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Stages Track Power Meter Issues

Old 05-23-14, 05:09 PM
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Stages Track Power Meter Issues

Is anyone else having trouble getting useful power and cadence data from a Stages Track Power Meter?

I have an old Paramount track bike with the Stages Dura Ace Power Meter, a Joule GPS computer (v19.087 firmware), a Bontrager Standard ANT + speed sensor, and a Magellan ANT + HR strap. Everything pairs and seems to play together nicely. The data are screen shots from Cyclops PowerAgent 7.7.5.

First pic is data from cadence drills on rollers. I'm on a tiny gear (43x22) working on leg speed. The exercise was 4 min warm up at 90 rpm, then a set of 6x of 30 sec at 120 rpm, 90 sec at 90 rpm, 30 sec at 140+ rpm, 90 sec at 90 rpm.



Here's a blowup of the 3rd 140 rpm interval at time 00:18



In this interval, I was probably doing ~160 rpm. I'm on rollers on a fixed gear bike trying to be smooth, but the power and cadence are bouncing wildly, while the speed (blue) is reasonably smooth. Dips in the power correspond with dips in the cadence.

Here's the next 120 rpm interval at 00:20:



The power looks reasonable, but the variability of the cadence is still a bit high compared to the variability of the speed. Other intervals show similar performance. Both cadence and power bounce wildly at higher cadence (>150ish).

So I took the bike to the track for a little training. Here's the data from a flying 200 (1.5 lap windup, sprinting down the back straight into turns 3-4) on a 47x14 gear:



More bouncy cadence data at higher rpm.

Here's data from a standing (actually slow roll) start at the velodrome on a 49x15 gear:



In this case the cadence starts bouncing around at a rather modest cadence, and the power with it since I'm assuming the calculated power is force*radius*cadence.

According to the Stages website, this power meter should be good from 0-220 rpm. I'm nowhere near the upper limit and my power is pretty modest as well.

I've tried re-zeroing the power meter (number doesn't move much) and replacing the battery (a problem I had with a powertap road hub).

Any ideas?

I contacted Stages 2 weeks ago and while they generated a 'support ticket', I haven't gotten any help.
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Old 05-23-14, 05:33 PM
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Might just be the chain bouncing around with the fix gear - especially on the rollers. Stages apparently uses an accelerometer for cadence; don't know how they get their speed data. It is not unheard of for different ways of measuring to show different variability, especially if you are measuring speed directly (via magnet or GPS) and cadence indirectly (via accelerometer). Since you are on a fixed gear drivetrain and the speed data looks cleaner than your cadence data, just scale your speed data by your gear ratio and wheel diameter to get cadence. These are directly proportional on a fixed gear. You don't need both numbers.
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Old 05-24-14, 12:11 AM
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I have noticed that the stages power meter does not seem to handle large amounts of power or cadence very well. For measured efforts it seems fine, but if I trying going fast, it seems to lose a bit. I also noticed that it seemed worse on the rollers during high RPM efforts. Rapid pace changes or high RPM work seem to throw it off more than my powertap, but I don't have enough meaningful data yet.

I am getting good data that it is useful for pursuit level efforts. I am hopeful to get a session at the track tomorrow to test out some more high RPM/power efforts.
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Old 05-24-14, 09:14 PM
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I'd recommend posting your question on the Wattage google group.
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Old 05-26-14, 03:55 PM
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Originally Posted by Brian Ratliff
Might just be the chain bouncing around with the fix gear - especially on the rollers. Stages apparently uses an accelerometer for cadence; don't know how they get their speed data. It is not unheard of for different ways of measuring to show different variability, especially if you are measuring speed directly (via magnet or GPS) and cadence indirectly (via accelerometer). Since you are on a fixed gear drivetrain and the speed data looks cleaner than your cadence data, just scale your speed data by your gear ratio and wheel diameter to get cadence. These are directly proportional on a fixed gear. You don't need both numbers.
The Stages power meter doesn't generate speed data. I've got a separate magnet speed sensor on the bike (Bontrager Standard ANT + speed sensor) for that. I could just scale the speed data as you suggest, except for two things: 1) I'd have to record the gearing for each effort since I've been known to change chainrings and cogs during a session, and more importantly 2) the cadence is used to calculate the power. The Stages power meter uses a series of strain gauges to calculate the force applied to the crank arm. Force times radius arm is torque, and torque times cadence is power. Bad cadence measurements makes the power measurement suspect.

I've got data from the track where consecutive cadence measurements (one second apart) are 142, 66, 128. My chain isn't that loose.
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