Filling Tires

[Ned]

And Charles' Law is an example of how not to apply physics to a specific problem.  Charles' Law relates volume and temperature, but when discussing tires, the volume is, for all practical purposes, a constant, so Charles' Law is of no use.  But you knew that 

 

[FastEagle]

Wow! Let's not forget Boyle's law.....................LOL

FastEagle

 

[Ned]

Wow! Let's not forget Boyle's law.....................LOL

Similar problem to Charles' Law but relating pressure and volume

 

[FastEagle]

?Generally, as a radial tire revolves during operation, heat is generated on the inside of the tire at 4 degrees per minute. However, the tire loses heat at the rate of 3 degrees per minute with dissipation throughout the casing and airflow around the tire. After 40 minutes of continuous operation, the temperature has increased 40 Fahrenheit. As the temperature inside the tire increases, the inflation pressure also increases. Thus, a tire inflated to 80 psi cold would now be 85 psi. Because the inflation pressure has increased , the amount of tire flexing gas decreased, which decreases the amount of heat generated per minute to 3 degrees per minute. Assuming the heat dissipation factor is still 3 degrees Fahrenheit per minute, the net temperature change is nil (0). This is called Thermal Equilibrium.?

From the Michelin truck tire maintenance manual.

FastEagle

 

[GIB]

I like things more simple and down to earth. I air the tires on a cool or rainy  day not in the sun, all at maximun recomendation. When you are talking 100 PSI. you don't get bead leaks so go around and hit the tires with a hammer . The boing should be all the same . If one sounds dramatically Different then get your gauge and check. Unless you are travelling long distance from south to north changing temperature by 60 degrees or so things will be OK. The door stickers and sidewalls are good starting points. I then monitor after 5,000 miles or so with a depth gauge. If they wear more in the centre add a couple of pounds. If they are wearing on the outside drop a couple pounds out and find where they wear evenly across entire tire. Then buy new tires and start over. I took physics 50 yrs. ago and liked it then but I'm not going back to school over adding air to tires.

 

[Just Lou]

....If they wear more in the centre add a couple of pounds. If they are wearing on the outside drop a couple pounds out and find where they wear evenly across entire tire.

Isn't this backwards logic?  Or have I had too much to drink tonight?

 

[GIB]

Sorry Lou: You're absolutely right . If they are wearing  the  centre drop a couple of pounds  and if they are wearing on the edges add a couple of pounds.  Wow, that was a senior moment.

 

[jlazar]

I have had my MH (2005 37ft Gas Allegro Bay) weighed but just on CAT scales.  Have not had a chance to have individual wheels weighed.  My weights for the front are 7530 and the rear 13740.  With an even distribution, that puts the weight on the front tires at 3765 and the rear duals at 3435.  The chart for my Toyo tires (the Michelin chart for the similar original tires is identical) shows a minimum of 80 PSI for the front (3860) and the duals (3525).  Adding 5 PSI for safety, I would run 85 PSI front and rear.  I know when my MH is weighed by wheel that I will see a difference due to weight distribution left and right.  I would like to make an estimate of this error and make and adjustment for it.  I was thinking of using a 10% figure.  That would make the heaviest weight of a front wheel at 4142 and the heaviest dual at 3779.  Using the charts, this would mean running the front at a min of 95 (4300) and the rear at a min of 95 (3970).  Adding 5 PSI for safety, this puts me at 100 PSI front and rear.  So axle weights with +5 PSI puts me at 85 and 10% factor puts me at 100.  That is a difference of 15 PSI between the two approaches.  Is this a good idea?  Should I be using a lower figure than 10%?  For those of you who have had individual wheel weighs, how much difference (%) did you see from side to side?  By the way, the max PSI on my tires is 110 PSI.  Using a 70% figure I have seen on the forum, the lowest PSI I should run with these tires would be 77 or 82 with a +5 safety factor.

 

[dave61]

The main thing is not to be underinflated which is bad for the tire and possibly dangerous. The good thing is your tires are not close to carrying the max load, the pressure you have been using is probably a little more than you really need but if it drives well...

You could drop all tires by 5 psi and see if you like the way it drives/rides better than currently.

 

[kenb1023]

An interesting twist, with Toyo. To sort of echo Sarge, a number of years ago I had my oil changed on my 4WD pickup, but there was new management (unknown to me), and when I left they'd filled the tires to the max on the sidewalls. It was snowing lightly at the time, and I had waaaaay less traction than I expected, so went back (a block or so) to get the pressure down. I had quite an argument with the manager of the place, who claimed that max was actually what you were supposed to always run. Needless to say, they lost my future business.

It was actually unsafe (slip and slide) to drive on that light snow with max, and was almost (not quite) like dry pavement with the tires at my normal 35 psi. All this is disregarding ride, which is also worse at high pressure, and tire wear which would have worn out the center tread well before the tread on the edges.

Dito on that.  When I take my Jeep Wrangler in they keep changing the pressure back to 32lbs.  Jeeps are a dangerous ride on the pavement at that pressure.  I go and air back down to 25 Lbs and my good ride comes back. 

 

[david73997]

One more reply here, If you are parked anywhere a Diesel Pusher, the owner will probably have an air hose on board and will gladly help you out by topping up your tires.

david73997
2005, 43`Newmar/Dutchstar

 

[Gary RV_Wizard]

Your 10% factor is generous, so I might not add that extra 5 psi as well.  If the axle weight plus 10% puts you at or below the middle of the 95 psi weight range, you already have some extra margin built into it, but if it is closer to the top of that range I would bump up to the next one (usually +5 psi).  The idea is to maintain some reserve capacity, but the amount of windage to be applied is a matter of judgement.

There is nothing wrong with running at 100 psi instead of 95, but the ride may be a bit harsher than necessary.

 

[jlazar]

Thanks.  The 10% puts me over the 90 PSI by 2# for the front.  So I will run 95 there with no +5 adjustment.  The 10% for the duals puts me over the 90 PSI by 24#.  That is only about 10% of the # range for 95 PSI.  So I will also run 95 there with no +5 adjustment.  As you may have noticed from another posting, I am getting ready to purchase a TPMS.  If I monitor the temperature of the tires at 95 PSI and don't see a significant increase in temp (for all the tires) from cold to hot, is that another good indicator that the pressure is correct.

 

[Gary RV_Wizard]

The normal increase from "cold" to "hot" is around 10 psi but can be as much as 15 in some cases.

Yes, a tire that runs hot or increases pressure more than the others probably has something wrong and may be underinflated. However, it could also have a dragging brake or bad bearings that are causing it to heat up. Be aware that the sunny side will run a few psi higher than the shaded side and that a banked highway can cause one side to run a few psi hotter than the other.

 

[jlazar]

Thanks.  I will watch for those.

 

[Wolf Pack Fan]

Just to muddy the waters more, if you've never chalk-tested your tires, you really should.  Particularly if you have a tire size different from OEM.

You use sidewalk chalk to mark a heavy line across the tire, then drive straight forward for 100 feet or so, then check the wear of the chalk. If the chalk is wearing in the center and not towards the edges of the tread block, you are overinflated.  I can just about guarantee you that you will have to drop air pressure to get a good, square contact patch.  When we did your rig, we had to drop 15lbs off of the manufacturer "reccomendation" to get things right.

The rig rides one heck of a lot better.  We haven't had our RV long enough to see what this does for tire life, BUT I did the same thing to my Jeep three years ago and I am at 54,000 miles on a 40,000 mile tire!

That sounds like a great idea!  Thanks for posting that.  I'm going to do exactly that before we get into our camping season.

 

[Wolf Pack Fan]

Similar problem to Charles' Law but relating pressure and volume

Lest we forget Murphy's Law, the one that get's us all to often and usually at the most inopportune time.

 

[Wolf Pack Fan]

I have a gas motorhome so I am planning on carrying a 150 psi Porter Cable pancake compressor so I can check tire pressure in the morning when cold and fill immediately if necessary.  I would appreciate any thoughts if there is a better approach.  Also, any recommendations on a good brand of dual head tool to fit at the end of the air hose for inflating.  Thanks

I've read and heard that the Bon-Aire i8000 Goodyear compressor is a good one.  A lot more compact than a pancake compressor.  I don't remember how to insert pictures, but here's a link to it on amazon.

http://www.amazon.com/Bon-Aire-i8000-Goodyear-120-Volt-Inflator/dp/B003XGYDE2/ref=sr_1_1?ie=UTF8&qid=1358384761&sr=8-1&keywords=120+psi+tankless+air+compressor

 

[Gary RV_Wizard]

There seems to be no spec for that compressor that indicates how much pressure it can achieve, or what the volume of air flow is. Reader comments on Amazon suggest that 30 psi is about what it can handle. If that's the case, it's not very useful for any but the smallest RVs (pop-ups and such).

 

[falconhunter]

Here is a chart I down loaded a long time ago that compares outside ambient temperature and recommended cold inflation pressure.