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Old 05-05-2007, 02:40 PM   #1
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Tire and Rim/Wheel Changes - Overall Effects on FE

I am at the bitter end of my tires service life and need to buy new summer tires, and am also considering getting new rims to save on the cost of remounting twice a year... So, now I am feeling drowned by the options and their unknown effects on FE.
Here's how I understand it: The Taller and Narrower the tires, the better, as it lowers engine RPM and overall tire to road friction and therefore raises FE.
Other than possibly not having enough power to drive the larger wheels as effectively (performance impact), I started wondering about other drawbacks that might reduce the potential gains in FE, or even lower it overall.
Here are the factors I was considering in my potential complete wheel replacement, so tell me how you all think that they may weigh in:

Larger Tires raise the vehicle - What effect on FE does the potential increase in aerodynamic drag have?

Larger Tires have more tire to road contact- More Friction?

Larger Tires weigh more- Doesnt spinning weight like that have a significantly bigger impact than stationary weight?

Tire to Rim/Wheel Proportion- If spinning wieght is something to worry about, whats lighter, a huge (17") alloy rim with short sidewalls or a small rim (14") with tall sidewalls?

??? ??? Thanks for the input!
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Old 05-07-2007, 08:37 PM   #2
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OK, I think I have definitely answered my fourth question on this list, and I have a cute little graph to illustrate it. The answer is: that given a fixed outside diameter, it is better (as in lighter which I think I can safely assume will improve FE) and less expensive to purchase, to have as small a wheel/rim as you can reasonably get and and make up the gap with a larger side walled tire.

Damn, how do I paste a .JPG on here if its on my computer?
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Old 05-07-2007, 08:37 PM   #3
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Old 05-07-2007, 09:16 PM   #4
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oh, sorry, I don't know what that means... Host with xs.to.?
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Old 05-07-2007, 11:26 PM   #5
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Quote:
Originally Posted by Phoenix View Post
Larger Tires weigh more- Doesnt spinning weight like that have a significantly bigger impact than stationary weight?
I'm gonna say only when accelerating (that is, speeding up/down, turning). Rotational mass, when constant (steady state), doesn't add anything to the party unless you're either 1) unbalanced or 2) operating at or near a natural frequency

Remember -- the further the mass is from the geometric center (rotational axis and ideally centroid), the bigger change in rotational inertia/momentum. Normally, I'd say SigmaF=ma - but that only applies to "linear sum of forces" -- for rotational mass, you'd use SigmaM=J*alpha where M is the moment, J is rotational mass (dependent on geometry) and alpha is rotational acceleration... I'm not sure if that's the same symbols used in classical physics though (they might use omega instead of alpha and something else for M) :/


Here's a page as it applies to cyclists... Where wheel inertia is a big big deal

http://www.analyticcycling.com/WheelsConcept_Page.html

But again -- for constant velocity... There's zero difference (unless you've terribly ended up in resonance at a natural frequency). But for accelerating.... Given constant mass, go for more of that mass to be near the center of the wheel
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Old 05-08-2007, 06:13 AM   #6
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My "gassaver" education is being applied to two vehicles, one sees a good mix of city and highway driving, although probably more city driving than average (EPA Mileage average that is, we are probably more like 35/65), and the other almost doesnt know what a highway is (more like 5/95)...

So, a think that for me personally that speeding up/down and turning factor in much higher than average and so may be more important than you had thought. I had a little trouble trying to apply those formulas that you gave me (I appreciate the technical and exacting nature of many at this site, but I am unfortunately not as mathematically adept as you ) and I understand the jist of what you can show with the math is that, yes, it plays a role in energy consumption, but I was stumped as to how to apply this to a real world FE change.

From what I gathered in random goggle searches, racers seem to think lighter wheels are noticeably important to power in acceleration, but they are racers, not so concerned with FE and also referring to super light, but also very wide, high performance tires. But, never the less, if you can notice the acceleration difference in the drivers seat of a high torque racer, I would think that a sensitive scangauge would show even more in a small econo box.

only testing will tell, but no one here has of yet, have they?
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Old 05-08-2007, 07:04 AM   #7
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Quote:
Originally Posted by Phoenix View Post
OK, I think I have definitely answered my fourth question on this list, and I have a cute little graph to illustrate it. The answer is: that given a fixed outside diameter, it is better (as in lighter which I think I can safely assume will improve FE) and less expensive to purchase, to have as small a wheel/rim as you can reasonably get and and make up the gap with a larger side walled tire.

Damn, how do I paste a .JPG on here if its on my computer?
I use this site, quick, easy and reliable.

http://allyoucanupload.webshots.com/

It would probably help advice givers to know the garage info on your cars. Some cars can barely pull themselves, much less taller tires.
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Old 05-08-2007, 04:09 PM   #8
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Awesome! So thats what hosting is! That was easy,... I'm pretty new to forums and HTML and the like, so thanks CO ZX2.

So I have been doing all this tire research and I had collected all this data to try and figure out what was lighter, a larger alloy wheel with short side walled tires or a smaller wheel with tall side walled tire? I thought it might interest you all...



As for the garage, I will check that out. thanks again.

(new and improved with edit, thanks a third time)
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Old 05-08-2007, 06:02 PM   #9
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Phoenix. If you copy the bottom link (direct link) after downloading your pic, then you can copy a full size pic into your post. Click "insert image" in toolbar then paste your link into the popup box that appears.

I was in the same boat as you when I started here and got help with the same thing from another member.
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Old 05-08-2007, 08:10 PM   #10
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Since nobody else has mentioned it, I thought I would...

If you do decide to change wheel diameter, keep in mind that will change the calibration on your speedometer unless you have some adjustment for that (and do in fact adjust for it).

The reason is simple. Most speedometers are keyed off how many times the wheels turn (and just indirectly turns that into how far you went in a given point in time). However, if you put bigger diameter wheels on the car, you will actually go further for each rotation of the wheels (and the reverse is also true, in that smaller diameter wheels move less distance for a given number of wheel rotations). Therefore, the speedometer (unless re-calibrated) will think you are going slower than you really are (or faster than you really are, if you go down in wheel size). And if/when your speedometer is off, you might get an unpleasant surprise from the cops (because you really aren't going the speed you thought you were going)...

So if you are going to go with bigger diameter wheels (or even smaller wheels than stock), you need to be aware that your speedometer will be off (or you will have to recalibrate it to get it accurate again). Not necessarily a reason not to change wheel diameter, but something to think about, if you are planning to do so.
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