[DeTomaso] Tire Pressure 102 Advanced

michael@michaelshortt.com michaelsavga at gmail.com
Tue Feb 10 11:51:26 EST 2009


Here's an engineers way of doing it, to at least give you a baseline for
your car.

This would be great if you had access to 4 corner scales and sat in the car
while it was being done with 1/2 tank of gas.

You need to be concerned about load capacity, impact resistance, and then
handling. Assuming you are dirving a street Z including you, a passenger,
full fuel, and the other crap that ends up in your car, you need to do some
basic math.

Gross weight: 3,000lbs
Weight dist: 52/48
Tire size: 185/70-14 (tires on my 810 as an example)
Tire max load: 1,235lbs @ 44psi.

How much weight can each psi in the tire support?

1,235 / 44 = 28lbs

How much weight on each front tire?

(3,000 * .52) / 2 = 780lbs

How much air pressure to support the 780lbs each front tire holds up?

780 / 28 = 28psi

So, we know that the MINIMUM tire pressure for each front tire is 28 psi
assuming a static weight distribution. Under hard braking we can see up to
70% of the weight on the front tires so that means each tire needs to be
able to support 1050lbs, which requires 38 psi. That can now be our MAXIMUM
tire pressure for the front tires (You guys do the match for the rear
tires.)

We now have a range of pressure to work with to adjust the handling of our
car (28psi to 38psi). What you'll want to do is start at the high number and
keep an eye on tire rollover. You want to see the tire scuffed to the top of
the little wear triangles marked on the side of the tread. Don't scuff the
tire any farther onto the sidewall. Lower the front tires pressures until
you see the scuffing to the point mentioned above.

In some extreme cases you may have to go above the high number of the range
due to suspension issues or driver problems, but normally this shouldn't be
the case.

then back to what I said in the first email, increase or decrease in 1 lbs
increments ( on warm tires ) to fine tune your car.

IMPORTANT.

once you get it there, keep in mind that the tires were Hot/warm when you
reached your "happy place", when the tire cools down, the cold tire pressure
will be lower, so when you measutre it again, that is your new baseline for
what the car should read when you check the pressures at home BEFORE you hit
the road each time out, knowing that they will come up to pressure as they
warm up.

and again this changes throughout the year, tires in the Georgia summer ride
on blacktop that can be 150 degrees as opposed to concrete highway in
Seatlle that measures 70 degrees on the same summer day.
so they won't grow as much as our tires and thus we need to start with lower
pressure because our tires will grow more.

Don't even get me started on Nitrogen in the tires, yes you can run it, we
did and yes it helps make control a great deal easier, the problem becomes
having it around when you need it ion a street car ( the Quickee Mart ain't
gonna have no stink'n Nitrogen at their coin operated air machine when you
find that your tire is low) .



Michael in Savannah

Michael in Savannah
-- 







Michael L. Shortt
Savannah, Georgia
www.michaelshortt.com
michael at michaelshortt.com
912-232-9390


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