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Old 04-27-2008, 06:42 PM   #11 (permalink)
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frank ????? The size is the same, even if you can measure it corectly of not. Its seems you want to change the discusion about the size of the patch, to your ability to measure it. Secondly I run my tires at 45 psi. when i said the best tires was a skinny tire, i was refering to FE only. Which is what this site is about. You might be right, but it doesnt affect the conversation. The conctact patch is the same, whether you can measure it accurately. the best tire for FE is a skinny tire. even if you have to air it to 50 psi.

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Old 04-27-2008, 07:44 PM   #12 (permalink)
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Cool

Quote:
Actually its an old racers trick. You can weigh your car by going out and using paper and a ruler. here is a link
http://www.rockcastle.org/activities.../weighcar.html
Thanks for the trick!

However, I wasn't actually trying to lower my contact area by changing the diameter of the wheel. I was trying to figure out which is better for gas in terms of rotational inertia: ie, which of the following options will the engine end up doing the least amount of work - having a wheel with a smaller diameter (so that more of the weight is in the center of the wheel) or having a wheel with a larger diameter (so most of the weight is towards the outskirts of the rim).

I had to pull out my book from highschool , but after brushing up on my physics a little bit, I think I figured it out.

While you want light wheels it matters where the weight is focused. If its towards the center of the wheel or spread out for that matter, its going to take less energy to accelerate because of the rotational inertia. However, with that said they will also tend deccelerate faster since they won't tend towards staying in motion. So if you plan on coasting or doing a lot of highway driving its not the best option.

Now if you could get light wheels that have a larger diameter where most of the wheel weight is focused towards the outer rim, then the wheel should stay rolling longer (kinda like it'd have more momentum). So this **should** increase mpg for highway driving and for coasting situations.

So I guess my conclusion at this point in the thread anyway would be that I want to find skinnier lighter rims that are larger than my current ones with most of the weight focused towards the outer wheel and then see what I can find for RR tires. this should potentially increase my mpg.

Thanks for all your responses so far! If you find anything more please let me know! I'll keep you posted on what I end up doing! Please keep on posting if you have more ideas!
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Old 04-27-2008, 08:24 PM   #13 (permalink)
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thank you for contributing to your own question. as corny as it sounds, it seems like we are all learning from each other. Damn that stings, I can't believe i said that. anyway, maybe you can find a set of used forged rims on ebay or craigslist. I know I'd take a pair if they were cheap enough! make sure they will fit your vehicle. there are a lot of variables to consider, like offset. Check out tire racks take on this:
http://www.tirerack.com/wheels/tech/...urrentpage=106
all of their tech articles are great
"To properly fit on a vehicle the wheel must have the proper bolt pattern, centerbore, offset, width and load capacity." -tire tack
might want to check this out too
http://www.tirerack.com/wheels/tech/...jsp?techid=108
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Last edited by toomuch; 04-27-2008 at 08:30 PM.. Reason: tirerack quote, more tire rack
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Old 04-27-2008, 08:50 PM   #14 (permalink)
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Forged lug nuts will help lower the weight at the center.
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Old 04-27-2008, 09:18 PM   #15 (permalink)
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Here's an important question: where does rolling resistance come from?

I think the contact patch in and of itself is irrelevant. An incompressible, perfectly smooth wheel rotating on a frictionless bearing rolling over an incompressible, perfectly smooth surface should see no friction (as long as it is not slipping)...whether that wheel is 1 foot wide or 1 mile wide.

As far as the tire is concerned, rolling resistance comes from flexing of the wheel. While the tread compresses, the majority of the flexing comes from the sidewall. Higher pressure doesn't work primarily by decreasing the contact patch, it works by decreasing sidewall flexure. Unlike balloons, radial tires have metal belts running under the tread, limiting their shape anyways.

Now, why would a skinny tire be more beneficial then a wider tire? Less flexing and surface area for slipping, but specifically how?

I think increasing wheel weight to aid in cruising is a bad idea. It's akin to putting extra weight in your car to tackle hills. Sure you will glide longer, but you'll take more energy to get up to speed in the first place. Unfortunately, the 2nd Law of Thermodynamics rules...you'll always get less energy out of a glide then you put into the acceleration. When everything is appropriately sized, light weight maximizes efficiciency.

Increasing wheel diameter may also be helpful by lowering engine rpms. Generally a useful feature, but must be appropriately sized.

Intuitively, I'd choose a wheel first based on weight, then diameter, then width.

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Old 04-27-2008, 09:57 PM   #16 (permalink)
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toomuch - thanks again for the post, that was a good article and a neat site!

LostCause - I think the sidewall flex would be another reason to pick a larger diameter rim and slimmer tires. There is less sidewall to flex, and as a result probably a lower contact surface as well. The main benefit though is the displacement of all the weight to the outside of the wheel, that has to be another factor of rolling resistance.

I also wanted to respond to your comment about the contact patch. The contact patch is relevant because its the amount of area that the car is putting energy into to make it move. It's like trying to run in the summer with snow shoes versus running with just sneakers. The sneakers have a smaller surface area and allow you to focus the energy into moving you forward on a smaller area. With the snow shoes there is a larger surface area and the force you apply is displaced accordingly so it takes more energy to move as fast as you would in sneakers.

I guess the problem is that we are not talking just about rolling, we're mainly talking about how much gas is used going from the engine to the tires, and if you have a smaller contact patch you will end up using less.
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Last edited by CorollaMaster; 04-27-2008 at 10:04 PM..
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Old 04-27-2008, 10:09 PM   #17 (permalink)
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Quote:
Originally Posted by CorollaMaster View Post
LostCause - I think the sidewall flex would be another reason to pick a larger diameter rim and slimmer tires. There is less sidewall to flex, and as a result probably a lower contact surface as well. The main benefit though is the displacement of all the weight to the outside of the wheel.

I also wanted to respond to your comment about the contact patch. The contact patch is relevant because its the amount of area that the car is putting energy into to make it move...
Larger diameter rim = more weight. A slimmer tire also doesn't necessarily mean less sidewall flex, as it could just be concentrated into a smaller area.

The contact patch is irrelevant in an ideal world. If nothing ever deformed/slipped, it wouldn't make a difference if you were pushing a car with a tennis shoe or the head of a pin. Therefore, in the real world, it is not the contact patch that is the issue, it is something else. The contact patch just manifests an issue concentrated somewhere else.

I'm not saying to disregard the contact patch, but to understand that rolling resistance does not lie solely in its geometric shape.

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Old 04-27-2008, 10:23 PM   #18 (permalink)
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Quote:
A slimmer tire also doesn't necessarily mean less sidewall flex, as it could just be concentrated into a smaller area.
Wouldn't that mean that less of the sidewall is flexing if its in a smaller area?

In addition, the amount of surface area that the power is being put into by the engine definitely effects the gas mileage. While the contact patch isn't solely responsible for rolling resistance, which I agree has a lot of different factors that add to it from what I've been reading (rotational inertia, design pattern of tread, skinniness of the tire, etc), it is definitely still very important to factor in. Unfortunately as you said, its not a perfect world with tires that don't deform.
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First Hypermiling experiment w/o ScanGuage 43.4mpg
Hypermiling with ScanGuage around 51mpg!

Last edited by CorollaMaster; 04-27-2008 at 10:35 PM..
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Old 04-27-2008, 11:14 PM   #19 (permalink)
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hey frank, if your contact patch is circlualar at high psi then u would figure area with pie r squared. using that formula 2.5 *2.5*3.14 gives a total of 1000. again i am not saying that this method is accurate. but the theory of contact patches being the same for the same psi and weight is. Just for the record. The weight method is not super accurate. There is lots of variables, (air gauge, measuring methods, even tread patterns, etc can effect the results) .
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Old 04-30-2008, 09:07 PM   #20 (permalink)
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I think we are both on the same page. Just we are on different subject. My point was that tire size does not affect contact size. Alot of people wrongly think that. Now the side note to what is what you said. That bigger tires CAN run at a lower psi and still be stable. Then (at the lower PSI) it would give a higher contact patch. Its kind of a circle. Kind of like octane. Higher octane does not increase performance or economy,BUT if you can increase the timing that will increase the perfromance. Therefore octane allows better performance. I have also been reading about this flywheel subject. My two cents, weight will hurt economy, no way around it. BUT a engine designed with a heavy flywheel might get great gas milage. Eg. a 440 big block in a 69 charger with a super light flywheel will probably get worse gas mileage than xfi metro with a super heavy flywheel. does that prove that a super heavy flywheel is fuel efficent NO. It is just that the design need a heavy flywheel for some reason. (maybe drivability)

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