wheel decision rotating mass vs. aero

[aggie sig]

ok so I currently have steel wheels with this cap on them:


I have an opportunity to get these alloy wheels that are the EXACT SAME SIZE OEM manufacture and weigh 4 pounds less that have this spoke pattern:


it looks like it might have more drag on it. but all else being equal, tires, backspacing etc.
do I have the potential to get better mileage with the second wheel???

thank you!

[t vago]

Get the alloy rims, then slap on aero covers on them. Then, you'd have less unsprung rotating weight, and better aerodynamics.

[Frank Lee]

The fe differences will be so slight you likely won't be able to measure them.

[MetroMPG]

Frank - that's true of many mods. I wouldn't discourage people from doing them though, since taken together they eventually add up to something measurable.

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Agreed on t_vago's comment: have your cake and eat it too. Put smooth covers on the lighter wheels.

[PaleMelanesian]

The mass only makes a meaningful difference if you waste their momentum by braking. If you adjust your driving to eliminate using the brakes, it won't matter. This is the real world, though, and that's not always possible. I like the solution offered above - light weight and aerodynamic at the same time.

[t vago]

Actually, it's more accurate to state that, given some rotational acceleration, more massive wheels take more energy to change their rotational momentum than less massive wheels do.

In other words, you'll use more gas if you speed up with the heavier wheels than if you used the lighter wheels, if you sped up at the same acceleration in both cases. Also, your brakes would work harder to slow down the heavier wheels than the lighter wheels, if you braked at the same deceleration in both cases.

Conversely, if you used the same amount of throttle to accelerate, the car with heavier wheels would take longer to get to speed. If you used the same amount of braking force, the car with heaver wheels would take longer to slow down.

Classical physics in action.

[PaleMelanesian]

Correct - it takes more to accelerate the heavier wheels. Since we're talking about angular momentum, it takes much more.

However, if you don't use your brakes, you get all that momentum back in increased coasting distance. If you're not using the brakes, they can't be working harder.

In the real world where we sometimes do have to use brakes, it's better to build up and then waste a smaller amount of momentum, so lighter wheels are a good thing.

[PaleMelanesian]

Even in the real world, you can get very close to zero brake usage. On a good day, I can do my 10 mile commute with 5 brake applications. 4 stop signs and one when I park at work. On a bad day if I hit red lights that adds 5 more for a total of 10. If I'm looking ahead and anticipating correctly, I can almost guarantee I'll arrive at a green light.

[aggie sig]

I notice that my worst fuel economy by far is when accelerating from a stop, I drive 40+ miles a day to work and 37 of those are on the freeway. My average mileage plummets when I start the vehicle and get the vehicle moving and slowly creeps back up as I'm cruising with very little gas and brake application. I figure lighter wheels will decrease the moment of inertia therefore allowing me to get up to speed with less energy expended. I also figure that aerodynamic drag on the whole vehicle will slow the vehicle down significantly more than and difference in rotational momentum.

keep typing, I'm liking the opinions/ ideas

[euromodder]

Originally Posted by PaleMelanesian

On a good day, I can do my 10 mile commute with 5 brake applications. 4 stop signs and one when I park at work.

I've done the 1 brake application thingy on my 30 mile commute - though a large part of that is on the motorway, and it was done using the less efficient technique of engine braking (DFCO) to slow down.


On Topic :

Last september, I've gotten a set of aerodynamic wheels designed by Volvo for their more frugal DrivE models, but they're a lot heavier - a whopping 7.5 lbs apiece !
Their weight can be felt in the steering even though it's heavily assisted.
So if you're planning to remove the power steering, get the lighter wheels !

I'm now getting similar fuel consumption on lighter rims with winter tyres.
Granted, my driving habits have changed a bit, but it's also a lot colder.

It appears the extra weight cancels out the aerodynamic improvement.

I'd go light - though it's unlikely you'll regain the cost in reduced fuel consumption.