FE with Steel v. Alloy Wheels?

[rpg51]

I just purchased some snow tires (Nokian Hak R) mounted on a second set of factory alloy 15 inch wheels for my Prius. I will install them this weekend. When I removed the mounted tires from the back of my car last night I noticed that they are extremely light weight - suprisingly so. I had just finishing the job of installing my wife's winter tires on her Corolla which are the same size as the Prius, which are on steel wheels, and the weight difference between the alloy and the steel rims and tires is remarkable.

I'm curious - how much does the total weight of the wheel and tire (steel v. alloy) affect fuel economy? Is it a significant factor? I'm no engineer but I do remember reading sometime in my life that rolling weight is much more significant that static weight - might have been in my old mountain biking days when we were all obsessed with minimizing weight on tires and rims.

[rpg51]

Nothing? Did I put this in the wrong forum?

Seems like this might be an interesting question for all those folks considering steel wheels for winter tire use. The question is - should I get steel or should I get alloy? Is there a fuel savings with alloy? If so, now much?

[efusco]

I think you're in the right forum, a lot of stuff goes through and sometimes a good question like this gets missed by those with the knowledge to answer.

I'm probably not one of those people. If you search "unsprung weight" you might find some info on the subject of rim weight. There are a few people who have purchased rims lighter than the OEM ones to try to save a little gas (and for a nicer appearance), and IIRC the differences are pretty darn small. That said, going UP significantly in weight, esp. when you stick a snow tire on there will definately give you a measurable impact.

[rpg51]

I think there are two issues here that could be relevant to FE - "Unsprung" weight AND "Rotational" or Rotating weight. I found this little tidbit out on the web-

"The use of light-weight alloys in wheels reduces rotational mass. This means that less energy will be required to accelerate the wheel. Given that each pound of rotational mass lost provides an equivalent performance gain as a 10 pound reduction in vehicle weight, the benefits of light alloy wheels on vehicle performance cannot be overlooked.
For example:
A reduction in the weight of the rim/tire assembly of 5lbs x 4 (all around the car) is equivalent to a 200lb weight reduction in vehicle weight"

Not sure if this is true - or if the analysis is this simplistic. But, I'd love to know what the weight difference is between Corolla steel wheels and Prius alloy wheels and I'd love to find a way to calculate the fuel economy hit likely caused by switching to steel.

[bwilson4web]

Originally Posted by rpg51

. . . I'd love to know what the weight difference is between Corolla steel wheels and Prius alloy wheels and I'd love to find a way to calculate the fuel economy hit likely caused by switching to steel.

If you have a bathroom scale, you could pull a tire off of each car and weigh them on the scale. This will give a gross effect. The next part of the problem is more difficult.

You want to measure what is called the 'moment of inertia' (See Wiki for a complete description.) One of the better ways is to measure the oscillation of a rim in a rotary setup where a spring is used to let the rim oscillate back and forth. The frequency is proportional to the inertial mass.

So here is the deal. When the wheel is spun up, the force applied to turn the wheels becomes angular momentum, stored energy. When the wheel slows down, that energy is either captured by regenerative braking or released as heat by the brake pads and rim or disk. You want to minimize the heat loss.

There is also a linear energy storage and loss. Look for 'kinetic energy' in the usual places. This is usually easier to calculate than rotational energy but it is another part of the equation.

The math is not trivial but it is something college physics courses often cover. But it takes some measurements and math to get an accurate answer.

Now I'm fonder of 'back of the envelope' calculations. So look at the relative weights:

• ~3,000 lbs of car mass including rims, gas and passengers
• ~14.3 lbs each rim, ~53 lbs total rim mass
• 53/(3,000-53) = 1.8%
• assumption: worst case angular momentum is 2x linear momentum
• 3.6% of vehicle kinetic energy is tied up in rims

To me, this is a potentially measurable effect but rather slight. If you are already achieving near peak vehicle efficiency, this would be detectable. At the limits of performance, small changes add up.

Bob Wilson

[subarutoo]

I went from stock OEM Goodyears to BFG G-Force Sport 205-60-15 (not snow tires) on stock rims. The tire weight is 5lb more per tire. My mpgs dropped from 49-50 to 43-44, with more than 5000 miles on the new tires. They are wider and a stickier tread, but the 5 lbs certainly didn't help. The car feels sluggish at slow speeds, but has a much improved highway ride and corners much better. My driving is 99% freeway.

[spitinuri]

Wider tires = more rolling resistance since more rubber is in contact with the pavement. Tire pressure is also a component. If you could put bike tires on a Prius at a very high pressure you would probably get very good mileage. But,.... you would trade mileage for safety which is what many of us to a lesser degree are doing.

Bikes are a really good example of this. Ride a racing bike with very thin high pressure tires and then ride a mountain bike with thick knobbys. You get more speed with less work on the racing bike.

[rpg51]

Obviously there are many factors involving wheels and tires that affect FE. My question was intended to be focused on one of those factors, wheel weight. All other things being equal - how significant a factor is wheel weight in FE for the Prius?

[Stefx]

rolling resistance, tire width and tire pressure are probably affecting FE more than rim and tire weight.

I go from my spring-summer-fall alloy 195/55R16 to winter steelies 185/65R15, but this change occurs as temperatures are falling. I observed some FE reduction but it was much more likely tied to the temperatures falling than rim+tire weight.

Also in my case, the steelies are smaller and the tire is narrower, therefore even in constant temp I probably wouldn't be able to answer your question. You'd need someone with same size steel vs alloy wheels and same size tires. But then again, those people would probably swap tires as temps are getting colder.

[rpg51]

I think an engineer could calculate the difference in FE. I understand that rolling weight has a much greater impact on FE than dead weight.