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Old 09-24-2008, 12:44 PM   #51 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
No. My understanding of footprint length is that at a given load/pressure the length is essentially independent of the width - everything else being equal.
I don't get why if I switch to a tire that is larger, the contact patch would be larger. The load on the tire being the same and the pressure too, the contact patch should keep approximately the same area. no?

Quote:
My understanding of RR is that most RR comes from the deflection of the tread and that the flexing of the sidewalls is coincidental. Put another way, if I take 2 identical tires, but one is pretty much worn out and the other tire is new - and then use the same load / pressure, not only would the footprint be the same size, but the worn tire will have substantially lower RR.

My understanding is that the thing that creates the most hysteresis (heat generation) is the thing that has the most mass - the tread rubber.
I can't disagree with the first part, as you remove material from the tread hysteresis can only go down (all else equal).

The second part I had problems with, so I went to look back at the 80% of total hysteresis from sidewalls figure I had in mind. Turns out I was wrong. In the transportation research board special report 286 there was the following :

"In particular, the tread contains much of the hysteretic material in
the tire. Not only is the tread made of rubber compounds that are
designed to improve wet traction, the tread band also contains relatively
large quantities of material to prolong wear life. Studies indicate that the
tread alone can contribute more than half of hysteretic energy losses in
a tire (Chang and Shackelton 1983; Martini 1983; LaClair 2005)."

So thank you for pointing out my error. Sidewall hysteresis plays a role, though it's not the most significant.

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Old 09-24-2008, 02:48 PM   #52 (permalink)
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its really difficult to know what tyres to buy. suggestions anyone? make/brand/product to go for?
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Old 09-24-2008, 03:05 PM   #53 (permalink)
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Quote:
Originally Posted by tasdrouille View Post
I don't get why if I switch to a tire that is larger, the contact patch would be larger. The load on the tire being the same and the pressure too, the contact patch should keep approximately the same area. no?

.......
First, let me state that the information available about contact patch is really sparse. So I am going on a lot of ..... ah ...... let's call it innuendo in the literature.

The best I can figure out is that you can envision the contact patch as a line bisecting a circle, with the length of the line being the length of the footprint. So you can go from being tangent to the circle, through barely bisecting the circle, and into the circle so you get a sizeable cresent. As you do that, the effect of the circumference diminishes. Put another way, moving further into the circle the same amount as previous, results in less than twice the length.

Part of the problem here is that when you go larger in diameter, you also have to change something else - the width, the load carrying capacity, etc. You just can not do anything without having a confounding issue.

So if you put those 2 pieces together and try to keep the same load carrying capacity as well as the diameter, the result is a narrower tire.

When you go to consider the same load (on the same load carrying capacity, but larger diameter) the % deflection remains the same and that means an ever so slightly longer footprint.


But this is really "picking nits". There is such a small difference it is not worth discussing.
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Old 09-24-2008, 03:36 PM   #54 (permalink)
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Your question has too many variables. There's no way to 'even-out' all the variables to give you a real-world 'yes/no' answer. For instance: If you regear to compensate for the tire size, you STILL have to compensate for the increased wind resistance from the vehicle sitting higher off the ground.

There are also 'rolling mass' issues to resolve, and so many other things that it's nearly impossible to overcome them all outside of a lab. Automotive engineers have been running these kind of numbers for years and still can't build a perfect car.

But I think you can get an idea of what's going on by looking at bikes. A guy coasting down hill on a 20" BMX bike won't be able to keep up with another on a 24" mountain bike. A guy with 700mm tires (27+") will blow them both away.

Now let's think about this on a car... If you kept the same width/psi/ kept the 'rolling mass' as close as possible, AND shoved the wheels up in the wheelwells so you're not increasing your wind resistance, you probably would get some kind of increase in milage. (don't forget to re-calibrate your speedometer) IMHO, a gear change isn't necessary because you would essentially be getting 'more gas friendly' gearing anyway. (in the Jeep community, we want the opposite)
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Old 09-24-2008, 04:54 PM   #55 (permalink)
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So it leads me to believe that I really need a 175/50R15.
Instead of the 155/80R13? (but 175/70R13 is the same circumfrence and has less rr?)
Is that even available?
Now I gotta get new rims.
Again
S.
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Old 09-24-2008, 05:51 PM   #56 (permalink)
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Honestly, use what you have or what you can get for cheap. I drove the last 2 summers with winter tires because I got 2 sets of wheels with the car and both were winters. Yet, my FE aint that bad.
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Old 09-25-2008, 01:57 AM   #57 (permalink)
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance -- Special Report 286

Anyone that is still confused should read this thoroughly as it explains answers more completely then mentioned here. IF you read this completely you can make an informed decision about rolling resistance and tires. I tried to cover everything here that was in the study, but it seems to be ignored.

And to answer someones question about the most efficient tire: Bridgestone 381 (It is a 14 inch tire).

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