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Old 05-23-2008, 12:25 PM   #11
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I have heard that a larger tire can have a significant affect on the brakes. this is part of the reason that the SUVs with the 24" rims seem to go through brake pads like nobodys business.

I don't know that for fact but figured I would add to the conversation. (it may just be because SUV drivers seem to drive fast and stop hard)
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Old 05-23-2008, 12:43 PM   #12
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That's true. A taller tire requires the brakes to work harder, just as it requires the engine to work harder. At least with the engine, you can change the gearing between the tires and the engine. With the brakes you can't.
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Old 05-23-2008, 04:36 PM   #13
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I would like for someone to challenge this piece of folklore. I question whether it's true or not. In bicycling, where such issues are FAR more important because its not only our own speed but indeed our own legs, muscles, and pain on the line, the most scientific and logical arguments I've seen were that it is in fact a myth.

I spent quite a bit of time in rec.bicycles.tech for awhile, where there are some real experts who have the credibility and research to back up their claims and arguments, and I do recall that such people did not buy into the "rotating weight is worth more than other weight" stuff. I also learned about tire pressure, tire width, and rolling resistance; and some of what I learned is counterintuitive but definitely true.
When our highschool team was designing the car, we appraoched it from the angle of a recumbent bike with two wheel in front. Our first year (my junior year), we tried driving at a constant speed of 25 mph (the required average speed) on one run, and accellerating to 35 mph and cruising down to 15mph after killing the engine. We only got 800mpg from burning and cruising. Our second year, we tweaked the body (which was constructed from aluminum J-channel and covered in heat-shrink plastic) and our only other changes were a smaller wheel made from carbon fiber that weighed half as much, special Michelin tires ordered from france to reduce rolling resistance, and ceramic bearings in the front hubs. That year we got 1500mpg with the same driver on the same track in mostly the same conditions (I think the humidity was 2% higher). Every time we reduced sprung (rotating) weight, we saw proportional increases in mpg. This past year in a national competition in California, the exact same body we built in 2005 got 6800 mpg in a test run. The only changes were yet lighter wheels and tires, lighter rear axle, lighter spokes than provided with the wheels, and a milled flywheel. That alone speakes volumes to me. Either the driver that they recruites this year somehow mastered the techniques, or reducing sprung weight is the key.
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Old 05-23-2008, 05:05 PM   #14
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I think you mean unsprung weight, not sprung. Anyway, your experiments certainly proved that your actions increased efficiency, but not 100% surely that saving rotating weight is any better than non-rotating weight. Don't forget that there are environmental factors to consider, too.

An experiment specifically to determine if rotating weight really means that much more than non-rotating weight would be to add some wheel weights, do some test runs, then move the wheel weights to the seat.

I'm not trying to be argumentative, it's just that one learns a lot challenging generally accepted but not thoroughly explored concepts like this.
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Old 05-23-2008, 11:02 PM   #15
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I think that a little here, a little there all adds up to maximum FE. Each little thing ( weight reduction, technique, aero mods, friction reduction, etc.) are not much by themselves but add up to max mpg when done together. Just my 2cents.
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Old 05-23-2008, 11:09 PM   #16
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if you live where the dump salt, keep the stock rims+ tires as winter rims/tires...
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Old 05-24-2008, 12:59 AM   #17
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I think you mean unsprung weight, not sprung. Anyway, your experiments certainly proved that your actions increased efficiency, but not 100% surely that saving rotating weight is any better than non-rotating weight. Don't forget that there are environmental factors to consider, too.

An experiment specifically to determine if rotating weight really means that much more than non-rotating weight would be to add some wheel weights, do some test runs, then move the wheel weights to the seat.

I'm not trying to be argumentative, it's just that one learns a lot challenging generally accepted but not thoroughly explored concepts like this.
I believe you are correct, and 1cheap1 is as well. I'm interested, but there would have to be a way to add the wheel weights in such a way that the wheels were balanced. Adding a weight to the wheel must be done carefully. I think we can all agree that an unbalanced wheel is a FE nightmare... I think the weights would have to be pretty significant to have any significant change show up in any tests us, the laymen, could do. Seeing how much fuel you put in the tank and reading the odometer isn't the most scientific thing ever...
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Old 05-24-2008, 05:01 AM   #18
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Quote:
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there would have to be a way to add the wheel weights in such a way that the wheels were balanced. [...] I think the weights would have to be pretty significant to have any significant change show up in any tests us, the laymen, could do.
For the 6800mpg car, the weights probably wouldn't need to be huge. The adhesive-backed wheel weights used to balance normal automotive wheels ought to do fine.

For a normal car, the OP could keep both sets of wheels, the 13lb ones and the 21lb ones. Carry one set in the vehicle and mount the other set, then switch. I suspect that, for this experiment, it will not be possible to measure accurately enough, and that the wheel switch will not produce any measurable gain even without carrying the old wheels in the vehicle.

If I were the OP, I might still get the new wheels anyway, under the following conditions: I'd have to be confident that I could sell the old set for nearly what I paid for the new set, and I'd have to be confident that the new wheels aren't going to be too weak and get damaged easily.
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