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Old 03-09-2008, 12:02 PM   #1 (permalink)
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Effect of weight of wheel+tire

Please confirm or correct me on this -
Weight of wheels + tires has an affects acceleration and affects mpg when accelerating, but has a nearly insignificant effect on steady-speed driving?

For steady-state driving, reducing the combined weight of all wheels by 20 lb. would be the same as removing 20 lb. of unneeded cargo from the car?

But for accelerating/decelerating reducing that weight would have a more significant result?

I ask because yesterday I weighed the 2 different wheel/tire combinations I have on hand. Nice 205/65-15's turned out to be 4.5 to 5 lb heavier (each!) than 195/75-14's, both mounted on oem steelies. The two have nearly identical outer diameters and so are swappable without affecting speedometer/odometer accuracy or effective gear ratios.

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Old 03-09-2008, 01:40 PM   #2 (permalink)
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It sounds to me like you have the right idea there just by thinking through it logically. The only thing I see that would be an advantage for heavier wheels would be while EOCing because they will have more momentum and thusly continue to role longer without assistance...but I don't have any proof I'm just thinking it might act similarly to a flywheel on a car. Heavier flywheel equals better gas millage then a lighter one because it can idle at lower RPM without stalling.
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Old 03-09-2008, 01:43 PM   #3 (permalink)
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Hi Bruce,

Yep, that is right, to a first order approximation. The issue with steady state driving is that its not really steady state due to road imperfections, merging and road hazard avoidance. So weight will effect the mileage there too, but as you say the difference will be nearly insignificant.

On a Prius the effect is about half as much, as about half the rotational (wheels) and linear (total mass) inertias can be reused. In a standard car however, all that inertia is turned into heat by the brakes.

There are very expensive forged alumimun and or magnesium alloy wheels that can give you almost the same weight with the bigger heavier tires as with the smaller tires on cast alumimum alloy wheels, let alone steel.

Shop around, there may be some lighter weight tires, too. Although, they tend to have the lower tread life.
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Old 03-09-2008, 01:49 PM   #4 (permalink)
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They also won't be as good at holding up to the higher pressures many of us run to help reduce RR. So if you want to run over the factory recommended pressure for your car make sure they didn't skimp on the thickness of the sidewall too much to reduce the weight.
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Old 03-09-2008, 03:25 PM   #5 (permalink)
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The 195's will give you lower aero and lower RR. Then the 205's.

185/80 would be even better for aero and RR.
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Old 03-09-2008, 07:40 PM   #6 (permalink)
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For a given overall weight the greater the distance from the centre of the mass ; the higher will be the centre of inertia.
So the greater the amount of energy needed to get things moving.

Also a factor but I think a fairly small one.

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Old 03-09-2008, 08:06 PM   #7 (permalink)
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Thanks all.

My winter tires which just came off are the 195's. Summer tires are on as of today, 205/65-15. Relatively new tires, spec'd for 44 psi that I ran at 50-53 last fall. Not a low profile tire by modern standards but they do handle a lot better than the skinnier taller-profile 195's. Same overall diameter.

This is a 3000 lb. car that came stock with 185/R 14's which is equivalent to 185/80-14. Size is not easily available at good prices. It's a very tall sidewall by modern standards. For improved handling many drivers of these Volvos have gone to much larger (and wider) wheels to get less flexing of the tires in hard cornering.

Nowadays any 3000 lb car will usually have tires size more like 205/55-16 or 215/60-16. That's as wide as my 205's or wider, and a shorter sidewall.
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Old 03-09-2008, 08:44 PM   #8 (permalink)
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So uh, would a heavier wheel coast farther than a same-sized lighter wheel on the same slope with the same environmentals etc?
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Old 03-09-2008, 10:26 PM   #9 (permalink)
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I coast a lot farther with a full Jeep, but I have to be heavier on the pedal when accelerating.

Too bad it's impossible to make a car that adds weight while coasting downhill and while slowing down and removes it while going down the highway and accelerating. :\
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Old 03-10-2008, 08:50 AM   #10 (permalink)
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Quote:
Originally Posted by hondaworkshop View Post
So uh, would a heavier wheel coast farther than a same-sized lighter wheel on the same slope with the same environmentals etc?
Yes.

The 205 is of course slightly wider, but it's only an extra centimeter. If you're mostly driving at cruising speeds, not a lot of stop-and-go, then there probably won't be much difference between the two.

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