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Old 06-03-2008, 04:01 PM   #81 (permalink)
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Quote:
Originally Posted by PaleMelanesian View Post
The car's recommended pressure is most definitely NOT right. My last set of tires, rated 44 psi, I ran them at 35, while the car called for 32. The wore out on the edges more than in the middle. That's above the recommended pressure, and it was still too low.

The question that remains is this: How much higher than this would be the best compromise between tread wear, traction, tire fatigue, and mileage?
I cite this website ad-nauseum :

the wheel & tyre bible - Page 2
http://www.carbibles.com/tyre_bible_pg2.html
Quote:
The Max. Pressure -10% theory.

Every tyre has a maximum inflation pressure stamped on the side somewhere. This is the maximum pressure the tyre can safely achieve under load. It is not the pressure you should inflate them to.
Having said this, I've given up using the door pillar sticker as my starting point and instead use the max.pressure-10% theory. According to the wags on many internet forums you can get the best performance by inflating them to 10% less than their recommended maximum pressure (the tyres, not the wags - they already haves inflated egos). It's a vague rule of thumb, and given that every car is different in weight and handling, it's a bit of a sledgehammer approach. But from my experience it does seem to provide a better starting point for adjusting tyre pressures. So to go back to my Subaru Impreza example, the maximum pressure on my Yokohama tyres is 44psi. 10% of that is 4.4, so 44-4.4=39.6psi which is about where I ended up. On my Element, the maximum pressure is 40psi so the 10% rule started me out at 36psi. I added one more to see what happened and it got better. Going up to 38psi and it definitely went off the boil, so for my vehicle and my driving style, 37psi on the Element was the sweet spot.
The emphasis is for performance. If you're cautious like me, it's a great place to start you tire PSI. I am now at max sidewall pressure (51 PSI) because I am comfortable with staying within the tire's design limits.

CarloSW2

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Old 06-04-2008, 09:27 AM   #82 (permalink)
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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.

This is an Society of Automotive Engineers paper presented in Feb of 1980. I don't yet know what it says, but I am hopeful it will address the issue.

************************************************** *****

I think I ought to correct some misconceptions about tire design:

Ax: I hope you don't think I am picking on you, but your post had a lot of stuff in it I could comment on.

Quote:
Originally Posted by AXMonster View Post
At a guess, I'd say that passenger car tyres are designed around the max inflation pressure that can be used at the tyre's rated loading at its rated speed.

i.e. 51psi at 1350lb at 130mph.
First let's start with this:

http://www.geocities.com/barrystiret...izingorgs.html

You should follow the link to the page on Load Tables, as well.

What you should get out of this is that passenger car tires are designed around this table (and tables like this) and that the design pressure is 35 or 36 psi depending on which standardizing organization is being followed.

Please note that the pressure on the load tables is NOT necessarily the max pressure indicated on the sidewall - and in most cases it is not. The max pressure listed on the sidewall comes from an allowable usage pressure as indicated on one of the pages of the design standard. (See the note about page 1-35)

Quote:
Originally Posted by AXMonster View Post

...............


These figures will be initially simulated during the design stage, then proven out with correlation testing, then real world testing.
A factor or safety will be designed in (possibly around 1.5 to 2, and I'd guess no less) to account for deviances in manufacturing tolerances and materials specifications between batches.

The testing would include kerb and pot hole strike cycles at specific speeds, with a required number of cycles before failure i.e. 10,000 cycles to failure pass rate. These would be tested on new and aged tyres to determine any age related degredation.

.................
Basic tire testing involves a tire loaded against a wheel with a diameter of 67" (and a fraction, so that the circumference of the wheel is a even fraction of a mile) in a room at 100F at a speed of 50 mph (so that every 2 hours of running is 100 miles). The testing machine is commonly called a Pulley Wheel.

The tire is first tested at the rated load and rated inflation pressure, and then different conditions are applied. This is standard throughout the industry. Typically, the load is increased in steps (so the test is called a Step Up Load test - SUL). If this testing is part of the design process, the tire is run to failure. However, if this test is for government compliance testing or QA testing, the test may be terminated before failure.

The machine and the tests were first used in the 1930's and while a lot has happened in the 70 years since, the basic test remains very similar and useful.

Sometime back, someone tried to simulate the effect rough roads would have on a tire and cleats were bolted to the wheel. The tires did not show any performance degradation, but the testing machine took quite a beating and the research was abandoned.


Quote:
Originally Posted by AXMonster View Post

...........

Testing would begin at a baseline predicted by the FEA simulation, then change according to the test results and failure rates. From all of this the result would be a homologated tyre that passes the required tests and would be given a load rating, speed rating and max inflation pressure based on the test results.

..............
The loads and inflation pressures are set by the tire standardizing organizations and testing would be performed to assure the tires perform addequately to that standard.

There are many parallels for other components. Bolts, for example, come in various sizes and the breaking strength (and a few other properties) are set by SAE - in the form of a table similar to the tire load table. It is the responsibility of the bolt manufacturer to meet those standards - and it is the design engineer's responsibility (in this case, the vehicle engineer) to use the bolt within that limitation.

There are decades of experience in using SUL and every tire manufacturer has its own take on how the test should be run / what the acceptance criteria should be / and how to design tires to meet the tire standard.

Finite Element Modeling of tires is a relatively recent phenomenon (oh, in the last 15 years) and while it is used to help design tires, it is more of a research tool to analyze why tires are failing the way they do and to shorten the time it takes to develop design criteria - rather than spending a lot of valuable time building tires, then testing them on the pulley wheel.

Quote:
Originally Posted by AXMonster View Post

.............

Its a bit of a wired science, but more and more tyres are being designed for specific vehicle models these days.

..............
Vehicle manufacturers specify a lot about the tires they select - rolling resistance, ride quality, handling, force and moment, dry traction, wet tractio, snow traction, etc. There are tests specified for each of these properties and it can take as long as 2 years to get through all the testing.

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.

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.

Quote:
Originally Posted by AXMonster View Post

............

If you fit a tyre rated to 1350lb and 130mph to a car that can only do 110mph and has a corner weight of 600lb, then at a guess you'd be reasonably safe increasing the max pressure over the limit on the sidewall...... by how much I don't know, but a few psi probably wouldn't put it into the danger area?

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.
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Old 06-04-2008, 09:31 AM   #83 (permalink)
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i may just test rolling resistance ,,, add air 2psi at a time till it doesn't change
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Old 06-04-2008, 11:42 AM   #84 (permalink)
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Quote:
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i may just test rolling resistance ,,, add air 2psi at a time till it doesn't change

Metro already has with his car.
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Old 06-04-2008, 01:50 PM   #85 (permalink)
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Metro already has with his car.
missed that thread
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Old 06-05-2008, 03:09 PM   #86 (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???
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Old 06-05-2008, 03:26 PM   #87 (permalink)
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http://www.geocities.com/barrystiretech/rrandfe.html

The effect of inflation pressure on rolling resistance. Nice graph showing % RR reduction with increasing pressure. Graph not clickable, but it shows going to 44psi from 24psi gives a 27% reduction in RR!
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Old 06-05-2008, 05:35 PM   #88 (permalink)
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Now if i could see a similar graph showing traction versus pressure (right up to sidewall) i might just shut up about this subject!!

ollie
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Old 06-05-2008, 05:51 PM   #89 (permalink)
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Quote:
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Now if i could see a similar graph showing traction versus pressure (right up to sidewall) i might just shut up about this subject!!

ollie
you got a strain gauge ??
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Old 06-05-2008, 05:59 PM   #90 (permalink)
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Hello,

Quote:
Originally Posted by AXMonster View Post
http://www.geocities.com/barrystiretech/rrandfe.html

The effect of inflation pressure on rolling resistance. Nice graph showing % RR reduction with increasing pressure. Graph not clickable, but it shows going to 44psi from 24psi gives a 27% reduction in RR!


Slight correction: the graph's range is from 24psi up to 40psi. And the curve certainly looks to be very flat at 40psi -- anything above 40psi seems to have greatly diminished returns.

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