February 2025 - The Tribology of Tires

In this month's episode I sit down with Anurag Warhadpande from Bridgestone Americas.   This podcast explores the critical role of tribology in tire performance, highlighting how factors like traction, wear, and noise are influenced by the tire-road interaction.  Anurag discusses the science behind tire friction, including the mu-slip curve and road texture effects, as well as how wear mechanisms align with Archard’s Wear Law. The conversation also delves into the evolving needs of tires for electric vehicles, the rise of smart tire technology, and sustainability efforts such as bio-based materials and recycling innovations. Looking ahead, we examine future opportunities in tire development, including AI and IoT advancements, while debunking common misconceptions about tires. 

Our guest:
Anurag Warhadpande holds a PhD in Mechanical Engineering from Purdue University and brings over 12 years of expertise from his tenure at Bridgestone Tire Company. A recognized specialist in tribology, his doctoral research focused on bearings tribology, providing a strong foundation for his subsequent work in tire tribology. At Bridgestone, he has leveraged this expertise to drive innovation in tire technology. A former member of the Society of Tribologists and Lubrication Engineers (STLE), Anurag is deeply committed to advancing the field through cutting-edge research and practical applications.

For more information on STLE, please visit https://www.stle.org/ If you have an idea for our podcast, or interested in being a guest, please Email STLE Director of Professional Development Robert Morowczynski at rmorowczynski@stle.org . Also, we love your feedback, please take a minute to provide us with your thoughts at Perfecting Motion Podcast Feedback.

Transcript

February 2025 – STLE Perfecting Motion Podcast – The Tribology of Tires with Anurag Warhadpande of Bridgestone Americas
 
 

Bob: [00:00:22] Hello and welcome to our STL podcast, Perfecting Motion. I am Bob Morrison, STL Director of Professional Development and on the heels of the great American race, the Daytona 500. This month, we want to focus on the tribology of tires, the grip, the shape, the innovations are all vital to their use, and tribology is at the center with us today. We have Anurag Wan Pandey, who is a strategic market and competitive intelligence specialist for Bridgestone Americas. How are you doing today, Anurag?

Anurag: [00:00:48] I'm doing great. How are you Robert?

Bob: [00:00:51] Doing wonderful. Thanks for being here today. We really appreciate your support. Why don't we go ahead and start by telling us a little bit about you and your background in tribology and how you got started down this path.

Anurag: [00:01:01] Absolutely. So first of all, thank you so much for inviting me onto this podcast. Really my pleasure and my honor to be able to connect with the audience at large. I'm actually a past Still member. I have a PhD in mechanical engineering from Purdue University, and my PhD topic was in the field of tribology. So fatigue of bearings is what it focused on. So at that time I was a member of S.h.i.e.l.d., so it almost feels like coming full circle right back to the folks here. So like I said, I did my PhD work on tribology. And then after graduating out of Purdue University, I transitioned into industry role here at Bridgestone about 12 years ago. And I've been doing a lot of trouble of tires as well. So traction, rubber friction and tire wear. So that has been my focus area here. What most people don't realize is that when you are going down the highway at 60 70mph, what is connecting the vehicle to the ground is just the tires, right? And the contact area that the tires make, the four tires of a vehicle can easily fit onto a standard printer paper, basically, so that seemingly small area of contact is responsible for your safety. For every turn that you make, every stop, every acceleration, essentially. So what I want to do today is talk about how that seemingly sophisticated or unsophisticated, overlooked piece of contact area can play such an important role in vehicle dynamics and tire design, basically.

Bob: [00:02:27] So I guess that's a plug to make sure that we keep our tires up to date and make sure we're rotating our tires on time.

Anurag: [00:02:32] Absolutely. Yeah.

Bob: [00:02:33] That little strip of rubber is all that's connecting us to the ground. And obviously the shape of that change is then to based on how fast you're going. Because obviously if you're going 30 miles, you have more traction. And if you're going 60 or 70, you have much less then.

Anurag: [00:02:46] Yeah, exactly. So so the shape and the design plays a very, very crucial role. And again tribology is central to all of it. Right. Because it's where the rubber is contacting with the road surface. So everything hinges on tribology okay.

Bob: [00:02:58] Well let's talk about tire tribology then. And many people don't understand how complex tires are. Can you break down what goes into making a tire and why tribology plays such a crucial role in the actual production of that tire?

Anurag: [00:03:10] Yeah, absolutely. So at first glance, tires seem like a simple combination of rubber and air, right? But when you start peeling the layers, just realize that how complex things are. I mean, in simple terms, tires are the three main components air, rubber, and car. But in reality it is far more intricate. There are composed of over 21 main components, including a more than 15 different rubber compounds. There are about 3000 cards that go into a tire. Some of them are steel cards. Polyester or nylon. Right. So these components often have like complex geometries. And that exhibit, you know, what we call as nonlinear history dependent material behavior. So that's basically what goes into making a tire right. What people will see typically is the outside tread pattern of the tire. But underneath there are various compounds which are specialized in nature, followed by we have what is called two layers of steel belts, which provides strength and stability to the tire, and these radial cords that go in a radial direction that maintain the shape of the tire. And then finally we have an airtight layer called as the inner liner, which holds the air pressure inside the cavity.

Anurag: [00:04:19] So that's basically the manufacturing of how you make tire right. Now let's talk about what are some of the demands that a typical tire that needs to conform to it must support the vehicle load right, withstand over 100 million cycles of stress and impact, provides steering and maneuverability. It needs to deliver ride comfort and ensure traction and a range of conditions like dry, wet, snow or even ice. In some instances, it also needs to offer low rolling resistance for improved fuel economy, minimize noise for comfort and deliver good wear performance for longevity. So you can see it's a lot of performance that a simple structure like a tire needs. Needs to provide. And this is where tribology again, right, which is essentially a study of friction, wear and lubrication, right comes into picture. It is essential for optimizing the interaction between these complex layers and the road surface. It also influences everything from traction to durability to noise and fuel efficiency. So in short, tires are one of the most sophisticated components on a vehicle, and no other man made product is asked to do as much under such diverse set of demanding conditions. Basically, I like.

Bob: [00:05:32] That you mentioned noise, because a lot of people don't realize when you're when you're next to a highway and you hear cars going by, it's not the engine of the cars, it's actually the tires contacting the road that's making those noises.

Anurag: [00:05:43] Exactly.

Bob: [00:05:43] Right. Right. And then and we'll maybe touch on that later. Right. But with EVs, when we don't have the masking sound of the engine. Right. The tire noise now is the only thing that you would probably hear. So minimizing and coming up with new technologies to minimize that noise become even more and more crucial. Important.

Anurag: [00:06:00] Great. Perfect.

Bob: [00:06:01] And we already touched on this. I know about the tire road contact patch, but that's where the action happens. So how does tribology influence that that traction wear and the noise in that critical area of that contact patch.

Anurag: [00:06:13] Yeah, absolutely. So the tire road contact is where all the action happens. Right. We talked earlier. Everything kind of fits into a one printer size paper. Right. And that's the only point of contact between the vehicle and the ground. And it's design is critical for effective load transfer and overall vehicle performance. So tribology of course plays a crucial role here. When we talk about traction, it all comes down to the friction between the rubber compound and the road surface. This friction will determine how the tire grips the road, how it impacts acceleration, braking and also the cornering performance of the tire where, on the other hand, is the result of frictional heat dissipation. So as the tire continuously interacts with the road, tiny amounts of rubber particles will wear away, and this process is dependent on the frictional characteristics between the rubber and the road surface. And then there is noise, right? Which is a byproduct of the road tire interaction. It can be caused by various mechanisms like the tread impacting the road, air pumping between the tread pattern slip stitch or adhesion happening at the contact patch. So all these kind of factors would impact the tire noise. So in a sense we can see like traction, wear and noise are all tribology driven phenomena that originate at the road contact patch. And by understanding and optimizing these interactions, we can significantly enhance tire performance, durability, and comfort for for our consumers.

Bob: [00:07:42] Great. And I think you had mentioned to going with friction where the mu slip is that am I pronouncing that right? Mu slip right. Okay. I think you would mention the traction depends on the mu slip curve. So what are some u slip curve and what does it mean and how does it impact vehicle performance.

Anurag: [00:07:56] Yeah exactly. So so mu slip curve is a very, very vehicle dynamic term. But it's in a sense what it defines is the performance of a tire under some kind of what we call a slip ratio. So it's really fundamental to understanding the tire traction and vehicle performance. So if I were to break it down into simple terms, the mu essentially represents the coefficient of friction between the tire and the road surface. And the slip basically refers to the difference in velocities or speeds between the tire rotational speed and how fast the vehicle is going, basically. So as the tire begins to rotate, the slip increases and so does the friction. So you would see the curve kind of going up, but it goes only up to a certain point. Right. And that point is called as the peak of the mu slip curve. Basically, beyond this peak, any increase in slip, what we see is that the friction will start decreasing, which can lead to loss of traction, something like skidding or wheelspin basically. So this curve is critical for vehicle dynamics because it influences acceleration, braking, and cornering. For instance, a high performance vehicle or an electric vehicle will try to optimize this curve in order to enhance the acceleration without losing grip, basically. So you know how fast to ramp up on this curve will define how much stability and grip you have when you are going through that motion. Another example, again, is many listeners might be familiar with the ABS system in our vehicles, right. So those are tuned in order based on the slip curve of a tire. Right. So understanding the slip relationship will help maximize traction without the wheel locking up for example. So in a sense, a mu slip curve is a balancing act. And its optimization is key to both safety, performance and safety and performance in modern vehicles basically.

Bob: [00:09:47] So it sounds like tires.

Anurag: [00:09:48] Obviously they need friction to to grip the road, but that friction also impacts fuel efficiency and the wear and tear on the car. So it's almost like it's a kind of a love hate kind of thing for the car and for your efficiency.

Anurag: [00:10:01] Yeah.

Anurag: [00:10:01] So friction is is good and bad. It's like walking, right? Essentially, you need friction to walk the same way. You need friction to go forward. Right. But friction is good when you want to, for example, come to a quick stop right? Then you definitely want high friction to be able to come to a stop in some kind of emergency scenario. However, think about, you know, fuel efficiency with EVs. You're driving range. You want less friction, right? Because you want to maximize that fuel efficiency or the driving range of your of your EV. So there are certain scenarios where friction is good. Certain scenarios where friction is bad. And like you said exactly. It's it's kind of almost like a love hate kind of relationship.

Bob: [00:10:41] So friction obviously is at macro and micro levels. So how does road texture affect both of those.

Anurag: [00:10:47] Yeah, that's a great, great question. And we might get a little bit into the weeds here. But rubber friction is fundamentally kind of driven by two main components, right? The first is called as the viscoelastic contribution. And then there is a second one called as the real contact area. So let's unpack these one by one right. So first of all, rubber in general is what we call as a viscoelastic material. And what I mean by that is when you deform rubber, a significant amount of energy that goes into deforming the rubber is actually lost or dissipated to heat, right. Now when you think of a road surface, it is not a perfectly smooth surface, right? If you start zooming onto the road surface, you would see that there are these hills and valleys on the surface roughness. And the more you zoom in, the more of these hills and valleys that start to appear, right? Now, when a tire rolls or slides over this textured surface, the rubber is actually deforming over these hills and valleys. Now, due to the viscoelastic nature of rubber, this continuous deformation and recovery process leads to energy dissipation in the form of heat.

Anurag: [00:11:52] Right. And this energy loss is what we call as the visco elastic contribution to friction. So whatever is lost as the rubber is going over these hills and valleys would be essentially friction. And that is attributed to the visco elastic portion of the friction. The second component I mentioned was the real contact area. Now this is driven by actual contact points or actual contact area between the rubber and the road surface at the microscopic level. Again, this contact point will be experiencing intermolecular forces including chemical bonds or dispersive. Van der Waals kind of interactions. Right. And the extent of this real contact area is influenced by how much the rubber deforms under load. So essentially rougher the road texture, the less real contact there is, but more elastic deformation. And conversely, a smoother road texture will have this increased real contact area but less of the the elastic contribution. So both visco elastic and real contact area components are heavily influenced by either the surface has more macro texture or more micro texture, right? But that is why road surface design plays a crucial role in vehicle traction and overall grip performance of a tire. Essentially.

Bob: [00:13:08] Right. And going with that friction, obviously there's rubber wear from the friction. So how does that compare to arcades where law where you can kind of explain the relationship and how it applies to real world tire durability?

Anurag: [00:13:21] Yeah, it's a fantastic question. So tire wear essentially manifests itself in the form of rubber loss, right. Which can be observed through abrasion patterns on the surface or formation of this sticky surface on the layer of the tire. This rubber loss is primarily driven by frictional energy generation during sliding events. Now, the key parameter that connects the frictional energy to the rubber loss is what we call as the upgradability of the tire or the rubber compound. It's a physical property of the rubber that determines how easily material wears away under friction. Now, this relationship is quite similar to our church wear law, and many of our listeners might be already familiar with it. In our church law, the volume of the wear debris is proportional to the frictional dissipation and is influenced by a material property called hardness of the material, and in the case of tires, it's a rubber of breathability that plays a similar role. It relates the friction energy at the tire road interface to the amount of rubber lost, much like how hardness influences wear volume in our church law. Basically, by understanding this relationship, we can better predict tire wear and optimize design for improved wear resistance, ultimately enhancing our vehicle safety and performance.

Bob: [00:14:41] Now going with that on the wear, and obviously we've already talked about electric vehicles already. I know electric vehicles weigh more, which obviously is going to impact the tires more. So how requirements evolving based on the rise of hybrids and electric vehicles. And what challenges do EVs pose for the traditional tire design that we've known since the birth of the vehicle?

Anurag: [00:15:02] Yeah, absolutely. Yeah. Of course, EVs, their adoption is kind of slowed down a little bit, but they are going nowhere, right? Evs are going to be more prevalent going forward. So it's important for us in the tire industry to to see what's coming and essentially understand how EVs would impact tire design. So there are three key areas. And one of the one you mentioned, right, that EVs are essentially heavier, but also they provide that instantaneous torque that we talked about. Right. So this this torque essentially, and the heavier vehicle would cause additional wear on our tires during particularly during acceleration. And that means that that poses a greater demand on our compounds from durability and lasting long point of view. Essentially the second again is the driving range. Range anxiety is a significant concern for EV owners, and tires play a crucial role in maximizing battery range by minimizing rolling resistance. The lower the rolling resistance of the tire, the less energy that it will consume, which translates to longer driving distance. Right? And the third most important thing is, again, what we hit on earlier was the noise, right? Without the masking noise of the engine, tire noise becomes even more noticeable in EVs. This makes it essential to design quieter tires, focusing on tread pattern and noise dampening technologies. For the tire industry, this means innovating in several areas, like developing reinforced structures to support the additional weight of the EVs, creating advanced materials that balance rolling resistance and wear resistance, and incorporating noise reducing technologies to ensure a quieter ride. So the rise of EVs is not just increasing tire design, but pushing the tire industry to rethink and innovate on multiple fronts.

Anurag: [00:16:55] Great, perfect.

Bob: [00:16:56] Now, I know we have obviously we have smart phones, we have smart appliances. Now we have smart watches. Now we can ask smart questions. You know, we've heard about the the concept of smart tires. So can you explain that to us. Is that something where we're going to be able to ask the tires questions that they're going to have AI built into them? What are smart tires going to be and how are they going to help the industry?

Anurag: [00:17:15] Yeah, absolutely. Again, smart tires or tires with sensors is not an entirely new concept. Most people are kind of familiar with the TPMS sensor in their in their tires. Right. Which would alert the driver when tire pressure is low. Especially you notice that during colder months, right. In fact TPM is is now a regulatory requirement in many region. But the concept of smart tires is taking this a step further, right beyond just monitoring pressure. The latest advancements include sensors that can track tire wear, temperature, and even road traction conditions right in real time. Now this information can be used to give advanced warning about changing road conditions, which is valuable not only for human drivers, but also for autonomous driving systems so that they can adapt quickly to the changing environment. This technology is particularly beneficial for fleet management. Right now, for commercial fleets, downtime is essentially lost revenue. So if sensors in tires can provide data for predictive maintenance like alerting fleet managers before a tire becomes unsafe or inefficient, it can prevent costly breakdowns and improve overall operational efficiency. So those are some of the things that, you know, smart cars can bring into into modern, modern mobility. So in a sense, like smart cars is not just about safety and efficiency of individual drivers. They're also about maximizing productivity and reducing operational costs. In commercial mobility, so not only just consumer but thinking about in commercial space as well.

Bob: [00:18:49] About friction wear obviously with tires. We've talked about smart tires. Now let's talk about which is obviously a growing concern in the industry. So besides making smart tires and besides obviously using different techniques to build tires, what are some of the innovative approaches that are being used to make them more sustainable? What type of bio based materials are you guys using and how are you recycling programs for the 21st century?

Anurag: [00:19:12] Yeah, definitely. So sustainability is a major focus across various industries. And tire industry is no exception, right? Even at Bridgestone, we have a commitment to achieving complete carbon neutrality by 2050. So when we look at a typical tire today, right, majority of the raw material or components are, for example, like synthetic rubber, carbon black silica and fiber, they're all fossil fuel based basically, or petroleum based. So our vision is to transition from this petroleum based materials to renewable or recycled alternatives, significantly reducing our environmental footprint. One exciting area of innovation is converting ethanol to butadiene using something called as a thermo catalytic one step process. Now, butadiene is a critical raw material for synthetic rubber that is used in our tires currently, right. So this technology will allow us to produce it from renewable sources. Currently we have to source the synthetic rubber from petroleum byproducts basically. But by using this conversion process we can rely on ethanol. Right. Which can be bio source. Right. And thus reducing our dependency on fossil fuels. Now beyond just raw materials, we are also looking at end of life solutions, such as advanced recycling methods that can break down tires into their base components for reuse. This circular approach is not only to minimize waste, but also conserve valuable resources. In short, sustainability is a driving way of innovation in our industry and we are excited to be at the forefront of it.

Bob: [00:20:45] Great!

Bob: [00:20:45] It sounds like you guys have a lot of progress and a lot of innovation that are going on over there and looking ahead, where do you think are the biggest opportunities? Maybe besides what you've already talked about for innovation of tire technology over the next decade and ten years?

Anurag: [00:20:57] Yeah. And we can maybe continue building on the sustainability theme, right? Basically. So I believe that one of the biggest opportunity for innovation in technology over the next decade is going to be in the development of sustainable raw materials. We are talking about creating new renewable, recycled materials that not only meet, but exceed current performance standards. The challenge isn't just developing this material right, but also scaling them up while maintaining manufacturability, tyre performance, and even economic feasibility. Imagine a tyre made entirely out of renewable sources or recycled components performing just well or even better than today's tire right now. Achieving that vision requires incredible innovation across materials science and manufacturing and even product design, basically. So in my view, sustainability is just not a trend. It's the future of technology and it presents the most exciting opportunities for groundbreaking advancements in the decade to come.

Bob: [00:21:56] Wonderful. So besides sustainability, as far as the innovation that goes on, let's talk about the buzzwords of today. And we have AI and we have the Internet of Things. How do they play in the future performance of mobility solutions and tires themselves?

Anurag: [00:22:10] Yeah, definitely. So yeah, like you said, it's a buzzword. But again, it's a game changer, right? It will play a larger role in nearly every industry and in the world of mobility. Its potential is enormous. I mean, the first logical application of AI in the tire industry. Mobility, for that matter, is in autonomous driving, right? By integrating tire sensor data with the vehicle's Adas systems, AI can significantly enhance the vehicle's ability to understand and respond to road conditions in real time. Another exciting application is in tire performance prediction and preventive maintenance. Ai powered analytics can monitor tire wear, predict when maintenance is needed and even suggest optimum tire replacement, improving both safety and efficiency. So I is also streamlining tire design cycle using machine learning algorithms so designers can quickly iterate on new tire design, reduce the time it takes to get a tire from concept to production. So those are some of the things that I can really make a huge impact from an engineering point of view, right? Beyond engineering, AI and IoT, for example, has a potential to transform nearly every part of the tire business from manufacturing and supply chain management to marketing, finance, and even retail space. Right? These technologies are not just revolutionizing how we make and design tires, but also how we manage our entire value chain, for that matter. Wonderful.

Bob: [00:23:37] So you've given us a lot of information about tires here. Seem like our tire expert. So here's your opportunity to debunk one misconception that people have about tires, whether it be about tires themselves or the tribology of tyres. What's something that you want to inform us of so that we don't have a misconception of it going forward?

Anurag: [00:23:55] Well, I'd like to bring it back to where I started. Right. So one of the biggest misconceptions is that tyres are just simple, round black objects. Even I had that same feeling before I got into the tyre industry. But as we learned today, right? In reality, tyres are incredibly complex, highly engineered product that play a crucial role in vehicle performance, safety and efficiency. From the rubber components to the steel belts and cords, each component of a tyre is carefully designed to fulfil a specific purpose, whether it's providing traction, reducing rolling resistance or ensuring comfort and noise reduction. There's a lot going on behind the scenes that most people don't realise. So while tyres might look like simple doughnuts on the surface, they're actually a sophisticated piece of technology that impact everything from your car's handling to fuel economy and now sustainability. Essentially, right. So hopefully after hearing this, people will have a little bit more appreciation for their attire than just being black round donuts, I guess.

Bob: [00:24:57] Certainly think about that as I'm driving home today about everything the tire is doing for me, keeping my car on the road, keeping me stable and so forth. Like they say, people think the steering wheel is what is actually controlling your car, but it's not. It's tires. And if you lose control, like you had said earlier, if you slip or if they lose traction, you have no control over that car. It's going to go wherever it wants to go. So we appreciate your your work and your diligence and keeping us all safe on the road. We that's obviously very important with everybody that drives, which is pretty much all of us. So thank you so much for this interesting take on friction and providing us with yet another way that tribology is present in the most important areas of our life. Hopefully we will see you soon in Atlanta at the STL Annual Meeting. If any of our listeners have an idea for the podcast, please feel free to reach out to the show notes to my email included in the show description. And for those interested in learning more about STL, please visit our website at AustLit. Stl. Org and thank you again so much for this. This was very interesting. We appreciate your take on tires and again helping us with tribology. Remember, keep your gears turning, keep your creativity flowing and keep our world in motion. Thanks for joining us. See you next month.