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Towing Towing with your TrailVoy

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  #21  
Old 08-30-2009, 12:09 PM
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I have been told by the tire tire store that all tires they install are inflated to 5psi less than the max tire pressure. I think I have 40psi in my tires now.
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  #22  
Old 08-30-2009, 02:32 PM
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Interesting discussion here.

First, being an engineer, I can comment on the 'max pressure' rating on tires. All tires have a bursting (or failure) pressure, which relates to sidewall strength and bead strength. The max pressure rating certainly is not the burst pressure, but it is based on a safety factor (established by the society of automotive engineers [http://automobile.sae.org/]) from the actual designed failure pressure (tires are designed with the same safety requirements of pressure vessels). So while going above that pressure will not pop the tire, it will reduce your safety margin, or safety factor... for instance, when you encounter a pothole. (Since tires are not purely elastic, such as a balloon, the pressures will change when you change the normal load on the tire.)

I agree with Chickenhawk that there is an optimum tire pressure. Higher pressure does not automatically mean better performance or better friction. However this is a double edged sword.

First, a lower tire pressure should theoretically produce less lateral stability. A tire gains much of it's lateral stability from internal tire pressure because sidewalls by themselves have little compressive or bending strength. You will notice a tire with underinflated tires will tend to wander on the road more. Higher tire pressures improve tracking purely because it increases the internal spring rate of the tire. (With more tire pressure, it takes more force to laterally deflect the tire tread in relation to the bead)

Secondly, tire contact area is related to tire pressure, but also related to vehicle weight and rim width. Taking these factors into account, there is a range of optimum tire pressure that will allow the entire tread to contact the ground (albeit with different ground pressures distrubuted across the tire contact patch).

A rolling tire is always in a state of partial sliding friction and partial static friction. By tuning the tire pressure, you can determine where you want the tire to slip and where you want it to be in static friction (center vs. outside). The danger zone is if you reduce the contact patch far enough to actually reduce your contact patch size. At a certain point, the friction generated at the tire (which is dependent only upon total vertical force and tire material) can be greater than the internal shear strength of the rubber at the contact patch. This limits your avaliable lateral forces because the rubber will actually shear away before you reach your max avaliable friction limit.

So, as far as actual answers... and no more of this theoretical mumbo jumbo... the max pressure rating on a tire can be exceeded without directly endangering you, but don't try to sue the tire company if your tire blows out. Personally I prefer to run at about 40 PSI on the road, but I drop my tire pressure to 20 psi when I travel off road because it improves dirt traction (due to a greater contact patch, less shear is induced on the ground) and because it greatly improves bumpy road comfort because the tire spring rate is reduced by a lower tire pressure.

To answer the OP:
Quote:
Originally Posted by ohp688
One last question. I have all of my tires filled with Nitrogen. I have them set at the psi per the Envoy recommendations. 35 rear and 30 front. Is this okay for towing appx 4000lb's or should I bump them up a bit? Any help or recommendations would be greatly appreciated!!!!
It depends almost entirely on your tongue weight. If you can lift the trailer tongue by hand, then your load is balanced on the trailer, and you won't need to up your tire pressure. If you put your trailer on and you notice your rear end swaying more and tracking less, I'd bump the pressure up to somewhere around the max tire pressure. A higher tire pressure should help with the lateral stability of the tire, and possibly improve your rear end tracking while towing because you reduce the lateral spring component of the tire.

Too many words again...
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  #23  
Old 08-30-2009, 02:36 PM
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Quote:
Originally Posted by regularjoe View Post
I have been told by the tire tire store that all tires they install are inflated to 5psi less than the max tire pressure.
If they put 75 PSI in my 80 PSI max LT Load range E tires I'd smack 'em.

I think that wins the idiotic rule of thumb of the day award.
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  #24  
Old 08-30-2009, 03:06 PM
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Peopleeeee!!!!!!

It's all about moneeeeyyyyy!!!!
The sooner you figure that out, the better off you'll be!


Car industry is tire industry and oil industry related. They are all budy-budy.

The lower the tire pressure you have on your vehicle, as per car manufacturer recommendations (KEY WORD - recommendations), the more often you need to change the tires, the more gas you are going to use.

A tire inflated at the lowest scale, even if the vehicle has crappy suspension, it gives people a more "comfortable" ride, thus the perception of "quality".
The higher the quality perception, the higher the sale volume.

Tires wear out faster - tire companies make more money
Use more gas - oil companies make more money

There are not any reports in the car history confirming that a perfectly good tire, used at the proper loading capacity, at the speed and type of road was designed for a tire "exploded" due to 10-15 PSI over the manufacturer recommendations.

Yet there are thousands of reports of blow-out tires using manufacturer's recommendations or below manufacturer's recommendations.

I have yet to see a bold tire right in the middle as per some people suggestions that an overinflated tire wears in the middle.

Does any of the people, condemning the practice of going for the side wall max PSI or over, have pictures of those bold tires in the middle?

I'll show you my tires that I am using 10 PSI over the maximum rated (41 PSI)
on my ride. .

They don't look anything like the tire shop told you. Do they?

Another thing, some people claim that with an over inflated tire they experienced lose of traction and poor handling. Let me tell you this.. .. this is an physical impossibility. (providing everything else is in the perfect working order on the vehicle)

"The Roadie" says, and i totally agree with him: "This conversation must focus on the vehicles and tires, not the assumed skills, background, or motivations of the posters."

Would the following verifiable background count?



=== Sgt. Dave Storton ===
Is the Director of the San Jose Police Academy, and he holds a Master's Degree in Adult Education. He is the lead instructor for the Emergency Vehicle Operations Course (EVOC) at the San Jose Police Academy, and is a lead instructor for the local regional academy. He teaches EVOC instructor courses, advanced EVOC instructor courses, off road EVOC, counter-terrorist / dignitary protection driving, and motion picture stunt driving. Dave has trained over 3,500 drivers.
================================================== =====

How about this?


=== Bobby Ore ===
A veteran stunt driver Ore, traveled the world for 25 years teaching military, FBI, CIA and Secret Service agents to drive in extreme circumstances.
He holds 13 automotive world records, including one for driving a London double-decker bus on two wheels for 810 feet.
================================================== =====

Do those people's skill and authority counts?

Well both of them are advocating 5-10 PSI over the Max PSI on the tire's wall.
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  #25  
Old 08-30-2009, 03:42 PM
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Quote:
Originally Posted by TacticalDriver View Post
Well both of them are advocating 5-10 PSI over the Max PSI on the tire's wall.
You probably meant to say "bald" not "bold", but your point is taken.

But I'm not sure it applies to us (mostly) amateurs.

Professional EVOC and stunt drivers who are striving for maximum performance to accomplish their challenging missions are advocating higher pressures. Fine.

Professional offroad drivers and rock crawling competition drivers are advocating and running 2-3 PSI to accomplish their challenging missions. Fine for them as well.

POINT: Neither one of those groups of professional driver populations is driving tires THEY pay for, and they aren't factoring life-cycle cost into their evaluations. We do.

Trailvoy members run the bell-shaped curve from straight line drag-racing enthusiasts to slalom rally fans to offroaders to snow and ice drivers to get to their ski cabins to pothole-avoiders where all they have is poorly-maintained roads to grocery getting moms and dads to comfort-seeking grandparents with stiff or injured backs who want a pillow-soft ride.

We cannot be categorized or stereotyped, and one size (or tire pressure) is not going to fit all.

I also don't believe a tire with excessive pressure (if taken too far) and a too-small contact patch can do anything EXCEPT lose traction and handle poorly. There must be an optimum tire pressure for different situations, but it's not the same for us as what the professional EVOC drivers advocate for themselves.

Now you're doing the opposite of the argument I advised against before. You're bringing in authority figures for testimonials, without giving them the chance to confirm that their advice is what they would give US, the general public. Find a testimonial from a NASCAR driver, and they'd tell us the left side should have 1" smaller circumference tires and less pressure than the right. See - inappropriate because that advice is meant for THEIR CONDITIONS, not US.

Not sure what your mission is here keeping this artificial debate alive. To get us all to adopt higher pressures to thwart the evil oil company's plots? To get us all to drive like we're being chased by zombies?
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  #26  
Old 08-30-2009, 04:02 PM
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  #27  
Old 08-30-2009, 04:25 PM
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Just to play devils advocate here:



However, I do agree that higher pressure will result in better lateral handling because the tire's lateral spring component is mainly related to tire pressure.

As I said, the only worries I have are reducing contact patch size, and reducing the safety factor.

It's all a compromise between ride comfort and handling up to a certain point, at a certain point over inflation does become dangerous... but I wouldn't say that the max tire rating is the danger level.

Also, you are greatly correct that rolling resistance is reduced by a higher inflation level. This is a scan from the textbook from my vehicle dynamics class. The upper right graph shows this relationship for a truck tire (don't inflate your own tires to 120 psi please):

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  #28  
Old 08-30-2009, 05:33 PM
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Quote:
Originally Posted by JamesDowning View Post
First, being an engineer, I can comment on the 'max pressure' rating on tires.
Duhhh...ohmmmm...welll...aaaa...anyone can comment on tire pressure, engineer or not.. ..

Quote:
Originally Posted by JamesDowning View Post
All tires have a bursting (or failure) pressure, which relates to sidewall strength and bead strength. The max pressure rating certainly is not the burst pressure, but it is based on a safety factor (established by the society of automotive engineers [http://automobile.sae.org/]) from the actual designed failure pressure (tires are designed with the same safety requirements of pressure vessels). So while going above that pressure will not pop the tire, it will reduce your safety margin, or safety factor... for instance, when you encounter a pothole.
Agree with that! But pothole seems to be the only risk when running on maximum or above maximum PSI indicated on the tire's sidewall.


Quote:
Originally Posted by JamesDowning View Post
I agree with Chickenhawk that there is an optimum tire pressure. Higher pressure does not automatically mean better performance
Based on what? Personal believes? Personal experience? Extensive experiments?



Quote:
Originally Posted by JamesDowning View Post
First, a lower tire pressure should theoretically produce less lateral stability.
Not only theoretically but practically as well.


Quote:
Originally Posted by JamesDowning View Post
A tire gains much of it's lateral stability from internal tire pressure because sidewalls by themselves have little compressive or bending strength. You will notice a tire with underinflated tires will tend to wander on the road more.
Amen to that!


Quote:
Originally Posted by JamesDowning View Post
Higher tire pressures improve tracking purely because it increases the internal spring rate of the tire. (With more tire pressure, it takes more force to laterally deflect the tire tread in relation to the bead)
Well... .. doesn't that mean a better performance? A more stable and precise handling?
There are 3 areas of lateral slip curve:
  • Linear
  • Transitional
  • Frictional


Linear
In a low speed turn the tire is flexing while providing lateral force but the contact patch itself doesn’t slide much, hence the lateral force increases linearly with slip angle.

Transitional
For the slip curve to enter the transitional area is just a matter of turning harder, thus the slip angle is increased further and the contact patch begins to slide.
Those forces have the tendency to stretch the tire and only the carcass is resisting causing to a part of the contact patch to start to slide along the ground.

Frictional
In the frictional area, most of the contact patch is sliding and the tire can no longer provide effective lateral force. Usually, unless you are a stunt driver, that's bad news.

On this note, one can understand why higher tire pressures feel more responsive. That's because the tire stretches less while the carcass resists stretching more, thus the tire reaches its peak grip at a lower slip angle.

Lateral Slip Curve explains why due stretching the steering gets light if the tires are over-driven. When the contact patch begins to slide, the contact's patch end let go first, hence only a smaller portion of the contact patch is resisting effectively and efficiently the twisting action.

Quote:
Originally Posted by JamesDowning View Post
Secondly, tire contact area is related to tire pressure, but also related to vehicle weight and rim width. Taking these factors into account, there is a range of optimum tire pressure that will allow the entire tread to contact the ground (albeit with different ground pressures distrubuted across the tire contact patch).
I am just curious how can be proved that a tire inflated at it's maximum rated PSI (let's say 41) has a decreased contact patch.

The springing characteristics you mentioned are indeed largely affected by the tire inflation pressure, but there are other influences as well. Carcass material and construction and the properties and tread pattern of the outer layer of rubber all have an effect on both the springing properties and the contact patch.

Quote:
Originally Posted by JamesDowning View Post
A rolling tire is always in a state of partial sliding friction and partial static friction.
You spoke like an engineer. LOL!!!

Quote:
Originally Posted by JamesDowning View Post
The danger zone is if you reduce the contact patch far enough to actually reduce your contact patch size.
And I assume you are implying (again) that a tire inflated at 41PSI vs 33 PSI
would reduce the contact patch. Am I right?

The contact patch surface pressure and inflation pressure are not the same thing. this is where a lot of people are getting confused.

Nevertheless there is a relationship between the two of them but there are few variables (at least four) that can modify it; carcass stiffness and shape, rubber depth and softness and last but not least road surface compliance. An extremely high carcass stiffness will reduce the influence of a high PSI in the tire.


Quote:
Originally Posted by JamesDowning View Post
So, as far as actual answers... and no more of this theoretical mumbo jumbo... the max pressure rating on a tire can be exceeded without directly endangering you, but don't try to sue the tire company if your tire blows out.
You are absolutely right. You can exceed the tire pressure indicated on the car's door sticker and the tire's sidewall and both manufacturers, the car and the tire manufacturer, know that.

However they have they own reasons not to encourage you to do that. One of them is political economy and the other reason is just that: They are trying to protect themselves from any eventual law suits that would result in a damage tire due to a road imperfection. At this point the only thing can be demonstrated against underinflated tires is the risk of damage due to a road imperfection.




Quote:
Originally Posted by JamesDowning View Post
Personally I prefer to run at about 40 PSI on the road, but I drop my tire pressure to 20 psi when I travel off road because it improves dirt traction (due to a greater contact patch, less shear is induced on the ground) and because it greatly improves bumpy road comfort because the tire spring rate is reduced by a lower tire pressure.
I once drop the pressure at 10 PSI to be able to drive away from a sandy beach


With best regards,

TD

P.S. People don't let you discouraged by anyone.. .. you can comment on the tire pressure topic even if you are not engineers
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  #29  
Old 08-30-2009, 05:53 PM
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I bet all your tires wear out in the middle way before the outside edges if you run ~50 psi in your tires (OH! Wait, that can't be since you've ben doing it for 15 years ). The 30-35 psi GM recommends for these trucks for everyday use is actually pretty darn good in my opinion. I've never seen one of these trucks wear tires on the middle or on the edges prematurely with those pressures unless alignment is an issue.
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  #30  
Old 08-30-2009, 05:59 PM
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Final warning to stay on topic if you want the thread to remain open.
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