Can Alloy Wheels Really Improve Fuel Economy?

[Dr Lots-o'watts]

Will switching to alloy wheels (typically 17% lighter than steel) noticibly improve fuel economy? It seems like we should be able to calculate a rough estimate if we had necessary data but i don't. Has anyone been there?

 

[AlephZero]

What proportion of the mass of a typical car is the wheels?

How much does a steel wheel (without the tire) weigh - say 20 pounds maximum? 20 x 4 x 0.17 = not much, compared with the total mass of the car.

The main difference would be the performance of the suspension, because you are reducing the mass being bounced around by bumps in the road. But if you can go faster round corners, your fuel consumption is likely to increase rather than decrease.

 

[Lsos]

Wheels not only have to be accelerated linearly, but also angularly, which makes them harder to accelerate than regular car parts. Reducing rotating weight has a larger effect on fuel economy and performance than stationary weight. How much though...I never caluclated that. I'd be interested to know.

 

[russ_watters]

I doubt it is substantial but because the savings is all during acceleration, most of the savings is in "city" driving.

 

[jimmy2222]

But city driving is all stop and go, constantly accelerating and decelerating. I always thought that reducing the rotational mass would have a much greater effect than the cars overall mass. Due to the fact the wheels are directly propelled by the engine (well, not all depending on the drive).

 

[AlephZero]

Wheels not only have to be accelerated linearly, but also angularly, which makes them harder to accelerate than regular car parts. Reducing rotating weight has a larger effect on fuel economy and performance than stationary weight. How much though...I never caluclated that. I'd be interested to know.

Suppose the radius of gyration of the wheel (without the tire) is = k. and the radius of the tire is r.

At a road speed of v, the translational kinetic energy of wheel = $mv^2/2$

Angular velocity = v/r
Rotational kinetic energy of wheel = $m(v/r)^2 k^2/2$
= $(k/r)^2 mv^2/2$

k/r must always be less than 1. I would guess a typical value would be about 0.7 which makes the rotational KE about half the translational KE.

If you add the rotational KE, 1.5 times "nothing much compared with KE of the rest of the car" is still "nothing much".

 

[Ranger Mike]

depending upon the automobile..the increase in MPG is big...
1. anecdotal - friend of mine had a Chrysler 300 which had wheels and tires each weighed 50 lbs.,,cut the weight in half when we went to aluminum centerline wheels..the acceleration improved dramatically..opinion only
2. every pound of rotating weight you can remove, makes the car think you removed 10 pounds.
so in the above case, removing 25 pounds per wheel made the driver ' feel" like it lost 1000 pounds, seat of the pants, of course..
empirically, the formula means your using less gasoline to accelerate the lower moment of inertia wheels

 

[AlephZero]

2. every pound of rotating weight you can remove, makes the car think you removed 10 pounds.

I think that number is based on reducing high speed rotating masses like the flywheel, crankshaft, conrods, pistons, clutch plates, camshaft, etc.

Unless you are measuring how much faster you can spin the tires, the math doesn't stack up for the road wheels.

Lower unsprung mass = better suspension performance = more power on the road without wheelspin, but that isn't going to improve your gas consumption.

 

[Ranger Mike]

Anytime you reduce weight of a vehicle , you get better MPG..it is a fact, even more savings is realized by reduction in ROTATION MASS...

High speed rotation parts is relative and the shear mass of 50 pound tires and wheels are a huge factor.

The calculations for the wheels are the same as regarding brake rotors, flywheels, and on a micro scale , crankshafts, cam shafts and axles..(gun drilled axles..yes, they do.)

Pistons and connecting rods are reciprocating weight and the rule does not apply. ( ok- 50% of a con rod would apply for big end only, if you want to get specific)

 

[AlephZero]

The "high speed effect" is proportional to RPM squared.

If your engine is doing 6000 RPM and your road wheels 600, the rotationg engine parts are 100 times more significant than the wheels.

Sure, people will go for ever last little saving. The old Mississippi steamboat racing captains used to make their crews shave their heads bald to reduce wind resistance, I believe. I would expect the psychological effect of gun drilled axles on the race team is much more significant than anything Newton's laws would predict.

 

[Ranger Mike]

AlephZero...good one! ...i got to remember that one!

i was in germany and saw th oil pump impeller for a Formulal 1 engine..it was carbon fiber..just to reduce rotation weight..now that is extreme!
back to the math, typical race car in high gear will have tires rotating at 2016 ( 2.91 rear gear) to 3120 rpm ( 5.17 gear gear)...the real reason for going with light weight wheels is to minimize unsprung weight which can not be controlled when weight transfer occurs..the reduced moment of inertia is a good way to rationalize the purchase of a $ 400 magnesium wheel t hat will get cracked and dinged by the driver..insane but true..
screw it ,,im going to shave my head now!

 

[AlephZero]

I deliberately chose a lower wheel RPM because I was thinking about the maximum acceleration coming out of a sharp corner etc. This is all about how fast the engine can spin up. Rotating mass doesn't have any effect on max speed. Neither does non-rotatiing mass, unless you change the shape of the car to reduce the air resistance.

Actually, air resistance from different shapes of wheel is another issue here - not my specialist subject, but I've been told that computational fluid dynamics models of cars include rotating wheels these days, to calculate the drag coefficient accuractely.

 

[Ranger Mike]

Excellent point AlephZero
i was not aware of the newer programs capabilities...tires account for 40 percent of aero drag on open wheel cars...huge
thanks

 

[Dr Lots-o'watts]

Thanks AlephZero and everyone.

 

[Mech_Engineer]

depending upon the automobile..the increase in MPG is big...
1. anecdotal - friend of mine had a Chrysler 300 which had wheels and tires each weighed 50 lbs.,,cut the weight in half when we went to aluminum centerline wheels..the acceleration improved dramatically..opinion only

Considering the Chrysler 300 weighs about 4100-4300 lbs, I seriously doubt it made any "dramatic" difference in the car's acceleration or fuel efficiency. A person's "butt-o-meter" is notoriously inaacurate in guessing a car's performance characteristics... My guess is there was no significant difference in 0-60, 1/4-mile, or MPG numbers.

2. every pound of rotating weight you can remove, makes the car think you removed 10 pounds.
so in the above case, removing 25 pounds per wheel made the driver ' feel" like it lost 1000 pounds, seat of the pants, of course..

This might be a general rule of thumb for an engine component rotating at thousands of RPM, but definitely not true for wheels and tires. Losing 1000lbs on a car is VERY significant, that would be a 25% reduction in total weight for that Chrysler...

empirically, the formula means your using less gasoline to accelerate the lower moment of inertia wheels

I think you mean analytically or theroretically rather than empirically. Empirically would mean you tested it and measured a difference.

Actually, air resistance from different shapes of wheel is another issue here - not my specialist subject, but I've been told that computational fluid dynamics models of cars include rotating wheels these days, to calculate the drag coefficient accuractely.

It's true, race teams these days in F1 and NASCAR actually use "rolling road" wind tunnels, because both the wheels rolling at high speed and the road moving at the same speed as the wind makes a difference.

 

[mender]

Here's a test:
http://www.carcraft.com/projectbuild/116_9905_light_vs_heavy_wheels_comparison/index.html

I assumed the numbers were accurate enough to be representative and used a 1/4 mile calculator to assess the changes in the quarter mile times and speeds. After baselining the calculator, I subtracted the weight reduction of the wheels (82 lbs) from the entered weight of the car (3882 lbs for simplicity) and the calculator predicted a drop from 12.14 sec @ 111.8 mph to 12.06 @ 112.2 mph.

Their result was 12.03 @ 113.11 mph; to get that same ET with the calculator required an extra 50 lb reduction. So the 82 lb reduction in wheel weight gave the same result as a 132 lb reduction in total vehicle weight, with 50 lbs worth due to reduction in rotating mass.

This is pretty crude and certainly shouldn't be taken as gospel, but it indicates that the effect is there, just not the magnitude that is usually stated.

 

[mender]

I would guess a typical value would be about 0.7 which makes the rotational KE about half the translational KE.

If you add the rotational KE, 1.5 times "nothing much compared with KE of the rest of the car" is still "nothing much".

The empirical numbers suggest 1.6; close enough!

 

[Ranger Mike]

Thanks , Mender..that puts some numbers to what we racers have known for years..reducing ANY rotating weight will help.. 0.08 second in 1/4 mile time is not a dramatic difference ( unless your bracket racing and have lost a lot of races by 0.05) but it is a going the right way.

 

[mender]

That's a car length in the 1/4 mile, which is a big deal for most racers!

It could be a difference of a couple of feet by the end of the straight even on a short oval, which in a lot of cases is enough!

 

[Mech_Engineer]

If you're racing there's no doubt that every pound counts. But let's be honest- the O.P. in this thread asked if lighter wheels will help gas mileage. The answer is probably not, unless you're measuring mileage at the drag strip.

 

[Ranger Mike]

I don't know if this is analytically or theroretically rather than empirically,,but it ought to put the nail in this coffin -

To accelerate the car, you would have to start the wheels rolling, or produce a change in the state of rotation of the wheels. Resistance of a solid body to a change in rotation is the moment of inertia. For a solid disk (such as a tire/wheel), this moment is given as I = mr**2, where m is the mass and r is the radius and I'm taking a huge liberty with the distribution of mass. Angular momentum is the product of the moment of inertia and the angular velocity (L = Iw). I'm assuming that angular momentum is what we mean by rotational weight. A change in mass produces a change in I that is directly proportional. A change in the radius, however, produces a change proportional to the square of the radius. Smaller tires with the same mass would reduce I much more than lighter tires with the same radius. At first glance, it looks like reducing the radius of the tire/wheel is the best bet.

However, a smaller tire means it has to rotate faster to produce the same translational velocity as a larger tire (circumference being a function of radius and all that). The faster rotation means that the angular momentum increases. And not just the tire either -- there are axles and crankshafts and other things connected to that wheel that are all rotating with their own moments of inertia and angular momentums. Reducing the radius of the wheel produces an decrease in its rotational weight, but it will increase the rotational weight of other components in the car. Hence my comment earlier on cutting down any rotating weight.
http://www.tirerack.com/wheels/tech/techpage.jsp?techid=108

Finally, we monitored our test car's fuel economy using their onboard trip computers during our road rides. Covering a total distance of just over 350 miles, the three vehicle's average speeds were virtually identical (all within 3/10 of 1 mph). During that time the BMW 3 Series equipped with its Original Equipment Grand Touring Michelin Energy MXV4 Plus low rolling resistance tires averaged 22.8 mpg. The vehicle equipped with the Plus Size Pirelli P Zero tires on lightweight wheels averaged 21.5 mpg, while the Plus Size Pirelli P Zero tires on heavyweight wheels dropped to a 21.1 mpg average.

Vehicle Combination Average Speed Average mpg
BMW 3 Series equipped with
Original Equipment (16") Stock 38.7 lbs. 22.8 mpg
BMW 3 Series equipped with
Plus One (17") Lightweight 39.0 lbs. 21.5 mpg
BMW 3 Series equipped with
Plus One (17") Heavyweight 38.8 lbs. 21.1 mpg

 

[mender]

The results are a little skewed because of the change in tire weight.

Despite having shorter sidewalls, the Pirellis weighed about one pound more than the Michelins. I assume that the extra mass is concentrated in the tread and that is a disadvantage. That would negate some of the mass savings of the wheel and have more effect per pound because of the location of that mass. The larger 17" diameter wheel also relocated the centre of mass of the wheel farther out compared to the 16" wheel.

That being said, when comparing only the otherwise identical light vs heavy wheels, the fuel economy improved by 2%; not bad, considering.

In stop and go, the difference should be at its highest; they didn't specify the driving conditions.

 

[Dr Lots-o'watts]

Well, 2% is significant. Over a year, that can some up to $40 in gas, so $400 in 10 years, which would pay for low-end mags. Would anyone bother? I doubt, but yet, some people do get convinced of filling their tires with nitrogen for similar reasons.

Surely losing weight that is constantly rotating and accelerating is more significant than simply losing linearly moving mass. It's funny the ecofriendly car-makers aren't marketing this more.