Lighter Weight Wheels --- Worth It?

[user removed]

Consider that many Insurance companies do not cover replacement costs of stolen wheels if they are not OEM.

I would look hard for used wheels off of a hybrid that might fit your vehicle, especially if they had the factory LRR tires.

Find some one who fell for the "bling" thing and get his take offs for cheap.

Lighter wheels especially with lighter tires will make a significant difference, especially if you can save 5+ pounds per assembly. The power required to double the speed of a flywheel increases exponentially not linearly. Each wheel is a flywheel that requires energy to accelerate. You may get some of it back but never as much as you spend getting up to speed.

I think of sprung to unsprung weight like this. The weight of the components that move up and down when your suspension works, versus the weight of that quadrant of the vehicle that dampens the energy of the rolling mass.

The greater the ratio the less the disturbance to the vehicle when you encounter bumps.
The best ride is when the ratio is greater and you do not need stiff springs and shocks to keep the tires on the pavement.

There are several other factors involved but that is the basic explanation.

regards
Mech

[Litotes]

This has been an interesting read. But I was left unsure of whether anyone has used light wheels to actually achieve a measured effect (in an urban environment) as I am thinking of changing car's alloys to "steelies" which are traditionally lighter.

I couldn't figure out why unsprung mass became a discussion point in this thread. It has a massive effect on vehicle ride/handling, spring arb choices etc but has no direct impact on fuel economy. A possible side benefit though.

Furthermore has anyone gone mad and fitted 10" wheels to reduce the wheel inertia significantly (and changed gear ratios, brakes and speedo to compensate).

[euromodder]

If unsprung mass has such a great impact on FE, I'm left wondering why Volvo changed their regular alloys (7 to 7.5 kg for 15") to the more streamlined Lybra rims that weigh 9.1 kg (15").

So maybe the aero-improvements can make up for the increase in unsprung weight ?


(See their accessories page for info on these wheels.)

[davidgrey50]

Maybe because your average Volvo weighs 2 tons, and 7 or 8 kilos (roughly 0.5%), is basically kinda irrelevant.

[some_other_dave]

Originally Posted by Litotes

... changing car's alloys to "steelies" which are traditionally lighter.

I think you have that backwards. Steel rims are generally heavier than aluminum ones.

-soD

[justinschmidt1]

Anyone find any new information on this topic?

[busypaws]

Bought old CRX-VX rims for my Mazda Protoge. Running 155/80/13s. Average commute is 22 stoplights 5.5miles. So since I never get above 40mph weight is where I'm ecomodding. 6 lbs per corner with change from steel, 175/70/13s. Check my garage/fuel to see other weight savings things I did.
Paid 200 for the rims. I don't think I'll ever make up that money in fuel saved but I'm here to learn - not just save money.

[zonker]

Originally Posted by tournapart

Totally new here, just signed up, but I have read this 8 page thread and am in the same boat as the original poster. I have a Mazda RX8, which yes, its a sports car that gets horrible mileage, and for its mid price range is probably amongst the worst. That being said, I am an avid car enthusiast and am studying to be an engineer. To take it further, I did complete a vocational program at a community college for auto repair, and have been quite involved with rotary engines for years. I am interested in making my car a little more efficient. To filter the BS so to speak this is what I have taken from this specifically and a mix of what I have read in the past.

Talking strictly wheels

Lighter wheels will promote greatest fuel effciency in more urban/stop and go environments.

More aerodynamic wheels will promote greatest fuel efficiency in more highway/constant speed environments.

Now dipping into how much more efficient as you get more aerodynamic or lighter is debatable. I share the same sentiment as someone who posted earlier, not sure if its the original or not, as saying, I would get them anyways, the fact that they are more efficient is just a plus.

I wont sit here and say that my intentions are strictly efficiency, obviously if that were the case I wouldn't own a sports car. But it is a hybrid affair, fun/daily not gas/electric, and I think I can try to squeeze some more MPG's out and enjoy the car even more.

After reading this thread I was up in the air about upgrading my wheels once again. Stock, my rims were 23-24lbs each. When I discovered that one of the rear rims was slightly warped when I was getting some tires installed I went out and found a set of cheap rims to get me over. These turned out to be more like 26-27lbs each. The rims I am thinking (and now am definitely) of getting are 18.6lbs each (Enkei RPF1). I think there are many things to be gained having lighter rims, less wear and tear on wear components, such as brakes, tires, clutch, suspension, etc. Acceleration improvements will be the most noticeable and I think there is something to be said about being able to put less power out to get to accelerate as much as it did before the weight reduction (more of as a real world application versus a study of efficiency), everyone has different driving styles that make them feel comfortable when they drive their cars.

I am unclear on one thing. From my readings today, and of years past, there is always some heated and unproved debates of the equal ratio of unsprung to sprung weight. Some say 1:2, some say 1:3 I have read as high as 1:10. What are the exact variables that determine this ratio? and how could you apply these variable to any vehicle? I guess what I am getting at is, is there a fill in the blanks calculator that will tell me for my specific vehicle (curbweight, HP, etc) what my unsprung to sprung weight ration is? or is it infact a fixed ratio that is applied to all vehicles and everyone is just giving it they're best guess?

Thanks again to all who contributed to this thread, it has help me immensely and I hope to get to know some of you smart people out there.

Well said
+1!

Tire weight does play a huge role too, as has also been pointed out, and the width and diameter of the tread is a large part of the consideration. Ideally, from a weight perspective, you'd want the smallest diameter metal wheel and the least diameter and width for your tread to achieve the most economical wheel/tire combination.

But since there's other factors, like traction, aero efficiency, ride quality, engine gearing, etc, the choice typically becomes some sort of intelligent compromise.

On my mustang, I'm looking to add wheel adapters to both move the tires out toward the body width to dam the arches better, and convert the bolt pattern to 4x100 so I can run a set of mazda miata hollow spoke 14" alloys (9.5 lbs) with a tire that is within 5% of stock diameter and hopefully a LRR rating. If I cannot find a LRR rated tire in the size i wish, then I will go for the tire that has the highest wear rating (400+) and the lightest weight (thank you tire rack). I hope to lose about 40 lbs of rotational mass this way.

[abell75]

The reason we see several rules of thumb for the translation from wheel&tire weight to 'normal' weight is that for a given wheel/tire combo, there can be various moments of inertia.

Rotational dynamics and linear dynamics don't translate so easily.

To figure out the difference that new wheels/tires would make to acceleration, you need to apply rotational dynamics to the before and after situations. The trouble is that while finding weights is easy enough, moments of inertia are not so readily available Others have already said it - for a given wheel/tire weight, you want the mass as close to the axis as you can get.

Hoop and Cylinder Motion
Rotational Work and Kinetic Energy

density of rubber is about 1500 kg/cubic meter (internet stats vary a lot here)
density of iron (steel) is about 7850 kg/cm
aluminum: 2600 kg/cm

[zonker]

Figure the two of a tire and wheel assemblies with the highest weight density would be the tire tread and the wheel rim.

So, I think as a general rule of thumb, assuming tire outside diameter is the same, that the smaller diameter the wheel, the better the power / economy benefit.

So if you had two wheel and tire packages that weighed exactly the same, the smaller wheel would most likely be the better choice, and the taller sidewall as well (again assuming tire OD's are the same).