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Old 11-11-2008, 02:53 PM   #17 (permalink)
MechEngVT
Mechanical Engineer
 
Join Date: Jun 2008
Location: Richmond, VA
Posts: 190

The Truck - '02 Dodge Ram 1500 SLT Sport
90 day: 13.32 mpg (US)

The Van 2 - '06 Honda Odyssey EX
90 day: 20.56 mpg (US)

GoKart - '14 Hyundai Elantra GT base 6MT
90 day: 30.24 mpg (US)

Godzilla - '21 Ford F350 XL
90 day: 8.69 mpg (US)
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Quote:
Originally Posted by jamesqf View Post
Not just the weight, either. There's the rotational inertia to consider.

Which brings up a point that I've wondered about, off and on, since I started seeing cars with large rims & thin tires. Which weighs more? That is, if you keep the same tire diameter, can you save weight/inertia by going to a larger, lightweight wheel?
You can definitely save weight without increasing the tire diameter. Many tire manufacturers will list the weight of a particular size/model tire on their spec sheets and you can look for one in the size you want and choose the lowest weight. You can choose a forged alloy wheel and they will be lighter than most cast alloys and steel wheels (again, pay attention to actual weight but also load ratings).

Rotational inertia is a different matter, and one that folks in the large rim/rubberband tire crowd must not understand. Polar moment of inertia (that is what resists acceleration/braking) is affected not only by the mass of the tire and wheel but by how far from the rotational axis the weight lies. A mounted tire with a cross-sectional shape of the letter "I" will have much higher polar moment of inertia than an equal diameter and equal mass mounted tire with a cross section of "<>" because the former concentrates mass far from the axis where the latter concentrates mass near the axis. I haven't seen wheel or tire manufacturers rate polar moment or radius of gyration (which helps calculate polar moment along with mass) data on their spec sheets.

It would be hard to say where the trade-off point is. I bet you'd be better off taking a larger radius of gyration along with a substantial cut in mass and come out of it with a net lower moment of inertia, but if your overall reduction in mass is very low or nets out going a +1 size with the same tire OD is probably a net loss to FE.
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