Rules of Thumb: fatter, wider, lower

December 11, 2006 at 11:17 pm Leave a comment

Basic car models have basic tyres. I guess you could call that a ‘rule of thumb’, which like all rules, are made to be broken.

One of the easiest ways to either dress up the appearance or the performance of your car is to put better tyres and wheels on it. Here are some basic guidelines that you can follow until you get to the point where you really do need to consult your tyre expert.

For every tyre section width increase of 10 mm, you can lower the tyre profile by 5.

An example 175/75 can go to 185/70, to 195/65 to 205/60 and so on, not to infinitum though. Yes, there are limits, occasioned by the need for a wider rim as the section width increases. This point is covered in the Tyre and Rim Association Standards. The section width of a tyre is measured on a specified rim (the measuring rim), but generally a maximum and minimum permissible range of widths that the tyre can be fitted to, is specified. Fortunately, there isn’t much demand today for the 175 to 205 section increase on the same rim diameter because these wide tyres had a short footprint, and required changes to the bodywork to accommodate them.  Their wet road hold was also not as good as it should have been.

These days, as the profile lowers, the diameter of the wheel increases. The latest sport oriented Holden for example, has 40 series tyres on either 18 or 19 inch rims, whilst the family sedan version can be obtained on either 16 or 17 inch rims.  See the examples later. What really matters is the total air volume contained in the tyre. Even if the shape of the air contained in the toroidal shaped tyre is long and relatively narrow, the tyre can carry the same load as a lower diameter, wider tyre if the volume is the same. Or perhaps the car engineer wanted to run the tyres at a lower pressure, to compensate for the low profile sidewalls and their shortcomings of lack of enveloping capacity for ridges and potholes. So he fitted bigger diameter tyres of lower profile to carry the same load at a lower pressure, but he still had to fit them in the same spaced wheel well.

So, somewhere along the way in the progression shown above, you come to the crossover point where a change in rim diameter is not only desirable but necessary. Many times, a peek at the rim specifications for the other models in your car maker’s catalogue, or even on the tyre placard on the car door pillar, will give a clue as to what the car will accept in bigger diameter, lower profile equipment.

However, obtaining a set of the correct wheels may prove a little more difficult, so at that point, it is wise to consult a wheel supplier. This is because as the tyre width widens, factors such as body clearances, suspension travel, and track width come into play. It may be possible, for example, to fit a rim to your car, but the offset of the wheel disc on the front suspension may not permit the fitment, because it fouls the body- work, particularly under full ‘jounce’- suspension travel in the vertical plane, or at full lock.

However, don’t let all the above deter you. Just make sure that you get good advice, fit a legal fitment, and be sure to advise your insurance company what you have done. Some wheel and tyre equipment can be worth more than the car!

Entry filed under: Choosing the Right Tyre. Tags: , , .

Care with Air Why don’t they make tread patterns deeper, so that I don’t have to change tyres so often?

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