difference in drag during trips ~ aero or brakes?

[j12piprius]

Originally Posted by 2000mc

Caliper, flexible brake hose.

Sticky calipers? Should the hose be rigid or flexible, and what difference does that make?

Thanks much for all of the comments. Based on these, and a low speed high mpg short run yesterday, the drag is most likely from my changes to the bumper cover and vent. This gives me incentive to cover the underside of the car, to add an adjustable vent, and to see what happens.

[3dplane]

Originally Posted by johnlvs2run

I'm not sure about the differences in fuel, as I think they're not shown on the pumps, but that wouldn't explain the different speeds for dfco coasting down the hill, which felt like a major difference. The previous trip the car was going full blast, and the last time hardly moving along.

Did you mean EOC down the hill? Because if you dfco down the hill that would introduce a whole set of driveline variables like transmission or engine criterias that can be totally diffrent for the runs.

About the brakes: Calipers can stick at the slides and the piston(s) can stick too.

Brake hose is not really a flexibility question but rather it will swell on the inside with age and what happens is when you apply the brakes the high hydraulic pressure will open up the way for the fluid to go toward the caliper but there is no real force for the fluid to return when you release the brakes so the swollen shut brake hose will keep the brakes applied.

Then in a really bad case it heats up more,fluid expands thus applying more brakes until lockup can occur.

[j12piprius]

The steepest downhill is 7 degrees, with a couple of 90 degree curves, all done dfco in 3rd gear. Now I'm quite sure my changes to the bumper and vent caused an increase in drag, and the reduction in speed / mpg. I previously had the bumper cover at 7" from the ground, slanted back, and now it is 8.75" from the ground, almost vertical.

Most of the belly of my car is 8" from the ground, 20% is 7" from the ground, and less than 3% is 6" from the ground. I should have left the bumper cover at 7", and will probably redo it to that height, but am going to work on an undertray first.

Thanks for your comments. I'm planning to change the hoses at the next opportunity.

[RedDevil]

Could it not just be caused by the higher air dam and bigger gap? They should at least have some effect.

The front wheel wells get some warmth from the engine btw.
My front brakes are usually warmer because they take most of the brake load.

Also, a 'slightly favourable' wind needs to be straight aft to really give you a benefit.
Side wind ruins the aero properties of your car much more than even straight ahead.
I get my best economy on windless days and not (even) when the wind is blowing aft.

On my morning commute the usual wind direction is 45 degrees left.
In the last few miles I turn into the wind, and fuel economy rises considerably.
Going back home the first part is pure bliss, then economy falls sharply when the wind strikes at 45 degrees aft.