LAC: Wow! Tire Pressure!

[Brett Dikeman]

On Jul 31, 2006, at 11:22 AM, SuffolkD at aol.com wrote:

>
> Before Brett Chimes in, you'll get expansion of air as the tire temps
> increase going down the road.

My reputation precedes me? :-)

> You need a tire pressure gauge (DIAL)

  After comparison between "professional" (ie $$$) race gauges and a  
cheapo digital gauge bought at Rat Shack, both are equally suitable;  
if anything the digital cheapos are better because you bleed off less  
air from the tire with each check.  Avoid the "pen" type stick  
gauges; they're lucky to be 10-20% accurate, whereas the digital ones  
are frequently under 5% (depends on the sensor etc) and usually have  
a precision of half a pound.  I have a "dial" "race" gauge I have to  
whack against the side of the tire to get a proper reading, because  
the gauge sticks.  The before and after readings are 5-6 psi  
different, sometimes more (at 36psi, a 30 psi reading is off by 17%!)

If tires are the same size as the originals or a proper "plus" size,  
the inflation should be the same as the what the manufacturer  
recommends- ie what's on the door jamb or fuel filler cap.  If they  
are not, then talk to a tire shop or call/email the manufacturer.  I  
emailed Sumitomo with this question with my 235/45-17 tires for the  
200q20v and the answer was, "that's the correct sizing for your car,  
so what the flap recommends."  'The flap' recommends 36 PSI all  
around.  More is not always better, and tires are the last place you  
should be looking to solve handling problems (ie, check bushings, get  
the car aligned, etc.).  If you have not been to a driver education  
school, you may not know "good handling" if it bit you on the rear,  
and many people kill overall handling capabilities "fixing"  
understeer or oversteer inherent in the car's layout or design.

Tires should be checked/filled "cold" (when the vehicle has not been  
driven for at least 15-30 minutes) and if the temperature outside  
changes dramatically, re-checked.  Ie, once a month should be a good  
idea if you live in an area with significant seasonal change.  Watch  
out for tires that have been in the sun vs. shade; even after 5-10  
minutes it caused a big difference in readings for me at the track.

Air from a big compressor will be best- ie something liable to have a  
moisture trap, or at least a tank between the compressor and line,  
where moisture will collect.  If you are a scuba diver, scuba tanks  
are a  perfect source of air for tires as the air is very filtered  
and highly compressed.  Look for a cheap first stage, plug everything  
except the low pressure port.  There are a variety of tire chucks and  
pool toy inflator nozzles sold online for just this purpose.

Brett