Pressure in spare

Hello All!

Does anyone know how much air to put in a spare tire while it's still on the spare tire rack?

The tire says max 50 psi, but that would be with the weight of the trailer on it.  What about with no load?

I've been filling it to about 40 psi thinking that should I ever have to put it on the camper,  I can either add a little or let a little out before driving.  I'd like to get a more accurate calculation to help narrow it down a little.
Thanks alot.

Gary

I always try to keep the max 50 psi in the spare.  Its easier to let some out than add some during a tire change.

Plus as infrequently as the tire is checked you can probably put more than 50 psi inthe tire, since 50 psi is max pressure for max load and the tire has no load.

The main thing is not to grab your spare and only have 25 psi in it on a rainy night on the side of the interstate....

Oilspot:

Thanks for the info.  I guess my only concern about filling it to 50 PSI at no load would be what the pressure would rise to when there is a load.  I'm not worried about letting air out when it's at 55 or even 60 PSI,  but I wouldn't want to put the camper down on the tire and find that it brought it up to 70+ psi !

I was wondering if anyone knew how much additional pressure the load would add (approximately).  

Gary

Gary,

There's a very easy way to determine just how much your spare tire's pressure will rise when it's mounted on an axle. On a cool morning jack up one wheel of your TM. Adjust the pressure of the tire on this side so that it is exactly 50 psi. Allow a few minutes for the compressed air in the tire to cool down. Then lower the tire completely and measure the pressure within it. I'll bet that you're going to find very little increase in pressure when it's resting on the ground, under the weight of the trailer.  

Although I don't have any scientific proof, I agree with Larry.  There should be negligible change in tire pressure on or off the trailer. :P

Rolling dynamics and heat aside, tires deflect (or flex) rather than changing volume.  At a constant temperature, the volume of the tire would have to change to affect the pressure.  That is why pressure affects the contact patch so dramaticly and too high or too low a pressure (in a radial tire) will reduce the contact patch.

If you really want to scare yourself about pressures, try to find an accurate gage!!  They are very hard to find and you would be surprised how far off your gage might be... ???
I do a little competitive driving on the side and 1 or 2 psi makes a big difference, so having a good gage is essential.  Most beginners at the competition have a "bargain" gage that can be off as much as 6 or 8 psi.

Off course the bottom line is that hopefully we aren't pushing the ragged edge of the envelope and operate within the limits of our tires and the reasonable margins of error designed by the engineers.

In other words, keep up with the pressures, but don't drive yourself crazy over them .

Oilspot,

For the benefit of the group, where does one find an "accurate" tire pressure guage and how much might one cost?

Wade

Wade,

I wish I had a good answer.  

I have one that I "adjusted" using a known standard of my brother's.  Although I cannot guarantee its accuracy I use it for my trailer tires.  I originally purchase this one from Brookstone and it was about 8 psi off of the standard.  The Brookstone was about $20 and my brother threw his away after testing it.

I also purchased one from a racing supply catalog.  It came with a +/- 0.5 psi guarantee.  Although I have not tested it against any known standards yet...  

I bought the middle of the road model for $35.  The inexpensive one was ~$25, and the professional versions were >$100.   I'll try to find the web page link for the catalog and post it later today.

The better ones usually have a long hose (8&quot and rubber armor to protect the sensitive parts inside.  They are also nice because you can let out air in controlled bursts from the gage and monitor the pressure as you go.

Larry & Oilspot:

Thanks for the info guys!  I'll make sure that my spare is up to the full 50 PSI before leaving on Saturday!

Gary

I keep the spare at 50 psi cold.

We recently returned from a July 4 round trip from Western NY to Indiana, about 550 miles each way.  We averaged 60-65 mph and got 12-15 MPG.

This is our second major trip since having the trailer weighed.  We continue to pack only the lighter items (clothing, pillows, and sleeping bags) in the trailer and keep everything else in the van.

Carefully monitoring the weight is the only change I can attribute to not having a repeat blowout.  Since I had recently replaced the brake magnets and repacked the bearings, I monitored the tires and axles frequently as we stopped.  The tires by far are the biggest source of heat.

There happens to be 2 different brands installed and one tire runs significantly hotter than the other, even though both started out at 50 psi cold.

I am convinced on the 3023 that there is very little margin between the 'dry weight' (3240# on a CAT Scale) with options and the GVWR (3500#).  Doesn't take much to exceed the limit, and then you don't know how long you've got until a failure.