iSport Power Oscillates on the Flats

[healdsbiker]

I have noticed consistently unreliable readings on my new iSport. After a bit of trial and error to get a correct Cal Ride, good tilt and Wind Cal, when I rode on flat or low-slope roads over the next couple of hours, the power would slowly go from high (sometimes 400W) down to zero (clipped, perhaps, to 0.00) and stay there before floating back up. This is while maintaining a constant speed and exertion level, with the wind 0-5MPH and not rapidly gusting; I also monitored the tilt slope during the ride and it seemed accurate.

While on a steady climb, the powers seem consistent and valid. In fact, on a recent 9% grade, I road along side a cyclist with a PowerTap hub, and he and I both were reading about 240W.

Some information about the bike and Cal Ride:
Trek Madone with Kevlar tires, 700x23cm
Riding on the hoods
Cal Ride: 4.6 miles (2.3 miles out, 2.3 miles back, no traffic, mostly flat, practically no wind)
Cst Aero: 0.372
Cst Fric: 10.123 (this seems high - could it be due to the Kevlar tires?)
Body weight: 156 lb
Bike weight: 182 lb

Note: Fast Start results, before Cal Ride:
Fast Start Estimates:
CSt Aero: 0.342
CSt Fric: 9.456

What can I do to get steadier readings on the flats?

[Velocomp]

Your aero and fric readings are completely normal.

When you climb a hill the effort required is pretty consistent--a result of gravity and relatively constant slope.

When riding on the flats the amount of effort required depends significantly on opposing wind, minor variations in hill slope, and drafting.

Here in Florida, where it is "flat" everywhere, I leave my house and cruise along at 100W for the first 0.5 mile because there is an extremely minor change in elevation--I drop about 5 feet over 1/2 mile. Then, the road truly levels out and my watts shoot up to 200W+

And if there is any tail wind, watts are even lower...

What I would do is this:

1) click your down arrow 1 time and watch hill slope as you ride. See if reported hill slope matches what your eye sees. Your iSport will tell you when there are extremely minor variations in hill slope; it is amazingly accurate.

2) Click the down arrow again and watch wind speed. If there is little wind then your reported wind speed should be very close to your bike speed.

If either of these two things seems "off" then there may be a problem.

Let us know...

[healdsbiker]

Thank you for the quick reply. I went out for a ride today that was mostly climbing (3100 ft. in 12 miles), but rode some flats/rollers to get there and back. I noticed that the oscillations were still occurring, though not consistently. During one period, though, I was going along and noticed that the Power would drop to zero for as long as 10 seconds, before climbing slowly back to 150-200W (which were believable numbers). Some more info:

- I pushed the down-arrow to monitor slope and power continually, and could see speed on my Garmin 705. I could then toggle to wind and back.
- I was riding on the hoods, steady pace with cadence of 85, pedaling was smooth, not hard/jerky.
- iSport mount is very secure and solid - the iSport does not move. I'm using a handlebar mount, and the wind port is clear of the brake cable.
- Traffic was light and I was riding solo, so no drafting was occurring.

During this stretch of road, grade moved slowly between 0% to -1%. Wind speed matched the road speed, at 15 MPH. Power dropped to 0W for about 10 seconds, before slowly recovering. It did this a couple more times as well, with no matching change in cadence, grade %, or wind speed. When it did settle down to a reasonable number, I tapped the iSport a bit, but didn't affect the Power reading.

My next ride is next week, so I can do more testing at that point if it makes sense. If you have any more ideas based on today's little test, let me know.

Thanks,
Steve

[Velocomp]

On your next ride look at wind speed too.

[healdsbiker]

I did monitor the relative wind speed (bike speed + wind speed), and it was showing numbers close to the bike speed (as seen on my Garmin), which was consistent with the low to no wind conditions. Would it be helpful to reconfigure the iSport to monitor the absolute wind speed?

Thanks,
Steve

[Velocomp]

From everything you are reporting it does sound as if your iSport is working properly. Wind speed is normal and slope is normal.

If you're on the flats and slope is going to -1%, that would result in a drop of watts. HOWEVER: it is nearly impossible for the eye to notice such minor changes in slope.

You might want to check to be sure your fork tube isn't loose in its bearings. If it is wiggling around at all that could affect slope readings.