Hubcaps, do they have a role in aerodynamics

The 17" wheel/tyre combo gets poorer mileage due to frictional loss. Bigger tire contact patch equals higher loss (but also better braking and cornering - maybe). There is also some extra aero loss because they are slightly wider and therefore push a little more air.

Pearl S has the 15", and the wheel covers look real nice hanging on the garage wall.

 

Additional Frictional loss due to larger contact area will be small. What will really affect the mpgs is the in reased moment of inertia of the larger wheel. The inertia of the wheel increases with weight and diameter.

 

Yes, easiest way to understand this is to twirl a small weight at the end of a length of string. It is easier to speed up and slow down the weight twirling at the end of the string if the length of string is made shorter. Additionally, larger diameter wheels/rims will have more of the weight of the wheel/tire at the outer edge as if you were twirling a heavier weight at the end of the string.

 

Nonsense! It's not 100% false, but it's "making a mountain of a molehill." The higher moment of inertia does slightly increase energy required to accelerate, but has zero effect on power required to maintain speed, whether on the level or uphill. In contrast, the higher rolling resistance and wind resistance of wider tires sucks additional power all the time the car is moving, and their additional mass has to be lifted over every hill.

 

Not nonsense at all. Acceleration is where vehicles loose most of their efficiency. And acceleration requires the most energy.The moment of inertia of the wheels significantly impacts the efficiency during acceleration. Once a vehicle is maintaining a certain velocity then it will be more efficient, since the forces acting on it are drag and friction. Friction is calculated as coefficient of friction*Normal force and the normal force will be increasing by a small amount (maybe 1 to 2 Lb_f per wheel) and the coefficient of friction will not change unless a different tire is chosen. In almost all cases surface area has no affect on friction.

 

I'll grant you that "acceleration requires most of the energy" in certain very adverse circumstances, such as rushing through a series of closely spaced stop signs or red lights. I'll even grant you that the moment of inertia of the wheel-tire combination significantly impacts the energy required to accelerate the wheels, to maybe roughly 150% the energy required to accelerate an equal amount of non-rotating mass. However, the wheels and tires are only a small part of the mass of the car, roughly 4-6%, and most of us, most of the time, manage to avoid that worst-case accelerate-brake-accelerate-brake routine.

You've confused friction with the rolling resistance of tires. The car is rolling, not sliding. Rolling resistance varies significantly between different tire sizes and models, and consumes energy continuously as long as the car is moving.

You're correct that "in almost all cases surface area has no affect [sic] on friction," but projected frontal area facing the wind definitely affects aerodynamic drag (although we're talking about a relatively small percentage difference in overall frontal area of a car).

 

This.

 

I had thought that the contact patch was wider but about the same area and that more rolling energy was probably lost in flexing the shorter sidewall on the 17".

 

Take nothing as a given when it comes to tire engineering. I've gone from a 215/45-17 HP all-season, to a 225/45-17 UHP summer with a higher load rating, and everything has improved. Handling was a given, but ride quality and fuel economy certainly would not be considered such. Tires seem as much a product of alchemy as they are science.

 

I also returned to my car to find a hubcap stolen. This is the second time since I got the car. Debating whether to replace or remove. To answer this question, it would be great if someone could conduct a real scientific test of mileage with and without hubcaps. Find a loop road with little traffic where you can set the cruise control to a set speed. Run the loop (at least 10 miles, so the battery effect will be evened out) with an without the hubcaps and record the mileage each time. THAT would be some real data- same weather same road same speed same wind. Etc. I live in Philly, so I'd have to do the test at 3am to not have traffic. If I get ambitious and manage it, I'll report the results.

 

I doubt there would be any measurable difference, and what difference there was could well be lost in the noise of changes in wind direction, how much the engine is warmed up, which one you did first vs second. Those plastic wheel covers are infentesimal in effect.

Yoink the rest off so it all matches?

I see a lot of third gen cabs around here sporting that look; they all just pull them off.

Those dorky wheel covers drove us to the 17" model, btw.

 

I represent that remark :



At least they didn’t drive you to drink.

I’ve been called worse too.