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Old 06-11-2013, 06:12 AM   #1
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Inflation - too much too little or is it all bs

I read your thread and its all sound advice and I agree 100% BUT when it comes to towing with an SMB there are several factors that become more important:

There are several things in play here that the theoretical analysis of high end inflation levels do not cover. Firstly the tire manufacturers are not bound by any universal standard that has been tested in the full sense of the word. Their internal conformance to the load index is a "trust me statement" that under ideal conditions usually in a test cell the tire will support a given load at the specified speed for the approximate life of the tire given perfect alignment throughout the test.

However unless the ambient and the surface temperature is STP as used in most Boyle's Law analyses the first variable for the load capacity of the tire being "cold inflation pressure" is already rendered incorrect. And in most US Summer areas (whether you believe in global warming or not) we start the day above STP and end up 20 or 30 degrees over the nominal 68F at which Cold is defined. At about 2 lbs per 10 degrees that means the Cold pressure needs to start at 3 or 4 psi above whatever the load calculation may dictate.

But then we get into the Load definition. And while I agree individual axle load measurement is essential, unfortunately this is a static measurement and once again assumes the perfect conditions of a test rig if the manufacturer even uses one which I have reason to doubt. In reality our SMB tow vehicles face a much harsher environment. The LWB versions have considerable leverage between the hitch point and the axle which combined with the dreadful condition of some of our interstates and secondary roads means that the load leveling on the trailers (if used) is combining to create considerable bounce on the tire. This bounce produces momentary increases in weight that are several hundred lbs above the nominal static level. Very hard to calculate and certainly way outside the manufacturers test cell but logically prevailing. In addition there are the surface imperfections such as pot holes or larger joints that batter the belts. All of this combining with a surface temperatures that may be as much as 50 deg F over the nominal Cold level.

So unless the load is 30% less than the nominal tire maximum including the hitch weight AND the roads are in good condition AND the temperatures are less than mid eighties AND the combined axle weight and the hitch has some flex, I cannot see why one would risk Under inflation by using anything but the maximum adjusted (for ambient) temperature inflation rate for that tire.

I have read and heard (from Michelin) that these factors are taken into account when they designate the tire load capacity but when asked how they test this, I get blank looks. So I investigated further. My company is the largest seller of tires in its sector and with gentle inquiry I have discovered that all the above is true and possibly worse than I am representing. Because we operate in a gray area of the market (converted vehicles, uncalibrated loading, uncalibrated tow loads, undocumented environments and usage) tire damage is such after the inevitable blow outs that legal recourse is going to very hard to obtain. I just experienced two blow outs under tow of identical Michelin E rated tires, carefully inflated at various speeds not exceeding the nominal maximum of 65 on a warm day. Both tires blew out within an hour of each other. My fourth failure of this tire type in a year and my fifth in 18 months. At 2910 lbs on that wheel (static with tow hitched) I am 140 lbs under the maximum and by your suggestion i should deflate slightly. I think that would be unwise on my or any one else's part. Instead I have to find a more reliable tire at these weights without changing the gearing. There are options but all are many $$$ but possibly much less than the damage caused by blow outs that have ripped out the plumbing, a/c and wiring that surround the rear tires.

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You are over inflating your tires and when they deflect due to load spiking, they are seeing unwanted momentary internal pressures.

Blow up a balloon until it is about to pop and then sit on it - BOOM!

You are way over thinking this.

I have towed extremely OVERLOADED loads thousands and thousands of miles on all surfaces.

Put three 55 gallon drums of Race Fuel, a spare 600lbs rear axle, bins of spare parts in the bed, a 3500lbs trailer with a 6000lbs Race Truck on it and half a dozen 130lbs spare tires on the hitch and the four 200lbs guys in the cab of my Superduty 4X4 6.0L on 315 D rated tires and tow to Cabo, Baja Sur, Mexico on their roads, including off-road and that is over loaded.

I've done it at least a dozen times and the only flat I have had is on the trailer when I clipped a boulder on the side of the road.

I typically run them at 45psi but don't pay that much attention to it. I do look a the profile and feel them for temp to see if anything is out of line when we stop for fuel - I have a ~46gal capacity.

By the way, most of the race teams down there are doing the same thing.
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Old 06-11-2013, 06:55 AM   #2
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Re: Inflation - too much too little or is it all bs

I posted the previously hijacked thread to try to make sense of this in the proper section.

Your comments about your load mean nothing without weights. I tow 10000 lbs worth of race trailer with car, tools tires etc. Hitch weight is 1300 lbs (independently measured) and I use a load leveling system (Hensley). Total rig is 19,500 and the van at 9500 with the spike effect of the hitch weight.

Nowhere, ever from anyone have I heard that D, E, F or G rated tires should be deflated to allow sidewall flex when at or close to max load. So that means we need to understand what max load really means. Hence my earlier effort to explain that the tire manufacturers have set a standard under perfect conditions. So given Southern clime summer temps of ambient and surface, I am suggesting that we cannot assume that an E rated tire can carry 3040 lbs when towing at 100 F and surface temps over 140 F. To suggest that we should deflate tires because the rear axle is carrying 5500 out of its 6080 maximum is frankly a unique suggestion.

However if your loads actually add up to (say) a rear axle load of say 4500 lbs or a full 30% below maximum you may have a point in two respects: 1) You will give the tire more ability to flex on the bad surfaces we often encounter whether at 65 mph or more and 2) the tress at these loads is actually that which the manufacturers were seeing when they did their "perfect condition" testing. And if you are right that means E tires need to be derated for loads on a curve as temperatures increase and surfaces deteriorate. Funnily enough just like aircraft tires for which standards are much tighter.

Of course finding F or G tires for these vans is next to impossible.
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Old 06-11-2013, 09:13 AM   #3
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Re: Inflation - too much too little or is it all bs

Over-inflattion can be dangerous because it reduces road grip. Tire coefficient of friction is not linear as many folks assume. It's actually a decreasing slope curve. Race cars don't run high tire pressures for this reason. They spend years establishing their tire loads under all foreseeable conditions, then pick a tire from that data that will allow for the lowest possible psi, while maintaining packaging constraints, and considering rolling resistance.

My advice if you are really worried.... Install a tire and temp monitoring system. Start at max sidewall pressure, and incrementally decrease until you start seeing an upward temperature trend. The real problem with under-inflation is temperature, so if you can control that, then lower is generally better (fuel economy aside)
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Old 06-12-2013, 01:52 PM   #4
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Re: Inflation - too much too little or is it all bs

I sympathize. I deliberately tried to keep my rig lighter because of issues like this. Folks seem to have vans that push the max weight, and then tow or drive in extreme conditions. Which is fine, but it is gonna stress the weak links, which might be your tires.

For a lot of us being able to lower pressures is important because we fourwheel. When purchasing someone needs to take that into account as well. And I would also suggest that if you're gonna tow with your SMB you should have it designed to do so for some of the reasons you cite. Simply looked at the OEM towing capacities and tire sidewalls isn't gonna give you a complete picture.
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Old 06-12-2013, 02:45 PM   #5
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Re: Inflation - too much too little or is it all bs

Quote:
Originally Posted by carringb
Over-inflattion can be dangerous because it reduces road grip. Tire coefficient of friction is not linear as many folks assume. It's actually a decreasing slope curve. Race cars don't run high tire pressures for this reason. They spend years establishing their tire loads under all foreseeable conditions, then pick a tire from that data that will allow for the lowest possible psi, while maintaining packaging constraints, and considering rolling resistance.

My advice if you are really worried.... Install a tire and temp monitoring system. Start at max sidewall pressure, and incrementally decrease until you start seeing an upward temperature trend. The real problem with under-inflation is temperature, so if you can control that, then lower is generally better (fuel economy aside)
I agree that both over and under inflation is problematic but I suspect that the bigger problem is that the tire load ratings are not a good indication of what you should expect in the real world of towing at max vehicle weights. And I am setting up a TPM system just to give me some warning of the blow out about to happen. And I have to retrain my right foot to floor it when it does!
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Old 06-13-2013, 10:25 AM   #6
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Re: Inflation - too much too little or is it all bs

I would hope you mean you'd accelerate to compensate for the loss of speed AFTER the blowout...not when it is imminent

On a more serious note, do TPM systems give warning of a blowout, as opposed to a more gradual leak? I suppose one scenario, where you have a small leak that deflates the tire enough that it heats up massively and fails, might be avoided with TPM but a major delamination like some have described might be different. I've never experienced even a flat with TPM so I'm curious.

Rob
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Old 06-13-2013, 10:50 AM   #7
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Re: Inflation - too much too little or is it all bs

I too do not have expereienc of how TPM will warn of an impending blow out but...I am hoping that the tire starts to delaminate and with that the pressure will increase above the 12% threshold of the system I have acquired. At that level I would be seeing 90 psi on my my E rated tires.

Which raises the question as to what the running pressures are at max load, 100 deg ambient and 140 deg surface. I am inclined to believe that the pressure will get closer to 95 or 100 psi on a 300 mile (between fill up) stretch.

This may render the TPM useless or it sgnifies what I have asserted in other threads that the Load ratings are far too optimistic in the Southern US summer conditions for towing.
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Old 06-13-2013, 12:13 PM   #8
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Re: Inflation - too much too little or is it all bs

Tomorrow we leave for a month-long trip out west from Pennsylvania. As a NE-USA person not accustomed to really hot SE conditions, should I decrease my tire pressure a bit in the really hot SE conditions? I usually run at "cold PSI" and noted in the door sticker, 80 in the rear (and I forget what in the front: 60 I think).

We're not towing anything and are a good bit under max gross weight.
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Old 06-13-2013, 01:03 PM   #9
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Re: Inflation - too much too little or is it all bs

Quote:
Originally Posted by rob_gendreau
I would hope you mean you'd accelerate to compensate for the loss of speed AFTER the blowout...not when it is imminent

On a more serious note, do TPM systems give warning of a blowout, as opposed to a more gradual leak? I suppose one scenario, where you have a small leak that deflates the tire enough that it heats up massively and fails, might be avoided with TPM but a major delamination like some have described might be different. I've never experienced even a flat with TPM so I'm curious.

Rob
My TPM does both. Hella TC-400, not available anymore.
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Old 06-13-2013, 01:31 PM   #10
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Re: Inflation - too much too little or is it all bs

Quote:
Originally Posted by BrianW
Tomorrow we leave for a month-long trip out west from Pennsylvania. As a NE-USA person not accustomed to really hot SE conditions, should I decrease my tire pressure a bit in the really hot SE conditions? I usually run at "cold PSI" and noted in the door sticker, 80 in the rear (and I forget what in the front: 60 I think).

We're not towing anything and are a good bit under max gross weight.
The book says that Cold means at 68 degrees. So at 2 psi per 10 deg you can do the math in terms of what your tire needs to be when the Cold morning temperature is 80 deg as it was in Dallas this morning or 90 in Phoenix. As to whether you should ride at max pressure there are two schools: mine which says that if you can stand a stiffer ride and you are not significantly below the max load of the tire (at say 50%) then you should inflate to max or close anyway. The other school says there is a linear relationship between the max inflation and the max load and you should ratchet down your cold inflation (as adjusted for temp) accordingly. So an E rated tire with a max at 68 deg of 80 psi would be inflated to 70 if the tire load was about 325 lbs less than the max load of 3040 lbs. Most advise not to go below the D rating of 64 psi irrespective of the load.

YMMV but thats the only sense I can make of this arcane subject.
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