for those interested, I found this listing of wheels weights (scroll down to "Aftermarket" section):

http://www.miata.net/faq/wheel_weights.html

I realize it's Miata specific, but the wheel sizes and bolt patterns are similar to the Civic...

 

Actually, let's go one more.

My new setup will actually be 29 lbs even. That's 3.6 llbs lighter per wheel, times 4 = 14.4 lbs unsprung weight. That's aprox. 115.2 lbs car weight, which by your calculations = 4.1% of vehicle weight. At 50 MPG, you'd expect a bump of ~2 MPG. Beats a poke in the eye!

 

I'm not sure if the bolt pattern is the same, but the wheels on the Insight are much more attractive however are spoked style wheels. I'm guessing the low rear fender skirt avoids the problem with wind resistance. I hate my HCH05 wheels.

 

Not what I'd expect. As noted, I think it would be half as much at best. And if you're getting spoked wheels instead of those plain looking flat disc covers, you'll lose at least some of the gain to air friction. And if the new tires aren't low roll resistance I'll bet that overall your FE will drop a bit.

 

Sorry - I misread you original post. However, I don't see how dopping more than 14 lbs off the wheels could do anything but increase FE. The tires, for those that are interested, are Continental ContiTouringContact CV95, which are one of the GreanSeal's low rolling resistance tires. In fact, they're one of the best on the list (actualy the CH95 is on the list - CV is just "V" rated version and takes 51 psi, weighs 18.3 lbs).

There are many other collateral benefits to lowering wheel weight. Better braking, accelleration, reduced strain on the suspension. I can't see that at the speeds I drive (40-55 MPH) spoked vs. flat would make any difference - no more than the 6 holes in the stock wheel currently do.

I'll have all this installed in about 2 weeks. I'll be sure to report back after the first tank.

 

Well, since I'm a bit of a physicist (I loved high school physics ), I just had to go back and look at the equations. I found this convenient site -
http://theory.uwinnipeg.ca/physics/rot/node1.html
and concluded that the weight factor of 8 - i.e. a 1 lb increase or decrease in wheel weight is equivalent to 8 lbs increase or decrease in car weight - is hugely exagerated. If I applied the equations correctly, the energy required to rotate a wheel, which for this purpose resembles a uniform disc, is 1.5 times the energy needed for linear motion. So the virtual weight of a wheel is 1.5 times its true weight. Reducing your wheel weight by 14 pounds will have a similar effect to reducing the vehicle weight by 21 pounds.

In other words, the effect of wheel weight on FE is a big whoopdedoo.

 

Excellent post ElanC. While I am not ready to completely discount people's claims that changing tires/rims greatly impacted their FE, I think we can get from this that we must appeal to something other than rotating weight to explain most of it. Also, car tires are not uniform discs, most of the weight is concentrated at the outer areas, still that could not more than double the energy cost over linear mass if all the weight was concentrated at the outer rim.

Things I could think of to better explain the reports that have come in:
o - heavier tires could account for about 1mpg if they were a lot heavier per tire.
o - larger tires confuse the computer, 2" dia more is an 8% loss or 4 mpg out of 50, and this is a virtual loss.
o - Loss of LRR could account for about 3-4 mpg according to reports.
o - increased wind resistance, no clue what that could account for.
o - exasperated people exaggerate problems, could double the size of the problem. :-)

So the hypothetical report of a 2" larger tire, 10-12 lbs more per tire, and loss of LRR, would account for about 8-9 mpg down from 50, and maybe 7-8 down from 45, that is pretty close to some of the bad reports we have seen. :-)

- Kurt

 

Only part of it is the fact that the wheel's weight has to be spun up with angular momentum for the car to go forward, the other part is that the wheel weight (in addition to part of the suspension system, brake system, electric motors mounted in the wheels on some concepts, etc) is "un-sprung" meaning that it does not rest on the suspension and can move independently of the chassis of the car. This has a larger effect on the equivalent weight then the rotational energy of the wheels.

I will leave it as an exercise to the reader to do the resulting computations (my physics books always said that at the end of a complex explanation ).

~Droid

 

Wow, that miata website says the stock wheels on the Civic HX (which are spokes) are 11.75 lbs. Assuming its accuracy, did Honda decide the wind resistance was such that it made since to add an extra ~3 lbs a wheel?

 

I can't speak to the impact on FE when it comes to changing rims and rubber but I can say this. The best thing I did was replace my stock HCH tires with only 2000 miles on them. They are just terrible. I bought quality Yokahama (sp) tires and LOVE them. Better ride, MUCH better breaking and MUCH better handling. Do yourself a favor, add $400 to the purchase price of the car and go directly from your dealership to the nearest quality tire store. I inflate to 40 pounds and get 40MPG in driving that is mostly city.