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Old 06-05-2008, 07:15 PM   #91 (permalink)
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CapriRacer -

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Originally Posted by CapriRacer View Post
Progress Report:

I may have found something pertinent:

SAE 800087 - The Effect of Inflation Pressure on Bias, Bias-Belted, and Radial Tire Performance - Coolier / Warchol - Feb 1980.

...

The net effect is that the tires that go onto new cars are somewhat different than what you would buy on the open market (sometimes, a lot different.) Typically car manufacturers want tires with lower rolling resistance (in order to meet CAFE requirements) and RR is bought by sacrificing wear and / or traction - especially wet traction.
I have read this elsewhere. The upshot of this is that if you can, getting the same tires that the car came with will get you LRR without having to do much homework.

Quote:
So anyone who tells you that the car manufacturers buy tires from the lowest bidder and that's why they don't perform well doesn't know what he is talking about.

...

There are many other types of lab tests - Step Up Speed, Plunger Energy, Bead Unseating, etc. - and each of these is designed to quanitify a particular aspect of tire performance.

There are also many on vehicle tests - but these are pretty expensive and controlling ambient conditions is impossible, so they become problematic.

But the point I want to make here is that the industry standard SUL test remains the backbone for tire durability - and more to the point, tire pressure is not one of the items that becomes a testing variable. The bursting pressure of a new tire is an artifact of the design based in these lab tests.
This is interesting. Once again, as Ecomodders we are using a component of the car (the tires) "outside the test domain".

Based on your description of the test, I am happy with my choice of "inflate to the max inflation rating". Because of this, however, I also think I have to invest in an accurate tire gauge.

What's the most accurate gauge out there? Can a gauge be "calibrated"? If I knew the % error, would that error be linear or ?!?!?!?!?

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Old 06-05-2008, 07:42 PM   #92 (permalink)
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I just bought one from Pep Boys that is +/- 2 PSI for $8, they had a +/- 1 PSI one for $10 but it didn't look as reliable. Either of them may or may not be entirely that accurate, but the one I got is accurate enough for me.
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Old 06-05-2008, 08:14 PM   #93 (permalink)
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Thanks Capri. I never worked on the rubber end of car design, just the performance & emissions end

So, I can't just say that my car has a corner weight of 228kg and the tyre's max loading is 387kg..... so the sidewall maximum of 51psi can't be scaled up to 72psi due to the loading being only 60%???
Shame its not that easy!
Plugging 36psi into the equation instead of 51 gives a maximum of ......... 51psi based on scaled pressure due to less than maximum loading!! Wierd!!

Edit - Have I got this back to front?? The tyre's maximum loading increases with increasing pressure.... but does its maximum pressure decrease with decreasing load???
You've worded the question a little funny. I think it would be better to state it this way: If you are using something less than the maximum load carrying capacity, then the pressure in the table is a MINIMUM pressure for the indicated load.

And it is common engineering practice to underutilize a product - and this is probably more true for tires as the load tables are predicated on some pretty ideal conditions - smooth roads, ambient temperatures below 100F, etc. - and this is substantiated by a lot of statistic gathering that says that most of the tire failures occur in hot climates.
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Old 06-06-2008, 12:26 AM   #94 (permalink)
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narrower might tolerate over inflate better, just a hunch. those racing bicycles run crazy high pressures.
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Old 06-06-2008, 06:25 AM   #95 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
Capri:

Slightly off topic, but: do you have any insights as to tire sizing for minimum R.R.? It is a popular notion to think narrower tires would have lower R.R., but I'm not sure about that.
Read this:

http://www.geocities.com/barrystiretech/rrandfe.html

What this says is that it is a lot more complex than just width, but tread volume is a factor - more being worse - and generally wider tires have wider tread. The trick would be to use the same or more load carrying capacity due to the tire size or you will negate any improvements you might receive.
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Old 06-06-2008, 07:47 AM   #96 (permalink)
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A picture is really worth a thousand words...I posted links to the following in this tread and others, but I guess I have to link straight to the picture.

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Old 06-06-2008, 04:06 PM   #97 (permalink)
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Am I reading that right? The P215/70/R15 had lower resistance than the 165/80/R13?
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Old 06-06-2008, 06:34 PM   #98 (permalink)
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Am I reading that right? The P215/70/R15 had lower resistance than the 165/80/R13?
i can;t make out the sizes
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Old 06-07-2008, 08:37 AM   #99 (permalink)
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Quote:
Originally Posted by JJW View Post
Am I reading that right? The P215/70/R15 had lower resistance than the 165/80/R13?
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.
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Old 06-07-2008, 01:13 PM   #100 (permalink)
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This is the source of the pic I posted earlier.

The force of rolling resistance is the RRC times the normal force. So it is load dependent. Take a fixed load and apply it to 2 different tire sizes, the rolling resistance force will depend on RRC alone.

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