Hi everyone,
This is my first post ever. I'll be purchasing an xA next week and this site has been very helpful.

I try to be very fuel conscious and I was wondering how wheel weight affects overall efficiency. I know extra weight in general will reduce mpg somewhat. But I was wondering if the wheel weight has even more of an effect or does the wheel weight simply contribute to the overall weight? --Does an extra 20 pounds in the car have the same effect on efficiency as an extra 20 pounds spaced out over the wheels????--

Being curious about this I called some Toyota guy to find out the weight of the 15" alloy wheels offered as an option for the xA. They are around 22lbs. I then asked him the weight of the steel wheels that come standard with the xA, he didn't know for sure but claimed they were 10-15lbs. Will this big difference in weight have an impact on efficiency???? Any help would be appreciated.

 

You should look into getting aftermarket 15" wheels. They are cheaper and lighter than the 15" Scion option alloys. I think Rota Slipstreams weigh like 11.9 pounds.

I don't know the stock steelie weight, but I've heard the total with tire is 37 lbs each (steel wheel w/ tire). You can definitely reduce that... I just got some new 17" rims, and the total weight with the tire was 35lbs each. So some lightweight 15s would probably be even lighter.

If the Scion 15" option alloy wheel weighs 22 lbs.(with no tire) , that's pretty heavy for a 15" wheel. You should shop around, or make sure you got the right info.

Good luck

-THE DON

 

Yes, 22 pounds for a 15-inch wheel is heavy, and it will have a negative effect on fuel economy. You should look into getting some lighter aftermarket wheels, or stay with the factory steel wheels.

For reference, my 17-inch Motegi Trak Lites weigh 14.3 pounds each, according to the manufacturer. There's also some good information on the Scion xA/xB Wheel and Tire forum about wheel weights.

 

Nice! Those are some light 17s


Anyway, 22 lbs for a 15" wheel just doesn't sound right. Can anyone with factory alloys confirm that?

If they do weigh that much, avoid them like the plague! My old 18" chromes didn't even weigh that much

-THE DON

 

from the "Fitment" sticky at the top of the xA/xB wheel and tire:

"...Stock Wheels/Tires
15"x5.5" +38 offset / 195/60-15 xB 185/60/15 xA
37 lbs per wheel & tire
Alloy wheel: 16.7 lbs..."

I've read on here that the steel rim alone weighs ~17.5 lbs.

 

yah, i remember seeing stock steelies @ 17.5 lbs too... which is fairly light for a steel wheel... i went w/aftermarket wheels (12.6 lbs) on the oem tires and saw my mpg go up from the high 31's to the low 33's... not much, but may have more of a positive effect on the xA's already better gas milage...

as for weight distribution... i'd say 20 extra pounds right on the wheels would be worse than 20 pounds in the actual car... i'm no physics expert, but think of walking 5 miles with 20 lbs of weight on your feet vs. 20 lbs in a backpack...

 

hey thanks for all the info. what you guys are sayin makes perfect sense. I think i'll get the factory steel wheels, and buy some light aftermarket 15s.

Thanks!

 

20 lbs added to the wheels is much worse than 20lbs in the car.

While everything IN the car has translational motion (meaning that it is shifting forward), weight on the wheels have translational and rotational motion. Anything moving has momentum, and momentum is your worst enemy. Wheels have two types of momentum, so it's twice your enemy.

Pick up a CD, stick your finger in the hole at the center and spin it. This demonstrates rotational motion. Now if you move your hand while the CD is spinning, it's rotational and translational motion at the same time. Holding the CD in your hand and shifting your hand around is translational motion only.

 

^^^ Good illustrations and analogies above.

Here's another way to think about it. Imagine carrying a 30-lb. child on your back or in your arms. That's easy. Now imagine the same 30-lb. kid hanging onto your legs. Not as easy.

 

now those car cling on's haha, get it

 

I haven't really checked my mileage since I got lighter wheels (17.5 lb steelies to 12.5 lb alloys), but I definately saw an increase in performance.

 

First, I want to say that I'm impressed by the OP's grasp of the proper use of 'effect' and 'affect'. It's very rare, as seen later in this thread.

That out of the way, it is very well known in the motorcycle community that there is a huge difference between adding weight to the wheels (chroming them is a huge nono) & having a passenger on the back. Google 'unsprung weight'...it has a huge effect on overall handling.

 

Futhermore, weight isn't the only factor. . .

Rotation Inertia = (mass) x (radius)^2.

A change in radius (the wheel size) has an exponentially greater effect than a proportional change in weight. 17's, although sometimes lighter, do not always outperform steelies. However, you do gain additional traction (via wider tires). Keep in mind that there is a huge MPG and power penalty.

Ideally, if you are looking for performance, you get a the smallest diameter wheel that will fit over your brakes. Second, you want the tires to be wide enough to keep traction, but not overkill. Third, you want to choose tires with the stiffest sidewalls possible (they exist, even for non-rubber band dub size tires). It's a common misconception that fatter tires have more flex. . . but rather fatter tires tend to be of poorer quality (which is why there is a price discrepency). You see those teeny tiny rubber-band-over-the-wheels tires on 19" rims. . . they aren't necessary for good handling. F1 cars do fine on the relatively fat tires they have. Finally, you want your wheels to be as light as possible, but sturdy enough not to deform over time.

 

djct_watt wrote:
----
Futhermore, weight isn't the only factor. . .

Rotation Inertia = (mass) x (radius)^2.
----
it's monday, so...
<\begin irrational irritation>

I think you mean 'geometrically' on radius. Something that varies exponentially with radius would go like e^(radius), not (radius)^2. The reason it matters is that something varying exponentially varies Alot more than something varying geometrically.

<\end irrational irritation>

Anyway, If you look up posts by "George", he makes these calculations. The change in effective weight is, worst case, not all that big. Less than the weight of a passenger. IT's something, just less something than you might think.

IIRC, he made the point that the tire width/rolling resistance has at least as large an effect on gas mileage, just as you say.

 

i appreciate the help! So basically, the consensus is that a wheel/tire combo if all factors remain the same besides the weight, the weight has a larger effect on mpg than just putting more weight in the car, but not an insane amount.

Lets say I have a set of 17lb rims. I then buy a set of 11lb rims. Tires and width and everything else stays the same. What would you guess my gas mileage would increase by???

 

yea... can i recommend a set a wheels?
Volk CE28N, get 15x7, you'll be set w/ those. don't forget to drop your car while your at it.

 

I disagree. Rotational inertia for a cylindrical shell is given by mass times radius squared. It's an approxmation since it doesn't account for the inertia of the spinning spokes, but it's a damn good one. For a solid disk, inertia is 1/2 mass times radius squared. A rim would be somewhere between that.

I'm not saying the equivalent weight savings from lighter rims is equal to the weight of a person, but the weight of a passenger is HUGE. Engineers would kill to be able to shave off 130lbs from a car. I promise you a full derivation of the effects of lighter rims in a later post.

 

1. Yes, I meant geometrically. . . but either way, the effect of reducing the wheel size is greater than simply removing weight (proportionally). I think you misunderstood his rebuttal, hnefrdo. It's simple vocabulary, e^n vs n^2.

All of that doesn't take away from the fact that mass is ONLY a linear relationship, and radius is a "geometric" relationship.

2. If that is too wordy for some of you, it means that keeping your wheels smaller has a greater effect in improving performance.

3. There is a reason why race cars don't use bling bling 19" wheels.

4. Keeping your wheels small (and the width suited to the traction limit of the chassis), you keep your braking and power efficiency at the max. Although lateral G's may not surpass those with rubber band tires, few people often race in a continuous circle. The gains in performance are substantial enough to drown out any gains acheived from increased lateral G's (which is further reduced if you compare equal composition tires with equal widths).

So what does this all mean? Keeping your wheels light is a great idea! But if you don't care about looks and want to maximize track times, stay away from big wheels. Keep them small, keep the light, and keep them just wide enough to keep you from flying off the road. Anything more is excessive and unecessary.

 

lol. My mistake. It just bothers me when people talk out their a$$es about the things they know nothing about, but this is my misunderstanding. It's all good here.

 

I was so intrigued by these ideas that I did the physics quickly. It turns out, I think, that these ideas are only partly true. The key question you should be asking is how much energy it takes to reach a certain vehicle velocity. This is directly proportional to the amount of gas consumed. And the kinetic energy of a rotating wheel (assuming most mass is at the rim) is

E = 0.5 * (mass) * (radius)^2 * (angular velocity)^2

So, while the rotational inertia varies with (radius)^2, it turns out that the angular velocity is (vehicle speed) / (wheel circumfrence), which varies with (radius)^(-1). When you multiply that all out, you find that the radius drops out, meaning that the kinetic energy of a rotating rim is independent its radius, and depends only on weight (of course, larger rims are more likely to weigh more).

E_(rotational) = 0.5 * (wheel mass) * (vehicle speed)^2 / (4*pi^2)

Now, referring to hnefrdo's comment, when you add in the energy required to also translate the wheel at the vehicle's speed, which equals

E_(translational) = 0.5 * (wheel mass) * (vehicle speed)^2,

you find that weight in the wheels is more dangerous than weight in the car, but only 2.5% more, because of the factor 1 / (4*pi^2) in the angular kinetic energy. So, as far as pure energetic arguments are concerned, wheel weight seems just the same as any other weight, and wheel size only matters as far as it affects weight.

However, other factors are surely at work. With larger rims, you're effectively lowering the engine rpms required for a given speed/gear. It's like switching into a harder bicycle gear. And while the energy needed to accelerate to a given speed remains the same, perhaps the engine can't deliver that energy as effectively in a "harder" wheel size. I'm just guessing -- anyone have any thoughts on how wheel size interacts with engine properties?