Okay, given the price and the lack of an audience, I'll post
my answer here.
Myoarin did a good job of explaining the essential principles,
which have to do with vectors and the fulcrum. The fulcrum is
the pivotal point around which a lever turns. In this case,
your bones act as the solid levers, and the fulcrum points
are the joints. So if you have a weight held directly over
your head with locked arms, the levers (your arm bones) bear
little tension, and the weight is pushing down on your
shoulder muscles. If you begin to lower your arms in front
of you, with your arms held straight, the bones take up
more tension, and the effective weight transferred to the
shoulders increases, peaking when your arms are parallel to
the ground. The further the weight is from your shoulders
(or the longer your arms), the greater the effective force
is on the shoulder (and back) muscles. If you were to bend
forward at this point, it would put unbearable pressure on
your lower back muscles.
The basic formula for levers and fulcrums is F1 x D1 = F2 x D2,
as seen in the illustration at the top right of this Wiki page:
I don't know the formulae to figure out the effective weight in
a precise manner, and it would be somewhat foolish to attempt to
do so, given that, when lifting the tire, your arms are "slightly
outstretched", so the amount of effective weight borne by each set
of muscles would vary with the length of your forearms, the angle
of your arm from shoulder to elbow, and more.
However, there is a general statement made on this page from
Steven Publishing which addresses safe lifting, which echoes
the estimate I have heard before:
"Encourage all workers to bend at the knee, not at the back.
Have them keep the product close to their bodies. A load held
at arm's length from the body puts 10 times as much stress on
Obviously, if your forearms are parallel to the road, but your
arms from the shoulder to the elbows are at, say, a 75 degree
angle to the road, the effective weight will be less than 10
times the actual weight, but you can get some idea of what it
might amount to. The closer you can keep the weight to your
body, the better.
By the way, I've found a way to mitigate the weight when doing
this maneuver, which is to place my elbows on my knees as I lift
the tire. This provides a good deal of leverage, and takes the
strain off the back muscles to a great degree. This effectively
places another fulcrum closer to the weight, and shortens the
distance from the weight to the first fulcrum, thus reducing
the effective weight and force.
As for using these principles of physics to assert that the
"weaker sex" will consequently be unable to reliably change
a tire, I'm afraid it would be presumptuous to assert anything
other than that it would make it highly unlikely that most
women would ever attempt to change a tire by holding the
wheel straight out from their bodies.
Children reach a weight ~40 pounds between the ages of 3 and
6 years old, and mommies are lifting them all the time at the
younger ages. They may be in better shape than you think!
Additionally, I recently watched a segment on the Rachel Ray
show where a female guest demonstrated how a woman should
change a tire. Included in the recommended kit were some
bricks. Two were to be used to place in front of and behind
the tire that was not being changed. The third one was used
to roll the spare tire up to the level of the lug bolts and
maneuver the wheel onto them. Add to that the possibility of
using the jack to raise or lower the car to assist in the
process of aligning the wheel with the bolts, using the brick
as an aid. What they lack in muscle can be supplemented with
Finally, most true spare tires these days are not full-size
tires (at least not in the US). They're half-size jobbies
that are smaller in diameter and also narrower in width.
They are specifically designed to get you to a gas station
and allow repair of the real tire, and you are cautioned
not to exceed 50mph when using them, as they aren't made
to take that kind of stress for long. Here's an image:
These specially-designed spares weigh more like 25 pounds,
and don't pose anywhere near the challenge of a regular
So, unless mom's driving the family Hummer (in which case
she probably has AAA club priveleges), I'd be reluctant to
draw the inevitably controversial conclusion that, with
training, a woman would have a much harder time changing a
tire than a man.
If you have any thoughts or questions, feel free to post a
Request for Clarification...
Searches done, via Google:
"replacing a tire" weight lifting
"physics of lifting"
"proper lifting" "effective weight"
"proper lifting" "effective weight" -training -management