Hyperinflating tires

[CapriRacer]

Originally Posted by SuperTrooper

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.

I realize it is easy to say someone is wrong, but when he is wrong, he's wrong. Referencing a textbook doesn't make him right. But more importantly, he never used that bit of information again to see if the conclusion is right. In fact, the rest of the paper deals with building a machine to test RR and he used it to test pavement. I think our Finnish friend would not qualify as an expert in tires, but based on his research he might be considered an expert on pavement.

Nevertheless, I am in the process of going through the reference to see where he found this conclusion and why he is mistaken about aspect ratio.

Originally Posted by SuperTrooper

.........

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?

.........

Yes, but I'll come back to that point.

Originally Posted by SuperTrooper

.......

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.

First, I'm going to reference a document that referenced in a previously posted link. Here's the whole document:

http://www.energy.ca.gov/transportat...%20Testing.pdf

The problem with this paper is that they are looking at a HUGE!!! volume of tires of all different types - large sizes / small sizes, all season/high performance, different speed ratings, OE / replacement, etc. - all of which cloud the issue and make it difficult to draw conclusions. We need to sort this out a bit.

On page 34, there is a graph that shows the relationship between RR and Max Load (I'm going to refer to this as "Rated Load") Notice there is a pretty strong correlation between RR and Rated Load - R squared = 86%. Considering all the different types of tires they looked at, this is a remarkable correlation.

If we want to compared the affect aspect ratio has on RR, then we need to hold the Rated Load constant.

-- As an aside - Look at pages 11 and 12, where they compare the RR values for the same size (and therefore, the same Rated Load). There is an incredible range of values - more than plus or minus 25%. This makes the data being published highly variable and we have to be careful with it.

Now look at page 30, where they publish the RR vs tire size (and tire size controls Rated Load). If we choose carefully, we can pick out tires that have about the same rated load, but different aspect ratios. Here's an example:

P195/75R14 (LI92)/ P205/70R14 (LI93)/ P205/65R15 (LI92):

From the graph on page 30, here's the order for RR: P195/75R14 (LI92) / P205/65R15 (LI92) / P205/70R14 (LI93)

Notice that this is more or less in the order of Load Index (LI) - LI being a quick way of referencing rated load - and NOT in the order of aspect ratio.

If you try to keep the width the same - P185/75R14 (LI89) / P185/70R14 (LI87) / P185/65R14 (LI85) - not only is the rated load going down, but the graph on page 30 shows the rack order to be P185/65R15 / and the other 2 are nearly identical. This is probably where the confusing is coming from.

The problem is that there are confounding issues. So if you want to define the role aspect ratio plays in RR, you are always going to get confounded by something. And since we know that rated load plays a HUGE role in RR, we have to hold it as constant as we can. The net effect is that every time you go to a lower aspect ratio, you have to go wider in section width (and therefore tread width) to maintain the load carrying capacity - and that means more volume of material - and that means more RR.

Please note: RR testing takes place at the rated load and rated pressure - which is not the way a vehicle using tires. A vehicle holds the load constant - another confounding issue.


Try a couple of other sizes and see what you get. Remember: The elephant in the room is "Rated Load".

[tasdrouille]

Originally Posted by CapriRacer

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.

That aint quite so.

RR = RRC * load

You've got a car applying a load X.
You have P215/70R15 with a RRC of roughly 0.0099.
You have P195/65R15 with a RRC of roughly 0.0113.
Both tires can handle load X.

Which tire has the lowest RR?

EDIT:

And since we know that rated load plays a HUGE role in RR

Where exactly do you see rated load in the RR equation? It's the actual load, we usually never reach the rated loads.

[CapriRacer]

Originally Posted by tasdrouille

That aint quite so.

RR = RRC * load

You've got a car applying a load X.
You have P215/70R15 with a RRC of roughly 0.0099.
You have P195/65R15 with a RRC of roughly 0.0113.
Both tires can handle load X.

Which tire has the lowest RR?

EDIT:

Where exactly do you see rated load in the RR equation?

This gets back to the issue of confounding:

A P215/70R15 has a Load Index of 97 and an OD of 26.9" , where a P195/65R15 has a Load Index of 89 and an OD of 25.0"

What this means is that a P215/70R15 would never go on a vehicle where a P195/65R15 is specified and vice versa.

As you have noticed, the RRC goes up as the Load Index goes up, but this isn't useful in dealing with an individual vehicle (unless you are either designing one from scratch or extensively modifying an existing one) As a general rule, a given vehicle is pretty much constrained to take tires within a small range of LI's.

[metroschultz]

Gentlemen,
I've been following this thread since day one, and I am still confused.
Bear in mind that not all of your readers are college trained, and research hardened.
So,
Would it be easier, perhaps, to pick a Vehicle and Style of Operation, and go from there.
For instance use Daox' Matrix for daily chores, then choose a tire (or tires) that best fit it for reduction in resistance?
Hmm mm?
Or use My metro, (tho I won't need tires for a few years, there are dozens on this forum who have Metros), up-size to 13 inch, used only for to & from work with the occasional errand, which tire would you recommend?
I believe that is what the end result of this thread should lead to. Thanks For Your Understanding,
Schultz.

[tasdrouille]

You have not answered my questions. Anyway, I'm done with this.

Bottom line it that the tire with the lowest RRC that will fit on your car (size and load) will have the lowest rolling resistance.

[SuperTrooper]

CapriRacer, I agree with your conclusions with regard to load. Indeed, I made the same type of statements in a thread I started trying to collect data on overinflation vs MPG.

The name of the sight is ECOMODDER. To me that means some here are willing to make modifications in the pursuit of higher MPG with the understanding that there may be a trade-off in function or aesthetics. Some remove seats, carpets, etc to save weight. Others give up opening trunks/hatches in the name of aerodynamics. It's no different than what I see at the dragstrip: street driven cars with fat tires in the back and skinny ones up front. They give up certain abilities to gain others elsewhere. It's all part of a balance we choose to make vs original design. I could easily see someone on this site choosing to try a different wheel/tire combo purely in the name of more MPG, and willing to give up things like load factors in the bargain. My favorite thread being about a member who wanted to cut his car in half lengthwise.

You just have to let your perspective adjust a little when you're here.

Cheers!

[ttoyoda]

COMP:

its all new and only started it in the last 1200 miles ,, when i upped the pressure,, i was watching for it ,,,,

Sorry, I should have been more explicit. The increase in tire pressure has unmasked a resonance between tire stiffness and the suspension spring, which your worn out parts (shocks, struts, etc.) are unable to control. Thus the tire cupping.

[COMP]

Originally Posted by ttoyoda

COMP:

Sorry, I should have been more explicit. The increase in tire pressure has unmasked a resonance between tire stiffness and the suspension spring, which your worn out parts (shocks, struts, etc.) are unable to control. Thus the tire cupping.

i did a check,,riding next to the truck at 50mph on a rough road,,,i am geting more bounce (like a basketball over presure) and i have HD shocks on all 4 corners,,,and yes the ride is hard,,,,the rears are leaving the ground on some

[ttoyoda]

i did a check,,riding next to the truck at 50mph on a rough road,,,i am geting more bounce (like a basketball over presure) and i have HD shocks on all 4 corners,,,and yes the ride is hard,,,,the rears are leaving the ground on some

How many of the suspension parts are newer than 1977?
FYI, the symbol indicates sarcasm, I don't think that is the emotion you really mean to convey.

[trikkonceptz]

Back in the early 90's I think a company marketed a wheel which was actually 2 wheels in place of one. It looked like two spares were bolted on to each side of the car. The company claimed it improved road handling, wet weather traction and reduced rolling resistance, does anyone remember those? Or am I having an acid flash back?