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20 Things You Need To Know About Tires - Tech
MM&FF's Essential Guide To Learning How The Rubber Meets The Road.
It doesn't matter what type of Mustang (or fast Ford) you drive or where you drive it-be it on the street, at the dragstrip, or on a windy road course-tires are an integral part of your car's setup, and in many cases they will help determine the overall handling characteristics. To say the tires are anything but an essential part of the performance puzzle would be a gross understatement. But for something that is an essential element of our automotive enjoyment, there seems to be a huge number of misconceptions and incorrect information surrounding our round, mostly-rubber buddies, all of which give their rolling lives for our fun and transportation.
With that in mind, we set out to piece together an insightful and informative story about tires in order to help you make the right choices when it comes time to fit your Ford with new rubber. We came up with 20 items for you to consider, and after diving into this guide you'll be better educated and rolling fancy.
1 - Tires Are Not Made Solely Out Of Rubber.
Modern tires are made from a range of materials, with the rubber being mostly synthetic. Carbon black is added to increase strength and toughness, as is Kevlar on some high-performance models. When used in the tire's tread, this combination gives long life. Natural rubber is weaker than the synthetic version, and today it is relegated to use primarily in the tire's sidewalls.
During design and construction, manufacturers balance sidewall stiffness and tread pattern and compound to come up with a tire that will perform in the desired arena. Softer tread compounds provide better grip but wear faster.
2 - Tires Are An Integral Part Of Your Suspension.
They act as a secondary spring during all driving conditions. Your tires absorb the hit on launch at the strip and all the loading in a corner. There is far more to the tire's interaction with the suspension than you may think. You can witness this fact just by watching a NASCAR race on TV, as teams adjust the car's handling by adding or decreasing tire pressure by as little as 1/4-lb of air pressure. Often, going with too short of a sidewall can hurt traction as it limits the tire's ability to remain compliant with anything but pool table-like road surfaces.
3 - What Do All Those Numbers Mean?
Today's tires are a veritable alphabet soup of numbers and letters. The sooner you learn how to decode all this information, the better off you'll be. Let's decode just one size, P235/55R/17. The first letter, P, means the tire is a "P-metric" size, which is designed to be fitted on passenger vehicles. The first set of numbers, 235 is the section width of the tire in millimeters. The next number is the aspect ratio of the tire's sidewall when compared to it's section width. In this case, the sidewall height is 55 percent of the section width. The next letter indicates the internal construction type of the tire, with "R" meaning "radial." The final number is the diameter of wheel this tire requires-17 inches in this case.
4 - What is a contact patch?
The contact patch is the actual "footprint" of the tire contacting the road or track. Believe it or not, the contact patch of most tires is only about the size of your hand, and it is dynamic, meaning it changes as the vehicle goes down the road. During acceleration, cornering, and braking, the area in contact changes shape and size.
5 - Tires Degrade With Age.
This is true, even if they have never been mounted on a wheel. This is due to exposure to ozone and sunlight. Keeping unused tires in a dark, cool, and dry environment will minimize their degradation, but not stop it completely. Poor alignment, improper inflation, and abusive driving can also accelerate tire degredation.
6 - How Much Air Pressure Should I Put In My Tires?
Proper tire inflation pressures are determined by a number of factors, including the tire's size, construction methods, and materials. It is also based on the axle load that tire was designed to carry. Bottom line: Read the tire, as they all have minimum and maximum tire pressures printed onto them.
7 - Check Your Tire Pressures Often.
As mentioned above, your tires have a lot of work to do, and they can't do their jobs properly if they aren't being operated at correct pressures. Buy a quality tire pressure gauge and use it often! Properly inflated tires are essential to maximizing fuel mileage, as well as providing adequate load-carrying capacities, all while providing you with the handling characteristics that your suspension was designed to deliver. This goes for your performance vehicle, commuter, and tow rig as well.
8 - What Is Dry Rot?
Tires lead a tough life. Manufacturers try to build tires that will last a long time, but over time, exposure to the elements will eventually cause rubber to lose some of its elasticity and allow surface cracks to appear.
Dry rot, weather checking, or ozone cracking are all names used to describe surface cracks. Most of the time, these small cracks develop on the tire's sidewall or in the bottom of the tread grooves. Depending on their severity, they may just be a minor cosmetic issue if they don't go too deep into the rubber, or they may be a reason to replace the tire if they are too deep or have otherwise compromised the integrity of the tires.
9 - Do Tires Require A Break-In Period?
Yes, since tires are manufactured using various layers of rubber compounds, as well as steel and fabric belts, new tires require a break-in period to ensure that they ultimately deliver the expected ride and performance. During the manufacturing process, a release lubricant is applied to prevent them from sticking in their molds. Some of this lubricant remains on the surface of your new tires, thereby reducing traction until it is worn off. Most manufacturers recommend 500 miles of easy acceleration, cornering, and braking to allow the lubricant to wear off.
If we're talking about all-out competition or racing tires, this is a completely different subject, as most of these tires are designed and manufactured to be maximized right away without any of the break-in time required on street/performance tires.
10 - Set Your Tire Pressure First Thing In The Morning According To The Manufacturer's Cold Tire Pressure Recommendations.
It's best to do this before rising air temperatures, sunlight, or even driving the tires a short distance, temporarily warms the tires.
The folks at Tire Rack have a wealth of information on properly maintaining tire pressures and explaining how some everyday variables can be taken into consideration when checking and adjusting inflation pressures. It basically comes down to "cold" inflation pressures versus "hot" pressures.
Indoor-to-outdoor temperature differences. Significant differences between the conditions tire pressures are set (the warmth of an attached garage, heated garage, or service shop) and in which the vehicle will be driven (winter's subfreezing temperatures) requires inflating tires 1 psi higher than recommended on the placard for every 10 F difference in temperature between interior and exterior temperatures.
Afternoon ambient temperature increase. Tire Rack recomends setting your tire pressures 2 psi above the vehicle manufacturer's cold inflation recommendations if the vehicle has been parked in the shade for a few hours or if you are having new tires installed.
Tire heat generated while being driven at speeds less than 45 mph. Set 4 psi above vehicle manufacturer's cold inflation recommendations.
Heat generated while being driven hard-speeds greater than 45 mph. Tire Rack recommends setting 6 psi above vehicle manufacturer's cold inflation recommendations.
11 - Compressed Air Or Nitrogen?
Most race teams inflate tires with nitrogen. Here's why: Nitrogen gas used to fill tires has had most of the water vapor removed. With the water vapor removed, the nitrogen gas will reduce the expansion and contraction of the gas inside the tires as they heat and cool. Bottom line is nitrogen-filled tires will maintain more even pressures during a use cycle, and this can be beneficial whether corner carving or launching down the dragstrip.
This is important in an all-out race car, where small changes in tire pressures will alter the handling of a high-performance car racing at tremendous speeds around a track. However, more consistent and even tire pressures are essentially insignificant to a consumer out on the road. Pure nitrogen in passenger car tires seems like a waste of money, as it costs more than $10 a tire when purchased to have them filled with nitrogen. If you want to keep them filled with nitrogen, you will need to find a tire shop that has the equipment to refill or adjust their pressures, or you can purchase small aerosol refill cans, adding even more expense to something that can be done with simple compressed air.
12 - What Are Tread Wear Indicators?
These are narrow bands built into the tread during manufacturing that begin to show when only 1/16 of the tread remains. Also called wear bars, treadwear indicators are there to provide an obvious visual warning that it's time to shop for new tires. That is, if you ever get to see them before the cords start showing on your old tires.
13 - Do You Know The Three T's Of Tires?
Each and every DOT-approved tire, even full-on competition tires, is required to have three separate ratings: Treadwear, Traction, and Temperature.
• Treadwear ratings differ from Traction and Temperature ratings in that they aren't a measure of a tire's built-in safety margin. Instead, these ratings, represented by a three-digit number, give you an idea of the expected useful life of the tire according to government testing.
• Keep in mind these ratings are relative to each other. For example, a tire with a treadwear rating of 150 can be expected to last about 1.5 times as long as a tire with a treadwear rating of 100. These are just guides, however. Your tires may last longer (or not) depending on such factors as how you drive, whether you maintain proper inflation pressure and rotate the tires per recommendations, and of course how many burn-outs or hot laps you put them through.
• Traction ratings run AA, A, B, and C, with C being the lowest on the scale. These ratings represent the tire's ability to stop on wet pavement under controlled testing conducted by the government. C-rated tires are marginal and should be avoided. Never buy a tire with a Traction rating that isn't at least equal to the minimum rating specified by the manufacturer of your vehicle.
• Temperature ratings are A, B, and C, with C being the minimum allowable for any passenger car tire. The ratings correspond to a given tire's ability to dissipate heat under load. Tires with lower ratings are more prone to heat-induced failure, especially if driven at high speeds (or when overloaded). As with Traction ratings, never buy a tire with a Temperature rating that's less than specified for your vehicle.
14 - What's The Difference Between Radial And Bias-Ply Tires?
Bias-ply tires have their underlying plies laid at alternating angles of less than 90 degrees to the centerline of the tread. Radial tires are constructed so that the plies are laid at 90 degrees to the centerline of the tread. That's the technical difference. The reason radial tires are dominant today is that they help improve fuel efficiency and handling; they also tend to dissipate heat better than bias-ply tires. Chances are, most people reading this article have never driven a car on bias-ply tires (drag slicks not withstanding); trucks maybe, but today, no modern passenger cars come with bias-ply tires. If you ever get the crazy notion to try some bias-ply tires, it's hugely important to never mix radial and bias-ply tires; dangerously erratic handling may result.
15 - What Is Hydro-Planing And What Does It Have To Do With Tires?
Hydroplaning can occur when the vehicle drives through areas of standing water, especially at highway speeds. If the standing water cannot disipate from under the tire quickly enough, the tire will lift off the surface of the road, supported by only the water, and hydroplaning will result. Because the affected tire will have almost no traction, cars can easily go out of control when hydroplaning. Avoid hydroplaning by slowing down in foul conditions. With the advent of computer-aided tread designs, most of today's street tires have been designed to maximize their ability to pump the water out of the tread. Most of these designs feature deep grooves running in the same direction as the tread, giving the water an extra channel to escape from under the tire.
16 - How Do I Set Tire Pressures For Autocrossing Or Road Course Driving In The Rain?
For starters, increase your tire pressure 6-10 psi from what you would normally run in dry conditions. As we just discussed, hydroplaning occurs when a wedge of water develops between the tire and road surface. Increasing the pressure "rounds" the profile of the tire by decreasing the amount the tire deflects when loaded with the vehicle's weight and cornering forces. This makes for a slightly smaller contact patch, which will put more of the vehicle's weight on a smaller area. It also helps keep the tread grooves more open so they can channel the water out from under the tire.
17 - How To Use Tire Pressures To Fine Tune The Handling Of Your Vehicle.
When racing on D.O.T. approved tires, air pressure is a major component you can use to make minor tuning adjustments to the handling characeteristics of your car, especially in situations where actual suspension adjustments are limited. Here is a handy chart you autocrossers and road course drivers can add to your "going to the track" notebook.
|Tires And High-Performance Handling|
|Adjustments||Decrease Understeer||Decrease Oversteer|
|Front Tire Pressure||Higher||Lower|
|Rear Tire Pressure||Lower||Higher|
|Front Tire Section||Larger||Smaller|
|Rear Tire Section||Smaller||Larger|
|Front Wheel Camber||More Negative||More Positive|
|Rear Wheel Camber||More Positive||More Negative|
|Front Wheel Toe||Toward Toe-Out||Toward Toe-In|
|Rear Wheel Toe||Toward Toe-In||Toward Toe-Out|
|Front Wheel Caster||More Positive||More Negative|
|Front Anti-sway Bar||Soften (Thinner)||Stiffen (Thicken)|
|Rear Anti-sway Bar||Stiffen (Thicker)||Soften (Thinner)|
|Weight Distribution||More Rearward||More Forward|
18 - Will Shaving Your Road Course Tires Actually Make Them Last Longer?
Yes, this is true and something I addressed in a recent "Contact Patch" column. To recap: One of the reasons radial racing slicks are so effective is because they feature shallow tread depths and their contact patch acts as a single unit-they don't have normal tread blocks. Basically, any tread design breaks the contact patch down into smaller elements, and additional deep tread depth (required to enhance wet traction) allows tread-block squirm, which will reduce dry performance.
Again, the folks at Tire Rack explain that shaving a tire is an effective means of permitting more of a tire's performance capability to be realized earlier in its life. A shaved tire's tread profile will usually result in a slight increase in the width of the tire's contact patch, putting a little more rubber on the road. The resulting shallower tread depths reduce the tire's slip angle, increases its responsiveness, and helps stabilize its cornering power by minimizing tread-block squirm. Minimizing tread-block squirm also reduces heat buildup and the risk of overheating its tread compound. In many cases, shaved tires used in competition or at driving schools may actually have a longer useful life than tires that begin being run at full-tread depth.
19 - What Are Tire Speed Ratings And Where Can I Find Them?
A tire's speed rating is expressed as an alpha-numeric symbol you'll find on your tire's sidewall that tells you the maximum sustained speed the tire is capable of safely handling. An H-rated tire, for example, is built to be safe for continuous operation at speeds up to 130 mph. Most current model year family-type cars have S (112 mph) or T (118 mph) speed ratings. High performance cars often have tires with a V (149 mph) or ZR (in excess of 149 mph) speed rating. A few ultra-performance cars have W (168 mph) and even Y (186 mph) speed-rated tires.
You will find this speed rating expressed one of two ways. One, as part of the tire sizing information, such as 275/40-ZR17, a ZR rated tire. Or, you may also find it expressed as a numeric-alpha "Service Description" such as 92 V, typically printed on the tire's sidewall behind the size info. This is a combination of load rating (92) and speed rating (V).
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20 - How Wide Of A Wheel Should I Use For My Tires?
Each tire has a specific wheel width recommendation, typically expressed as a range. Failure to follow these wheel width recommendations may result in poor tire performance or possible wheel and/or tire failure.
Here is a quick reference chart listing each tire size and the acceptable rim widths for that size. (The chart does not show all sizes, but gives a good example) Choosing a wheel near the middle of the range will give a balance between ride quality and handling. A wider wheel will improve handling at the expense of ride quality, while a narrower wheel will improve ride quality at the expense of handling. Consider these compromises when selecting wheels.
|Tire Aspect Ratios|
Tire Sizes By Mustang Chassis Types
When it comes to maximizing your Mustang's performance, there are many factors to consider. Power, suspension package, brake setup, and of course, to put it all to work, the proper wheel and tire package are all essential elements to getting the most out of your performance investments. Here is a quick look at some tire and wheel recommendations for the various years of Mustang.
Fox-Body ('79 - '93)
The Fox-body cars witnessed a huge growth in the number of wheel and tire packages available. Of course, upgrading the Fox cars to a five-bolt configuration greatly expands the number of combinations available. Typically, fender clearance and quad-shock interference are the limiting factors for tire packaging under stock Fox fenders. 225/60-16 are an easy fit under most stock Foxes. Making the move up to 255/45-17 tires will fit under the stock fenders, as long as they are mounted on wheels with the proper backspacing and offset. The real hot set up for road course going Foxes is the move up to 275/40-17 tires mounted on 17x9 wheels. However, this package will require some "clearancing" inside the rear fenders, including rolling the fender lips. Up front, it will take some seriously "tweaked" front fenders to make room for the big 275 section width. It can be done by an accomplished body shop or via the installation of some flared front fenders.
SN95 Body ('94 - '04)
When they first came out, the Mustang faithful claimed our new, lithe, more sculpted ponycars looked like 4x4s. OK, so at their stock ride height, maybe they did have a little too much fender clearance around the tires. But, as modified suspensions became the norm for these cars, those same large fenderwells provided excellent room for a staggering array of tire and wheel combinations, be it street, strip, or road course. When a 17x9 wheel and 275 section width tires fit under a stock ride-height car, you know good things await those wanting to maximize their tire contact patch. Personally, I like the looks and performance envelope of a 275 tire mounted on 17x9 wheels on this car in all four positions. Yes, you can fit 11-inch wide wheels under the rear, but that eliminates the possibility of being able to rotate your tire and wheel package from the rear to the front. Another nice tire and wheel combination for SN95 cars is the 18x9.5 wheel and 275 tire package that debuted on the '00 Cobra R.
S197 Body ('05 - current)
If you thought the SN95 fenderwells were large, take a look at how small 16-inch wheels and tires look under stock V6 Mustangs. Once again, the engineers and designers at Ford gave us some huge fenderwells, made to look even larger with their exaggerated exterior fender arches. The only wheel and tire drawback to these cars is that they need larger wheels and tires to deal with the extra mass (and horsepower) the S197 cars bring to the party. The SVT/Shelby GT500 utilizes 18x9.5 wheels front and rear, but uses slightly larger tires out back (285/40-18 versus the front 255/45-18) to try and cope with the 500+ horsepower these cars put out. For me, I would most likely stick with the 18x9.5-inch wheels, but look to a more balanced tire package, something that could be rotated front to rear.