Tyres, Tires, and More Tired - Part II
by Jim Lunson
wrote last month about tires on our MGs and the danger of their aging
long before the tread wears out. We'll look at what replacement
options are available in this day and age for a car at least now 30
The first step is to determine the age of your tires.
Because this is a safety issue, the government requires all tire
manufacturers to date stamp their tires as they are made. This
allows you to check your tires and see just how old they really are.
This stamp can be read on the sidewall of every tire, although they try
to make it hard for the average customer to read the code.
The date of manufacture code consists of 3 digits
located in a rectangular box after the noted DOT letters. This
box is found on the sidewall, usually near the rim, usually only on one
side of the tire (often the inside where it is very hard to find) and
surrounded by other numbers (these numbers are not in a box). The
first two digits of this code signify the week of the year (01 to 52)
and the last designates the year. This means a code for example of
236 in a box means the tire was made in the 23rd week of the year
2006. The safety people assumed no tire lasted over ten years so
there would be no way the 6 meant 1996 instead of 2006. Find
this box; start counting the weeks from January and you can figure out
pretty closely when the tire was made. And with older tires try to
remember at least in which decade they may have been put on the car so
you dont lose ten years in the calculation. A few manufacturers
also provide a year stamp which is much easier to read, but look for the
code box first. A tire with no code is really old and should not
This date stamp in the tires can give you a good idea
how old your tires may be and indicate when you should think about
replacing them, even if the tread is still good and there is no
cracking. It can also be a good guideline to check when buying new
tires as sometimes tire sizes for the MG may not be the fastest moving
stock and the dealer may have just the size you need, only to find out
they have been sitting in his storage or warehouse for 10 years.
It happens and needs to be checked.
In selecting new tires, available today is a wide
option on profiles. In the current size designation, the profile
or height is presented as a percent of the width. Thus, two tires
of the same width, but one with a profile of 50 will be a lot lower than
one with a 75 or 80% designation. The lower profile tire will be
much more responsive to turns, will feel the road better, and will yield
faster more positive cornering ability; all positives for a sports car
such as an MG. The downside to lower profiles is that the ride
will be harsher as there is just not as much air between the tread and
the rim. The car will not take potholes and rough road surfaces
nearly as well. I have seen many a dented rim from the streets in
DC on cars running with low profile tires. A second problem with
low profile tires is that they alter the speedometer reading as the
overall diameter of the tire is changed. Think you are rolling
along at 60 MPH per the speedometer, only to find you are actually only
going 50. So then you speed up to 70, and guess you are going
about 60. A policemans radar will often then tell you exactly how
fast you were actually going. This factor can be checked as most tire
dealers have a chart that measures the rolling diameter of various
size tires. For the MG, compare the old stock size tire that
originally came with the car to the new tire and try to keep them close.
One way to get lower profile tires and not affect
either the appearance or the speedometer is to do a substitution called
one upping. This term means increasing the wheel diameter one
inch (say 14 to 15) and then using the lower profile tire to gain the
handling. For instance, a tire size 175x70x13 (23 rolling
diameter) is the same as a 185x60x14 (22.9 rolling diameter). The
problem in doing this on an MG is finding rims of the same appearance in
the larger diameter. To do this change, the car either needs wire
wheels or aftermarket sport rims as there are no matching rims (either
the stamped steel or rostyle) for our cars in a larger size than the
stock wheels. So if you plan to do the one upping improvement,
check closely for the rims you need.
In addition to the diameter, two items to check when
changing wheel rims are to match the bolt pattern and wheel offset.
The bolt pattern is the layout of the wheel lugs on the hub. For
cars, the patterns are usually either 4 or 5 lug holes. The 4 lug
pattern is measured as the distance across the diagonal of the lugs.
For example, the MGB has a 4-4.5 pattern (4 lugs measuring 4 1/2 across
the diagonal). Five lug patterns are measured by the diameter of
the lug circle. It is measured by taking the distance from one lug
bolt to the imaginary circle that lies between the opposite two bolt
holes. Bolt patterns for aftermarket sport rims are noted in
either inches or metric, but the pattern can be converted by simple
mathematics (4-41/2 equates to 4-114).
The wheel offset is a bit trickier to measure.
This dimension is the horizontal distance from the vertical plane at the
exact center of the wheel to the mounting surface of the bolt holes.
For example, MGs have a very low offset (about 1), meaning the surface
of the bolt holes is close to the center of the wheel and tire.
Most modern cars have very large offsets (2-4), moving the mounting
surface far outboard to where it is almost flush with the outside edge
of the wheel and tire. This gives modern rims a very sleek look
while the MG wheel shows a deep recess (thats where you use chrome rims
to hide it). The problem with using a wheel rim with a large
offset on an MG is that when it is mounted on the hub, the increased
offset moves the entire tire and wheel assembly much further inboard
toward the center of the car. The narrowing of the wheel track
dimension will cause dangerous handling problems with the car steering.
Also, by moving the wheel far inboard, the potential for interference
with the steering and brake assemblies is much more likely. On the
MG, there is not a lot of inboard clearance and even a slight change
will hit the steering ball joint, especially when the steering is turned
far in one direction. So be careful when switching rims to check
the offset and then watch for interference with the brake and steering.
A last item to note when changing the offset of wheel
rims is in the use of spacer plates. These plates, usually 1/4"
thick, have bolt pattern holes to match the lugs and can be inserted
between the hub and the rim, moving the wheel further outboard and
negating the effect of the large wheel offset. These spacers do
the job but reduce the protrusion of the lug length through the wheel.
The DOT safely people require that on car wheel assemblies, the lug nut
has to have a minimum of eight (8) full turns on the shaft before
becoming tight. So, if you use spacers, check the turns as you
install wheels and make sure this minimum is maintained.
So, in buying new tires for your MG, there are a lot
of things to consider, and they all make a difference in the car and
your satisfaction with it. Happy tire and wheel hunting.