AeroPod, PowerPod and Newton

I did my long 4 mile calibration a while back and noticed that on current rides my numbers would go beyond the +/- 20 suggested parameter. So I do a quick coast down and after the rides I've been seeing CRRs in the realm of -0.0036 to -0.0069. I feel like my power numbers for the ride are accurate for about 10 mins and then seem quite low for the remainder of the ride.

My long calibration had 0.0012 CRR

I'm using carbon tubulars. Any thoughts as to how to interpret the above?

Crr cannot be negative. If it were, you might coast forever if you could overcome your aero drag.

The broad range of Crr is about .003 to .007, although most people fall in the .005 to .0065 range.

I have found that tilt calibration errors will mess with Crr values. So double check your tilt after calibration by doing a 180 with your bike - tires in the same place just swap front/rear and make sure the fork is straight( front/back wheels aligned) and the frame is vertical.

5-10 coast downs - gives you more of an opportunity to see if one or two are not giving good/similar results than the others and can be dropped. Doing just one CD gets you little to work with.

Other items mentioned often on this forum include doing coast downs on a declining slope (you don't slow down very well) and windy days - including just cross-winds - they don't have to be tail winds.

Tom

lorduintah wrote: Other items mentioned often on this forum include doing coast downs on a declining slope (you don't slow down very well) and windy days - including just cross-winds - they don't have to be tail winds.
Tom

I've just redone my calibrations for one of my bikes and I notice that the Crr is .0044 which is much lower than it used to be. When I did the coast downs, I used a different road to normal (with less traffic) but the road undulated a little. If it was a slight downhill, would this produce a false reading or does the iBIke take the slope into account during the coast down?

For fun I messed around with overriding the CRR in the software to see what effect it would have putting it up to. The answer was surprising little ~2 Watts.

The iBike takes slope into account during the coastdowns.

The best road I have found for coastdowns is a bowl shaped road. You can use the downhill to get up to speed, start coasting on the flat, and use the uphill to slow you down. I have found great results with coastdowns that last about 15-25 seconds (so before you do your first coastdown, test one out and time how long it takes).

I know Travis has also said that if you use a bowl shaped road, you should do them in both directions, as long as the wind speed isn't over 8mph. If the wind is 8mph though, it's probably not a great day to do a coastdown. In fact if it were possible to get everybody to do an equal number of coastdowns in both directions, you wouldn't even need to do the calibration ride. . .but it's easier just having people do the calibration ride afterwards.

The road surface, the wheels and the tires plus tire pressure will influence the Crr (as well as the overall shape of where you do coast downs) - add to that bumps in the road and there will be some variance in Friction and Crr.

I personally do not think 0.0044 is out of line with a good set of wheels and tires. Others may say it is a little on the low side for a chip seal road or some other riding surface that is relatively rough. To be sure - everyone is correct in saying that whatever value you get it must be positive. (no black holes in your neighborhood? I'd ask about dark matter, but we don't have anyway of really detecting it - yet.)

Tom

coachboyd wrote:The best road I have found for coastdowns is a bowl shaped road. You can use the downhill to get up to speed, start coasting on the flat, and use the uphill to slow you down. I have found great results with coastdowns that last about 15-25 seconds (so before you do your first coastdown, test one out and time how long it takes).

If I understand how this is done (curve fit using the equations of motion) I think a flat road would be better than a bowl shape. The Crr begins to dominate only at low speeds, and if this also includes an increase in slope, then there won't be much time to extract Crr from the overall drag. Plus the unit might experience a time lag in slope determination that could throw off the Crr quite a lot. 0.1% in slope error is ~20% error in Crr.

Seems like flat, smooth, and no wind would be best... ideal spots for doing CDs are sometimes hard to find though.