Tire pressure question

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FleetProwl

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I just put new tires on my 5th wheel yesterday.  The old Goodyear Marathons called for 35psi cold max pressure.  The new tires call for 50psi cold max pressure.  When I picked up the tires yesterday after having them mounted, I asked the guy who mounted them if he put 50psi in the tires.  He told me that he put 35psi and that you should never put the max pressure during the hot summer, because the tires will build pressure while on the road and could possibly blow out.  When I got them home and put them on the 5th wheel they do look a slight bit baggy but its really hard to tell for sure.  The tag on my 5th wheel says to run 35psi, but that was with the Goodyears that I am almost certain came on the trailer from the factory, and their max pressure was 35psi cold.  Should I leave them at 35psi or should I increase the pressure (and by how much)?
 

Carl L

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Increase it to the 50 psi max pressure*.  Underinflation, which 35 psi is for your new tires, actually increases tire heating while running.  You also decrease the load carrying rating of the tires by reduced inflation. 


*Rule of thumb.  Actually you should balance the pressure against the load the wheels are actually carrying.  That requires a mfr's table of pressures v. loads.  If you have that, follow the table.  If you don't go with the maximum and watch for patterned tread wear.  By the way, trailer tires suffer a lot of scrubbing in turns so they normally wear at the edges, a sign of underinflation in a motor vehicle.
 

2006F350

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I know I'm gonna get jumped on (again), but

There is a quick method to determine approximate proper inflation - not all of us have access to mfr specs ... with your trailer loaded as you would on a vacation, etc. This process assumes the tires are cold (driven less that a mile) Take it to an area where you pull straight for a few hundred feet. Take a piece of sidewalk chalk and draw a line across the tread using the side of the chalk. Now, jump in, and pull the trailer for that hundred or so feet. Stop, Look at your chalk mark. Ideally, it will be scuffed evenly across the width of the tread. That would mead that your tires are inflated to what is needed to give you the largest footprint. More scuffing in the center indicates over-inflation, sidewall scuffing - underinflation (that is assuming you pulled straight). It is not scientific, but it works.

I would be hesitant to inflate tire to the max rating on the sidewall. Anything over that and you are stressing the tires beyond what they are rated. When you drive, the laws of science says rotating tires produce friction. Friction in turn creates heat which makes the tires hot. The hot tires in turn heats the air in them, and heated air expands, so your 35PSI after a few miles down the road is now approaching 40PSI. No matter what web mfg website I've visited, everyone of them will specifically state to not air their tires to the max inflation on the sidewall for this very reason.

Case in point, the tires on my truck have a max sidewall pressure of 75PSI. Ford recommends 60PSI. At 60, using this process, the outer 1" of tread does not even touch the road. I lower that to 50PSI, now, only about 1/4" of the outside tread does touch. Even at 60PSI with the 2400Lb pin weight of my trailer, I still leaves the outer 1/4" unscathed. If I were to put the 75 Max PSI, dang, I'd hit a bump and it would take me 3 miles or 3 days which ever comes first to get the backend back under control.

Just my 2 Lincolns (cents)

Larry
 

Ron

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Then ONLY proper and correct way to determine the correct tire pressure for a given load is to weigh each axle, weighing the end of each axle is better and preferred, then using the tire manufacturers charts determine the correct pressure for the load on the tires.? Using any other method is just a guessing game.

1. load rig as you normally travel

2. weight each axle with rig loaded

3. Determine correct pressure using your tire manufacturers pressure charts. ( You can probably google for your tire manufacturers web site or go to
? ? your tire dealer.)

4. Air up tires to pressure determined from charts.

The pressure found on the side wall of tires is the pressure required for maximum load the tire is rated for.? While it would be better to be overinflated than under inflated neither condition is desireable.

If you are right at the maximum load for the tires to reuire the maximum pressure posted on the sidewall of the tires very likely you are over the GVWR for the rig anyway.


 

Carl L

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I would be hesitant to inflate tire to the max rating on the sidewall. Anything over that and you are stressing the tires beyond what they are rated. When you drive, the laws of science says rotating tires produce friction. Friction in turn creates heat which makes the tires hot. The hot tires in turn heats the air in them, and heated air expands, so your 35PSI after a few miles down the road is now approaching 40PSI.

And indeed, every tire manufacturer allows for that in their tire design.  It is the reason that all universally recommend inflating tires only before driving, or, at least before driving more than 1 mile.    It is also the reason that they all caution against trimming pressures after the tires have heated up.

No matter what web mfg website I've visited, everyone of them will specifically state to not air their tires to the max inflation on the sidewall for this very reason.

That is curious.  The Michelin and Bridgestone sites I have visited have no such caution.  Thye both caution against underinflation quite strongly tho.  Could you cite the web pages where you found that advice?
 

2006F350

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I knew this was coming ......

My response is aimed at those that just automatically reply to any tire pressure question -- "aw, just inflate to the max sidewall pressure - you'll be OK", or "The vehicle mfg say to inflate to XX PSI".

It doesn't always work as I exampled with my truck bouncing worse than a basket ball at an NBA game when even going to Fords recommended 60 PSI and not having the load that they say I'm capable of carrying (that 5400Lbs incidentally).

I agree that the method you outline is no doubt the preferred method, but isn't the ultimate goal here to achieve a full width footprint. This goal can be achieved using the way I was taught also - it just isn't as time and labor intensive, and because you don't have to locate a scale that give ample room to weight each axle, will allow you the time to do 4 or 6 individual weights, etc., etc., it is probably a tad bit less expensive (unless you happen to own your own scale or have access to one free).

I will admit I got a bit carried away on what I've seen and where, but as long as I driven vehicles (40+ years), the one thing I remember being pounded into my head by my father, mechanic shop instructors, tire reps when I worked at a gas station, was to never inflate tires to sidewall max psi. Granted this was back in pre-historic days of the 60's when we had wooden spokes between the hub and rim and had to adjust breaks with a break spoon and regular gas sold for $0.23 a gallon at Texaco ... but I've always followed that advise, and it has never let me down.

If my way of doing things rub you wrong, I am sorry but as the old saying goes - "Such is Life". I am just stating that there can be and usually is more than one way to skin a cat and the end result is the same.

Larry
 

Carl L

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I agree that the method you outline is no doubt the preferred method, but isn't the ultimate goal here to achieve a full width footprint. This goal can be achieved using the way I was taught also - it just isn't as time and labor intensive, and because you don't have to locate a scale that give ample room to weight each axle, will allow you the time to do 4 or 6 individual weights, etc., etc., it is probably a tad bit less expensive (unless you happen to own your own scale or have access to one free

No.  We are talking about trailer tires here.  The ultimate goal is to safely bear the weight of the trailer.  To the extent that footprint contributes to that, fine. 

Anybody hauling a trailer should have weighed it before goiing too much further.  While side weights are nice to have with a trailer, the total weight divided by the number of wheels is close enough for most purposes. 

Ride comfort is no priority at all in a trailer, no one rides in one.
 

2006F350

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Once again, no doubt, my pre-historic knowledge is outdated, but I have been under the impression for the past 40+ years that when a tire is inflated so that the entire width of the tread in on the ground, that tire is correctly inflated to support the weight of what ever it is being asked to support as long as what you are asking it to support does not exceed the load capacity of that tire.

I'm not trying to get the last word in (I know it sounds like it though), I am just having a hard time in my mind attempting to determine why one why is any better than another as long as the same goal is achieved - in this case, inflating tires to properly support a trailer.

I know ride comfort is not the issue here, but isn't the desired footprint (full width tread usage) the same regardless of what the tire is used on? I know there are some exception - but none of the exceptions include trucks, trailers and motorhomes.

Larry
 

Karl

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Larry,

Assuming that your 'footprint' reasoning is correct (and I'm not saying it is), with very few exceptions, tires are now manufactured with tread going part way up the sidewall for grip during turning and side load. Granted, years ago tires had a clearly defined tread, but that isn't so today. If you inflated new tires so the whole tread were on the ground, you'd be seriously underinflated, and a portion of the center of the tread probably wouldn't be touching the ground at all. An extreme example of not being able to tell where the tread begins and ends would be a motorcycle tire - they're convex and there's no flat tread anywhere. I know you're going to say that I'm comparing apples to oranges, but I think you'd have a hard time determining the actual tread width of almost any modern tire. O.k., yes, the manufacturer tells you what it is. 
 

FleetProwl

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Thanks for all the info guys.  Even though my trailer tag as axle bearing weights on it, I don't know what the tire manufacturer recommends for those weights.  Even though the tires max cold pressure is 50psi, I have decided to run 45psi in them.  That way, if they do build pressure on the hightway, I'll have a little margin for that.
 

Ned

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FleetProwl said:
Even though the tires max cold pressure is 50psi, I have decided to run 45psi in them.  That way, if they do build pressure on the hightway, I'll have a little margin for that.

That's not necessary.  The cold inflation pressure already takes into account the increased pressure when the tire heats up.  If you don't know that actual weight and pressure for each axle, I wouldn't run the risk of underinflation.
 

Gary RV_Wizard

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As Ned says, it's not necessary or desirable to allow for pressure build-up due to heat.  And if it was, 5 psi would be  nowhere near enough - on a hot day at highway speeds the pressure will likely climb much more than that.

What brand and model are your new tires? Chances are fair-good that the inflation table is available online.
 

raedmunds71

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I just installed new Goodyear tires on my Class B Jayco Weekender yesterday and it felt like I was driving a lumber wagon.
After reading this discussion, I went out to check the tire presure and there was 80 lbs in all 4 tires.
When I look at the tire presure label on the door post (from Ford I assume) it says to put in 50 lbs in front and 80 lbs in back.
Would that make sense to you guys ???
 

Ned

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That seems like a big difference from front to back, but if that's what Ford recommends it's probably right.  Are the new tires the same as the original equipment?  Are the rear tires single or dual?  If dual, I would expect lower pressure for them than the front.
 

raedmunds71

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Ned,
The dealer I bought the tires from said they are the same tires Ford used as original equipment. I guess I will have to take his word for it as I didn't buy the van new so I don't know what was originally on it.
The rear tires are single.
 

Ned

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The best method is to get the tire manufacturers inflation tables and actual axle weights for the RV.  Use the tables to find the correct cold inflation pressure for each axle.
 

GaryB

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Slight diversion - but hated to start a new thread just to ask a simple question.  Regarding pulling a 5er, I've read that recommended tire pressure (for truck tires) is lower when not towing than when towing.  So my question is - once people unhook the 5er at campground, do they normally reduce the truck's tire pressure before driving around the area in just the truck?  Of course they would then have to increase it before leaving camp.  That would seem to be a pain in the neck.  I'm guessing people don't worry about it and just keep their tires inflated to recomm. towing pressure even when driving around in just the truck itself (at least until they get home).  Is this correct?

Thanks
Gary
 

Gary RV_Wizard

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Yes, that's pretty much correct.  There's no harm in leaving the tires inflated higher, but the ride can be softened a bit by reducing pressure when unloaded.  We are probably talking at most 10 psi difference, though, and more likely only 5 psi.
 

Lowell

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The tires on our TT show a maximum tire pressure of 50 PSI.  When starting out last week on a 500 mile plus drive, I had 40 PSI in the tires.  This was a relatively level drive between Phoenix-LA and on to Buellton, CA. I used cruise control set at 60 MPH except on the hills and averaged 10.2 MPG.  Before returning, I increased the TT tire pressue to 45 PSI.  On the return trip from Morro Bay to I-5-LA -Phoenix, using the same procedure, I averaged 11.9 MPG. I attribute the better milage to the increased tire pressure on the TT and slightly less city traffic that I didn't have the first time.
 

rbell

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I wonder if Lowell went downhill going home or if it's uphill both ways like when I walked to school when I was a kid. LOL ;)
Not to rain on anyones parade but, here is what Goodyear says about the subject. These are the people who make tires, not the town where Bernie lives. Some where else in reading all this they say not to lower pressure as it increases while driving. Also somewhere I read that you get about 1 lb increase for every 10 deg temp increase.
 
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