Tires have come a long way since their inception. Although the design hasn't changed much, the modern tire allows for ever greater safety, particularly in foul weather, along with better comfort and longer treadlife.
Still, there is one technology that added an ability that conventional tires didn't have – to continue driving in the event of complete air loss. It is called run-flat, and it took the OE market by storm, particularly among premium automakers.
BMW was the first company to adopt run-flat tires across its range, but other companies, like Audi, Mercedes-Benz, Lexus, Mini, and more, also started employing run-flat tires as original equipment.
But what are run flat tires, and are they a good solution for everyone? Should you switch to these tires for the added convenience of not having to change or fix a tire? I will answer these questions in-depth in this article, along with the pros and cons of run-flat tires, how they work, and what effects they have on the driving experience.
So, without further ado, let's unveil the mysteries of run-flat tires and see whether they are the future of mobility!
- Understanding the Concept of Run-Flat Tires
- Types of Run-Flat Tires
- Pros and Cons of Run-Flat Tires
- Comparing Run-Flat Tires with Conventional Tires
- Is Switching to Run-Flat Tires Right for You?
Understanding the Concept of Run-Flat Tires
Run-flat tires, as their name suggests, are designed to maintain their structural integrity when there is almost no pressure inside the tire. As such, these tires allow the driver to continue driving for a limited time and to a limited top speed in order to find a tire shop to repair the puncture.
This engineering feat is made possible through the integration of specific design elements and materials in the sidewall, which allow the tire to carry the whole weight of the vehicle, even without any air inside. Thanks to this, run-flat tires are considered a more convenient solution, though there are quite a few reasons why they are not as widely adopted in the industry.
1. What are Run Flat Tires?
Run-flat tires, also known as zero pressure or ZP tires, are a specialized type of tire with unique characteristics that distinguish them from regular tires. These tires have a built-in heat-resistant and resilient material in the construction to help them cover ground with zero pressure inside.
Moreover, depending on the model, run-flat tires feature a reinforced sidewall or an internal support ring, helping the tire carry the weight of the vehicle and deal with the forces of driving without the help of pressurized air.
2. How Do Run Flat Tires Work?
The operation of run-flat tires can be categorized into two principal systems: the self-supporting system and the auxiliary-supported system.
Self-supporting run-flat tires use a reinforced sidewall construction, which helps them carry the weight of the vehicle without any pressure inside. Thus, even when the tire is completely devoid of air, these tires would appear inflated from the outside.
Meanwhile, the auxiliary system only adds a supporting ring, which only carries the weight of the car in the event of a complete pressure loss. It is a more complex system that solves some of the cons of self-supporting run-flat tires (like the worse ride), but also a much more complex one and less common.
Regardless, both technologies allow the driver to cover up to 50 miles (80 km) and speeds of up to 50 mph (80 km/h) with zero pressure inside the tire. This should be enough to reach a tire service center in most places, but sometimes it might not be enough. You could cover more miles, but the tire manufacturer doesn't recommend that, i.e., it doesn't guarantee safety.
It is important to note that cars equipped with run-flat tires from the factory don't come with a spare tire and not even a puncture sealant. Thus, if you are very far from a tire service center, it might be a good idea to call roadside assistance. Or, even better, purchase a cheap tire sealant from the gas station and always keep it inside your trunk. It's cheap and tiny, and it might save your day.
Types of Run-Flat Tires
I already mentioned both run-flat tire technologies used in the industry today, but here I will delve into more detail. Self-supporting run-flat tires (SSR) and auxiliary supported run-flat tires (ASR) employ a distinct mechanism to achieve the fundamental objective of a run-flat tire – to retain functionality when air pressure is lost. Both systems work in the same manner when pressure is lost, though they change the overall characteristics of the tire.
There is also another technology that offers quasi-run-flat capabilities – self-sealing tires. Although not considered run-flat tires by many, self-sealing tires provide almost the same benefits.
An important thing to note about all run-flat tires is that your car's TPMS (Tire Pressure Monitoring System) should function properly. That is because, with these tires, the driver needs to be alerted when a loss of pressure occurs, as he/she might not notice that through driving.
1. Self-Supporting Run-Flat Tires
Self-supporting run-flat tires are the most common type available on the market today, with most premium brands using this technology. These tires employ a thicker and stiffer sidewall, engineered from a heat-resistant material that allows them to maintain their structural integrity and carry the load of the vehicle even when air pressure is completely lost.
Thus, when a tire is punctured and loses its pressure, the thicker sidewall will carry the load of the vehicle, allowing mobility. Meanwhile, the carcass of the tire will be intact, i.e., it will still largely carry the same forces compared to an inflated tire. I say largely because run-flat tires are limited in operation and don't allow driving at very high speeds or for a very long time.
Although self-supporting run-flat tires are a great idea, their design also brings some obvious compromises. For instance, the thicker sidewall adds weight to the tire, which makes the suspension work harder to control the wheel and increases fuel consumption.
Moreover, the stiffer sidewall doesn't absorb road impacts, as well as a softer sidewall on a regular tire, resulting in a harsher ride.
2. Auxiliary Supported Run-Flat Tires
Auxiliary-supported run-flat (ASR) tires, also known as support ring systems, are a different breed in the run-flat tire realm. This technology is actually a system of a tire and wheel. In other words, you will need special tires and special wheels. For that reason, auxiliary-supported run-flat tires are only installed as original equipment on armored vehicles and aren't readily available on the market.
ASR tires include a robust support ring that is attached to the inside of the wheel. The support ring is usually made of hard, heat-resistant rubber or another composite material. Just like the stiffened sidewall on SSR tires, it is designed to carry the maximum weight of the vehicle in the event of a complete pressure loss. When an ASR tire deflates, the support ring will come into play and keep the wheel from touching the road.
This technology is better than SSR in some ways, particularly when it comes to ride comfort. However, the ring adds even more unsprung weight, and the technology is more complex and harder to deal with. As a result, ASR tires/wheels are only used in armored vehicles and trucks.
3. Self-Sealing Run-Flat Tires
Another lesser-used run-flat technology is self-sealing. While not a run-flat tire in the traditional sense, it certainly holds its ground in the realm of puncture resistance.
Self-sealing tires have an extra inner layer of sealant material inside the tire carcass. Just like a tire puncture sealant, this material will flow to the puncture and form a seal in a matter of seconds. As a result, further air loss will be prevented, and you will be able to reach a tire service center or at least the nearest gas station to re-inflate the tire.
It is important to note that while self-sealing tires can auto-fix a puncture, it is necessary to bring the tire to a professional shop to re-seal it properly. Also, this sealing action typically works for punctures of a certain size (usually up to 1/4 inch in diameter), which covers most common road hazards like nails or screws.
With that said, while self-sealing tires can't cover all the contingencies offered by self-supporting or auxiliary-supported run-flat tires (such as a severe blowout or larger puncture), they offer an extra line of defense against typical tire punctures, reducing the likelihood of sudden air pressure loss and, subsequently, an unplanned roadside stop for tire repair or replacement.
Pros and Cons of Run-Flat Tires
1. Pros of Using Run-Flat Tires
Run-flat tires offer a plethora of benefits, with perhaps the most important one being safety. They are also way more convenient because they allow you to continue driving in the event of a puncture – no need to stop! Not to mention the peace of mind and confidence the technology brings on long journeys.
- Enhanced Safety Features
One of the primary reasons drivers opt for run-flat tires is the significant safety enhancement they provide. Namely, on a traditional tire, you would completely lose control of the vehicle in the event of a puncture because the tire would lose its structural integrity. This can be dangerous if it happens at higher speeds and could result in an accident.
Run-flat tires, on the other hand, largely maintain their structural integrity after a complete loss of pressure. Therefore, they allow the driver to retain control of the vehicle, make an evasive maneuver, and safely stop. This feature is particularly beneficial at high speeds or on busy roads, where a sudden tire blowout can lead to severe accidents.
Oh, and while we are at the blowout issue, run-flat tires are much less likely to suffer from it. This is again thanks to the stiffer overall construction – a run-flat tire retains its shape after a complete air loss and has better heat resistance, both acting against a blowout.
Finally, the fact that you don't need to exit your vehicle in the middle of a busy highway also increases your safety. Run-flat tires allow you to drive for 50 miles, enough to reach a safe place to stop or to bring the tire to a service center. This also helps in severe weather, when stopping might not be safe.
- Convenience During a Puncture
In the past, changing a tire was something that every driver knew – even the ladies of the time. Today, though, most drivers don't want that inconvenience and instead just want to continue driving. For instance, a driver with run-flat tires doesn't need to worry about changing a tire at the roadside in the rain or dark or in potentially dangerous locations like the side of a busy highway. It is a luxury, in a sense.
- Potential Weight and Space Savings
Cars that were equipped with run-flat tires from the factory don't come with spares, tire repair kits, jacks, or any other tire-changing equipment in the trunk. This opens up cargo space but also saves weight, potentially improving fuel efficiency.
2. Cons of Using Run-Flat Tires
Run-flat tires seem like a no-brainer, but wait until you learn about their disadvantages. I mean, there is a reason why many car manufacturers still don't put run-flat tires as original equipment on their models and that tiremakers don't produce many run-flat tires. Here are all the cons of run-flat tires.
- Considerations Regarding Comfort
The single most notable disadvantage of putting run-flat tires on your car is worse ride quality. Although the latest run-flat tires have become much better at absorbing impacts, they are still miles behind a regular tire. Moreover, the rigid design results in more road noise than a regular tire would produce.
This is because you can't beat physics – the rigid sidewalls that allow run-flat tires to perform even without air pressure also result in a stiffer ride compared to standard tires. Moreover, the inflexibility of the tire can transmit more road noise and harshness into the vehicle's cabin, especially when driving over uneven surfaces or potholes.
It is important to note that only self-supporting run-flat tires suffer from this issue, but they are also the only ones available for passenger cars.
- Limited Repair Options and Replacement Costs
One less talked about aspect of run-flat tires is that they are harder to repair than regular tires. Not all punctures or damages in run-flat tires can be repaired safely, and some tire manufacturers and service providers don't recommend repairing run-flat tires at all.
The reason behind this is the difficulty in assessing the full extent of internal damage once a run-flat tire has been driven on with low or no air pressure. Namely, the technician will repair the puncture, and the tire will hold pressure, but there is no way to tell whether the self-supporting sidewall was damaged.
Even small damages or driver negligence (like driving for prolonged periods without any pressure) could compromise the tire's structure and cause it to lose its run-flat properties.
Therefore, a punctured or damaged run-flat tire often means that a replacement is necessary, which can be an expensive proposition, as run-flat tires cost much more than their standard counterparts.
- Limited Availability
Run-flat tires aren't as common in tire shops as regular tires, especially for less common tire sizes. If you have a car that comes with run-flat tires from the factory, you might be able to find a set at your local tire dealer, but if you want a more exotic size, you are out of luck.
Even more importantly, run-flat tires are usually late on the market. Namely, when a tiremaker launches an all-new model, its run-flat alternative might take years to arrive at shops, and in some cases, it won't arrive at all. Just have a quick look at the run-flat offerings online, and you'll see that they are based on older tire models.
- Impact on Unsprung Weight
Unsprung weight refers to the mass of the suspension, wheels, and other components directly connected to them that move up and down with the irregularities of the road. Due to the reinforced sidewalls (more material), run-flat tires are heavier than regular tires, and this increases the unsprung weight of the suspension.
The effect of unsprung weight on a vehicle's performance and handling can be significant. Higher unsprung weight can negatively impact the vehicle's ability to maintain consistent contact with the road surface, particularly on uneven or bumpy terrain. This happens because the suspension won't be able to control the movement of the wheel as successfully, resulting in compromised handling, a worse ride, and a decrease in traction.
Furthermore, the added unsprung weight puts more strain on the suspension and will wear the components faster.
However, it is important to note that the effect of the higher unsprung weight is generally an issue on cars that weren't equipped with run-flat tires from the factory. Vehicle manufacturers who equip their models with run-flat tires often tune their suspension to account for the increased unsprung weight, mitigating some of the potential negative impacts.
Comparing Run-Flat Tires with Conventional Tires
Before switching from regular to run-flat tires or vice versa, it is important to know the various differences between the two different designs. Each type of tire has its specific benefits and potential drawbacks, which can impact vehicle performance and cost.
1. Performance Differences
Conventional tires will provide a smoother and quieter ride than run-flat tires, particularly on cars that were equipped with such tires from the factory. However, run-flat tires will feel sportier – the stiffer construction allows for quicker and more precise steering response. That said, the handling agility will largely be the same, with run-flat tires losing their advantage due to the added weight.
With that said, there is no question that run-flat tires are safer, as they allow control of the vehicle in the event of a puncture. Moreover, they are far less susceptible to a blowout. Regular tires, on the other hand, will deflate rapidly when punctured, necessitating an immediate stop for repair or replacement.
2. Cost Implications
There is no getting around the fact that run-flat tires are much more expensive to buy than regular tires. The specialized construction and materials used in run-flat tires can make them more expensive to produce, and this cost is typically passed on to the consumer.
But the cost difference increases down the line. Namely, run-flat tires are not as easy to repair, and tire shops charge more to repair them. In some cases, you might need to buy a new tire, even after a small puncture. In addition, run-flat tires have shorter treadlife, meaning more frequent replacements and even higher running costs.
Of course, you won't need a pricey spare tire when dealing with run-flats, and you might save on towing costs. Still, overall, run-flat tires are a much more expensive (albeit more convenient) solution.
Is Switching to Run-Flat Tires Right for You?
Whether or not to switch to run-flat tires is a personal choice. Still, I recommend assessing your driving routine and particular car model before switching because it's not a cheap transition.
1. Assessing Your Driving Needs
Before you even splurge the cash on run-flat tires, you need to ask the question – do you really need them? If you typically drive in urban areas or on highways near the city/settlement where you live, there is really no point in investing to buy run-flat tires.
That said, if you frequently travel in remote areas or on highways where roadside assistance might not be promptly available, run-flat tires' ability to continue functioning after a puncture can be a significant advantage.
Also, you should be prepared for a harsher ride after installing run-flat tires, particularly on broken tarmac. Run-flat tires tend to produce more noise, too, which can be felt at higher speeds. That said, the idea of not having to change a tire in the middle of a busy highway is somewhat comfortable and luxurious.
2. Considering Your Vehicle's Design
Self-supporting run-flat tires will fit just about any wheel, but they aren't a good fit for every car. Namely, if you have an older vehicle without a TPMS (Tire Pressure Monitoring System), it is inadvisable to put run-flat tires. That is because you won't be notified that one of the tires has lost pressure, and you might drive your car past the speed limit (50 mph) and distance limit (50 miles).
Also, it is important to note that cars that weren't designed for run-flat tires will have bigger issues with comfort and handling because their suspension components were tuned for regular tires. With that said, by opting for run-flat tires, you will free up some space in the trunk, as you won't need that spare or tire repair kit anymore.
Switching to run-flat tires can significantly alter your driving experience. It also involves a significant investment, as run-flat tires are much more expensive and harder to find. Still, the idea of not having to change a tire is attractive to modern drivers, especially since, today, most of us are in a hurry. Thus, even with all their compromises, run-flat tires are becoming more widely adopted across the industry.
At the end of the day, the decision between run-flat and conventional tires is a personal one, requiring thoughtful consideration. Regardless of your choice, ensuring your tires are properly maintained, regularly inspected, and inflated to the correct pressure will optimize their performance and longevity.
I’m Ivo Gievski, the content writer for Tireer. We built our website with over 15 years of experience and extensive research in the automotive and technology sectors. My dedication to delivering high-quality content is unwavering, and I strive to continuously hone my skills to stay ahead of industry trends and provide readers with informative, engaging, and valuable insights.