Washington, D.C. Centre
the Spark

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 secure ride.

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.


Back to Technical Tips