Wheel weight.. does it Matter?

[KONVERTER]

Lighter wheels do not store as much inertia so you can not coast as easily. At cruising speeds lighter wheels hurts your mpg. But it is a small amount....

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[ZDan]

Originally Posted by KONVERTER

Lighter wheels do not store as much inertia so you can not coast as easily. At cruising speeds lighter wheels hurts your mpg. But it is a small amount....

No no no no no. At constant speed cruise it does not matter at all how much the wheels weigh. For in-town stop/go driving, more rotational mass does hurt fuel economy a bit (extra energy spent to spin up heavy wheels/tires is totally lost to braking heat).

[Anaxilus]

Originally Posted by ZDan

I *am* a "seasoned engineer". I've done performance models for road vehicles including rotational inertial effects.

Mind sharing the math as to how you got 1.7 since you are an engineer? I find it odd since the factor based on physics should be dynamic and not a static figure. The larger and wider the wheel and the faster you spin it the greater the rate of increase. I'm not questioning your credentials, just find the lack of a more accurate answer or range odd w/ the available math and tests you have done.

If you happen to figure out 86 wheel rates sometime feel free to share those as well. The thread I started on it fell completely silent which was surprising for how many track this car. Granted the car is new, but the Integra and Civic boys have all the data on wheel rates they need pretty much.

Originally Posted by KONVERTER

Lighter wheels do not store as much inertia so you can not coast as easily. At cruising speeds lighter wheels hurts your mpg. But it is a small amount....

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If the wheel is wider w/ a larger section width it increases drag and rolling resistance. Most fuel is consumed in start stop traffic so the benefit is much larger there.

[SurfAndSand]

Originally Posted by mit_peid

I went from the stock wheels/tires to one of the lightest wheels ever made and feel the car accelerates a bit better but my mpg still went down from around 23mpg to closer to 21mpg (I might be driving it a little bit harder now too).

Stock: wheel + tires (~21 lbs + 22 lbs = 43 lbs)
Now: Volk SE37ks 18x7.5 +48 + Goodyear Eagle F1 Assy 2 - 225/40/18 (~16 lbs + 21 lbs = 37 lbs)

Note: I also know the 225/40/18 have a slightly larger diameter compared to the stock 215/45/17s affecting the mpg calculations too.

The larger rolling diameter means that your car is traveling more per rotation, so your car thinks it's getting less MPG. You'll have to get the exact calculations to see if that's actually the case.

If anything, the wider aspect ratio will increase rolling resistance, which can lower MPG. Right?

[Anaxilus]

Originally Posted by ZDan

No no no no no. At constant speed cruise it does not matter at all how much the wheels weigh.

Because weight is independent of rate of deceleration/gravity per DaVinci I presume? But it would matter under braking because that's F=MA.

[KONVERTER]

Originally Posted by ZDan

No no no no no. At constant speed cruise it does not matter at all how much the wheels weigh. For in-town stop/go driving, more rotational mass does hurt fuel economy a bit (extra energy spent to spin up heavy wheels/tires is totally lost to braking heat).

So an object at a constant speed has no inertia?

Lighter weight equals less stored energy so the motor works more to keep a constant speed

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[Anaxilus]

Originally Posted by SurfAndSand

If anything, the wider aspect ratio will increase rolling resistance, which can lower MPG. Right?

Correct. But you actually mean section width, not aspect ratio unless you mean it relative to section width and then you also have aerodynamic drag with a larger frontal area pushing through air.

[BRAWL]

Get the wheels you want and can afford. Then buy the best tyre you can afford as that is where you will feel the biggest difference

[mit_peid]

Originally Posted by SurfAndSand

The larger rolling diameter means that your car is traveling more per rotation, so your car thinks it's getting less MPG. You'll have to get the exact calculations to see if that's actually the case.

If anything, the wider aspect ratio will increase rolling resistance, which can lower MPG. Right?

x = Diameter of stock 17" wheels w/ 215/45/17 = 625.3mm
y = Diameter of 18" wheels w/ 225/40/18 = 637.2mm
% Error = (|x - y|/x)*100 = 1.9%

So assuming I'm getting 21.0mpg with my new aftermarket 18" wheels/tires, I'm actually getting 1.9% better, or 21.4mpg.

Does that math sound correct or close enough? Or I should I circumference instead of diameter?

Update: Just had to look up some old high school formulas to remind me C = 2R(Pi) = D(Pi). So the error would still be 1.9%.

[fatoni]

Originally Posted by Draco-REX

Grassroots Motorsports Magazine (reminds me, I need to renew) did a comparison using the same tires but different wheels of differnet weights and they made very little difference.

i think they actually went faster with the wheel that was 3 lbs heavier on a miata, a car that is much more responsive to wheel weight.

[ZDan]

Originally Posted by Anaxilus

Mind sharing the math as to how you got 1.7 since you are an engineer? I find it odd since the factor based on physics should be dynamic and not a static figure. The larger and wider the wheel and the faster you spin it the greater the rate of increase.

That's not how it works. For a given wheel mass, a given wheel rotational moment of inertia, and a given tire diameter, it is indeed a specific ratio, whatever the speed or rate of speed change.

I'm not questioning your credentials, just find the lack of a more accurate answer or range odd w/ the available math and tests you have done.

There is no one "answer". It DEPENDS on the specific wheel and/or tire design. For wheel models I have played with in this size range (~17x7), for a 25" tire diameter, it works out that as far as outright vehicle acceleration is concerned, saving 1 lb. wheel weight is effectively the same as saving about 1.5 lb. times 4 (four wheels) or 6 lb vehicle weight.

If the wheel is wider w/ a larger section width it increases drag and rolling resistance.

Not so much. Tire section width doesn't amount to much frontal area and it is somewhat overwhelmed by the sheer bulk of the car (I looked into this when figuring out what tire/wheel sizes to run for the Texas Mile).

Most fuel is consumed in start stop traffic so the benefit is much larger there.

Yep. Wheel/tire weights don't affect highway mpg, but they can hurt in-town mpg.

[Anaxilus]

Originally Posted by ZDan

That's not how it works. For a given wheel mass, a given wheel rotational moment of inertia, and a given tire diameter, it is indeed a specific ratio, whatever the speed or rate of speed change.

Actually I was referring to increased rate of effective weight/force, not speed at the polar moment.

[Anaxilus]

Originally Posted by fatoni

i think they actually went faster with the wheel that was 3 lbs heavier on a miata, a car that is much more responsive to wheel weight.

This is because you need to calculate for contact patch and traction versus polar moment if you want to determine the fastest way around a track. Larger wheels have bigger contact patches. Adding weight increases the psi on each contact patch. So that's traction.

Polar inertia is something independent of traction, and far more relevant to the street than to track. Track cars maintain more high speed even in deceleration than street cars, so they have less work to do to spin the wheels back up. Plus they tend to have much more power/torque than street cars and move less mass overall.

It's a question of balance versus your driving environment. In terms of absolute timed track performance, you always go with the most rubber you can put on the road (unless the weather is bad). Street is different and will depend on user preferences and driving style. The ideal track car above will absolutely suck to drive in street conditions.

[R3d]

Originally Posted by ZDan

I *am* a "seasoned engineer". I've done performance models for road vehicles including rotational inertial effects.

where did you get 1.7 from? Because all the research I come up with say that there is no number and that the gain is proportional to the part, its location and how soon it breaks inertia. For example the tire and wheel have a bigger impact then the brakes and pads would. Apparently the real life gains after removing 1lbs from a wheel ``are`` greater then removing 1lbs of non rotating sprung mass ( something that doesn't spin and is after the suspension) but nowhere can I find any kind of value for sprung vs un-sprung. Also I read that removing 20lbs from your wheels would be more of a direct improvement on acceleration and breaking compared to shaving 100lbs from the body even tough loosing 100 lbs will help more ``over all``!?!?