Bizarre Powertap issues

Laflore said:

Since last weekend, I've been experiencing some very strange behavior from my Powertap Pro. The CPU is a couple of years old and the hub is last year's model.

There are 2 issues that are probably related:

1) When I'm heading in an easterly direction (roughly NE to ESE) the signal (power, speed) zeroes out, both in the display and the saved data. When not heading in this direction, the unit seems to function normally. This is completely deterministic as opposed to intermittent/random, and I was able to demonstrate the behavior in my LBS's parking lot. Strange!

2) After a short period of coasting the signal drops altogether, and then comes back immediately when I resume pedalling. Not direction dependent, and I see "--" in the display instead of zeroes as in 1)

My LBS mechanic, who usually services my PT, has also observed this weirdness and is at a loss to explain it. The next step for us is to send the unit to Cycleops. I'm wondering if somehow the signal strength has been weakened due to how the cable is routed, pinching, etc. But I haven't messed with the cables in over a year and this just started happening.

Has anyone experienced anything like this? Any guesses?

Edit: the first thing we tried was replacing all batteries - no luck.

How about your wiring harness is shot?

I think anyone who owns a SRM or PowerTap should have an extra wiring harness if not a another bike completely setup and ready to go with a swap of cranks/cpu or wheel/cpu respectively. This makes it fairly simple to isolate most wiring harness issues.

The other suggestion is to find a friend with a PowerTap and swap a single item for testing, put your CPU and wheel on their setup, and see what happens. Then try their CPU with your wheel or your CPU with their wheel.

I have had my SL for16 months now all fine. Last Saturday I go out and I start getting short drop outs --- the next day still drop outs but not as many lasting 1-4 sec. Next day only one short drop out. Last 2 days been working great. All the while i'm looking for the problem in the wires by moving and inspecting them. I was ready to ship it for repair and buy another, but its working great again. I recall some months back a few days of short drops and then all good. Sorry no help to you, just thought id share my bizarre experience.

By the way. Saris rebuild is $175.00 a bargain I think.

We'll take a look at the harness again. Everything looked fine before, though. Testing the CPU and hub on another PT-equipped bike is a good idea to try, thanks.

Here are some graphs of a ride up towards the local ski hill a couple of days ago. The first half hour of the ride, heading east to the climb, the problem is very noticeable. Although the data stops many times during the ride, the only time I actually stopped riding was to put on arm warmers at 1:13 into the ride, before the descent.

Part 1
Part 2
Part 3

LoL my bike is basically like a compass now. Last night I was holding it in the air and spinning the back wheel. Speed was displayed normally until I pointed the bike east, which zero'd the display. Then back to normal again when I pointed the bike in a different direction. Odd to say the least.

Laflore said:

2) After a short period of coasting the signal drops altogether, and then comes back immediately when I resume pedalling. Not direction dependent, and I see "--" in the display instead of zeroes as in 1)

We used to have several threads about PTs dropping the signal while coasting, there was an issue with the vibration frequency of the pawls in the freehub interfering with the pickup while coasting - sounds absurd, but even Saris agreed. The fix is to put some extra rubber under the rear pickup, and not pull the zipties too tight (i.e. you are trying to prevent the pickup from picking up every frame vibration.) This was one of the issues Saris addressed with the newer "shark fin" pickup. Your "compass effect" is much more interesting, almost as if you are living within a directional magnetic field!