Lesson 4: Tires

This lesson is all about tires. There may be more to these rubber doughnuts than many people realize. In this lesson we’ll discuss tire specifications, how to check tire pressure, wheel alignment, the wear bar, how to change a tire, and old tire disposal.

Tire Specifications

There are two sets of specifications for tires including the tire manufacturer specifications and the vehicle manufacturer specifications. Whenever you take your car to a tire shop to get the tires changed, they have to meet these specifications by law. You can get a tire that goes above these specifications, but never below.

The vehicle manufacturer’s specifications include speed rating, load and size. It is located on a sticker in the driver’s door jamb of the vehicle and in the vehicle owner’s manual. When buying a new tire you’ll want to stick with a tire that meets the manufacturer’s specifications because that’s the kind of tire that matches the build of the vehicle.

The tire manufacturer specifications include the tire size, load, pressure, temperature, speed rating and tread wear. The tire manufacturer specifications indicate what the tire is made to withstand. These specifications can be found on the wall of the tire itself.

Unless you become a tire technician, you don’t have to know all the numbers on the tire or on the sticker/owner’s manual because every time you get your tires replaced, the tire shop will make sure you at least have a tire that meets the required specifications.

Probably the most important number for the common person to recognize is the recommended tire pressure on the vehicle manufacturer’s specifications on the driver’s door jamb sticker. (The tire manufacturer’s specifications on the tire itself include the maximum pressure that the tire can have, yet the maximum pressure isn’t necessarily the best pressure for the vehicle).

How to Check the Tire Pressure

In order to check tire pressure you’ll need a tire pressure gauge. Gauges can be purchased at auto parts stores and service stations (sometimes a little as $1, but could be $3-$7). They’re usually about the size of a large pen, but can come in other forms. Sometimes a gauge is included with the air pump hose at the gas station where you can inflate your tires.

Follow these steps to check and correct the tire pressure:

• Remove the valve cap of each tire
• Align the gauge up to the valve
• Press the gauge onto the valve with firm direct pressure and then release. You’ll see the measuring stick get pushed out of the other side of the gauge (on a pen gauge).
• Fill the tire with air, recheck the pressure, then repeat as needed until the desired pressure is obtained.
• Replace the valve cap

Tire pressure is measured in PSI (pounds per square inch) or KPA (Kilopascals) which are simply units of measurement for pressure. (Pressure is a force distributed over surface area)

Wheel Alignment

Tires can wear rapidly if they are not aligned properly, or the vehicle can pull to one side while driving. There are three angle measurements that are used in tire alignment: camber, toe, and caster. Knowing these angles is necessary for any tire technician. In addition, knowing these angles can be helpful for anyone trying to communicate with tire technicians.

In order to understand these angles one must consider the planes of the vehicle, which are ways of dividing the vehicle in each of its three dimensions (length, width and height). The horizontal plane, often depicted in math as the X plane, divides the vehicle into top and bottom parts. The vertical plane, often depicted in math as the Y plane, divides the vehicle into right and left parts (or driver’s side and passenger side). A second vertical plane which intersects the first vertical plane at 90 degrees is often depicted in math as the Z plane, and it divides the vehicle into front and rear parts.

The camber angle lies on the “Z plane” (in this description). A camber angle of zero is when the wheel is vertical. If there is any camber, then the wheel is deviating from the vertical plane, or the Y plane. In many vehicles the camber should be zero, but some may deviate by 2-3 degrees.

The toe angle lies on the “X plane” (in this description) and deviates from the Y and Z planes. (Imagine the vehicle from a bird’s eye view or a top view). A toe angle of zero is when the wheels are parallel on the Z and Y planes. This isn’t quite how the wheels should be, at least not the front wheels. When thinking of toe, consider either the front wheels as a pair or the rear wheels as a pair. The distance between the front of each pair of wheels is measured and the rear of each pair (i.e. the distance between the front of the front wheels and the rear of the front wheels when considering the pair of front wheels). When the front wheels are properly aligned the front of the front wheels will actually be slightly closer together than the back of the front wheels, meaning the front wheels should have a slight inward toe. When the rear wheels are properly aligned they will be parallel (i.e. the distance between the front of the rear wheels is the same as the distance between the rear of the rear wheels).

If a right wheel is turned inward while the left wheel is straight, or if both wheels are turned too far inward, or both are turned outward, etc., then there is an abnormal toe angle present and the wheels are not aligned. (Technically when you steer the vehicle and the front wheels turn to the left or right, they are deviating from the Y and Z planes, but they stay parallel to each other with only a slight variation to account for the fact that one wheel is on the outside and one is on the inside while turning).

The caster angle is an angle which lies on the “Y plane” (in this description) and deviates from the Z plane. (Imagine looking at the vehicle from the side). Most devices with steering capability do not have a caster angle of zero. When the deviation is towards the rear, the caster is considered positive. When the deviation is towards the front, the caster is considered negative. On motor vehicles the caster is usually positive, (the higher ball joint will be closer to the rear than the lower ball joint). This helps the vehicle to drive straight even when the steering wheel is hands-free. Since the view of the caster is blocked by the wheel on vehicles with four wheels, it is most easily visible on motorcycles and bicycles. Think of the two braces that come down on both sides of the front wheel. Notice how the handlebars are closer to the rear than the center point (fulcrum) of the front wheel. This is a clearly visible positive caster. The front wheels of shopping carts have a negative caster. The physics of a negative caster allows for the front wheels to turn counter clockwise as the cart is turning clockwise so that the cart is easier to maneuver throughout the store.

Wear Bar

The wear bar is a horizontal bar molded into the tire between the treads. The wear bar indicates safe tread depth. It is usually about 1/16 of an inch measured from the valley of the tread, but not as high as the tread itself when the tire is new. As the tread wears down, the wear bar becomes more visible. When the tread is even with the wear bar the tires should be changed. Tires should be rotated with each oil change and kept in alignment to ensure even tread wear.

How to Change a Tire

Knowing how to change a tire is mostly helpful in case you get a flat while driving on the road. Every other time your tires get removed, rotated and/or replaced it will be by a tire technician.

Every vehicle has instructions for using the jack and other tools as necessary to change the tire. Some of the details can vary by make and model, such as the storage location for the jack and spare tire. Consult the owner’s manual of the vehicle. Also, there will often be a sticker with instructions located with the jack and spare tire.

Here are some general guidelines for changing a tire (in order):

• Make sure the vehicle is on level ground (see Lesson 1, Safety, proper lifting points)
• Apply the parking brake
• Remove the spare tire and tools from the vehicle
• Use wheel chocks to block the wheels opposite of the wheel you’re changing (i.e. if you’re changing a rear tire, then put the chocks in front of the front wheels).
  • Wheel chocks are similar to triangle-shaped door stoppers. When chocks are included with the tire changing kit, then they should be used. However, they are not present with every vehicle. Cases where they’re not present may include vehicles with a rear parking brake and front wheel drive. When changing a rear wheel in these vehicles the front wheel drive keeps the vehicle stable, and when changing a front wheel in these vehicles the rear parking brake keeps the vehicle stable.
• Loosen the lug nuts before lifting the vehicle, but do not remove
• Pump or crank the jack to lift the vehicle using the proper lift points (see Lesson 1, Safety, proper lifting points)
• Remove the lug nuts
• Remove the flat tire
• Place the spare tire
• Replace the lug nuts snug
• Lower the vehicle
• Torque the lug nuts (tighten them as much as possible after the vehicle has been lowered)
• Replace the tools in the appropriate location for future use.
• Carry the flat full-size tire to the tire shop with you.

Spare tires are often smaller than the full-size tires that are regularly driven on. Spare tires are only meant to transport the vehicle to a tire shop when needed and should be changed as soon as possible. They should not be driven on regularly.

Tire Disposal

Any time you purchase a tire, the tire shop is responsible for charging a disposal fee and disposing the old tire properly. If for some reason you have to dispose of the tires yourself, you can take them to the city dump. The dump may charge you a fee. You can try taking it to a tire recycling facility where you might be able to dump them for free or even get paid for them.

Lesson 4: Tires is one of twelve lessons that are also available as a free ebook download which is complete with illustrations. These lessons are based on the outline for the Boy Scout Automotive Maintenance merit badge and adapted by Crawford’s Auto Repair for a general audience. This article is for informational purposes only and the author does not assume responsibility or liability for any accident that may occur when working on motor vehicles. By reading this page you accept the condition that you are ultimately responsible for your own actions.

Copyright © 2014, Jeff Crawford. Permission is granted to republish this article for personal or commercial use as long as the content, citation, and copyright notice all remain intact and unchanged. There must be an active “follow” link to CrawfordsAutoService.com.

Lesson 4: Tires is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.