Welcome to Tires 201!
As everyone is coming here from @Peter_Black 's first lesson, I think we can jump right into it, yes? For our first deep-dive into the world of rubber (back row, stop giggling), we will be looking into the construction of tires…
Peter and I conspired to bring you this content, based largely on my experience as an enginerd in 'the industry' in a previous life. We’ll both be commenting throughout the article(s) in this format, so it’s quasi-conversational.
In this particular lesson, we'll be looking more at what makes a tire a... well... a tire. We went over the basics of different types of tires before in Tires 101, and now we will dig a little deeper into tire science. This is a for-credit Opponaut course, right? Let us begin...
Disclaimer: We’re just two regular doods… or, I guess Peter and Dood… There are a ton of resources out in the world to get smart on tires, but be careful and consider the source. We’ve just taken the time and interest to share some of our extensive knowledge with you here in the Opponaut-verse.
We’re going to talk about the Black Art of Tire Design (pun intended), Construction, and how tires influence Vehicle Dynamics. Unless you are a tire engineer or vehicle dynamicist, these phrases don’t mean what you think they do… Stick around and find out.
Tire Design involves a whole lot more than what the tread pattern looks like and those pretty molded-in graphics on the sidewall, although both of those - unfortunately - might be the primary selection criteria for a tire purchase by a non-tire-nerd - humans tend to judge a book by it’s cover, right ? While tread design is a critical factor in water evacuation, wet/snow/dirt traction, noise, ride comfort, and steering feel (among other things), it is important to know that everything (in tire construction) counts in large amounts.
(Any other Depeche Mode fans here ? <crickets> Damn. Tough crowd.)
The design of the tire as a system in itself is critical to its success.
Peter: Come on, Dood… Aren’t tires really just round and black ? Tires can’t be that complicated, right ?
Dood: They’re complicated - trust me. There are lots of crappy ‘round and black’ tires out there.
I’m going to hit you with the heavy stuff - design element by design element, then you’ll have a better appreciation for how complicated tires can be on your motorcycle, car, truck, or flying thing.
Tire Construction goes well beyond bias-ply versus radial construction - although they are popular descriptors of two basic tire construction casing ply configurations - more on that later. Between the wheel and the contact patch, there are six primary areas in tire design: Bead Area, Sidewall, Inner Tire Liner, Casing Plies, Crown Plies, UnderTread, and Tread.
Peter: blinks You wanna run that again by me, maybe a bit slower?
Dood: ‘S okay… I got you, man… Pitter-patter; Let’s get at ‘er.
The bead area of the tire is the critical interface between the tire ‘system’ and a vehicle’s wheel. This ends up being a structural interface between the relatively (very) rigid wheel and the purposely compliant-by-design tire system. This portion of the tire incorporates the tire bead rubber which is molded over the bead cables as well as the bias or radial plies which wrap around the bead cables. The bead rubber not only seals the tire to the wheel to allow the tire to hold air pressure, but it also forms a semi-rigid interface between the tire carcass and wheel which can influence both vertical and lateral stiffness of the tire
Peter: Stiffness that influences ride comfort and steering feel respectively.
I say ‘semi-rigid’ because there is some flex here, but it is very limited as the force to maintain the integrity of the tire-wheel interface is critical for safety. The results of de-beading a tire at speed during high g-force and/or evasive maneuvers are less than desirable.
Peter: He means you could likely crash, and in the case of an SUV, your wheel might dig into the pavement and flip your vehicle. Bad News Bears.
For a motorcycle, the contact patch loads are such that you’re less likely to debead a tire on the street, but dirt riders with lower tire pressure beware on the trails - that’s why you’ve got rim locks. Rapid deflation of a tire on a motorcycle at speed is no picnic, either.
(Figure 1: Motorcycle Tire and Wheel Cross Section)
(Figure 2: Figure depicting the importance of tire & wheel bead seat matching appropriately.)
Because of the critical nature of the tire/wheel interface, the tightness of this bead fit to the wheel is very closely controlled in design and manufacturing. The European Tyre and Rim Technical Organisation (ETRTO) standardizes wheel and tire geometry in this area (and elsewhere in the tire construction, BTW) which ensures that every wheel design has a tight manufacturing tolerance on the bead seat geometry (there are multiple profiles), and every tire has a tight manufacturing tolerance on the mating bead seat geometry (again, several bead profiles). This means that every brand and style of tire meant for a specific wheel size should fit properly and the bead should seat precisely and tightly. The bead cables are sized precisely circumferentially, and the amount of bead rubber and ply material between them and the wheel (radially) is tightly controlled such that the tire will literally snap or pop into place during the initial inflation after tire fitment to the wheel as the tire bead stretches momentarily over the bead hump before snapping into the bead seat. The bead is mostly seated at this point, but several rotations of the wheel under load and subsequent temperature cycling will fully seat the bead. Some tire balancing machines actually have a bead seating sequence during the balancing process to aid in bead seating before measuring road force variation (and residual self aligning torque to properly pair car/truck tires on the front and rear axles) then the tire is spun at higher speed to measure the imbalance. (We’re not going to debate the use of car tires on touring/cruiser motorcycles here. Do that on your favorite owner’s forum.)
Peter: So, basically, the bead / seat area is more or less a standardized item, yes? NEATO.… Can we debate spinnerz on motorcycles ?
Dood: Yes, the bead and bead seat areas are standardized. No... No spinnerz talk..
Peter: Rude! Anyways, the only part of the tire that touches the bead is the sidewall, so lets chat about that next!
The tire sidewall is a simple looking portion of the tire, but as you are probably realizing at this point, tires are more complex than them ‘round and black things’... The sidewall has to
- transmit drive/brake torque,
- withstand lateral cornering forces,
- serve as a vertical ‘spring’ to cushion sharp impacts (hello toronto potholes),
- the rubber protects the tire carcass plies from curbs and debris,
- serve as a prime location for tire branding.
The transmission of drive/brake torque seems simple enough, but the truth is that extremely high torque drivetrains (Peter: Uhhhh, he means Top Fuel, Funny Car, ProStock dragsters. No your boosted K20 civic isnt a high torque application) challenge the very limits of sidewall resistance to these forces, so they select bias-ply construction for the rear (drive) tires… Drag racers are more concerned with literally ‘not tearing the tire apart’ than ride comfort or steering feel in a 4 second squirt of a race. Sure, we’re not all Larry McBride on our rides, but the truth is the sidewalls are important. A motorcycle tire - owing to it’s curved tread profile necessitated by motorcycle kinematics - generates less lateral force (relatively) than a car tire does from the perspective of the tire bead (the place through which the tire forces are reacted through into the wheel to the axles, and into the chassis). Cornering forces on a car/truck tire application can be approximated by forces orthogonal to one another and reacted through an axle that is essentially parallel with the ground at all times. In the car/truck example, the rotational (drive/brake direction; accel/decel traction/slip), lateral (cornering load direction; steering feel), and vertical (road impact direction; ride comfort) can be tuned quasi-independently with creative carcass ply arrangements and fiber bundle sizes & densities. Let’s take a look:
(Figure 3: Car/Tire Force/Moment axes diagram)
Not that complicated, right ?
On a motorcycle, the wheel axle is not parallel with the ground during cornering because motorcycles must lean into the corners to generate centripetal force to keep from falling to the outside of a corner.
(Figure 4: Motorcycle cornering forces diagram)
Simple, right ? What that means is that motorcycle tire sidewall construction is even more critical to the three important tasks of torque delivery, lateral compliance, and vertical compliance as the lateral and vertical compliances influence cornering stability which imparts confidence on the part of the rider that they won’t have a bad day.
Peter: They won’t crash.
Right ! Not crashing is good. The relative stiffness of the sidewall area of a motorcycle tire carcass (Peter: and tire inflation pressure) can influence the size and uniformity of the relatively small tire contact patch on the ground, too. Remember that a motorcycle only has two contact patches, leans during turns, and has an inherently unstable static stability factor (SSF) - they are great fun ! Anyway…. Sidewalls: also important.
Dood: Hey. Peter - you OK, man?
Dood: He’s still breathing. We're good. On to the NEXT...
The inner tire liner is probably the simplest tire construction component that we’ll discuss here, but it is important nonetheless. The inner tire liner is exactly what it sounds like, too ! It’s job is to primarily act as an air-tight membrane to maintain inflation pressure. Air pressure is good in a tire because the air inside the tire gives it strength to support the weight of your vehicle and payload.
Peter: Flat tire = bad.
Yes, Peter. Flat = tire bad.
The inner tire liner seals from bead to bead and circumferentially around the tire carcass in an often seamless, but not always smooth surface. Why not smooth, you ask ? Good question ! Firstly, during the ‘lay-up’ of the tire in manufacturing, the inner tire liner material requires some compliance to allow it to exactly fit - without folds or wrinkles - the inside of the tire carcass casing plies. A dimpled or surface texture on the material aids in this fitment. Secondly, a textured surface allows for better heat transfer between the tire and the air inside. Thirdly, a textured inner liner could identify the liner for specific tire constructions in a tire factory along with colored stripes added to the material. Fourthly, (OK this is getting more complicated than I thought) a textured inner liner can attenuate or reduce shell ring or cavity resonance - a noise produced by the excitation of the ‘doughnut’ (Peter: Bagel?; Dood: I’ll allow it.) of air by the act of rolling the tire at a critical speed. This noise - produced by a standing sound wave inside the tire and transmitted through the air and chassis components into the cabin - can be objectionable in some vehicles with certain tire fitments.
ALSO… Some ‘puncture-resistant’ tires will have a rubber-cement-like material applied to the inside of the inner tire liner from the factory. In the event that a nail or other sharp object punctures through the tread and casing/body plies of the tire, the compliant sealant material will conform to the object and (ostensibly) seal it to minimize air leakage. The drawbacks of these puncture-resistant tires is they are heavier than non-puncture- resistance tires, and they also run hotter due to the increased thermal mass relative to the surface area through which heat can be rejected. Tires get hot while rolling down the highway, let alone on track under heavy cornering forces (with more tire slip). Did you know tires don’t generate grip without some percentage of slip ? More on that later….. Like in Tire Tread Compounds in Tires 203…
Peter: How are they supposed to know that ?
Dood: I dunno, but they’ll know soon enough.
Sidebar / Teaser: Tire Slip vs. Grip is a critical relationship for racing vehicle dynamicists and chassis tuners. The grip/slip relationship is so critical, even the powertrain developers/engineers get in on the action to maximize grip. In the mid-2000’s, the WSBK bikes were pumping out over 200 HP. That’s a huge amount of power to put down through a contact patch area smaller than the base of your coffee cup. So, you can imagine how difficult it is for a Pro WSBK rider to modulate the throttle so as not to overcook the rear tire and lose grip under power. The tire guys were all out of black-magic (Peter: Was that a pun?), so the team race engineers turned to the engine developers for help. Not to reduce power, but to change the firing order of their inline four engines. You see, the inline-four engines put down sewing-machine smooth power at 19,000RPM, and the V-twins had the advantage of putting power down in pulses. (Peter: Yeah, so?) Well, the smooth 200HP power delivery of the inline-fours was causing the tires to break traction and slip too much causing reduced power transmission at the contact patch, and tire temperatures to skyrocket. The V-twin powered superbikes realized an unexpected benefit of pulsing power to the contact patch, which allowed the tire to grip, slip-a-smidge and propel the bike forward, then relax and ‘re-grab’ the track surface before the next power-pulse. Solution for the inline-fours to remain competitive: Change the firing order of the cylinders such that power output is pulsed - not smooth. Some of these I4’s - it is rumored - had firing orders which had all four cylinders firing within 90⁰ of crankshaft rotation. This bra-a-a-ap - pause - bra-a-a-ap - pause - bra-a-a-ap power output was such an advantage that the WSBK outlawed the big-bang engines in 2005. Cool beans, right ?
Peter: So cool.
Yep! Anyway, grip/slip curves, and lateral compliance, vertical spring rate, Residual Self Aligning Torque (RSAT), and more are very important tire data for racing chassis tuners. When consulted about chassis setup by fellow F1 chassis setup engineers, French vehicle dynamicist, Claude Rouelle, often famously replied, “Where are your tire curves?”. Funny guy. Like I said. More on that later… In Tires 203…
Back to Tires 201...
The Casing/Body Plies of a tire really define its behavior and suitability for different load conditions and usages. Yes, we finally get to talk about Bias-Ply and Radial-Ply tire construction. WOOOOOooooo.. hoo.. <cough> Still not feelin’ it? OK.. on with the nerd-babble. Casing plies run from the outside of the sidewall, under the first bead wire, up the inside of the sidewall, around the tire profile under the tread, down the opposite sidewall, under the opposite bead wire, and up the outside of the opposite sidewall. Phew ! Check out the (radial) casing plies in Figure 5 below. You can trace the casing/body ply path exactly as I described.
(Figure 5: Tire Construction Cross-Section)
This ply arrangement ensures the structural integrity of the tire for what could be thousands of miles on a motorcycle (I have gotten 6,000 to 9,000 miles out of sport and Sport-Touring Tires on my VFR800), or hundreds of thousands of miles(!) on an over-the-road truck tire. Bias-ply tire construction is usually employed on taller sidewall tire sizes and special purpose applications (Like the aforementioned drag racing and agricultural uses).
The Crown Plies are laid down on top of the aforementioned Casing/Body Plies and form the underlying tread area shape/profile (reference Figure 5 above). The Crown Plies (including belt, edge cover, and cap plies) wrap around the tire circumferentially and influence the ride comfort, steering feel, and Residual Self Aligning Torque (RSAT). Without crown plies, your tires would look like olde tyme pneumatic tyres with nearly circular cross section. To make the contact patch much larger, and improve steering feel (lateral Stiffness), these crown plies act as a structural (but compliant) hoop to contain the tire shape when inflated. The steel belts and aramid fibers in this area of the tire have a huge influence on ride comfort and steering feel. The number of plies, material type, fiber density, and the belt/fiber angle are carefully selected (and tuned during tire development) to produce a tire that matches with the vehicle chassis kinematics and compliance. (Peter: He means that the OEM car manufacturer engineers work closely with the tire manufacturer engineers to test multiple constructions of a tire brand/model and select the best tire suited to the vehicle.) Right-o, good chap!
Residual Self Aligning Torque (RSAT) has been mentioned a few times, so let’s talk about that for a quick minute. This RSAT is mostly applicable to car/truck tires, but it’s an important part of tire science. After tuning the tire construction, (Peter: If you haven’t caught on yet, ‘Tire Construction’ is an industry phrase to describe all of the design elements of a tire.) the resulting ply arrangement/recipe will undoubtedly result in some amount of residual self aligning torque. Many times, RSAT can be tuned - especially on directional performance tires - to improve on-center feel and initial turn-in performance of a tire. The RSAT is the tire’s tendency to steer itself about the Z-axis (see ‘aligning torque’ in Figure 3 above) while it is rolled across the ground. If the left-side tires (front and back) are trying to steer to the right, and the right-side tires are trying to steer to the left, then the car will have some inherent stability due to the RSAT and toe-in geometry in the chassis. The RSAT also keeps the tire carcass ‘pre-loaded’ such that when the tire is steered in that preloaded direction, turn-in is nearly immediate. Of course, you shouldn’t run directional tires on your vehicle in the wrong orientation, or the vehicle will feel slow on turnin, or down-right unstable. Neat, huh ?
Peter: So, the tires on Moms Volvo weren’t likely high performance tires?
Dood: Right you are! And the ride-comfort focused tire construction meant that the steering feel of dad’s Buick was akin to a big boat with a small rudder - vague and slow.
That doesn’t mean that non-directional non-performance tires don’t have RSAT. RSAT helps a car/truck track straight down a crowned road. That’s right - car/truck tires (and vehicle alignments via slightly asymmetric camber) are tuned such that left-hand-drive vehicles steer left up the road crown, and right-hand-drive vehicles steer right up the road crown. If manufacturers didn’t do this, then there would be thousands of warranty claims for vehicle alignments because “CS: vehicle pulls right/left” complaints. I have seen it happen.
On the other hand, Motorcycle tire crown ply constructions are almost always symmetric, such that steering feel, turn-in, and grip are predictable and progressive throughout the vehicle performance envelope. As such, the RSAT is almost always minimized on motorcycle tires. I say almost always, because I wouldn’t put it past the motorcycle tire manufacturers to bake a special batch of tires that might optimize left or right cornering performance and/or endurance on a particular track with more left or right turns or mother speed sweepers. *wink wink
Peter: Which they do actually. Watch MOTOGP, and you will notice they have asymmetric tires with SLIGHTLY different compounds left/right for the track.
Imagine riding a motorcycle that always wants to turn and fall over… at 200mph.
Yeah, and it’s a well known fact that some of the superbike race slicks were extremely optimized for cornering grip and stability - so much so that the motorcycles were down-right scary unstable on the straights as the tires squirmed under acceleration loads and caused the tire carcass to do all kinds of interesting things. You can see this as the superbikes launch out of the corners and down the straights twitching around with each shift and ripple in the track surface.
Well, folks... We’re finally getting to the sexy part of the tire.
The Tread Area of a tire is designed for all manner of road and track conditions. Tires can be focused on only one terrain / condition, or designed for multi-condition use. Dirt, mud, asphalt terrain with dry, wet, snow, or ice conditions cover most of the scenarios. The two major features of the treat area are 1) the tread design, and 2) the tread compound. Let’s start with the latter: Tread Compound. All of the rubber built onto the tire up to this point is not optimized for traction - it’s more structural and durable. The tread rubber on the other hand is critical to dry, wet, snow grip and influences treadwear and rolling resistance. In fact, multiple compounds are routinely tested and evaluated with the same tread block design to tune the balance of all season performance and durability. Tire compounds that are great in the dry (summer tires) are ‘OK’ in the wet (and terrible in the snow). Conversely, tire compounds that are great in the wet are ‘OK’ in the dry (and not great in the snow). All season tires have a tread compound (or compounds) that attempts to strike a decent balance of performance in all conditions, but it means that their performance in each of these conditions is not as good as a tire with a compound more appropriate to each of those conditions. Several top-tier tire manufacturers employ multiple compounds in their tire constructions to improve all-season performance. In the motorcycle tire world, multi-compound tires have gotten pretty amazing with two (or more) compounds across the tread area. The compound in the center is the hardest for long distance tire endurance (treadlife). The compound(s) towards the side-wall are softer and optimized for cornering grip. Tread design certainly influences tire performance in these different scenarios, but the ultimate grip is dominated by the tread compound. The difference in performance can be stunning on two wheels or four. In one winter tire test, I was sliding all over the place trying to gather the composure of a 4500lb RWD car on the ‘rally-track’ loop in the snow on “Tire construction A”. The ESP and Traction Control were going bonkers, too, reacting to my ham-fisted inputs to test traction limits. On the other hand, 20 minutes later - after swapping on “Tire construction B” - I was giving the 400+HP V8 all of the beans in 3rd gear down the straights. The only difference between the two tire constructions was the tread compound. The carcass and tread block were identical. Yep. Full-chat at 80mph on the snow is fun, and an important part of vehicle testing. Very important.
Peter: I bet. LOL.
Tire Tread Block design is a critical design element as it influences water evacuation, dirt/mud/snow self cleaning, noise, and comfort and handling to some extent. No doubt, if you have shopped for tires, you have seen the myriad of tread designs from the manufacturers. Tread design has become one means of instantly identifying a particular brand/model of tire. Motorcycle tire nerds will be able to instantly identify Michelin Pilot Sports or Pilot Road, Bridgestone Battlax, Conti *-Attack, and Dunlop Sportmax tires by their tread design. Similarly, Car/Truck Tire nerds can likely ID several brand/models by their tread also.
Motorcycle tire offerings from the top-tier manufacturers are dizzying… Michelin has twelve different tire categories. TWELVE ! And then there are several individual models within each of those categories - each with unique tread area designs - block design and compounds…
Yeah, Wow. The other manufacturers aren’t much different - choices abound. Lucky - as you mentioned in Tires101 - motorcycle and scooter tires have fewer size variations than car/truck tires.
(Figure 6: The twelve product categories for Michelin Motorcycle Tires)
And just for street-bikes, Michelin has a huge range of tires with unique tread designs.
(Figure 7: Several Michelin multi-compound street-bike tires from the mid-2010’s)
So there you have it… The six main tire construction design elements, outlined for you to digest here. We’ve got a few more Tires20X classes up our sleeves, so stay tuned for more in the coming weeks. We’ll dive deeper into Tire Geometry, Compounds, Mud/SnowWater Evacuation, and Performance Measurement.
Peter: 100%. Tires, much like the cars and bikes that use them, have a LOT of different parts and they all work as a cohesive system. We will be looking into each of these later, as my brain right now is mush.
If you have some suggestions or corrections, please be sure to message one of us and we’ll try to accommodate them in our mini-series. If you have some first-hand experience that you’d like to share in a mini-series writeup, please contact us as well so we can incorporate it into Tires 20X series...
-Dood & Peter