Technical questions

If you have a technical, application, pricing, or fitment question, or something that may fall between the cracks and just need an ear and an educated opinion? Call our sales/tech line at (213) 746-4040

 
Section 1 :Tire and Wheel Installation Instruction

Care and attention to detail are key concerns whenever dealing with the installation of custom wheels, especially alloy wheels. Taking the time to do the job correctly will greatly reduce the potential for customer complaints.

Preparing for wheel installation
NOTE: Prior to installing any wheel (steel or alloy), verify the condition of the fastening system’s threads (nuts and studs or bolts and hub threads). All threads should be clean and free of dirt, grease, grit, etc. If burrs or flat spots are found, replace the offending Lug Nut. Before attempting to install the wheel to the vehicle hub, first verify that you have the correct size Lug Nut, and check thread condition. Finger-install all Lug Nuts. Each Lug Nut should be able to be easily threaded into place without the use of a tool. If not, the thread size (diameter or thread pitch) may be incorrect, or thread damage may exist on either the stud and/or nut; or the wheel bolt and/or hub’s threaded hole. Resolve any Lug Nut issues before attempting to install the wheel(s).

Please be aware and to see if you have Asymmetrical or Directional tires. It is very important that they are going in the correct rotation. Asymmetrical or Directional tires will always have some type of indication to show which way the tire should rotate, this indication will be on the sidewall of the tire. Here are some samples that show the different types of indicators that tire manufactures use.

Sample 1	Sample 2	Sample 3

Once you’re satisfied with regard to the Lug Nuts, if this is a first-time installation (new aftermarket wheels), it’s a good idea to first test-fit the wheels before mounting the tires to the wheels. Make sure that no obstructions exist on the hub face that would prevent the wheel from flush-mating to the hub (check for OE stud clips, etc.).

Install the wheel to the vehicle hub, tightening at least three wheel Lug Nuts. There’s no need to tighten to full torque value, just make sure the wheel fits flush against the hub face. Check for disc brake clearance and for clearance between the wheel rim and any steering/suspension components.
Also check hub fit. Some wheels are designed to be centered to the hub by means of the bolt holes (lug-centric), while others are designed to center to the hub via the hub center-to-wheel hub center hole (hub-centric).
If the wheel’s center hole is larger than the outer diameter of the hub’s centering housing, a hub-centric ring may be needed to properly center the wheel to the hub. If you don’t see any hub-centric rings with your shipment call your Rushwheels.com salesperson and they will send them to you. Hub-centric rings are not always available for some of the older vehicles.
There’s nothing wrong with using a hub-centric ring. Some wheel makers may use the same center hole diameter to accommodate a variety of hub sizes. Using a centering ring allows a single center hole size to function on smaller hub diameters. Simply check this first to save time and aggravation. The rings, if needed, must be installed to the hub before the wheel is installed.

Wheel Lug Nut torque
Many folks don’t realize it, but all threaded Lug Nuts are intended to stretch slightly when fully tightened to specification. In the case of wheel studs and nuts (or wheel bolts), this creates the correct preload required to properly secure the wheel to the hub. If the wheel Lug Nuts are under-tightened, they will eventually loosen, resulting in wheel damage or separation from the vehicle. If the Lug Nuts are tightened beyond their design limit, the wheel stud or bolt can permanently stretch (fatiguing beyond its designed elastic range) or even break during installation. While the use of an impact gun (or the use of an impact gun equipped with a torsional wrench) may be tempting in terms of saving time, let’s face it: the only way to ensure correct clamping loads is by taking the time to tighten wheel Lug Nuts with the use of a quality and properly-calibrated torque wrench.

Never use an impact gun to tighten custom wheel Lug Nuts. Not only will you not be able to accurately control the level of Lug Nut tightness, but use of an impact tool can easily damage the Lug Nuts or the adjacent wheel surface. And when we’re dealing with attractive (and expensive) alloy wheels, such damage, even if only cosmetic, is simply unacceptable. Use sockets that fit the wheel!
Before attempting to engage a socket to the Lug Nuts, first check to make sure that the socket is clean to prevent damaging the Lug Nut finish. Also, verify that the socket of choice will comfortably fit into the wheel’s Lug Nut hole (in those cases where the Lug Nut sinks into a recess at the wheel’s bolt holes).

Using a socket that is too thick will cause the socket to jam and gall inside the recess, damaging the wheel finish (flaking off chrome, galling a powder coat finish, etc.). My preference is to set aside a dedicated set of thin wall sockets specifically for use on custom wheels. This provides a set of sockets that are kept clean, and that you know will not provide a too-tight fit into bolt hole recesses. The use of a pneumatic (or electric) impact gun offers the potential of damaging the Lug Nut’s exterior surfaces and/or the wheel’s Lug Nut hole area, in both removal and installation procedures. In a nutshell: If you’re dealing with alloy wheels, leave the impact gun on the bench. Here’s another tip: When tightening the wheel’s Lug Nuts, don’t make the mistake of finger-tightening, then lowering the vehicle to the ground to continue tightening. Instead of fighting vehicle weight, it’s best to perform your complete tightening procedure while the tire is off of the ground. If the mating surfaces (wheel to hub) aren’t fully compacted together, placing the weight of the vehicle against the tire results in then overcoming the resistance of the sidewall deflection (due to vehicle weight), which could possibly result in inaccurate torquing.
INSTALLATION TIP: In order to prevent the wheel from sticking to the hub in the future (when an aluminum wheel is mated against a steel hub, this can result in electrolysis), it’s a good idea to apply a thin coating of an anti-seize paste to the hub face where the wheel makes contact. A thin application of this compound will make it easy to remove the wheels in the future.

Re-torque
While opinions on re-torquing vary, the best suggestion is to re-torque all wheel Lug Nuts after the first 50 to 100 miles, specifically after installing new wheels.
This is especially true of alloy wheels, since the initial Lug Nut tightening may result in a slight compression of the wheel material (at the hub mating face).
If a bit of material compression occurs, this will directly result in a lower torque value at the Lug Nuts, decreasing the clamping load.
The best approach is to initially tighten all wheel Lug Nuts to specified value, drive the vehicle for 50 to 100 miles, and then re-torque the Lug Nuts.
When re-torquing, raise the vehicle to lift the tires away from the ground (removing vehicle weight). Loosen all of the wheel’s Lug Nuts (in a crisscross pattern), then re-tighten in the proper sequence to full specified torque value.

Wheel Lug Nut torque values
Always follow the vehicle maker or wheel maker torque specifications. Just remember that tighter is not necessarily better.
While you should always adhere to the torque specifications listed by either the vehicle make or the wheel maker, the box below offers a broad guideline of torque values for common wheel Lug Nut sizes.

Tightening sequence
Always tighten any wheel in the proper sequence pattern in order to evenly distribute the clamping load between the wheel and the hub. Especially considering today’s comparatively lightweight hubs and brake rotors, if the wheel Lug Nuts are tightened improperly (in terms of torque value and tightening pattern), the risk of creating a hub/rotor warpage increases, resulting in a brake pulsation. The goal in using the proper tightening pattern is to avoid concentrated areas of clamping force. You want to evenly distribute the clamping load across the hub surface.

Four-bolt hub
With the hub/wheel positioned so that one Lug Nut is at the 12-o’clock position, tighten the 12-o’clock position first, followed by the 6 o’clock position, followed by the 3 o’clock position, followed by the 9 o’clock position.

Five-bolt hub
With the hub/wheel positioned with one Lug Nut at 12 o’clock, tighten the 12 o’clock position first, followed by the 7 o’clock position, followed by the 2 o’clock position, followed by the 10 o’clock position, followed by the 5 o’clock position.
Basically, from the first Lug Nut, move to a Lug Nut that is furthest away from the first Lug Nut. Then move to a Lug Nut furthest away from that second Lug Nut, etc. Always move to the Lug Nut that is furthest away from the previous Lug Nut.

Six-bolt hub
The same rule applies. After tightening the first Lug Nut, move to a Lug Nut that is furthest away from that first Lug Nut, and so-on. Always move to the Lug Nut that is furthest away from the previously-tightened Lug Nut.
An example of a six-bolt pattern would be: With the hub/wheel positioned to place one Lug Nut at 12- o’clock, tighten the 12 o’clock position first, followed by the 6 o’clock position, followed by the 2 o’clock position, followed by the 7 o’clock position, followed by the 5 o’clock position, followed by the 10 o’clock position.

NOTE: For ideal clamping results, it’s best not to fully tighten the Lug Nuts in one step. Instead, tighten in at least two steps. For instance, if the specified torque value is 100 ft.-lb., tighten all Lug Nuts (in the proper sequence pattern) to an initial value of 25 ft.-lb. Then perform a second tightening to full value (using 100 ft.-lb. as our example). By tightening in multiple steps, you greatly reduce the chance of initiating excessive clamping force in isolated areas. Taking this extra care is but another way of increasing your chances of achieving an optimum wheel-to-hub clamping load.

Section 2: A Few Facts About Wheels
Here are a few measurements you will need before you buy wheels for your rod.

• Wheel Diameter
• Wheel Width
• Back Space (also consider tire bulge)
• Front Space (also consider tire bulge)
• Bolt Pattern
• Center Bore Diameter

In addition to the above measurements, it is recommended that you check the clearance on your existing wheels.  Areas to check are;

• Outside fender clearance
• Inside frame, fender braces and bumper brackets
• Suspension and Steering components (turn wheels with steering lock to lock)

Remember, always make sure the wheels are load rated to your vehicle.  Check the GAWR (Gross Axle Weight Rating) of the heaviest axle of your vehicle.  The wheel load rating is based on 50% of the heaviest GAWR of your vehicle.

Section 3: Measuring Your Bolt Pattern
Here is a simple but accurate way to measure the Bolt Pattern on your Rod.

• Remove the existing wheel (the rear wheel is usually the easiest to measure because the hub normally won’t be in the way).
• Use a ruler, or tape measure that shows 32nd’s of an inch.
• Measure from the inside of one stud to the outside of another stud (see diagram to the right) (4 lug and 6 lug use opposing studs; 5 lug use 2 studs next to each other).
• When measuring a 4 lug or 6 lug, the P.C.D. (pitch circle diameter) or bolt pattern diameter will be the distance measured.
• When measuring a 5 lug, use the distance measured then refer to the “5 Lug P.C.D. Chart. The chart shows the “Actual Dimension” in a 3 place decimal. The chart also shows the measurement to the “Nearest 32nd of an inch”.
• Find the measurement you took on the chart and that will indicate the Bolt Pattern of your vehicle.

In addition to the above measurements, it is recommended that you check both the front and rear to verify they are the same bolt pattern.
Remember, most wheel companies (and ours is one of them) will not accept returned wheels if they have had a tire mounted. Do a fit check on your vehicle to make sure the wheels fit before you have tires mounted.

Section 4: Bolt Pattern Conversion Chart (mm to inches)

Section 5: A Few Facts About Lug Nuts
Here are a few facts about wheel fasteners you should know before you bolt up your wheels on your ride.

• Matching your lug nuts or bolts to your wheels.

There are 3 basic type fastener seats;

• Conical Seat – cone shape
• Spherical Seat – round or ball shape
• Flat Washer – commonly known as Mag Type

Be sure the fastener seat matches the seat in the wheel.

• Proper Thread Size.

There are several different thread sizes used today by vehicle manufacturers;

• 12mm X 1.25
• 12mm X 1.5
• 12mm X 1.75
• 14mm X 1.5
• 14mm X 2.0
• 7/16" X 20
• 1/2" X 20
• 9/16" X 18

Two other things to remember; 1) do not put oil or lubricant on the threads of either the stud or lugs nuts/bolts, and 2) re-torque the lug nuts/bolts after 25-50 miles.

• Proper Thread Engagement.

This is critically important. Make sure you have a minimum thread engagement of the diameter of the vehicle stud (as recommended by SAE). An example is, if the stud size of your vehicle is ½" then you will need a minimum of ½" of threads into the lug nut. If for some reason you do not have this minimum then it is recommended that you use an ET Type nut (Extended Thread) (see illustration). ET Type nuts are useful when using spacers (that will be another subject in a future TECH Stuff).

• Proper Torque (tightening)

This is also very important. Over tightening lug nuts/bolts can fatigue the vehicle studs or lug bolts. Use the SAE recommended torque listed below as a guideline for passenger cars and light trucks;
12mm, 7/16", 1/2" = 85 ft/lbs (+/- 5 ft/lbs)
14mm, 9/16" = 115 ft/lbs (+/- 5 ft/lbs)

Section 6: Types of Aftermarket Wheels

There are several types of construction that are used to make wheels for the aftermarket.  The type of construction does not necessarily mean that one type is better than the other.  Some types of construction allow for more elaborate styling or finishing, others reduce weight, while others are for the purpose of duplicating the original equipment wheels.  In any case, the strength and safety aspects of the wheel are based on the design and manufacturing quality built in by the manufacturer.  Remember, all the wheels sold should be tested to a recognized specification or standard by the manufacturer to assure that they are safe and reliable regardless of what type construction they are.
The following are the most common types of wheels offered by the aftermarket wheel industry.

• Steel Wheels – This is where the aftermarket wheel industry got started.  They consist of a steel outer/rim and a steel center.  Chrome plating is very easy on this type because the rim and center are polished and chrome plated separately then pressed together and welded.  Painting is also easy but is done after the wheel is assembled.  Backspacing/offsets can be varied when the rim and center are pressed together.  Trim rings and hub caps finish off these wheels nicely.  Wire spoke wheels would fall in to this category but require much more maintenance in cleaning and runout (balancing). 
• Steel/Composite – This method of building wheels was started back in the 60's.  It consists of a steel rim and a cast aluminum center.  This allows for more styling in the center.  The aluminum centers are cast with steel cleats or inserts on the outer edges so it can be pressed into the steel rim and welded just like the Steel Wheels.  These wheels are offered primarily chrome plated.
• 1 Piece Cast Aluminum – This is now the most common wheel sold in the aftermarket.  It offers the most variety of styling, sizes and finishes.  These wheels can be painted, machined, polished or chrome plated.  They are offered in sizes from 13" to 26" (28" & 30" are in development).  These also offer a reasonable weight savings over Steel or Steel/Composite.  The disadvantage of the 1 Piece Cast is the backspace/offsets are fixed in the mold.
• Forged – This is the type that offers the lightest weight.  Because of the forging process, the wheel can be built using much less material than a 1 Piece Cast.  The styling is limited in comparison to a 1 Piece Cast and the tooling and manufacturing costs are much greater.  These too can be painted, machined, polished or chrome plated.  Because of costs the sizes offered are limited.
• 2 Piece Aluminum – Also known as Billet, this type of wheel has been around for about 20 years.  Like the Steel Wheel, it is made of a rim and center.  The rims are rolled or spun and the centers are cast, forged or machined billet aluminum.  The rim and center are pressed and welded together which also allows for a variation in backspace/offset.  Forged and billet aluminum centers are typically stronger than cast centers because of the density of the aluminum.  Finishing is limited to painting, machining or polishing.  Chrome plating is difficult because the welding has problems created by the copper, nickel and chrome from the plating process.
• 3 Piece Aluminum – These are also known as a Modular wheel.  The rim is made up of 2 pieces, the front rim section and the rear section which are either spun or formed.  The center can be of a cast, forged or machined billet type.  The 3 pieces are assembled with the center sandwiched between the front and rear rim sections with bolts or rivets securing the pieces together.  Styling is again limited, but finishing choices are like the 1 Piece Cast.

There are a few folks out there in the industry that will say wheels made by forging or from machined aluminum billet are stronger than cast wheels.  That would be true if both wheels were made to the exact same design and dimensions.  However, the strength or load carrying capacity and durability of a wheel comes from the design and manufacturing quality, based on the type of construction.  The biggest advantage gained from forging or billet is weight.  Because they are stronger, less material is needed.  Regardless if it is steel, cast aluminum, forged or machined from billet, the wheel should still meet performance testing standards to be deemed safe and reliable.

Section 7: Definition: Staggered Wheels

Staggered wheels refers to an arrangement of wheels on cars, trucks, and other vehicles in which the pair of wheels on one end of the vehicle are a different size or shape than the wheels on the other end. Usually, this means the back wheels are larger than the front wheels, often a couple inches wider across the tread or larger in diameter. This is done for practical and aesthetic reasons. Although larger rear tires (also called a staggered fitment or a staggered application) can help vehicles move over uneven ground, they also affect the build and frame design of the car, making it look sleeker or more powerful. Many

Section 8: The Scoop on Wheel Billet Adapters and Spacers

There are a lot of theories floating around about the use of wheel adapters and spacers and their safety.  Here are a few facts and thoughts that will help you make your decision whether to use them or not.
Wheel Adapters
There are 2 primary uses for adapters:

• The first and most common use is to change the bolt pattern.  Adapters are available that will change a 5-4.50" to a 5-4.75" or to a 5-5.00", or the other way around.  There are several companies that make adapters that can offer about any combination within reason.  There are also adapters that will change a 4 hole pattern to a 5 hole.  Changing a 5 hole to a 6 hole is pretty much out of reason.  The drawback to using adapters is the thickness required to make them work properly.  The minimum thickness required is about 1.125" (depending on the length of your studs).  This will move your mounting surface out 1.125" and could cause some fender clearance problems. 
• The second use is if you need to move your wheels out from the existing mounting surface.  Adapters are available that have the same bolt pattern (i.e. 5-4.50" to 5-4.50").  Adapters are the best way to accomplish moving your mounting face out.  Adapters can allow you to move the mounting face from 1.125" up to 2.500".  Anything over 2.500" would require a little engineering, design and load calculation.

How they work:
Adapters are machined with a P.C.D. drilled with lug holes (60 deg. Conical Seat) to accept an open end lug nut.  There are studs inserted in another P.C.D. between the lug holes (see drawing).  The adapters are installed on the vehicle with lug nuts and tightened to the proper torque. Make sure the vehicle studs or lug nuts do not extend past the mounting surface of the adapter.  The adapter becomes an extension of your original mounting surface.  Then mount your wheel and tighten it to the proper torque.
Spacers:
Spacers have gotten a bad rap over the years.  Some of the stuff is well deserved, but for the most part, spacers are not a bad thing and if used properly  are a very useful tool to make your wheels fit properly.  A spacer over .313" (5/16) is not recommended.  That makes it tough when you need to move the wheel out from .313" to 1.125".  That's no man's land (between a spacer and an adapter).  Oh well.  There are 2 basic rules to consider when using spacers;

• Always make sure you still have proper thread engagement of your lug nut and stud (TECH Stuff 4) after you have mounted the wheel on the vehicle with the spacer.
• Choose a spacer that has a lot of mounting surface.  There are spacers being offered that are a "fit all" (4, 5 & 6 hole) type.  These look like a piece of swiss cheese and work about as well.  Also make sure the material is adequate (billet aluminum is best) (pot metal is worst).

Adapters and Spacers can be very handy tools that can help you get just the look you're trying for with the wheels on your Rod.  When used properly, they are safe and reliable.

Section 9: Proper Care of Your Wheels
There are several different types of finishes available for Aftermarket Wheels. Most wheel companies offer a warranty on the finish. In most cases, painted wheels have a 1 year warranty. Most chrome wheels have a 2 year warranty, and in some cases a 3 year warranty. Polished aluminum wheels are only warranteed until you take them out of the box. In any event, the buyer has the responsibility to properly maintain the wheels. There are a lot of folks out there that pay a lot of money for custom wheels then only clean them when they send the car through the car wash once in awhile. It takes time and elbow grease to keep custom wheels clean and to protect the finish. I've seen chrome wheels that customers wanted to return because they were pitted or rusting so bad that they look like the car was parked on the bottom of Lake Michigan for the winter. If you don't maintain your wheels, don't expect the manufacturer to take them back and give you a new set.
Let's cover the 3 types of finishes and how to maintain them.

• Painted Finish – There are 2 types of paint used. First and most popular is Powder Paint. This is a paint that is applied in a powder form then baked at about 400 degrees. The powder actually melts and becomes liquid for a short time then adheres to the wheel and creates a very hard surface with excellent coverage. The second type is Liquid Paint. It is sprayed in the conventional method and then baked in an oven. Liquid paint normally results in a better finish with little or no orange peel effect. Most high quality painted wheels are powder painted with the color and then liquid painted with a clear coat that gives the overall finish a lot of depth.
  
  Maintaining Painted wheels can be accomplished with soap and water and a soft brush or cloth. Make sure that the brake dust is rinsed away prior to using a brush or cloth because brake dust is very abrasive and can put small scratches in the finish. Be very careful when sending your vehicle thru a car wash because they use coarse bushes to scrub the tires and they can get into the wheels and chew them up in a flash.
• Chrome Plated – Chrome wheels are by far the easiest wheels to clean, if they are done frequently. If they are not at least wiped clean with a damp cloth every 2-3 weeks, they accumulate road dust, brake dust and what ever else is out there, and then it requires chrome cleaner to remove the mess that hides the shine. Chrome plating is a very tough and resistant finish, but it is not impervious. Steel wheels that are chrome plated can still rust if they are subject to a lot of moisture or constant moisture. Chrome plated aluminum wheels can oxidize from under the plating and cause pitting if not properly maintained. The new technology used in chrome plating uses a 4 layer system. First the wheel is plated with copper as a base and filler. Then a semi-bright nickel is applied, followed by a bright nickel layer. The final layer is the chrome. The chrome is the thinnest of the layers but the hardest and the most resistant to the outside environment.
  
  Maintaining chrome wheels can be done with soap and water and a soft brush or cloth. Again make sure to rinse away any brake dust prior to using a brush or cloth. There are several chrome cleaners and polishes available to really put a shine on your wheels after washing them with soap and water.
• Polished Aluminum – Owning and maintaining polished aluminum wheels is a real test to ones devotion and love of his ride. After the wheel has been polished, that's it. There is no protective coating put on the wheel. Aluminum oxidizes and dulls after a time. The length of time depends on where you live, and how much moisture is in the air. The dryer the climate the longer the finish will stay bright.
  
  Maintaining polished wheels has only 2 requirements. Elbow grease and a lot of time. I will admit that a highly polished set of alloy wheels looks really bitchin. I prefer chrome because I don't want to work that hard and I'm too old to get down on my knees for that long, and it's too hard to get back up. Again there are several aluminum polishing products available and they pretty much all work. That is with you supplying the muscle.

There are a few other finishes that I didn't cover that should be mentioned. One being a machined finish aluminum wheel. Maintenance is accomplished with soap and water and a soft brush or cloth. Then there is a machined finished aluminum wheel that has a liquid clear coat finish applied after the machining. Maintenance and care would be the same as a painted wheel.
Remember, care and maintenance is your job not the manufacturer. Don't use harsh chemical cleaners and be careful of products that say "just spray on and rinse off". They most likely have some acid or etching chemicals that will destroy your wheels finish

Section 10: Formula for Calculating Tire Dimensions
FORMULA FOR CALCULATING TIRE DIMENSIONS

USING SIZE 235/35-19 FOR EXAMPLE

The dimensions of a 235/35-19 tire are
9.3” wide (section width) & 25.5” tall (outside dia)
The section width is 235mm wide
(There are 25.4mm to an inch)
STEP 1) 235mm divided by 25.4” = 9.25” (section width)
STEP 2) Multiply 9.25 (section width) by .35 (aspect ratio)
9.25 x .35 = 3.24” (section height)
To find the outside diameter
Multiply 3.24 (section height) x 2 then add rim diameter
STEP 3) 3.24” x 2 = 6.48” + 19.0” = 25.48” round out to 25.5”  (outside dia)

Section 11: Tire Pressure Monitoring Systems (TPMS)

There are two basic types of Tire Pressure Monitoring Systems (TPMS) currently available that can alert the driver while driving that the tire pressure is low: direct measurement systems and indirect measurement systems. A direct measurement system measures tire pressure directly. A variation of the direct measurement system (a direct measurement system with a pump) will soon be available that can inflate the tire when it gets low, relieving the driver of that responsibility.
An indirect measurement system measures wheel speed or something factors other than tire pressure. Most current ABS-based systems are indirect measurement systems. They measure wheel speed and then compare the variance in wheel speed from one wheel to another to determine whether a tire is under-inflated.
Although not currently in production, we believe that it would be possible to produce hybrid TPMSs with performance characteristics of both direct and indirect TPMSs.

 
Direct measurement systems
Most direct measurement systems have pressure and temperature sensors in each tire, usually attached to the inflation valve. They broadcast their data to a central receiver, or in some cases to individual antennae that transmit the data to the control module, which analyzes them and sends appropriate signals to a display. This display can be as simple as a single telltale, or as complex as pressure and temperature displays for all four tires (or five if the spare is included).
Direct measurement systems advantages include: (1) much greater sensitivity to small pressure losses, with claims ranging from +/- 0.1 psi to 1 psi; (2) the ability to directly measure pressure in any tire at any time, including before starting the vehicle, and including the spare tire. The disadvantages include: (1) the higher cost; (2) possible maintenance problems when tires are taken on and off the rim (sensors have been broken off). These systems have not been installed on many vehicles, although they have been used on cars with run-flat tires and as accessories on high-end luxury vehicles.

 
Direct measurement system with a pump
A direct measurement system with a pump has the same qualities as a pressure-sensor-based system, except that it also has the ability to pump the tire back up to the placard tire pressure. Each tire has a separate sensor and a pump. The system display is designed to give a warning when a particular tire needs to be continuously inflated and if the tire pressure gets too low, indicating that a particular tire has a problem and needs servicing. Unless there is a catastrophic failure or a rapid loss of pressure due to a nail or puncture, the pump can keep the tire inflated to get the vehicle to its destination. However, once the vehicle stops, the pump stops, and the tire may deflate. The advantages of these systems include: (1) driver convenience, (only need to worry about tire inflation when a warning of a continuing problem that the pump has to continue working to control); (2) better fuel economy, tread wear, and safety by keeping tires up to correct pressure. The disadvantages include: (1) the higher cost; (2) maintenance considerations - when rotating the tires, the pumps must stay on the same side of the car. These systems have not been installed on any light vehicles, although they have been used on a number of heavy trucks for several years. Because of cost issues, a direct measurement system with a pump has not been considered in further analyses.

 
Indirect measurement systems
The current indirect measurement systems utilize the wheel speed sensors of Anti-lock Brake Systems (ABS). They take information from the ABS wheel-speed sensors and look for small changes in wheel speed that occur when a tire loses pressure. Low pressure results in a smaller wheel radius, which increases the speed of that wheel relative to the others. The systems work by comparing the relative speed of one tire to the other tires on the same vehicle.
The advantages for these systems include: (1) low costand (2) the need for only minor changes to the vehicle that has an ABS system, including a new dashboard telltale and upgraded software in the electrical system. Disadvantages include: (1) not all vehicles have ABS, so costs are significantly higher for vehicles without ABS; (2) the indirect system cannot tell which tire is underinflated; (3) if all tires lose pressure evenly, it cannot detect it, since it works on the relative wheel speed; (4) in some current systems, some combinations of two tires being underinflated cannot be detected (e.g., two tires on the same axle or the same side of the vehicle). (Regarding #3 and 4, current ABS-based systems cannot detect certain conditions of low tire pressure. To meet the proposal, the ABS-based systems would need to be improved.) (5) they cannot check the spare tire; (6) the vehicle must be moving; (7) they require significant time, sometimes hours, to calibrate the system and several minutes, sometimes tens of minutes, to detect a pressure loss; and (8) they cannot detect small pressure losses. (Regarding #8, the best claim is that they can detect a 20 percent relative pressure loss differential between tires, but others state they can only detect a 30 percent loss, e.g., a tire properly inflated to 30 pounds per square inch (psi) would have to deflate to 21 psi before the system would detect it.) (9) some systems cannot detect a pressure loss at vehicle speeds of 70 mph or higher.

 
Hybrid measurement systems
The agency believes that an indirect measurement system supplemented with direct tire pressure measurement in two wheels and a radio frequency receiver, a "hybrid" system, could meet the proposal. This system was first discussed by TRW in its docket comment. To date, no such systems have been produced.

 
ALTERNATIVES
In contrast to the June 5, 2002, final rule the agency is not proposing alternative levels of stringency. The proposal is that the driver must be given a warning when tire pressure is 25 percent or more below the placard pressure for one to four tires, or when tire pressure is at or below the defined minimum activation pressure (MAP).
The MAP presented in Table II-1 shows the level at or below which the warning must be activated. The floor is different depending upon the tire type. All tires are required to have a single maximum inflation pressure labeled on the sidewall and that pressure must be one of the values indicated in the table. If a vehicle has p-metric tires marked 240, 300, or 350 kPa, it is a standard load tire that will be tested at 25 percent below placard, or 140 kPa, whichever is higher. If a vehicle has a p-metric tire marked 280 or 340 kPa, it is an extra load tire that will be tested at 25 percent below placard, or 160 kPa, whichever is higher. (Extra load tires are marked "XL" or "extra load" on the sidewall). LT-tires on light trucks have higher maximum inflation pressures and, therefore, have been assigned a higher floor below, which the warning has to be activated. The values in Table II-1 are the only values that can be used for maximum inflation pressure.

* The standard is based on kPa, the psi values have been rounded to the nearest whole number.
Currently, the lowest P-metric tire recommended placard pressure is 26 psi. At 26 psi recommended placard pressure, the 20-psi floor would come into play.
The rationales for the minimum activation pressure are:
A 20 psi floor for p-metric tires is proposed because the agency believes that below that level, safety in terms of vehicle handling, stability performance, and tire failure is an issue. The agency ran a variety of p-metric tires in what it calls a "low pressure endurance test" at 20 psi with a 100 percent load at 75 mph for 90 minutes on a dynamometer. None of these tires failed. In a second set of test it calls a "low pressure high speed test" at 20 psi with a 67 percent load for 90 minutes, in 30 minutes steps at 140, 150, and 160 km/h (87, 93, and 99 mph), about 30 percent of the tires failed. Since tires could pass the "low pressure high speed test" at 20 psi, this leads the agency to believe that there will be a safety margin, in terms of tire failures, if a TPMS warning is provided at or above 20 psi, that will allow consumers to fill their tires back up before the tire fails, unless the vehicle is driven at very high speeds (above 140 km/h or 87 mph).
The lowest inflation pressure used in the 2000 Tire & Rim Association Yearbook is 140 kPa (20 psi) for P-metric tires. In the 2001 Tire & Rim Association Yearbook, the 140-kPA pressures have been deleted, apparently because the Association believes they are too low for P-metric tires. The agency agrees that 140 kPA is too low and believes a floor is needed to assure that drivers are warned when tire pressure gets to or below that level. For the LT tires, we used the 2000 JATMA yearbook for the lower limits for Load Range C, D, and E tires. For most cases, the floor is about 58 percent of the maximum inflation pressure.

Section 12: Tire Glossary
Adjustment - An allowance given to the customer to be used toward the replacement of a tire because of warranty or service issues.
Alignment - The checking and adjustment of camber, caster and toe angles of the vehicle’s suspension to maintain proper steering and handling of the vehicle for maximum performance.
Air Pressure - Force exerted by air within a tire, expressed in pounds per inch in USA and Bar in Europe.
Aspect Ratio -A numerical term which expresses the relationship between the standing height of the tire and the cross section width. (Aspect Ratio of 70 means the tire section stands approximately 70 percent as high as it is wide between the sidewall.)
Balance -The equal distribution of the mass of the tire and wheel assembly for smooth driving. Balance is achieved by fitting weights to the wheel rim to off-set uneven weight distribution of the tire, wheel or brake assembly.
Bead -The part of the tire that is shaped to fit the rim at the rim flange and the bead seat. Made of high tensile steel wires that are wrapped in woven fabric and then held by the plies and molded rubber. On clincher tires or (beaded edge tires) the bead is a hard rubber core reinforced by cord material in a specialized shape.
Bias Tires - Pneumatic tires having crossed layers of ply cord (usually two, four or more) running diagonally from bead to tread.
Bias Belted Tires - These tires have a body similar to that of bias tires, with the addition of two or more belts under the tread to strengthen and stabilize the tread. The belts improve tread life by reducing tread movement during contact with the road.
Bolt Pattern -The pattern of holes on a rim or wheel which the bolts that mount the wheel to the vehicle are affixed.
Camber - The measurement of tilt of the tire and wheel assembly on the front end of the vehicle off vertical measured in degrees.
Carbon Black - Substance used in tire manufacturing as a reinforcing material in the rubber mixing process which gives the rubber a higher resistance to premature wear.
Clincher tires - Early type of tire construction (as used on Ford Model T pre-1926). Clincher beads have a hard rubber core reinforced by cord material in a specialized shape with no steel in the bead. Air pressure is very critical to proper fitment of these tires. These tires are also referred to as Beaded Edge tires.
Contact Patch - The area of the tire that comes in contact with the road.
Cord - The twisted fiber or filament of polyester, rayon, nylon or steel which gives the tire body and belts strength.
Cross Section - The linear distance between the exterior sidewalls of an inflated tire at the maximum width.
Crown - The center area of a tire’s tread footprint.
Detachable rim - Early style automobile rim permanently fixed to the wooden wheel. This type of rim has one or two side rings and possibly a separate lock ring to secure the tire assembly on the rim (see diagram on page 49 under Stanweld rim).
Demountable/Detachable rim - Same as above except this rim removes from the wood wheel and is secured to the wood wheel by a series of lugs and lug bolts and clamp rings. (See diagram on page 49 under Stanweld rim)
DOT Number - Number molded onto the tire indicating the Manufacturer and date of Manufacture as registered with the United States Department of Transportation. A tire with a branded number near the DOT number classifies the tire as a retread. Responsibility passes to the Retreader.
Footprint - The area of the tire tread that is actual contact with the ground or road surface.
Drop center - The drop portion of a rim cross section between the bead seats or rim flanges in which a bead is placed during the mounting process.
ECE Number - ECE (Economic Commission of Europe) developed testing standards for vehicles and tires which apply to dimensions, loads, tire markings and speed ratings.
ETRTO - European Tire and Rim Technical Organization.
Green Tire - A tire that has not yet gone through the process of vulcanization or curing in the tire molding process.
Gross Vehicle Weight - Total weight of a vehicle including all fluids, cargo and passengers.
Inner Liner - The layer of rubber which is laminated to the inside of a tubeless tire to insure the air retention quality of the tire body.
Hub centric - Wheels manufactured to match and true the diameter of the bead seats off of the center hole of rim.
High Pressure tire - 100 percent aspect ratio tire with cross section equaling section height. This type tire was used on example would be 35x5. The height of the tire should be 35 inches, the cross section should be 5 inches on a 25 inch rim.
Kilopascal - Metric unit for air pressure specifically in European use tires. One psi equals 6.9 kPa.
Lateral runout - Side to side movement of a wheel assembly.
Load Carrying Capacity - Maximum load for which the tire is designed under standard conditions.
Load Index - Relative load carrying capacity as originated by European community, refers to the maximum load carrying capacity in kilograms and/or pounds.
Load Range - A system of designations, which identifies the carrying capacity range of a tire. These markings shown on the tire indicate the ply rating established for the tire such as B (4-Ply rating), C (6-Ply Rating) and D (8-Ply rating). This system was established by the Rubber Manufacturers Association.
Lug centric - Wheels manufactured to match and true the bead seat diameters off the lug holes.
Matched slicks – Rear drag tires that when inflated to recommend PSI, have matching circumferences.
Matching balance system - A computer balancing system used to match the high point of a rim to the low point of a tire diameter, creating a more perfecting round total assembly.
Maximum Inflation Pressure - The maximum air pressure in kpa/bar or psi that a tire can be inflated while cold. Usually found on sidewall of tire.
M+S - The designation of a tire that meets the requirements given by RMA to be used for all seasons.
NHTSA - National Highway Traffic Safety Administration (www.nhtsa.dot.gov).
Overall diameter - The diameter of an unloaded inflated tire, measured from the crown on one side to the crown on the opposite side. The free radius equals one-half the overall diameter.
Overinflation - The condition that exists when a tire is inflated beyond the pressure corresponding to the actual load or beyond the vehicle manufacturer’s recommendation.
P-Metric - A tire sizing system where the section width is shown in millimeters, the aspect ratio, type of construction and finally rim diameter are shown in inches example; (P205/75R15).
Plus One - From original equipment size tire and wheel, choosing to go up in rim diameter one inch while keeping the same overall diameter. Usually always adds width to rim and tire and decreases sidewall section height. Key is to keep same exact diameter to keep rolling circumference same.
Ply - A layer of rubber-coated parallel cords forming the tire body or carcass.
Polyester Cord - A synthetic fiber that excels in maintaining strength properties at high heat levels, and eliminates flat spotting.
Post Inflation - When tires are removed from the mold after the curing process, they are mounted on a rim and inflated to their proper inflation, during the cooling period, to allow the tire to cool keeping its correct shape and size.
PSI - A measure of air pressure commonly used throughout the world signifying pounds per square inch.
RMA - Rubber Manufacturers Association. (www.rma.org).
Radial Ply - A tire with cords running radially from bead to bead (90 degrees to centerline of the tire). Radials have the addition of one or more belts under the tread to strengthen and stabilize the tread.
Radial runout - Up and down movement of a wheel assembly.
Rayon - A fiber used in tire construction made from cotton or wood pulp by chemical process.
Ribs - Part of the tire tread design created by grooves running circumferentially around the tire.
Revolutions Per Mile - (R.P.M.) - The number of revolutions that the mounted tires will make in one mile, at rated load and inflation.
Rolling Resistance - The resistance of a tire to free rolling.
Roll out - Term used for drag tire circumference, and is the number of inches the tire rolls before it has traveled back to its initial starting point.
Section Height - The height of a tire measured from the rim to the outside measurement of the tread.
Section Width - Measurement of distance through cross sectional width of a tire at widest part, exclusive of scuffing rib when inflated to normal pressure and not under load.
Shoulder - The edge of a tire’s tread where the tread joins the sidewall.
Single Tube Tire - This is one of the earliest types of tire constructions as used on early automobiles, motorcycles and bicycles, where the tire and tube are made together and are fitted onto a rim with a semi-circle cross-section. The cord angles of the construction allow for the tire to tighten on the rim as it is inflated. These tires often have lugs for securing to the rim. They are often glued on the rims as well.
Sipe - To cut across a tire tread to produce biting traction edges. Also refers to the thin slots in the tread of a tire designed to aid in wet traction and water removal.
Speed Rating - A speed designation of S, T, H, V or Z shown in the size marking of some tires.
Synthetic Rubber - Rubber made by man using chemicals as opposed to natural rubber harvested from trees. Most tires today are made of this type rubber.
Tire Storage - Tires should be stored horizontally