRM averages over a pedal stroke, where the PT averages over an arbitrary time period (1 second or 1.26 seconds, new versus old firmware). Since the pedal stroke includes peaks and valleys in power, if you happen to get one more peak than valley, that boosts your power for the sample, The SRM always gets the same number of peaks and valleys.

I suspect MetriGear will average over the pedal stroke, so be similar to SRM. This is the only peak power which really matters, with the possible exception of a standing start on the track, where the actual peak might be of some interest to some people.

Tunafish
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by Tunafish

This looks like a great power meter. Have the designers talked about the choice of measuring each leg separately and comparing the data. It would be beneficial to know your weakest leg and have the ability to correct that weakness.

For everyone debating on where the power numbers come from ie crank,hub or now pedals who cares.
What you truly need for the training purpose is reliability and consistency each session. I could care less if my Quark reads this # while also sitting on a Computrainer I get this #.
Whether your buddy makes # watts and you make # watts is a useless comparison unless your buddy happens to be your identical twin.
I train at my # and that is all that matters.

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djconnel
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by djconnel

If I compare PT to SRM/Quarq, I multiply PT by 1.03.

Yes -- the plan is to compare left and right foot. Quarq Qollector will handle this sort of data: it basically grabs and stores any ANT+ data it finds, including data from nearby bikes :). But I'm not sure of any head units. So L/R balance might initially be left to post-hoc analysis.

tranzformer
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by tranzformer

If the unit is open source, couldn't someone just write a program that would allow L/R comparison in real time?

2 wheels
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by 2 wheels

mythical wrote:There's only one way to solve this! Mount the new Metrigear pedals onto an old SRM that fits a Ergomo BB. Then have a PowerTap rear hub and to top it off the Polar power meter. Then measure all the data up against each other and see which one comes out most accurate! :twisted:

No, measuring all the data up against each other does not reveal anything about which device is most accurate. You can compare how close to each other they measure, but there's no way to tell which one is most accurate by doing this.
To do this you need to compare them to a know accurate reference or apply a known and accurate power input.

2 wheels
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by 2 wheels

Raxel wrote:Even simpler method to get cadence is putting a simple rotary encoder at the pedal axle.

That will not be simple at all, since the elctronics is inside the axle and the rotary encoder will have to be outside the axle. This will require a way to get the signal from outside the pedal to the electronics inside the pedal. This also means they can't use stock pedals anymore but will have to use custom pedals. So not a simple solution at all and also harder to make weather proof than something enclosed inside the axle.

KDub
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by KDub

Having both could be useful for testing wheel/frame stiffness and power transfer efficiency when testing a new frame out. That way you can tell if the bb and chainstay designs are as efficient as they claim and in some cases see if you would really gain anything by spending $1000 extra to go from a pro sl to an sl3 or from an r3 to an r3sl.

2 wheels
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by 2 wheels

I think Brim Brothers cleat based system discussed here viewtopic.php?t=54198 sounds more promising than pedal based systems as you can use it on more than one bike without moving pedals with you + it does not depend on a specific pedal brand it seems (+ it's moved closer to the rider output so pedal loss is not included in the measurements if this matters to you).

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53x12
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by 53x12

2wheels, but it isn't like removing pedals is that hard of a job. 45 seconds and you are done. Now the issue of using a specific pedal brand is one drawback, but sure better than only being able to use PT with one set of wheels, and using a SRM on one bike.
"Marginal gains are the only gains when all that's left to gain is in the margins."

2 wheels
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by 2 wheels

True it doesn't take much time to move pedals, the only thing you have to remeber is which pedal you have to turn the opposite way to loosen.
Both pedal and cleat based systems have a problem if you want to use it for both road an MTB as you would normally use different shoes and pedals for MTB.

But in any case it's a good thing with at least 3 new power meter brands in the prototype phase + recently released devices like iBike and Quarq CinQo. Power meters are very expensive so hopefully more competition can lower prices.
The market for powermeters isn't that big so it's a question if all manufacturers will survive with possibly 3 new brands coming to the market in 2010. But if the prices will drop a lot from the SRM price-range and quality will still be good, then I also think a lot more will start using power meters, some just because it gives them a new electronics gadget to play with without necessarily using it in systematically in a scentific way to get better training.
A lot of recreational runners/cyclist buys a pulse meter without knowing what they are really going to use it for, but's it's not that expensive so they just buy it to have a gadget to play with. Power meters are still too expensvie for most people to just buy them to play with like they do with pulse meters.
Last edited by 2 wheels on Fri Sep 25, 2009 8:14 pm, edited 1 time in total.

tranzformer
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by tranzformer

@ 2wheels- I am one of those people that you mentioned in your post who will probably get a power meter in 2010 if price is right (looks like it will) and if the quality is good. I have always wanted to get an SRM, but I just couldn't justify the cost on a students budget. Also I wanted to use the power meter on several frames which SRM would make difficult do to different setups on the bikes. Also a PT is nice, but I wanted to use it on race wheels, training wheels, disc wheel. So if 2010 really turns out to be the year of the power meter as it looks like it might be, I will be one happy new customer of a power meter. :)

youngs_modulus
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by youngs_modulus

KDub wrote:Having both could be useful for testing wheel/frame stiffness and power transfer efficiency when testing a new frame out. That way you can tell if the bb and chainstay designs are as efficient as they claim and in some cases see if you would really gain anything by spending $1000 extra to go from a pro sl to an sl3 or from an r3 to an r3sl.


Unfortunately, that wouldn't really work. It sounds like you're referring to marketing claims like, "the new SuperDuper 1000X has a beefier bottom bracket and huge chainstays for more efficent power transfer." When marketing people make claims like this, they don't really mean anything. A stiffer bike feels "more efficient," it's true. But neither the claims nor the feel have any real effect on efficiency.

A reasonable definition of "efficiency" in this case is the ratio of output power to input power. It's true that an SRM and a PowerTap would help you get numbers, but even if the two power meters were perfectly matched to one another (i.e., the delta between the SRM and the PT is exactly equal to the actual drivetrain losses) you wouldn't be able to gather useful data on the power transfer efficiency of frame designs.

That's because you can only lose power in the frame in certain ways. One way is the conversion of force to sound. Another way is by plastically deforming the frame--and if you can pedal hard enough to bend your frame, you don't care about chainstay efficiency; you're more concerned about your legacy as the best match sprinter the world has ever known.
.
The largest "power sink" in a frame--by several orders of magnitude--is the material's damping coefficient. And the power lost to material damping is tiny(1); it's certainly much smaller than the precision of either power meter.

So when you hear a marketing rep (or anyone else) blathering on about how X bike's new chainstay/bottom bracket design is good for "efficient power transfer," just know that the new design is stiffer--or claimed to be, anyway.

Cheers,

Jason

(1) For what it's worth, carbon composites have much higher damping coefficients than most metals. (Magnesium is a big exception). The specific *amount* of damping in a carbon structure, however, is dictated by many, many factors. These include laminate schedule (layup), the direction of the vibration to be damped and the vibration's frequency and amplitude. Even accounting for all of these variables, the amount of pedal stroke energy damped out of a bicycle frame is vanishingly small.

by Weenie


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djconnel
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by djconnel

Wow -- that was awesome :).

The usual argument is the flex is damped by the body, which absorbs the energy. And it's true when move limbs around there's frictional losses. But the body can only respond to applied forces, and I don't see how those are particularly different with a stiff frame than with a flexy one.

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