SRM power meters use spider-mounted strain gages that are attached with copper wires to a circuit board below
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German power meter company builds instrumentation by hand
This article originally published on BikeRadar
Back in the mid-80s, a young Uli Schoberer created a cycling power meter so he could better measure his training efforts as an amateur rider. Fast forward to 2013, and SRM is an international company boasting a dominant presence among professional cyclists. SRM established an American office in Colorado Springs, Colorado, which first began producing mountain bike power meters, but now also assembles about half of the company's product, according to sales and marketing director Mike Hall. Hall recently gave BikeRadar a tour of the Colorado Springs facility.
Below is an illustrated overview of the assembly process. For more details, check out the photo gallery at right.
The heart of the SRM system is a system of strain gages that is mounted on a CNC-machined aluminum spider. The spiders are made both in Jülich, Germany and in Colorado Springs. In Colorado, SRM moves through about 2,500 units a year.
In Germany and in Colorado, pairs of strain gages are hand laid on the spider, then compressed by custom gigs at a set pressure to be cured inside an oven. Most units get 16 gages, but a few ( FSA Gossamer 130 BCD, FSA K-Force 130 BCD, Rotor 3D 130 BCD, and 3D+ 130 BCD) get eight. "Basically on those models, the chassis provides enough stiffness to get the same accurate measurement as the lighter weight chassis with 110 BCDs," Hall said.
Next, German-made circuit boards are mounted onto the spider with silicone, which allows the boards to 'float' on the spider and not add or reduce torque on the system. All this work is done by hand, again either in Germany or Colorado, to ensure precision, Hall said. While outsourcing to Asia could reduce labor costs and perhaps the end unit cost — which start at $2,045/€1,952 for road and $1,799/€1,892 for mountain — SRM prefers to keep production in-house.
Once affixed to the spider, the circuit boards are connected to the strain gages via copper wire, the excess of which is carefully trimmed off.
Once inspected, the spiders are then ready to be mounted onto cranks and have chain rings attached.
SRM calibrates its meters on both the large and the small ring with two weights that are themselves carefully measured at a nearby university. The cranks are checked radially, laterally, and for trueness.
SRM meters have internal batteries, which last for between 1,600 and a claimed 3,000 hours. Then they have to be sent back to be recharged, at which time they are overhauled and inspected.
The PowerControl head units are similarly built to last, with refurbishing of old units a standard practice.
Mountain bike power meters are continuing to evolve, and SRM's latest generation features a USB recharge port.
The vast majority of power meters are still road units, and the Shimano Dura-Ace 9000 11-speed version is among the latest from SRM. Shimano's new four-arm spider design gave SRM engineers more room to work with, and they decided to capitalize with a larger battery for longer (a claimed 3,000 hours) life before the unit has to be sent back for an overhaul. Shimano makes a custom Dura-Ace-level crank for this product, but declined to add the Dura-Ace moniker to a product that isn't 100-percent Shimano.
SRM loaned BikeRadar a test power meter for a review, which you can look for soon along with a general comparison of many competitors including Stages, PowerTap, Pioneer, and Garmin Vector. In the meantime, check out the photo gallery at above right for more detail on SRM's Colorado Springs facility.
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