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Old 10-04-2009, 01:26 PM   #21
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Re: Good thing I had both hands on the wheel !

Wow Rob, not to hi-jack the thread but I have the same compressor and I don't think it took that long. I aired down to 45 on the last trip and back to 70 but never really saw much of a difference than when running the D's. Guess I'll have to time it someday. Maybe getting from 40 to 80 can make the difference? I do have an air tank on the system.
I suppose (or I hope) that it's OK to run 70 PSI highway speeds with the E rated tires.
Dave
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Old 10-04-2009, 01:54 PM   #22
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Re: Good thing I had both hands on the wheel !

How much pressure is ok for the highway depends on the weight of your van and the tires you are running.

First, you need an accurate weight, preferably for all 4 tires. Or, settle for front/rear weights and round up in case one side is heavier than the other.

The calculate how much pressure you should be running. This from an earlier thread:

Old school answer:

The pressure rise from cold tire to hot tire should be 4-6psi. Less than 4psi rise means the cold pressure is too high, more pressure rise means the cold pressure is too low.

Best answer:

Get the actual weights on the tires and ask the manufacturer for the proper pressure for that weight.

My method:

Again, get the actual weight. Compare it to the max weight on the tire. Do some math.


Actual weight/Max weight X Max Pressure = Desired pressure

If you have 2500# on a tire rated at 3000#@80psi you get:

2500/3000 = .8333 .8333 x 80psi 66.66psi, which I round up to 70psi


Then I always round the pressure up. Much better to err on the side of too much pressure than on the side of too little pressure.


As a general rule, D and E rated tires carry the same load at the same pressure. The big difference is E rated tires can handle more pressure, hence more load.

My van weighs in at 9800#, fairly evenly split front/rear side/side. The tires are LT275/70-18E rated at 3640# @ 80psi. For 4 tires, that would add up to 14,560# at 80 psi. My math says I have 9800/14560 of full load. That is about 67% of the max tire ratings. 67% of 80psi is about 55psi. I run 65 psi. A good safety margin. I will never run a tire at its full capacity. Just too much at stake for me.

4 tires rated at 3000# would be too close for me. Going down the road with some lean to the road, the outside tires could be overloaded for many miles. I wouldn't do it. I always want at least a 10% safety factor, 20% is better. Hence, my 65psi in my tires.

I always round up with tire pressures since higher pressures (within the max tire ratings) will reduce tire temps and might tend to wear the center of the tread. Lower tire pressures will cause higher tire temps and can easily cause a tire to fail.

Mike
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Old 10-04-2009, 03:18 PM   #23
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Re: Good thing I had both hands on the wheel !

How does one get their rig weighed, and then by each tire?

My RB-50 comes in at just under 10K lbs, and I've always run 50-55 on my 315 'D' rated AT/KO's. Maybe I could go higher for hiway use only?
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Old 10-04-2009, 03:59 PM   #24
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Re: Good thing I had both hands on the wheel !

Just find a public truck scale and tell the weigh master what you want. I usually weigh the total then the front or the rear axle. After that each tire. I've read Mike's method before and figure you can get close, but for me I'm always adding or pulling out equipment and all I can do is get close. I'm not too sure how critical the weight vs pressure has to be with the E's compared to the D's which were right at my limit. The tire shop I bought mine at suggested running them at between 70 and 75. My van weighs about 11,000 +/- 800lbs depending on equipment and/or passengers. I weighed in with a normal load, full of fuel and water, oh and me not in the mix.
April 2008
Total = 11,080
F axle = 4,320
R axle = 6,760
LF = 2,340
RF = 1,980
LR = 3,340
RR = 3,360
I plan to pull out everything I haven't used out to shed weight if those items are not needed.
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Old 10-04-2009, 04:39 PM   #25
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Re: Good thing I had both hands on the wheel !

Quote:
Originally Posted by Jeffrey
How does one get their rig weighed, and then by each tire?

My RB-50 comes in at just under 10K lbs, and I've always run 50-55 on my 315 'D' rated AT/KO's. Maybe I could go higher for hiway use only?
Probably not. The max pressure (cold) on those tires is 50psi.

Mike
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Old 10-04-2009, 04:41 PM   #26
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Re: Good thing I had both hands on the wheel !

Quote:
Originally Posted by daveb
Just find a public truck scale and tell the weigh master what you want. I usually weigh the total then the front or the rear axle. After that each tire. I've read Mike's method before and figure you can get close, but for me I'm always adding or pulling out equipment and all I can do is get close. I'm not too sure how critical the weight vs pressure has to be with the E's compared to the D's which were right at my limit. The tire shop I bought mine at suggested running them at between 70 and 75. My van weighs about 11,000 +/- 800lbs depending on equipment and/or passengers. I weighed in with a normal load, full of fuel and water, oh and me not in the mix.
April 2008
Total = 11,080
F axle = 4,320
R axle = 6,760
LF = 2,340
RF = 1,980
LR = 3,340
RR = 3,360
I plan to pull out everything I haven't used out to shed weight if those items are not needed.
With that big a weight difference front to rear, I'm surprised you run the same pressure front and rear. 60-65 front and 80 rear would seem more appropriate.

And you were over the weight rating in the rear for the 'D' rated tires you had on. I'm glad you went to 'E's.

Mike

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Old 10-04-2009, 05:22 PM   #27
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Re: Good thing I had both hands on the wheel !

Yes Mike I figured I could lower the front. For over a year I was running E's on the rear only. I freaked out when that gal rolled her van and bought a couple E's. Now those are the spares and I have E's all the way around. I actually vary the pressure from time to time. I think I have 75 in the front now and close to 80 in the rear. When I up in the hills I keep the fronts about 62-65. While off road I take everything down to what I need and then back to the 60's and lower 70's. On the trip home I was taking everything back up to about 75/80. Maybe I'll just leave the fronts at 65 from now on. Thanks.
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Old 10-04-2009, 08:06 PM   #28
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Re: Good thing I had both hands on the wheel !

Quote:
Originally Posted by daveb
Wow Rob, not to hi-jack the thread but I have the same compressor and I don't think it took that long. I aired down to 45 on the last trip and back to 70 but never really saw much of a difference than when running the D's. Guess I'll have to time it someday. Maybe getting from 40 to 80 can make the difference? I do have an air tank on the system.
I suppose (or I hope) that it's OK to run 70 PSI highway speeds with the E rated tires.
Dave
I know Dave, my situation seems to differ from others with onboard air. Maybe I'm not using the system correctly? I know there is also a small air leak in my hose too. But for whatever reason, it takes me a long time to air back up. Oh well , it still get me there eventually.
R
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Old 10-04-2009, 09:15 PM   #29
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Re: Good thing I had both hands on the wheel !

Rob, I had a problem with my Extreme Air the very first day I had the van (new). I later found out it was the check valve from the comp output to the tank. It was a broken cheap chinese valve. I replaced it with a real good brass check valve and the system just rips now. From 25 to 50 psi in a tire in just a few minutes.
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Old 10-04-2009, 11:03 PM   #30
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Re: Good thing I had both hands on the wheel !

Quote:
Originally Posted by Jeffrey
Rob, I had a problem with my Extreme Air the very first day I had the van (new). I later found out it was the check valve from the comp output to the tank. It was a broken cheap chinese valve. I replaced it with a real good brass check valve and the system just rips now. From 25 to 50 psi in a tire in just a few minutes.
Thanks Jeffrey, I'll give that a look. How were you able determine that that particular valve was defective? Was there an air leak or some other sign of a problem?
Thanks
R
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