Quote:
Originally Posted by jimepting
Jim,
I've been studying your data and I have one suggestion as to a possible error source.
We all know how very sensitive the Insight fuel economy measurement is to tire pressure. Looking over the part of the year you were testing, I'm wondering if you may have had significant tire pressure variations. Tire temperature changes tire pressure. On sunny warmer days, the road would have been warmer, and on cold overcast days the road surface would have been colder. Cold stiff tires have higher rolling resistance.
I have a slight hill in front of my house which runs for .4 miles. When I give my Insight a little push, I gain speed to 8 MPH at the first intersection in the summer. In the winter, I can barely make 5 MPH at that point. You say that the car is thoroughly warmed up, but perhaps the tire temperatures are still varying significantly on different days.
From my racing I also know this. We use a bit more pressure on cold days to put the traction into the sweet spot.
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Hi Jim,
You could very well be correct about this.
I simply do not have enough data or experience with coast down testing to say that tire pressure is an issue or not, but I would think that this is indeed a major factor in Crr.
There are several bicycle sites that I have investigated, and rolling resistance seems primarily a "road/tire" function. Tire carcass deflection is a large factor.
I typically keep tire pressure from 55 to 60 psi, but still ambient temperature, road conditions, dust and so on *could* be a big factor and I may not even be aware of the many factors.
All this testing and data collecting is just part of our aero "research and development" process here at EcoModder.
Jim.