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Next time you’re driving, take a look around and I’ll bet you spot a set of 45 aspect, low profile tyres on a car that didn’t roll out of the showroom with them. Their popularity from both an aesthetic and performance enhancing perspective shows no sign of waning.

There are some common (and correct) perceptions about the benefits of changing from a standard profile tyre to a low profile including increased steering response, and improved dry handling and cornering ability.

However, unless you do your homework and involve an expert to specify and fit your new tyres, you risk doing some serious damage to your car beyond the commonly spruiked problems like smashed up rims and pinched sidewalls.

So how low can you go? Should you buy a car with OEM low profile tyres? Or is it ok to change them later when you’ve saved up enough dosh?

I know what I think, but its best to hear it directly from those in the know. I had a chat with two such people, Bridgestone’s general manager of technical field services, Claudio Sodano and Kia’s ride and handling consultant, Graeme Gambold. Both have extensive knowledge on the subject, yet each approaches it from a slightly different perspective.

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WHAT ARE LOW PROFILE TYRES

Low profile tyres have a shorter sidewall or a lower aspect ratio than normal tyres.

They also have larger tread blocks, a stiffer, wider apex and can be made from specialised compounds.

Generally a tyre with a 50 aspect or less is considered to be low profile. To find out if you have low profile tyres fitted to your vehicle, check our guide to reading tyre size here.

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DESIGN

A lot of time and effort is put into the development, engineering and design of OEM tyres that are either fitted as standard or listed as optional equipment on new vehicles. Tyre manufacturers like Bridgestone liaise directly with manufacturers and they work together to meet the engineering, design and performance requirements for each model and variation.

This delicate equation has numerous factors to consider, as Bridgestone’s Claudio Sodano explains.

“Manufacturers look at many things, not just vehicle handling, dry braking, noise and rolling resistance. There are many attributes they look at to find what they believe to be a good tyre for overall performance. But sometimes the driver may be happy to compromise in a certain area, they may want ultimate performance in terms of dry grip and steering response and move to a low profile tyre,” he said.

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Kia consultant Graeme Gambold also knows just how important it is to get that balance right.

“There’s a huge amount of engineering involved. Tuning the car for the particular tyre that’s fitted to it, tuning the particular type of tyre, or the particular size and brand. There’s a plethora of things we change; the chemistry of the rubber, the belt height, the tread depths, the apex height of the bead where it maps to the rim. There’s a million things that get tuned and you could take two tyres from a tyre store both marked exactly the same, Bridgestone, Michelin, Pirelli doesn’t matter, the same tyre, the same size, but if one’s an OEM tyre and one’s an aftermarket tyre they’ll be of a different construction almost certainly.”

These days, many vehicles come with two or three tyre sizes available across different trim grades and models within the range. Gambold explained how that affects the process.

“At Kia we typically tune nominally for the medium sized tyre, or the highest market volume tyre, it might be something like a 17-inch with a 16-inch low grade option and an 18-inch high grade option. We’ll test both sides of that but most of our work will be done around the nominal mid sized tyre specification or the volume specification,” he said.

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Sodano says sometimes the compound itself doesn’t change, but when it comes to tyre development and construction, every tyre is different.

“In terms of dynamic vehicle handling they (car manufacturers) may want to increase the cornering power of the tyre simply to reduce the amount of understeer or get a more neutral biased tyre. So we would work with them to basically change the construction of the tyre. For example increasing the sidewall thickness or tread thickness to provide the performance that meets their requirements,” he said.

“In some touring patterns we may have the same compound that goes through from 60-series down to 35-series, from low profile up to standard profile. Sometimes the compound may not change depending on what the target is for that particular pattern tyre. In some circumstances, for ultra low profile tyres, 30 series say on high-end performance vehicles, we’ll adopt a compound with ultimate grip.”

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TYRES AND RIDE COMFORT

Between the road and the driver there are three effective ‘springs’ in a car - the tyres, the suspension and the seat cushion. These elements combine to make up the car’s measurable spring rate. As Gambold explained, when the tyre profile goes down, the spring rate of the tyre goes up drastically.

“We talk about the spring rate of the tyre being about ten times the spring rate of the suspension. If you go from a reasonably normal profile tyre like a 65 or 70 series, down to a 55 or 45 profile tyre there’s a huge change in the spring rate and that’s what most people feel; the low profile tyre ride is harder. As the spring rate goes down the ability of the tyre to envelope and cushion bumps and resist impact damage reduces.” he said.

The challenge for manufacturers is to find that balance between the amount of grip that’s needed, and maintaining ride comfort.

“In the quest to have better lateral performance or cornering power and cornering grip, you’re reducing the spring. So the cornering power goes up in a low profile tyre but the vertical spring rate or the cushioning or the ride comfort, goes down,” Gambold said.

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LOOKS AND PERFORMANCE

These are the two main reasons people opt for low profile tyres when they purchase a new car, or fit them after purchase. As Sodano outlined, low profile tyres have an increased tread width, which in turn increases the contact area between the tyre and the road, which is positive in terms of handling and braking. Another performance benefit is that the shorter sidewall height aids performance by sharpening the steering response.

“You get less deflection and in turn increased steering response, improved dry weather handling and better cornering ability. Basically a low profile tyre, in general, will have higher cornering force than a standard profile tyre. It’s really all about performance but obviously there are some people who just like the appearance aspect of a low profile tyre,” he said.

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Form and function need to be seamlessly integrated to ensure optimal performance.

“To get better dry surface grip they (low profile tyres) will have less negative ratio, or more contact area, and less void area in the tyre. There’s that balance in terms of having a contact area with enough rubber on the road, but also having the ability to push the water out of the way of the tyre,” Sodano siad.

But Gambold warns the gains on the performance side of the scale can come at a cost.

“From an engineering viewpoint for passenger cars you always want to use a higher profile tyre, but people don’t want to buy those these days. They want their cars to look good, be on low profiles with big wheels and that sort of thing. But that obviously comes at an expense, and the main issue, particularly in Australia and countries that have rough road conditions, is the tyre damage and tyre durability. From a practicality point of view its better to stay with a higher profile tyre, but from an ultimate handling and aesthetics point of view everybody wants lower profile,” he said.

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Which begs the question, what can go wrong?

DAMAGE & TYRE WEAR

If you didn’t buy your car new with the low profile tyres you wanted, what are you putting on the line if you make the change after purchase?

Sodano says the aspect of the tyre will have an impact on tyre wear, but there are other contributors.

“Tread wear is governed by the tread compound, the contact area or negative ratio of the tread area and the tread width. There is some small influence because when you reduce the sidewall height you increase the cornering force of the tyre. Essentially it doesn’t have a big affect on tyre wear, but because low profile tyres are geared towards performance there may be some sacrifice.” he said.

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When it comes to one of the biggest contributors to tyre wear – it’s actually you, the driver.

“It can be a by-product of the way you drive, driving habits and driving style have the biggest effect on tyre wear. Put the same tyre on the same vehicle in the hands of five different drivers and you’ll get five different wear lives,” Sodano said.

“You’ve got to be a little bit careful when you start getting into ultra low profile, 30 or 35 series, there’s not a lot of rubber between the rim and the road. If the road conditions are poor and you have a lot of potholes it’s a lot easier to damage the tyre. The rim could push through the sidewall of the tyre, what we call pincher cut. Something to be careful of.”

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Gambold also pointed out some of the potential issues, including damage to the rim and sidewall.

“As the spring rate goes down, the ability of the tyre to envelope and cushion bumps and resist impact damage reduces. So it’s very easy to get rim cracks and sidewall damage, and cracking on the inner edge of the sidewall. The compliance of your tyre to withstand the rigours of the road is a measurable factor so they’re all cost issues because your tyre won’t last as long,” he said.

If you attempt an extreme change, for example from a low-grade car on 15-inch tyres, and put on whopping 20-inch tyres, that’s a huge difference and Gambold warns the problems could be far more serious.

“Things like speedo accuracy will be out and most importantly these days, the Electronic Stability Program (ESP) calibration can go out. The car’s ESP system may not react as quickly because it’s seeing fewer counts per metre travelled. The suspension has a pulse generation system that the computers look at and if those pulses aren’t within a certain calibration window it can wreak all sorts of havoc with the logic in the computer. It can be a direct safety problem to the point where in Europe now they’re actually developing regulations to cover this because everybody’s so concerned about the calibration variability of ESP with aftermarket fitment of tyres,” he said.

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CHANGING TYRE SIZE & PROFILE

If you’re still hell-bent on putting low profile tyres on a car that came standard with higher profile rubber, here are the rules.

Gambold recommends keeping two things front of mind. Don’t go outside of the manufacturer’s sizes as specified on the tyre placard or in its type approval, and don’t opt for a cheaper tyre.

“Let’s say you’ve got a base model car and it’s got 15′s but the higher grade car has 17s, generally you’d be ok stepping up to the 17-inch rim. Generally speaking, if a manufacturer offers something as wide as the range from 15 to 19-inch, there’ll be other engineering aspects in the cars that differentiate them. There’ll be a different ESP tune going from 15 to 19, there’ll be a different spring rate. In going to something extreme, certainly if you see a car on 19s and 20s, but it was only ever designed to be on 16s and 17s, then you almost certainly have performance degradation. I would recommend if somebody’s buying a low-grade car and they want the big wheels on it negotiate with the dealer to fit them at purchase. Or get them fitted soon after buying the vehicle.”

The decision to change brands or size after purchase can also come down to cost, but it’s always important to engage an expert.

“A lot of things happen when someone wears out their first set of tyres. For example a BMW fitted with genuinely tuned star marked Michelin’s, those tyres are tuned specifically for that BMW. Then the owner finds out it’s thousands of dollars for a new set so they go and buy a cheaper brand. Personally I don’t recommend doing that, because the tyre grip levels and the ride comfort and a whole lot of factors in the tyre and the wear rate have been specifically tuned to those high end cars. You’re giving away a lot even to put the equivalent size of lower brand on. In the aftermarket I’d always recommend changing like for like – or change up in brand, but don’t change down in brand. Generally speaking the car will be at its optimum best performance in terms of that balance between ride comfort, durability, fuel consumption and everything on the tyre that its originally engineered for,” Gambold said.

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Sodano agrees that price will be a major consideration for many people, and also advises seeking expert help on the options available. The reality is circumstances change and when it comes time for new tyres, your pockets could be shallower than you thought.

“We (Bridgestone) make high performance tyres, it’s our bread and butter, but there are some second tier approaches for customers that are a bit more price conscious,” he said.

“Something else to consider if you’re changing from a standard to a low profile tyre - the sidewall height has been reduced and it’s become stiffer, so it can be more difficult to fit to the rim. You need to make sure that you have the correct fitting equipment so you’re not damaging the tyre when fitting it to the rim. That’s quite important from a tyre retailer side. They have to have the latest equipment so they can fit low profile tyres without damaging them. If you haven’t got the correct fitting and balancing equipment and you’re fitting them to a high performance vehicle, it’s worth taking them to a professional who can give you the correct advice, can make sure that the tyres are fitted within the vehicles’ requirements and ensure you’re maximising the performance of the vehicle.”

And it gets even more involved when you start looking at things like the load carrying capacity.

“There are things to be careful about, things that if you don’t consult a tyre professional you may overlook. In some circumstances the internal volume of a low profile tyre could be less, so the load index may be different to the original. This means it may not have the correct load carrying capacity or it could need a higher inflation pressure to carry the load of the vehicle,” Sodano said.

When it comes to run flat tyres, Gambold says there are factors that must be kept in mind.

“Run flat tyres have a very hard construction, the technology is getting better and closer to normal tyres in terms of spring rate, but run flat tyres have very high spring rates. Normally the spring rate in the suspension is softer so if you go and put a non run flat tyre on, even of the same size, the car will be a lot lighter sprung than it was in the original guise.”

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LEGAL

If you change the aspect ratio, the overall diameter of the tyre can change, and therein lays the legal issue.

The allowable variation in diameter varies from state to state, but most are plus or minus 15mm from the original equipment tyre. You need to be careful to stay within those limits, or the aforementioned problems could become yours. If you change the diameter significantly, it could have an affect on the speedo calibration, ABS, traction control and ESC – all of which are potentially dangerous problems and there is legislation in place to help govern that.

The other factor to keep in mind is the load carrying capacity. The tyres are essentially there to bear the load of the car, so if the load index isn’t high enough to carry the load of the vehicle there could be big problems.

“The car is type approved and most tyre optional sizes are placed on the placard. If you go outside the tyre placard sizes by more than the legal level of deviation, technically your car is un-roadworthy and technically it’s uninsured,” warned Gambold.

Sodano also urged drivers to ensure they maintain the overall diameter and correct load index.

“The diameter of the original fitment to the vehicle becomes the base. The variation does change from state to state, but there’s always a strict specification,” he said.

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MY STORY

I learned the need to be careful when changing tyres the hard way. I was in my late teens and had a Subaru Impreza RS that needed two new tyres. Instead of changing all four at once, or making sure I purchased exactly the same tyres as the two that were remaining on the vehicle, I fitted a different brand. Despite being the same size, because my car was all-wheel drive, it was a huge mistake.

I’d noticed a whining noise and a strange smell, and then it all went wrong. While driving at 100kph on the motorway late one night I noticed smoke billowing from under the car – the centre diff had fried itself. Within minutes fire engines were on the scene, lanes of traffic were blocked, and my little Subaru was in need of expensive repairs.

I told my story to Gambould, and he knew straight away what had gone wrong.

“Oh no. Well see there you go. Even though it’s the same size tyre, one will be a bit softer so it’s a couple of millimetres smaller in its rolling diameter. Just millimetres will make all the difference,” he said.

“We always say that to people with 4WD/AWD vehicles, change all tyres at the same time and put the original brand and size on. With 4WD/AWD it’s particularly important because you’ve got the differentials, and with things like torque vectoring VSC systems where we actually accelerate wheels as well as put the brakes on. You know torque vectoring is very sensitive to rolling diameter of the tyres and European cars often have split or staggered sizing now so the front wheels will be smaller than the back. It’s hugely important to keep the original tyres on those. It can create serious driveline problems like you’ve just explained. Tyres are a really complicated thing, there’s a lot of engineering that goes into them so shortcutting that is taking a bit of a risk I always feel.”

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ANSWERS

So how low can you go?

This is dictated by the legal requirement to maintain the overall tyre diameter approved for a particular car. To decrease the profile or the aspect of the tyre, you’ll need to increase the rim width and wheel size. But be warned, changing to something lower than the manufacturers original scope of offerings will mean you’ll need to consider what could change in regards to tuning and calibration. You’ll also need to be willing to sacrifice a more comfortable ride and longer wearing tyres. No matter what always seek expert advice.

Should you buy a car with OEM low profile tyres? Or is it ok to change them later when you’ve saved up enough cash?

If they are on offer, and you can afford it then this is the best way to ensure the car and tyres are the best match they can be. But the reality is that our pockets are often shallower than what we wish they were. There are ways to get something a bit closer to what you want down the track. Keep in mind you may need to compromise and fit tyres somewhere between what you purchased originally and your dream super low profile tyres.

The key thing is, if you deviate from the tyres specifically designed for the car, there will always be a trade-off.