Tyres, Tires, and More Tired - Part I
by Jim Lunson
I recently examined the tires on my MGB with the thought that they may
need replacing. A quandary becomes what to buy. Tires have
greatly changed from when the car was made presenting many options.
Our MG cars were all made back before radial tire construction took over
the market. Designed for bias ply tires, the MGB for example,
originally came with 5.60x14 size tires. These numbers equate to
5.60" width of tread, height profile (from rim to tread top) a standard
80% of the width and 14" diameter. MGAs and earlier cars had
different sizes, but followed this same numbering system. You
would have to check your manuals for the exact requirement.
Today's tires bear no resemblance to this nomenclature.
Using the MGB as the standard for this article, tires
now come in sizes like 165x70x14. This equates to 165mm width of
tread, height equal to a ratio of 70% of width, and still 14" diameter.
The comparison of sizes is not hard doing a little math: width is 165mm
(vs. 5.60x25.4 =142mm); and height is 165x70%=115 (vs.142mm x .80
=113mm). So, within a few millimeters, these sizes are comparable.
The new tire is slightly wider, but is basically the exact replacement.
So check your manuals and do a little math and you can determine what
current size fits your MG.
The problem in buying new tires today is two-fold.
First, finding the size you need is difficult as most manufacturers only
make select sizes that are a popular fit for most modern cars, and
secondly, with advances in tire design, much more performance can be
achieved by changing profiles. The size selected has four goals;
fit the rim width, fit the car mechanics, look proper on the car, and
provide the handling desired by the driver. For the MGB, this
usually equates to a size of 175 (or 185) x 70 x 14 in tires made today,
but lets look at the details.
Rim width: most MGs use a rim width of 4" to
51/2" width. This includes both earlier cars and either wire,
steel or rostyle wheels. The maximum width that can be safely put
on this width rim is 185mm. Anything wider has a danger of popping
off the rim during a sharp turn. Aftermarket rims such as
Panasport, Minilite, Aero and others have wider sizes that of course
will take wider tires. And the early MG T series had narrower rims
that take narrower sizes. But the wider the tire, the more tread
contacts the road improving corning, and making for a smoother more
Car mechanics: The tire has to fit in the wheel
well without dragging against fenders when turned sharply or hitting any
of the brake or steering mechanisms. Extra wide tires and rims can
lead to interference with many of these front end mechanisms, especially
the steering tie rods on the front. It is essential that these
dimensions be considered before making changes as handling and safety
can be affected. There is not as much room inside the fender as
you may think.
Proper looks: The MG designers produced
beautiful body shapes for all their models and the tire size they used
formed an integral part of this design. Tires too small, too tall
or too wide will have a severe effect on the appearance. Ever see
a modern Chevy Suburban with oversize 20" rims and 45 ratio series
profile tires on it -certainly changes the vehicles appearance?
Handling: A show car needs one type tire, a
racer needs another and the daily driver or interstate cruiser still
another. Todays automobile tires are usually listed as
passenger, performance, racing or light truck. The
application of each is obvious based on which handling characteristics
you desire for your car and carry different features. Shorter
height tires (65 and lower ratios) yield much better cornering
characteristics, although the ride suffers as there is less give going
over bumps or potholes.
My thoughts turned to tires after putting two new
tires on my Oldsmobile and having the technician tell me that the old
tires coming off showed a lot of rubber deterioration, in addition to
the obvious tread wear. Sure enough, in looking closely at the
sidewalls, there were thousands of hairline cracks in the rubber on the
sides facing outward. There were also long cracks in the bottom of the
grooves of the tread running around the tire. This situation
prompted me to take a look at the tires on my MG. The tread on
these tires is still great and in looking back over my records, this set
has only racked up 18,000 miles, but with further check of the old
invoice, I found that they are also now 11 years old. In looking
closely at the tires, sure enough I spotted the same rubber cracking I
found on the Olds. Not good.
I chuckle at the response I get when asking
someone how long should tires last. The reply is usually something
along the lines of 40,000 to 60,000 miles on a good set of radials.
Not an answer. That is how far they will roll, not how long they
will last. The actual time limit on a tire lifespan depends more
on where it is kept. Sunlight is the big culprit as it breaks down
the rubber compounds causing cracks and failure. My Olds sat
outside and had never been garaged. The worst cracks were on the
outside sidewalls while the insides were fine. In looking at my MG
which stays inside except when being driven, these tires lasted 11 years
and have considerably less damage to the sides, although it is starting
to show now. Rubber does not last forever like steel.
There is the story of one club member who took his MG
out for its first run in the spring after sitting all winter and blew
out two tires in the first 30 miles, leaving him stranded and scratching
his head. The tires still had lots of tread. Seems that his
car sat outside in his driveway every winter, diligently covered by an
expensive car cover, but one that covered all but the bottom half of his
tires. After doing this for about 5 years, the tires were ready
for disaster. And the tires on the shady side of the car during
the winter are the ones that survived.
So as we go into the winter months and a lot of our
cars get stored, try to get the tires covered as well as the rest of the
car, and be ready next spring to check the tires and for more than just
looking at the tread wear. Cracking of the sidewalls and in the
tread that occurred during the winter can be serious, especially on that
first spring run. We don't need to have anyone stranded or in an
accident due to a blowout caused by old tires.
I'll discuss in more detail the selection of new
tires in the next issue, including date stamping, handling profiles,
speed ratings, one-upping, castor angles, rim offsets, and other
exciting tire related information that applies to our MGs.