Can Form == Function?
 

I know i'm going against the grain here on EcoModder.com. I like things to look good. My last vehicle was completely form over function. the wheels rubbed the fenders, it had 3" straight exhaust off the turbine. 10" wide wheels with 225's stretched over them. I know looking good is relative and in this community duct tape and coroplast looks damn good when it raises your MPG.

The near future plans for my civic are to both make it look good (relative to normal car nuts :)) and to extract the most MPG's out of it as I can without resulting to coroplast and duct tape!

I've always lowered cars and ran flush wheel fitments for looks, but as I'm wanting to modify my car and am leaning towards the same route, I can't help but think it will HELP my mpg's.

After this upcoming winter, I plan on lowering the car and putting it on some alloy wheels.

The wheels I have picked out are 8" wide, 2.5" wider than factory. I will also be running a 195 sidewall, vs 185. Now you guys are thinking.. He's adding wider wheels, with wider tires.. He's killing his small contact patch, and adding to his rolling resistance! Here is where form can equal function. Yes, i'm running a wider wheel, with a slightly wider tire. What i'm doing to offset that, is stretching a skinny tire, over a wide wheel. This strengthens sidewalls for cornering, as well as tightens down the tires contact patch significantly. Stretching a tire on a rim allows you to run more effective air pressures as you aren't losing air due to sidewall flex.

These wheels are very aggressive offsets that will require negative camber to fit in my fenders. Once again, negative offset's and aggressive fitment looks great, but also serves as a functional mod to my car. Reducing camber allows EVEN MORE tire to be taken off the pavement, thus reducing RR even more. (Safety nuts out there do not criticize, ackerman says I'll get my full contact patch back during cornering) I also believe that pushing the wheels out to the fender opening will reduce drag by diverting most of the air flow down the car, and not into the back of the wheel well. Last but not least, the 16x8" forged wheels that I'm picking up are 3lbs lighter per wheel than the stock 14x5.5" steelies. Once again, FORM equaling FUNCTION.

Does the EcoModder population agree with my theories, or am I just trying to justify making my car look good.. Also, I am open ears for any other FORM == FUNCTION mod's you have conjured up as an excuse to modify your vehicle outside of the FE realm.

Lighter wheels, but, you are moving the mass of the wheels outward by increasing diameter. Therefore, you may run into the issue of using more power to accelerate at the same rate you did with the smaller diameter wheels. More power = more gas.

Larger wheels also usually increase braking distances.

I was under the assumption that the contact patch was determined by the air pressure and probably somewhat affected by tire stiffness. A tire with 40 psi supporting a 400 pound load would have a static contact patch of 10 sq inches. If you ran 20psi you would have 20 sq inch contact patch. If your tires are angled and running straight then they would deflect on the inner area to maintain their contact patch.

I think there are a few holes in cmj's theories

cmj,
maybe you should try using the search function and read about tires in other posts before asking hypothetical questions that have been adressed before.

also, there are people on here who have done some amazing mods that didn't use coroplast and duct tape. again try using the search.

Finally, the only reason to 'lower a car and run flush fitment' is for looks. There is no proven reason otherwise. Cars are each built for specific performance and setup. There is a balance built in. When you start moving things around you throw all the design limits uot the window. Bushing wear uot prematurely, each pound over factory wheel/tire weight is multiplied by 4 as moving mass.
You can absolutely do what ever you want with your car. afterall this is America. But false justifications won't validate.

Good looks and low drag can only go hand-in-hand if you fully understand aerodynamics AND see beauty in a well-executed piece of engineering.

Pursuit of the latest automotive fashion trends will only make your car uglier in my eyes, until all the ball-players have Xhibit ecomod their Prius, or until Dave Cloud becomes a celebrity coachbuilder.

I have done research, and the only amount of info even relevant to this discussion is heavier wheel/tire combo multiplied by 4. I'm using a larger diameter wheel, yes the actual wheel mass is moved 1" further out (radially) but I'm also losing 1" radial sidewall. Less tire (smaller sidewall), Lighter wheels == less rolling resistance. Also, who can argue with lowering a car to reduce air flow underneath? Premature bushing wear is always a consideration when changing suspension geometry, but with proper camber adjustment kits and steer spacers, proper geometry is maintained.

Please enlighten me to my false justifications. I created this thread to voice some ideas that I haven't seen posted.. A different way of doing things. I'm looking for specific feedback, not just general reasons as to why my hypothesis are incorrect (no more calling them theories, because they have not been even remotely proven)

I checked some random brands. I used a 225/60R17 and a 225/55R18 which are about the same width and height. They happen to be the about the same weight. Again, you are moving the weight outward, which takes more power to turn. Did you check the weight of the tires?

The 4 to 1 pound is for unsprung weight. It has nothing to do with changing wheel diameters. Unsprung weight is tires, wheels, axles, brakes, anything under the springs. So, adding heavier wheels will use the 4 to 1, but is different all together from the rotating mass.

Rotating mass. Think of it this way. If you put a one pound weight on the end of a one foot pole and a one pound weight at the end of a two foot pole, which one is easier to lift? The one with the weight at the end of the one pound pole. One pound at one foot is one foot pound of torque. One pound at two feet or two pounds at one foot is the same, two foot pounds of torque required to lift. (We are also assuming this pole has no mass) So, if you go from 17" wheels to 19" wheels and add one pound to the rim portion, the engine must now work harder to accelerate the wheels because the mass at the end of the "pole" weighs more and the "pole" is longer. Add that to 4 pounds to 1 pound ratio for unsprung weight.

Now, since your new wheels are lighter, they may have a rotating mass similar to the smaller, heavier wheels. It all depends on the design of each individual wheel.

Rotating mass really makes a difference. Why do you think slicks are all on 15" wheels? Because who wants to slow down their car in the quarter mile to run 17" wheels? That power that is used to get those wheels moving could be used to accelerate the car faster.

There was a fellow named CRXGator on InsightCentral who "upgraded" from the 11lb Insight wheels to wider, 9.5lb wheels. He also replaced the 13lb OE tires with 20lb units. So he replaced 96lbs of rotating mass with 118lbs that has more of the mass farther from the axis of rotation - a BIG step in the wrong direction.

He also moved from a wheel that has a good selection of summer and winter LRR available, to one that has almost nothing but grippy, high rolling resistance summer tires.

Wider tires have everything to do with conformity, and nothing to do with fuel economy.

Do you mean you will be running a 195 section width? What size tires and wheels will you actually be using and what were the sizes of the originals?

Wheel weight does not effect rolling resistance. That is a function of the tires and their air pressure and the weight that the tires support. Wheel weight does effect rotating mass, which must accelerated, but once accelerated conserves that energy - more weight will let you coast further but you may lose that energy when you stop if you don't have regenerative braking.

With a heavier tire and lighter wheel you may be better off, the same, or worse off than the stock setup, depending on what the total weight is and where it is distributed in relation to the center of the wheel (the polar moment of inertia).

Lowering the car may help some by covering part of the frontal area of the tires and may help by reducing airflow under the car, but there is a point where further lowering increases drag.

Negative camber will probably help your cornering if you corner hard, but your tires will wear out faster on the inside edges.

I will be running 16x8" wheels.

New: 195/45/16
Old: 185/60/14

I will most likely be running wan-li, or some other hard china compound since these are summer wheels, won't see much lateral G, and don't need to be sticky at all. As far as tire weights, I won't have any good comparisons until I figure out what factory tires weigh versus the new shoes.

This is something that I have remedied by flipping tires once every season. I consistently get 30k miles out of a set of wan-li's by flipping them every 10k miles or so. Not ideal.. but it works. I realize some of you cringe at only getting 30k a tire.. But I only pay $30 a tire so I'm not terribly upset.

Additional wear is probably a good indicator of additional friction.

OK, so your OD is about the same, the new wheels are lighter and the new tires may be heavier, the same, or lighter. So your moment of inertia may be slightly better, the same, or slightly worse. You've increased section width by 10mm, but covered some of the frontal area by lowering the car. I predict a slight increase in mileage. Just my humble SWAG at it.

Drag from wheels is one of the biggest single drag sources in a car. Widening tires with all other things the same will almost certainly increase your drag. There is one reference in Hucho which shows a .29 to .30 change going from 175-70-15 to 185-65-15.

Lowering a car may or may not increase drag. It depends on the car. Most wind tunnel tests show a reduction in drag with increasing road clearance. Belly pan it and you may offset the increase.

RR seems to be much more compound related than section and width related. The differences in compounds in the tires you choose will have a much greater effect on RR than differences in section.

The rotational mass issue is really trivial AFAIK. It may make a slight difference in acceleration but almost none in FE. Your engine's efficiency in converting heat to kinetic energy will have a much greater effect on FE that how quickly you can convert that energy into velocity. The extra weight will have a small effect on RR but that would be almost unmeasurable. If you are drag racing (or stopping quickly) then it will make a difference.

My two bits.

The Tire Rack lists the mass of almost every tire they sell on the Specs page. Assuming most of a tire's mass is in the belts and tread, the small increase in section width will result in a small increase in mass, and a moderate increase in inertia. It may be too small of an effect to notice, though.