Tire Pressure Question

I recently purchased a used TT and have used it exactly one time.  Everything went well and in the next few weeks I'll be using it a lot.  One question I had though concerning tire pressure.  The owner's manual said that I should inflate my TT's tires to their maximum PSI.  This seemed kind of strange to me and definitely not what you typically do for a car or truck.  I did what they asked and didn't have any problems.  However, the manual also said to overinflate my tow vehicle's rear tires.  I don't recall the exact amount amount of overinflation they recommended (I don't have the manual in front of me), but I think it was something like a x # of PSI on top of the normal PSI.  I didn't feel comfortable doing this on my tow vehicle.  Is this normal and do most of you out there do this?

thanks!

In a word, NO.   

Most people underinflate or allow their tires to run low, so your owner's manual is erring on the side of caution by telling you to use high pressures.  There is nothing wrong with using max inflation values, but it is usually unnecessary and often gives an harsh ride.  However, in extreme cases overinflation can cause loss of traction and wear down the center of the tire tread.

The proper way to inflate your tires is to weigh the trailer axles while it is hitched to the tow vehicle. Ditto for the rear axle of the tow vehicle, with the trailer hitched on.  Once you know the weight on the axles, you can consult the inflation tables for your make and model of tire and inflate to the psi indicated for the weight being carried on each axle.  [Tire manufacturers publish tables for each model of tire, showing the pressure needed to carry a given load.  A dealer for that tire brand will have them, though many don't even know they do! Many tire inflation tables are available online as well.]  You can weigh each side of the axle individually, or just divide the total axle weight by 2 and add a 100 lbs to allow for possible side-to-side imbalance.

While this is not a difficult procedure, it takes a bit of individual effort to get the data and find the proper inflation, so your owners manual was taking the easy route and recommending the max psi.

Most of us here inflate to 5-10 psi above the pressure recommended in the inflation tables, on the theory that it is better to be over than under.  Underinflation causes blowouts, while modest overinflation mostly just affects the ride.

tettnanger said:

I recently purchased a used TT and have used it exactly one time.? Everything went well and in the next few weeks I'll be using it a lot.? One question I had though concerning tire pressure.? The owner's manual said that I should inflate my TT's tires to their maximum PSI.? This seemed kind of strange to me and definitely not what you typically do for a car or truck.? I did what they asked and didn't have any problems.? However, the manual also said to overinflate my tow vehicle's rear tires.? I don't recall the exact amount amount of overinflation they recommended (I don't have the manual in front of me), but I think it was something like a x # of PSI on top of the normal PSI.? I didn't feel comfortable doing this on my tow vehicle.? Is this normal and do most of you out there do this?

thanks!

Click to expand...

Beg to gently and slightly differ with Gary.  On a trailer, ride is irrelevant, there is no one in the trailer to "enjoy" the ride.  A tire is rated to bear its maximum load at that pressure, lesser pressures lesser loads.  And yes there are tables that give a pressure vs. load figure for an acceptable range of pressures.  However, for a bloody trailer that is cutting things awfully fine.  I just use the max load pressure in all axles.

As regards the truck tire inflation, the recommendation to raise the pressure of the rear tires should be unecessary just from the aspect of the trailer's presence.  A weight distributing hitch system does just what its name suggests -- it distributes the tongue weight of the trailer evenly over the front and rear axles.  It returns the relationship of the axle loads to the same proportion as before the trailer is hitched on.

What raising the pressure of the rear tires over the front does is to increase the understeer of the truck.  That is the truck will try to steer out of turns.  Reducing the pressure relative to the front increases oversteer.  That is the truck will steer into turns.  Understeer is the more desirable condition in a truck and in fact in most street vehicles.    A vehicle that oversteers tends to feel, and be, unstable responding disportionately to steering inputs.  A vehicle that understeers responds less to steering inputs and wants to resist turning motion.  In extremes it will 'plow' into turns.  This is  safer than an oversteer situation where the vehicle snaps in to the turn.  Snapping into turns is great in a sportscar -- it is really bad in a loaded truck.

OK where this leaves us is that you should follow the truck mfr's. recommendations on tire inflation and the differential between front and rear.
Myself, since my mfr recommends no differential, I just inflate to the maximum sidewall pressure.  I am using all terrain flotation gumballs with a LT rating.  These have stiff 4-ply sidewalls so that a max inflation does not do too much to an already stiff ride.  But then your experience may differ.

So

Here is a trick I learned for truck tires because of varying loads vs what the mfg recommends. I would think that this would also work like a champ for anything with tires. Take a piece of sidewalk chalk and draw a line across the tread using the side of the chalk, not the end. Drive forward a dozen feet or so, then look at the chalk mark. If the sides are? more scrubbed than the center, you are under inflated conversly, if the center is more scrubbed, you are over inflated. You are looking for even scrubbing across the width which indicates a full footprint and proper inflation.

According to this, the proper inflation on the read on my truck when not pulling is about 45PSI instead for Ford's recommend 60. But I keep them there for less rolling resistance in a vain attempt to keep Exxon from becomming overly rich (but I think I'm fighting a loosing battle).

Larry

If you do that when the tires are cold, they will indicate underinflation, even if properly inflated.  As the tire heats up, the pressure rises and changes the profile.  I'll use the charts and my known weights to calculate inflation pressure.

Over inflated tires no matter the vehicle is a not a good idea.

http://www.rma.org/tire_safety/tire_maintenance_and_safety/tire_safety_brochure/tire_care_and_safety.cfm

http://www.rma.org/tire_safety/tire_maintenance_and_safety/tire_safety_brochure/rv_faqs.cfm

Ned said:

If you do that when the tires are cold, they will indicate underinflation, even if properly inflated.? As the tire heats up, the pressure rises and changes the profile.? I'll use the charts and my known weights to calculate inflation pressure.

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I'm with you on this Ned.  The only proper way to correctly determine proper tire inflation pressure is to get weighed and determine correct tire pressure using the TIRE manufacturers inflation tables.  To do otherwise is nothing more than a guessing game that could end up very expensive or dangerous if one guesses wrong.  One cannot depend on a vehicle manufacturer to recommend the correct inflation pressure either.  From what I understand Ford recommended a tire pressure for the Explorer that was below the tire recommended tire inflation pressure and the results were not good. 

Know your vehicle weights as you normally load it as determined by weighing and inflate the tires to the correct pressure as determined from the your TIRE manufacturers inflation tables.

Ron and Ned,

The info on the sidewall of my truck tires state 44 PSI maximum.  That's what I run in them towing or empty.  Are you saying this is unsafe and I should be using some other figure based on the weight on each wheel? If so, where do I find a pressure/wheel load curve for my tires?

Jake

Jake,

Tire manufacturers will have the inflation charts on their web site. Here for example are GoodYear's inflation charts.

Jake said:

Ron and Ned,

The info on the sidewall of my truck tires state 44 PSI maximum.? That's what I run in them towing or empty.? Are you saying this is unsafe and I should be using some other figure based on the weight on each wheel? If so, where do I find a pressure/wheel load curve for my tires?

Jake

Click to expand...

YES.
The 44 PSI maximum shown on your tires are for the maximum load that tire is rated at.  If your are less than the maximum load then you should use the tire manufacturers inflation table to determine correct tire pressure for the load you have.  One should never exceed the maximum load on any tire.

The only way to determine correct tire pressure for your load condition is to have the vehicle weighed then use your tire manufacturers inflation table to determine correct air pressure.

I have a 2004 Outback 28RSS.  It is showing unusual tire wear patterns.  The sides of the treads are very worn and the middle of the tread looks almost new.  I check the tire pressure every time we head out and make sure it is at the maximum tire pressure on the side wall (50psi). 

Any idea what is causing this tire wear on all four tires?

See my reply to your other message.