The art of setting tire pressures has changed significantly over the
past 30 years. In the sixties a non-belted bias ply tire was common.
Now almost everyone runs belted radials. The rules are different for
each type.
If you are running non-belted tires and wear is your major
consideration, then factory specs are what you should use. These are
usually set to make sure that the face of a non-belted tire has even
pressure across the tread and will wear well.
If you are running belted tires then air pressure has less to do with
tire wear. If you are looking for maximum handling, then chalk your
tires. Buy some childrens' driveway chalk (better known as "hop chalk"
in our family) and make a mark that extends over the outside edge of the
tire. Then go out and do some spirited cornering- slide her around a
bit. Look at how much of the chalk is worn off. If it is worn off well
up the sidewall then your tires are rolling over from under inflation.
If chalk is still left on the face then you are overinflated. A good
rule of thumb is to inflate such that the chalk disappears almost down
to where the tread ends on the sidewall. Once this is done then you can
change the pressure slightly on one end or the other to tune
understeer/oversteer. You can also deflate a bit if you find the ride a
little rough for daily use. This technique can also be use for
non-belted tires to maximize handling but you will probably wear them
out in the middle quite quickly.
One other thing to watch out for is strange suspension harmonics. On
the MGA I have found that setting the front tire pressure by chalking
causes front tire bounce under certain conditions. If I take a few PSI
out then the bounce disappears. Maybe it is from learning to drive in
old sports cars on dirt roads but I do not like the feeling of
understeer. As long as the car is still responding to the front wheels
I feels I have a chance but once understeer sets in I feel like control
is lost. As always, my opinion, YMMV, shake vigorously before use.
Regards,
Bill Eastman
61 MGA driver
http://www.british-cars.org.uk/kimber/MGA/mgapics.html
58 Jag fhc project
http://www.jag-lovers.org/xk-lovers/gallery150.htm
A Morgan is next
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