Understanding Tyres Part 2: Debunking Tyre Myths

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In the first part of our Understanding Tyres series, we took an in depth look at the information that can be found on a tyre. By now, you should have an appreciation of how different tyres can be once you look deeper than the size. Before you waltz into your local tyre dealer and choose tyres like a pro however, now is a good time to clear your mind of some common misconceptions about tyres. In this chapter, we will identify and assess some of the most popular “tyre myths”.

 

All-Season Tyres out-grip Summer Tyres on wet roads (False)

This is by far the most universally believed myth about tyres. So much so, that when wet season comes around (or spring and fall for those readers not in the tropics) you’ll likely see a tyre dealer advertising their all-season range as ideal for wet roads. It may surprise you therefore, that the truth is the opposite. An all-season tyre will have less wet grip than an otherwise equivalent summer tyre from the same manufacturer. Contrary to popular belief, ‘All-Season’ does not mean the tyre can perform excellently in all conditions. Rather, All-Season indicates a compromise – some wet grip has been traded for mobility in snow. Where the softer, grippier compound of a summer tire will harden in below freezing temperatures, the harder compound of all season tires will remain flexible. However, the soft and grippy tread compound of a summer tyre is actually better for traction on damp or lightly wet roads.

It should be noted that a tyre’s hydroplaning resistance – that is, its ability to maintain contact with the road in deep water or at high speeds in wet conditions – has nothing to do with whether it is an all-season or summer tyre. Rather, the main factor for hydroplaning resistance is the tread pattern. There are plenty all-season tires with excellent tread pattern designs as well as many summer tyres with mediocre tread pattern designs. The above statement in bold stays the same though. While a manufacturer can focus solely on dry and wet grip in their summer tyre tread pattern designs, some compromises are made to an equivalent all-season tyre’s tread patterns for traction in winter conditions.

While both are very good tyres, the only advantage the Bridgestone Potenza RE970AS Ultra-High Performance All Season Tyre (Left) has over the Bridgestone Potenza RE760 Sport Ultra-High Performance Summer Tyre (Right), is traction in cold, winter conditions. The RE760 will out-perform its All Season cousin in both wet and dry weather. 

When replacing two tyres on a FWD car, put the new tyres at the front (False)

NOTE: we are assuming that two of your tyres are only half worn and in good condition otherwise (i.e no bulges, dry rot, uneven wear etc.). If more than two of your tyres need replacing then don’t cheap out, replace them all.

The front tyres on a FWD car transmit the acceleration and steering forces and most of the braking forces to the road. As such, the front tyres will wear faster than the rear. If you don’t regularly monitor and rotate your tyres to ensure even wear, they will tend to wear in pairs; the rears having around half the original tread depth by the time the fronts need replacing. The common belief is that the new tyres should be mounted at the front of the car to provide more traction in wet weather. However, the truth is the opposite in this case as well. When tyres are replaced in pairs, the new tyres should always be mounted at the rear. This rule applies to RWD and AWD cars as well. Having new tyres at the rear axle will provide cornering and braking stability on wet roads because they have better hydroplaning resistance. You will likely feel the car understeering – since the front tires have less grip – but this is easily remedied by easing off the throttle or gently applying brakes if necessary. Conversely, if the new tyres are mounted at the front, the rears will lose traction first, causing oversteer. This is much more difficult for a driver to control, as the first instinct of trying to slow the car may actually increase oversteer.

 

A tyre will burst if it is inflated past the Max Press. Number on the tyre (False)

A new, quality tyre will not burst even if inflated to well over the maximum pressure number on the tyre. To understand why you need to understand how tyres carry load. Air pressure is what carries the load, the wheel and tyre being the container. Increasing air pressure increases the load the tyre can carry. At some point however, there will be a pressure where adding more air to the tyre will not increase its load carrying capacity. This is what the Max Press/Load numbers indicate. They do not indicate the limits of the tyre. If the tyre and/or wheel are damaged, however, anything can happen.

 

A tyre’s grip and handling is optimal at the maximum pressure (False)

The car manufacturer determines the optimal tyre pressure for your vehicle. This number is usually found in your car’s door jamb as mentioned in Part 1. The car manufacturer will do a lot of testing to determine the pressures that offer the best traction, handling and ride comfort, taking many factors such as weight and weight distribution and suspension geometry into account. When you move away from the factory wheel and tyre specifications or make changes to your car’s suspension, weight or weight distribution you will have to perform your own testing to find the tyre pressures that work best for your setup. It is never as simple as filling your tyres to the maximum air pressure number.

Always use the recommended tyre pressures found on the driver’s side door or door jamb. If you are moving away from the factory wheel and tyre specifications, use the recommended pressures as a starting point to find the pressures that work best for your new setup. 

 

Larger diameter wheels and low profile tyres will improve handling (Sort of…but it’s not so simple)

It is true that switching to a larger diameter wheel and low-profile tyres (known as plus-sizing which we will cover in Part 3) can increase steering response. However, after the initial ‘bite’ you feel when the tyre quickly responds to your steering input, it’s the tyre compound that will determine how well the tyre grips the road. Therefore it is very possible that a ‘sticky’ high performance tyre in the factory specifications on factory wheels will out-grip a lesser low profile tyre on larger wheels, even if the initial turn in may not feel as fast.

It is also possible to overdo it, fitting very low profile tyres onto wheels several sizes larger than the factory diameter. Super-low profile tyres are very shock sensitive and offer little compliance. These tyres can skip over rough surfaces (bumps, potholes etc.) instead of holding onto the road and providing traction.

Fitting low profile tyres to larger wheels can improve your car’s handling IF you choose a quality high performance tyre and plus-size your new wheel and tyre package correctly.

 

All tyres with the same size markings are exactly the same size (False)

Bet you weren’t expecting that. To prove it, let’s take a look at the three tyres below. These measurements were taken from TireRack.com, which is actually a very useful tool for tyre research. They provide very detailed specifications on the tyres they sell as well as thorough and unbiased comparison testing. The three tyres below are all designated 205/40 R17. All three tyres have a recommended rim width of 7″ – 8″ and were measured by the manufacturer when mounted on a 7.5″ wheel. Generally, for every 0.5″ increase in rim width, the section width increases by 0.2″ or 5mm; so we will subtract 5mm from the recorded measurements to get the section width on a 7″ rim, giving the lowest possible section width.

Kumho Escta PS31: Section Width = 8.3” = 210.82mm  –  5mm = 205.82mm (on a 7″ rim)

Goodyear Eagle F1 Asymmetric 3: Section Width = 8.4” = 213.36mm – 5mm = 208.36mm (on a 7″ rim)

Yokohama S.drive: Section Width = 8.5” = 215.9mm – 5mm = 210.9mm (on a 7″ rim)

As you can see, not only are none of these tyres the same size, but at the smallest recommended wheel width, only the Kumho is around 205, with the Goodyear and Yokohama tyres being increasingly wider. Tyres of a specific model can be slightly narrower or wider than the stated section width and have a slightly narrower or taller sidewall than the stated percentage. Manufacturers may make their tyre slightly narrower and shorter in order to use less material and therefore reduce cost. On the other hand, manufacturers may make their tyres slightly wider and taller for performance purposes.

 

With that we’ve reached the end of our myth busting chapter. Now that we’ve addressed the common misconceptions that car owners tend to have about tyres, we can go on to the third and final part of our series, where we will outline the steps that should be taken when choosing a set of tyres for your ride. Until then!

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