Tuesday, September 29, 2009

MetriGear Vector Pt 1: Introduction

MetriGear is a new company in Silicon Valley California, which has developed a very promising product: a module of acceleratometers and piezeresistive strain gauges which can be inserted in the hollow axle of a bike pedal, provide force, pedal velocity, and power data via ANT+ Sport communication to any sufficiently cooperative ANT+ Sport head units. It is being developed first for Speedplay pedals, although there are other candidates.


Schematic of MetriGear Vector

Velocity Nation, one of the best techie cycling web sites on the web called the MetriGear Vector "by far and away the most exciting thing at (Interbike)". So what's the big fuss?

Well, first of all the Vector looks to be the Weight Weenie leader in power metrology. When coupled with shoes drilled with 4 holes (versus the more common 3) such as Bonts, Speedplay X1's and Zero Ti's, especially when tuned with aftermarket Al bowties, probably have the best combination of functionality and low weight of anything presently on the market. The first version of the Vector will add around 25 grams to the weight of a normal Speedplay pedal (50 grams for the pair). When coupled with, for example, the Specialized SpeedZone ANT+ Sport computer, easily the smallest power-equipped head unit announced so far, power metrology may be available for under 100 grams total. Truly fanatical weight weenies might even get by using the Vector in only one of the two pedals, assuming power from one pedal can be doubled to yield no power (obviously not valid with a L-R pedal imbalance), shaving 25 grams from that total.

Another advantage is the Vector is the most promising solution for tandem bicycles. Tandems drivetrains are more complex than single bikes. Related to the advantage for tandems is that it works well with track bikes. I'll write more about these later.

Still another advantage is Vector's ability to separately report power to the left leg and the right leg. It's generally considered optimal that power be equally partitioned between the two legs. Since the body is relatively biaxially symmetric, an equal division of labor limits the preferential overload of muscles on one leg. Additionally, a left-right imbalance may be an indicator of poor form which has other misaligment implications. So that Vector is sensitive to this is a big deal.

Yet another advantage, and one which got Velocity Nation excited, is Vector's capability for a high sampling rate. This is hardly a unique capability to pedal-based systems; there's no reason hub-based or crank-based systems can't also use a high sampling rate. (On the other hand, the chain-based Polar power meter, and the wind-based iBike, are incapable of high sampling rates). But previously systems have sampled at a fixed rate, for example one per second. If a higher sampling rate were used, for example 10 times per second, it would reduce the riding time which could be stored, for example 1 hour instead of 10 hours. But Vector promises to offer a variable sampling rate. Velocity Nation provides the example of the start of a kilometer time trial on the track as an application for a high sampling rate, where the first pedal stroke could be analyzed in detail.

But there's a lot more than all of this. Vector's catch-phrase is "direct your forces". The idea, they say, is to shift from a paradigm of power to a paradigm of forces. I'll discuss this further next post.

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