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rob's garage

Rob

What is the "aspect ratio" of a tire? Some time ago you mentioned this but didn't go into detail. I figure it has something to do with the look of a tire because there are a lot of cars running around with different thickness of rubber. What's the point to all this?

Sam

Sam, aspect ratio (A/R) is yet one more measurement of a tire. I say one more because there is a lot of information necessary to classify tires. When you have nothing better to do - and I mean nothing better to do - read the novel that is imprinted on the sidewall of your tires.

The aspect ratio is not easily identifiable but it's there - literally it is a ratio that compares the width of a tire to the sidewall height. I'll use a common tire size to illustrate:

P225/55R 15 87S

This number, other than the manufacturer's brand name, is the largest set of figures that you will find on a sidewall. Without going into the details of the other numbers, because to describe them will take up most of our bandwidth, they are articles that, put together, would make a great blog thread.

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Anyway, this A/R is one of the important sizing calculations because, without knowing the A/R, automotive styling studios and vehicle engineers would have a tough time fitting tire and wheel combinations to make a vehicle look just right.

So, in this example, the number 55 happens to be the A/R. This states that the sidewall of this tire is 55 per cent of the width.

The sidewall is measured from the contact point at the wheel (or where the tire's bead meets the side of the tire), to the outermost circumference of the tread. This (and all) measurements in this example, are taken while the tire is not mounted on a vehicle.

The width of the tire is measured in millimetres across the widest cross-section of the two sidewalls, but - this is important at the design and engineering level - the tire must be mounted on its Original Equipment Manufacturer (OEM) wheel and must be inflated to its recommended pressure.

The interesting thing about the A/R is that no matter the diameter of the wheel on which it's mounted, nor how wide the tire is, this ratio never deviates from its intended description. In this example, the sidewall is 55 per cent of the width. This tire could be 150 mm wide or 300 millimetres wide - 55 per cent is still 55 per cent. What will be noticeable is the perceived thickness - as you mentioned Sam - of the sidewall. The A/R gives a tire its "Cool Factor". That is, if a tire is not very wide and it has an A/R of 35 per cent - imagine 35 per cent of a tire that is 200 millimetres across - the sidewall height would only be 70 millimetres. We're looking at a serious "Rubber Band" tire, Sam.

From the side, this tire will look like a wheel with a black band around it.

For those that require lots of tire for ground clearance, knobby tread design or protection of the wheel or chassis, a high A/R would be called for. Using the same width of 200 mm, but an A/R of 70, the sidewall will be 140 mm tall - almost a square.

To answer your last point, Sam, much of the aspect ratio consideration takes place during the design phase of a vehicle. Many dynamics are calculated into the final decision on the tire profile - things like handling, speed requirement, vehicle weight and carrying capacity.

There is no hard-and-fast rule that dictates tire make-up, design and profile. But as Michelin says in its commercials, "There's a lot riding on your tires", it's good so much care goes into this mostly neglected and misunderstood part of our cars and trucks.

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