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Old 06-08-2008, 04:36 PM   #101 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
NO!! - If I am reading the graphic right.

What this is saying (I think) is that the RRC (Rolling Resistance Coefficient) is lower for larger tires.

RRC is RR divided by the load on the tire. I've been searching for the source document to see how they tested the tires - and haven't found it (so far). But Smithers is well known and well respected in the tire industry, so I'm going to take an educated guess that they tested all these tires at the rated conditions - the logical way to do this.

So a P215/70R15 - which has about 50% more load carrying capacity also has about a 30% lower RRC - which means that the P215/70R15 has more RR - just like you'd think.

But what I think this graphic points to is that large tires could be used in such a way that they would generate lower RR.
The other factor is sidewall height. Low aspect ratio tires have stiffer sidewalls = less RR. Look at the tires on mileage contest cars: tall, skinny, short sidewall. The tricky part is balancing low RR vs handling requirements.

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Old 06-08-2008, 07:22 PM   #102 (permalink)
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well two weeks at 2 psi over the front tires on my truck are starting to cup
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Old 06-08-2008, 07:59 PM   #103 (permalink)
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Quote:
well two weeks at 2 psi over the front tires on my truck are starting to cup
From here:
http://www.procarcare.com/includes/c...dtirewear.html

Cupping
Cups or scalloped dips appearing around the edge of the tread on one side or the other, almost always indicate worn (sometimes bent) suspension parts. Adjustment of wheel alignment alone will seldom cure the problem. Any worn component that connects the wheel to the car (ball joint, wheel bearing, shock absorber, springs, bushings, etc.) can cause this condition. Worn components should be replaced with new ones. The worn tire should be balanced and possibly moved to a different location on the car. Occasionally, wheels that are out of balance will wear like this, but wheel imbalance usually shows up as bald spots between the outside edges and center of the tread.

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Old 06-08-2008, 08:11 PM   #104 (permalink)
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Quote:
Originally Posted by ttoyoda View Post
From here:
http://www.procarcare.com/includes/c...dtirewear.html

Cupping
Cups or scalloped dips appearing around the edge of the tread on one side or the other, almost always indicate worn (sometimes bent) suspension parts. Adjustment of wheel alignment alone will seldom cure the problem. Any worn component that connects the wheel to the car (ball joint, wheel bearing, shock absorber, springs, bushings, etc.) can cause this condition. Worn components should be replaced with new ones. The worn tire should be balanced and possibly moved to a different location on the car. Occasionally, wheels that are out of balance will wear like this, but wheel imbalance usually shows up as bald spots between the outside edges and center of the tread.

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its all new and only started it in the last 1200 miles ,, when i upped the pressure,, i was watching for it ,,,,
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Old 06-09-2008, 07:45 AM   #105 (permalink)
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Quote:
Originally Posted by SuperTrooper View Post
The other factor is sidewall height. Low aspect ratio tires have stiffer sidewalls = less RR. Look at the tires on mileage contest cars: tall, skinny, short sidewall. The tricky part is balancing low RR vs handling requirements.

Ah .... Mmmmmm .......Ah ........ Not exactly.

First, isn't this phrase contradictory? ....... "tall, skinny, short sidewall."

If you have tall, you don't have short sidewalls.


And this is certainly not true: ....... " Low aspect ratio tires have stiffer sidewalls = less RR."

Quite the opposite. Low aspect ratio tires have wider tread widths and that is the wrong direction for RR. Not to mention that lower aspect ratio tires generally have good grip - and that means poor RR. Contrary to popular belief, sidewall stiffness doesn't affect RR much.

BTW, based on the graphic we were discussing, you can't draw any conclusions. about aspect ratios - they are pretty mixed up in the data. Plus the lowest aspect ratio tested was a 60 series - and I don't know anyone who would call those a "Low Profile".
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Old 06-10-2008, 01:27 PM   #106 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
Ah .... Mmmmmm .......Ah ........ Not exactly.

First, isn't this phrase contradictory? ....... "tall, skinny, short sidewall."

If you have tall, you don't have short sidewalls.


And this is certainly not true: ....... " Low aspect ratio tires have stiffer sidewalls = less RR."

Quite the opposite. Low aspect ratio tires have wider tread widths and that is the wrong direction for RR. Not to mention that lower aspect ratio tires generally have good grip - and that means poor RR. Contrary to popular belief, sidewall stiffness doesn't affect RR much.

BTW, based on the graphic we were discussing, you can't draw any conclusions. about aspect ratios - they are pretty mixed up in the data. Plus the lowest aspect ratio tested was a 60 series - and I don't know anyone who would call those a "Low Profile".
I was using a conclusion contained in the Finnish study of RR on roads:

http://www.motiva.fi/attachment/f16d...ce_on_road.pdf

"2.1.7 Size of Tire
Both the radius of the tyre and the aspect ratio H/W influence the rolling resistance. Rolling resistance can be reduced by increasing the radius and decreasing the aspect ratio of the tyre. Lower aspect ratio means increased stiffness of the sidewalls and thus lower hysteresis losses."

By taller I meant a larger radius. The trick is finding a larger radius tire with a lower aspect ratio (on a larger wheel) that doesn't widen the tire.
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Old 06-10-2008, 02:16 PM   #107 (permalink)
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RR is mostly about hysteresis. Reduce hysteresis and you reduce RR. Since roughly 80% of the hysteresis in tires happens in the sidewalls, make the sidewalls stiffer and RR will go down. The easiest way to do so is to increase the pressure. Btw, if you're adding material to make the sidewalls stiffer (same material, just ticker sidewalls), that won't work nearly a well as added pressure. Even though it'll flex less, more material will be flexing so overall hysteresis will pretty much stay unchanged.

Re tread width. It's not about skinny or wide, it's about finding the right contact patch size. Long contact patches from skinny tires increase RR due to increased hysteresis. Make the tread wider and RR goes down, up to a point where RR goes up again.

You can validate that in the graph posted earlier.
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Old 06-10-2008, 04:53 PM   #108 (permalink)
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Quote:
Originally Posted by SuperTrooper View Post
I was using a conclusion contained in the Finnish study of RR on roads:

http://www.motiva.fi/attachment/f16d...ce_on_road.pdf

"2.1.7 Size of Tire
Both the radius of the tyre and the aspect ratio H/W influence the rolling resistance. Rolling resistance can be reduced by increasing the radius and decreasing the aspect ratio of the tyre. Lower aspect ratio means increased stiffness of the sidewalls and thus lower hysteresis losses."

By taller I meant a larger radius. The trick is finding a larger radius tire with a lower aspect ratio (on a larger wheel) that doesn't widen the tire.
Unfortunately, our Finnish friend is wrong.

First, it wasn't a conclusion. It was in the introduction and he was referencing another paper - one on vehicle modeling - and I'm sure he misinterpreted what was written. I'm in the process of locating a copy of the reference paper in order to understand what where he went wrong.

Second, if you are stuck with a certain vehicle - as opposed to designing one from scratch - you are going to have trouble with the size / aspect ratio thing. The fenderwells limit your ability to go with larger diameter tires. Out another way, you can't go with a larger diameter tire and go lower in aspect ration without also affecting the load carrying capacity - and as we can see in the graphic that Tasdrouille posted - load carrying capacity (as evidenced by the tire size) greatly affects RRC.


Quote:
Originally Posted by tasdrouille View Post
RR is mostly about hysteresis. Reduce hysteresis and you reduce RR. Since roughly 80% of the hysteresis in tires happens in the sidewalls, make the sidewalls stiffer and RR will go down. The easiest way to do so is to increase the pressure. Btw, if you're adding material to make the sidewalls stiffer (same material, just ticker sidewalls), that won't work nearly a well as added pressure. Even though it'll flex less, more material will be flexing so overall hysteresis will pretty much stay unchanged.

Re tread width. It's not about skinny or wide, it's about finding the right contact patch size. Long contact patches from skinny tires increase RR due to increased hysteresis. Make the tread wider and RR goes down, up to a point where RR goes up again.

You can validate that in the graph posted earlier.
I'm afraid you are mistaken in this. I think the source of the misunderstanding is that you are trying to look at too big a picture.

If you imagine a tire with a given load on it - and you can change the tire, but you can't change the load carrying capacity - then narrow treaded tires work better - less volume of rubber moving = less hysteresis.

BTW, the graph posted earlier is RRC (Rolling Resistance Coefficient) and not RR (Rolling Resistance) - and that might be part of the confusion.
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Old 06-10-2008, 06:07 PM   #109 (permalink)
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tasdrouille -

Quote:
Originally Posted by tasdrouille View Post
...

Re tread width. It's not about skinny or wide, it's about finding the right contact patch size. Long contact patches from skinny tires increase RR due to increased hysteresis. Make the tread wider and RR goes down, up to a point where RR goes up again.

...
The following is an exaggeration. Ignore for the moment that the wide tire would probably "sag" in the gaps :



I think that in the above, the wider tire has a better LRR. Is that true?

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Old 06-10-2008, 06:51 PM   #110 (permalink)
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It's easy to say someone is wrong because they don't reach the same conclusion as you. The text book he references is 539 pages. He did get the ISBN number wrong.

At the beginning of Section 2 he references a couple of papers from the SAE defining the factors in RR, of which 90% is due to material hysteresis. Sidewall stiffness contributes to hysteresis inversely. Shorter sidewalls are stiffer. Less sidewall = lower hysteresis. Any disagreement here?

I was speaking more of a theoretical nature. As I noted in my posts it would be tricky to achieve this. I referenced the tires/wheels on supermileage racers as the ideal, not as a practical solution for daily drivers.

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