(Words by Dave Nutting) – Hello and welcome to the second installment in our on-going Pro-touring tech series, a little something that we like to call “Pro-touring 101”. Think less magazine bench-racing and more real-world results, our plan to is practice what we preach by taking the lessons learned and applying them to our long-term project car, Buford T Justice. Not for the faint of heart, there will be more than a few defenseless autocross cones murdered along the way…
In our last Pro Touring Tech installment, we discussed the theory of traction and how skid pad numbers are derived. Building on this, today’s article will focus on tires and the integral part that they play in the performance of your vehicle. We’ll also touch on wheels, but the real tech info on that will be saved for its own article at a future date.
As mentioned before, tires are arguably the most important component of your vehicle. The fury and power generated by your engine results in tire smoke and not much else without the right tires. Same goes for the thousands of dollars spend on the latest suspension goodies. All those geometry-correcting NASA-spec chrome-moly parts will not do much good for your pride and joy if it is still riding on 215/65/15 white letter “Sears-special” tires. While we don’t mean to discount suspension modifications (Quite the opposite as we’re going to focus heavily on them in coming articles), end of the day, all of those modifications are made with the goal to maximize the amount of tire that is on the road at all times.
Along with improved geometry compared to their older counterparts, much of what allows today’s vehicles to perform as well as they do can be attributed to the tires that they wear, and it’s an unfortunate reality that even an otherwise well thought-out Pro-Touring machine can be handicapped by the wrong wheel and tire selections. As part of our daily service to you loyal Bangshift readers, we’ve worked with Richard Winchester, one of the experts at BFG, to get the low-down on tires and help you make an informed decision once it’s time for you to buy new rubber. There are a lot of numbers being advertised, and we’re going to go over the ones that we feel matter the most for performance.
Today’s radial tires are somewhat of a modern technological marvel in regards to their construction: We could spend pages upon describing their beginning to end construction, but that’s beyond the scope of this article given that we’re focusing on the performance of, and not the common build process for, a radial tire. Suffice to say that the creation of a radial tire is not as simple as pouring rubber into a mold, and aside from some basic shared underpinnings there are vast differences in the design and quality of the materials used between a common passenger tire and a high-performance tire. These material differences result in more potential traction for acceleration, braking, and cornering.
Section Width and Aspect Ratio
In terms of numbers related to tires, it’s safe to say that almost every automotive enthusiast is familiar with what I call the “Big 3”: Section width, Aspect ratio, and Diameter.
In addition to the other specs that we’ll be covering, these so-called Big 3 (Which I just coined on the spot; completely original phrase…) are all part of the ISO Metric Tire code. Section width and diameter are pretty self-explanatory, being the width of the tire from inner to outer sidewall (In millimeters), and diameter of the wheel that it is designed to mount on (In inches), respectively. The aspect ratio is a relatively simple concept as well, being the ratio of the tire’s sidewall height to its width.
As an example, a 245/50/16 tire is 245 mm wide (Roughly 9.65 inches to those that grew up with a school system which feared communism and the metric system), has a 122 ½ mm (4.82 inch) sidewall height, and mounts on a 16 inch wheel. All in all a pretty simple concept to understand, and explains why these numbers are often the only ones that many people look at when buying tires. After all, a big component of traction is getting as much tire on the ground as possible to maximize the contact patch, so wider is always better, right?
Right, in the same way that pulling up a camshaft sheet and finding the entry with the largest lift and highest duration will be the best fit for your motor. Just as a massive lumpy camshaft sounds great at idle but may be a huge disappointment at anything but wide open throttle and high RPMs, overly wide tires with bulging sidewalls on your 15×7 Rally wheels may look tough, but in the end will have more bark than bite (Or squeal in this case) in regards to handling. Here is where proper wheel sizing comes into play.
Remember that 245/50/16 tire mentioned earlier? Now, that “245 mm” wide tire isn’t actually 245 mm wide on every possible 16 inch wheel that it can be mounted on. Being made primarily of rubber, the flexible sidewalls of a tire allow it to be mounted on a number of different widths of wheel; this recommended range is often called the “wheel width range” and generally ranges an inch or so in either direction from the measuring wheel width. The advertised section width is the width of the tire at that specific measuring wheel width, and the section width will increase/decrease roughly 2/10 of an inch for every 1/2 inch of deviation of the width of the wheel from that benchmark width.
In real-world terms, this means that your 295 width tire that you just purchased for your hypothetical 15×7 wheel is now not only putting as much tread down as advertised, it’s also mounted on a wheel narrower than recommended and has some serious sidewall bulge, neither of which is performance oriented.
Richard Winchester: “Every tire has a recommended wheel width. Just for easy numbers, let’s say it’s from 7 to 8 inches. If you put it on a 7 inch wheel, you’re going to get better ride quality, but if you put it on an 8 inch you’re going to get the best handling, because it stiffens up the sidewall and makes the sidewall stand up straight. You don’t want to get out of that range, as once you get out of that range the tire is possibly flexing where it’s not supposed to flex as much, and it’s building up extra heat. So stay within the range, but lean it as wide as you can for handling purposes.”
Wrapping up tire section width, we’re on the right track with “wider is better” for better handling, but you also want to be aware of the wheel width range and tend towards using a wheel towards the wider end of the recommended range to keep sidewall bulge to a minimum.
This brings us to aspect ratio and sidewall height. You can think of a tire as acting like a spring, in that the flexible sidewall will absorb bumps in the road similar to the rest of the car’s suspension. Taller sidewalls (Higher aspect ratio) are more flexible, and therefore tend to help give a cushier ride than a shorter sidewall, which is stiffer. They also tend to flex more when cornering and have a narrower, taller contact patch, neither of which is conducive to performance driving.
(This photo is an illustration of the point. We don’t recommend these tires for your car.)
Richard Winchester: “What’s happening with a shorter sidewall is you’re changing that contact patch; you’re making that contact patch wider, not as long, but wider, and that shorter sidewall is making it more rigid, so your handling is much better because of the rigid sidewall as the taller the sidewall the more flex you’re going to have.
When you get down to the lower profile tires, the only negative would be the ride quality and chuck holes [Editor’s note: AKA “Pot holes” for those of us without a nifty Southern accent] will bend wheels and will pop tires if you don’t watch out. When you hit a chuck hole or a curb, the tire will flex and a lot of times you’ll think that you just bent your wheel. Well, you may have also pinched the internal construction of the tire. It may not show up that day and may end up showing at a later date.”
Both the section width and aspect ratio define the contact patch of the tire, which is literally where the rubber meets the road (You knew that phrase was going to works its way in eventually…)
Richard Winchester: “If you think about a sheet of paper, 8 ½ by 11, that sheet of paper represents about how much contact patch you have on the ground with your four tires. It doesn’t matter if you have a Chevelle or a Mustang or whatever; the contact patch is about that size. If you want to do an experiment, think about a contact patch on a ’53 Corvette: It’s going to be long and skinny. If you were to stand up right now and put your right foot directly in front of your left foot, you’re connected to the ground about the same as one tire of a ’53 Corvette. How stable are you? [Author’s note: Both Brian and I failed this test, sober] Editor’s Note: Who said I was sober?
Now, put your feet side by side and this is a brand new Corvette: Today’s Corvette. You still have about the same amount of contact patch, but the shape has changed and now you’re much more stable. So when you’re looking at tires, wider is good, but you’re really not getting that much more patch as much as you’re changing the shape of it. So, sometimes going with a much bigger tire may not help you as much as you’re getting wider but you’re also getting shorter in your contact patch at the same time.
Some of this has to do with the weight and how much you’re able to transfer to that contact patch. Think about the guys with hot rods with 700 to 800 horsepower. That thing may weigh about 1000 pounds and have Mickey Thompsons on the back: They’re not really putting much weight on that tire to really make it connect to the ground.”
To expand on Richard’s last point above, a wider footprint in general helps with stability when cornering, but it is also possible to go so wide that the vehicle may not be able to transfer weight when needed for traction. A snow shoe is a good example of this: Spreading the weight out creates less downward pressure and allows a person to walk on soft snow without sinking. While this is a good thing when attempting to trek through the Arctic, it’s less than desirable with a car and a familiar sight during a New England winter is a late model Mustang or Camaro with wide tires struggling to make forward progress on a snowy street. In this very specific case, a narrower tire which allows for vertical load (Weight pressing down on the tire) in a smaller area will keep the car planted and allow for better traction. All of that said, with the weight of a typical muscle car it’s more than likely safe to say that enough vertical load will be created for almost any wheel and tire combination that can be fit within the wheel wells.
Load index and Vertical Load
Vertical load was briefly touched upon in the last section and we’ll expand upon its definition further here. As mentioned above, vertical load can be thought of simply the weight of the car pushing down on a particular tire. This weight creates traction through friction between the tire and the road, which in turn also creates heat.
Richard Winchester: “When you put more and more weight on the tire, you’ll have more and more grip up to the point where the tire is overloaded. At this point the tire is basically overheating and it will die. Now there isn’t a magic formula for [grip versus vertical load] that I know of, as it’s going to change for every vehicle and every tire size.”
In general, tires that are able to dissipate heat better will have a higher speed rating among other attributes, something that we will discuss later on in this article. Keeping focused on the present topic for a few more moments, vertical load is one of the elements crucial to traction, and will be mentioned repeatedly in future articles that discuss springs, dampeners, and suspension geometry.
Before moving on, the load index needs a brief, if important mention: While not a performance metric per se, it is important to purchase a tire that can comfortably withstand the weight of the vehicle, as both overloading and under-inflation of a tire can and will kill it and possibly you and your car. The higher the load index number, the more weight a tire is rated to carry.
Tire Speed Ratings
Last but certainly not least in terms of importance, the speed rating of a tire is, not surprisingly, the max speed at which a tire has been deemed safe based on its load index (This is the maximum weight that each tire is rated to carry). What is surprising is how little attention is paid to this specification by most buyers when purchasing a tire, as the higher speed ratings are an insider’s look into how well a tire can dissipate heat; as we just learned, better heat dissipation results in a better performance tire.
Tires with higher speed ratings typically have more traction than their lower-rated counterparts, which translates into more grip for cornering, acceleration, and braking at the expense of tread wear. My opinion is that higher speed rated tires will typically make for better performance tires, and this is one of the most important specifications to review when comparing two tires.
The federal government has their own ratings system called the Uniform Tire Quality Grading (UTQG) that is meant to rank tires based on Tread wear, Temperature, and Traction, but truth be told we have it on good authority that these tests really aren’t a good metric for gauging performance thanks to advances in modern tire compounds that often exceed the tests themselves. As an example, “Traction” is rated on a scale from AA to C, with AA being the highest and C being the worst. Given a random sample of “performance” tires, it would be easier to find Waldo at a striped shirt convention than it would be to find a tire NOT listed as A or AA for traction. The same goes for the Tread wear and Temperature ratings as well.
Richard Winchester: “Tread wear ratings are part of a system that was designed by the federal government so that an uneducated tire buyer would be able to compare tire A to tire B. Racing tires, performance tires, Mom ‘n Pop tires, they all have to fit into this bucket for measuring purposes, which means it’s not a good system: There’s too many items in this system and too many variables as well. With the rubber compounds that we’ve got today, we can almost put whatever rating we want on the tires. When we put a 300 rating on a Goodrich KDW, we’re not saying it’s a great mileage tire; it’s a good mileage tire, and because of the treadwear, we’re not saying it’s a good performance tire when in actuality it’s a hell of a great performance tires and gives pretty good mileage for what it is.
If we put a 200 rating on it, the guy that thinks this rating has something to do with performance may buy it, but the guy that’s looking for mileage may not want it because he thinks that 300 is better, so we lean the tire more towards 300 for the people looking for more mileage out of a tire.
Now, the new Sport-Comp 2 has a rating of 340 and it’s a better traction tire than the KDW! Again, these numbers do not necessarily correspond to the rubber compound.”
Keep in mind that Richard is speaking for BFGoodrich, but again, we have it on good authority that the above holds true for every tire brand, so make sure to take the UTQG rankings for a particular tire with a grain of salt, and also refer to both independent testing, reviews, and speed ratings for a true indication of the performance potential of a tire.
Choosing the correct tire compound
Going back to our camshaft analogy, it’s all too easy to find yourself paying careful attention to all of the specifications and still choosing the wrong tire for your application if you’re not mindful of tire compound as well. Tire compounds themselves are pretty much a trade secret, but at a high level it’s fair to say that a race tire from Brand A will always be stickier than an All Season or even “Max Performance” Summer tire. That said, unless the only street time that your vehicle is seeing is the drive from the trailer to the race track, a race tire is not something that we would recommend for the majority of cars out there.
Richard Winchester: “The spirit of pro-touring, as far as I’m concerned anyway, is building your and enjoying it. If you’re going to enjoy it chances are you’re going to have to do some street driving, and when you go to those special compound tires and you’re on the street, it can be right hazardous if you hit a damp spot. It doesn’t have to be rain, it can be slightly wet. Those special compound tires with the low tread wear or no tread wear basically are for dry surfaces; it’s what they’re designed for. The closer to a race tire you get, the more you need to be an expert and the car needs to be really set up. A race tire, if you can drive like a Richard Petty or a Mario Andretti, you can control a tire at its limits. Let’s face it: Most of us guys out there on the street, when we get up to the limits of the tire, we need the tire to help us, because we’re at our limit as well. I know with our tires, when you get to that limit, the tire will give you a feeling of “Hey, there’s something not right here and you better be letting up a bit”, here as an R1, with a real sticky compound that’s made for road-racing, it might wiggle just a little bit and give you a little bit of notice, but if you’re not on your toes you’re just about ready to lose it. So when you get into a street able tire that’s really a max handling street tire, it’s a lot more forgiving in that area.”
In addition, depending on where and when you’re driving your car, it may make more sense to purchase a good performance All Season tire than a summer tire, especially in the areas of the country where even summer mornings can be relatively cold. I know that with my ’87 Monte Carlo SS I had a few moments with my summer tires on a chilly Fall morning that resulted in some serious pucker factor as the tires were not in their “sweet spot” in regards to temperature.
Richard Winchester: “Technology has taken us to the point where we can do a lot of different things with rubber compounds. We’ve got a tire called the Super Sport A/S, A/S for “All Season”. The rubber compound on that tire gets up to operating temperature much quicker when it’s cold, so if you take that tire out in Maine in December and compare it to an R1 and its very sticky compound for racing, that all season tire is going to kick that racing tire’s butt that day, just because of the outside temperature and the fact that the all season gets up to operating temperature very fast in cold weather. Do you need that all season tire in Miami, Florida? Not really, but it wouldn’t hurt you as it’s still a good compound that likes working in cold weather as well as hot weather. We do a lot of testing and prefer to wait until the temperature is above 40 degrees unless we are testing all season or winter tires.”
Again, this is not to say that tires designated as “Summer tires” aren’t a wise choice (In fact, it’s possible that they’re the best choice for your application and performance goals), but the intended temperature range of a tire is something to be aware of when making an educated purchasing decision.
A few general words of advice
Before concluding, there are a few general comments from Richard in regards to the direction in which the market is going, along with words of wisdom on project planning that we felt important to pass along:
Richard Winchester: “The 17, 18, 19, and 20 inch wheel, that’s where the OE’s are right now and 15’s and 16’s are dying out quickly. In fact, right now the biggest market share is 18, 19, and 20 inch for both domestic and import. Look at the new Camaro with 20s, Mustang with 19s, and Challenger with 20s. A lot of the Pro-Touring muscle cars are using 18 inch and that seems to be working really well.
I would say the present is more 18, 19, and 20 and it’s going to be that way for the next few years.”
(Keep this in mind when buying your next set of wheels and tires, as you don’t want to find yourself unable to purchase replacement performance tires for a 16 or 17 inch wheel in a few years. This is advice that we will definitely take into consideration for future project cars)
Richard Winchester: “Any time you’re starting a project car, the thing you need to be thinking about is the complete package. If you have a tire and wheel in mind, buy those, and then start buying your suspension and brakes, you make come to find out that you have the wrong wheel fitment. So, you go ahead and buy the right wheels and find out that you can’t find a tire in the width that you want. I’d like to say that there’s a good rule of thumb, but you really need to work with companies that have experience; If you’re into first and second generation Camaros, get ahold of a company like Detroit Speed and they can give you a lot of information about what to do and what not to do. Even then, sometimes we still mess up!”
“The other thing about wheels is, there’s a lot of them that I’m going to call “bling bling wheels”, that really look cool, but weigh A LOT. If you’re serious [about handling], stick with a wheel that’s lighter weight or not much heavier than what came on the car originally. It’s one thing to look cool, but it’s another to actually have really good performance.”
“With getting a new tire purchase wrong, I think that a lot of times it’s actually getting the wheel purchase wrong in terms of things like the wrong offset. In fact, this happened to me recently: I found the wheel that I wanted and was told that it only came in one of two offsets. Well, I gambled and tried them. They almost worked in that they fit the car, but I couldn’t enjoy the car to its max with them on, so I ended up going with a different set of wheels that fit properly and now I can enjoy the car.”
“Do some good homework before grabbing a wheel and tire that look cool, as you don’t want to end up with something that you can’t use. Keep in mind when you’re measuring that you need to consider that you’re measuring a static item; as soon as you start going down the road and weight starts shifting around, you may have some issues. A good rule of thumb is to look at the total height of whatever wheel and tire you have that’s working now and use that as your base. If you know that works well and you have another ½ of clearance and end up with combo that’s an inch taller, chances are that’s not going to work.”
To wrap it all up, there a heck of a lot more to buying a performance tire than finding the biggest one you can stuff in the wheel well. Keeping in line with our promise to put our money where our mouths are, we’re going to upsize the wheels and tires on Buford T Justice from the stock 15s and steelies to more modern wheels and rubber, but not before finding out just how well (Terribly?) a stock 1987 Chevy Caprice can navigate an autocross course on stock suspension and tires. I predict large amounts of body roll in our future…