Tire Flatspotting

Do you ever feel a ride disturbance or shimmy during the first few miles of driving after your vehicle has been parked for a few days, weeks or months? Then, after you drive a couple of miles, the ride smoothes out and feels OK. This condition is often called flatspotting because it is used to describe the tire flatspots that can occur when a vehicle is parked.



Many heavy duty, high performance, high speed rated and racing tires have a memory because they continue to remember the position in which they were last parked after they begin to be driven on again. Unfortunately, their memory can become a problem when the tires experience big swings in ambient temperature, have been parked overnight in cold temperatures, or parked for an extended period of time...because it's a lack of use that can cause tires to flatspot.



As they roll, tires go from a relaxed state to a loaded state about 800 times every mile. This constant

deflection generates heat that makes the tires more flexible. But once they are parked, the spot in contact with the ground (the tire's footprint) flattens as it is pressed against the road's flat surface as the tires cool. This is what generates flatspots. And until the tires "warm up" again, the flatspot on each tire can cause a ride disturbance that will be felt for the first few miles the next time the vehicle is driven.



Flatspotting can be temporary (the tire will round out as driving warms it up) or in the most severe cases, permanent (in which the tire's memory effectively destroys its ride quality). A flatspot's severity is often a function of the tire size, internal structure, load, ambient temperature and time.



Low aspect ratio tires have less sidewall flex due to their short sidewalls and much of their load carrying capacity is absorbed by the deflection of their wide footprints.



The tread compounds and firm, nylon reinforced internal constructions used on high performance and high speed rated tires are more susceptible to flatspotting.



Heavy loads and too little air pressure in the tires (underinflation) will allow them to deflect more where they come into contact with the ground. This allows even more deflection, increasing the severity of the flatspotting.



Cold ambient temperatures make rubber compounds stiffer, increasing their tendency to flatspot.

The longer tires remain stationary, the better they remember the position in which they were last parked. Tires on vehicles stored on the ground for many months can be permanently flatspotted.



Minimizing Flatspotting



While there is no way to completely avoid tire flatspotting, knowing what to expect under different conditions will help minimize its inconvenience.



NOTE: It is important to check and reset tire inflation pressures to those recommended by the vehicle manufacturer on the vehicle placard or owners manual when taking a vehicle out of storage.



Tire flatspotting would be most noticeable when beginning to drive a vehicle that has been stored incorrectly (with the weight of the vehicle pressing down through the tires to the ground). When storing a vehicle for more than a few weeks, it is best to drive the vehicle until it is thoroughly warmed up and then immediately put it up on "blocks" after arriving at the storage location. Doing this takes the load off of the tires completely. Not doing this on a vehicle that will be parked for a few months runs the risk of permanently flatspotting the tires.



Tire flatspotting may also be noticed when beginning to drive a vehicle that has not been driven for a few days, or during the colder winter months after the vehicle has been parked overnight. However, these types of flatspots will usually disappear during the first few miles of driving.



Usually during the day, the warmer ambient temperatures and more frequent vehicle use will not allow noticeable flatspots to form. However, anytime a vehicle goes in for ride-related services (tire rotation, rebalancing, or to diagnose ride disturbances), the vehicle should be driven for 5 to 10 miles immediately before being raised in the shop to make certain that temporary flatspots are not preventing the source of the ride complaint from being isolated and corrected.



And finally, tire flatspotting will also be noticed at the beginning of each session when attending a driver's school, track day or race. Whenever the car is returned to the paddock, the vehicle should immediately be lifted off of the ground to prevent flatspotting (this will also allow the car to feel more stable at the beginning of the next track session). This practice also allows debris to be cleaned off of the hot tires while they are inspected for any punctures and cuts. If you watch the professional race teams at an event, you'll see that they always remove the race tires immediately after stopping in the pits at the end of a session (if they plan to continue using the tires)

Tire Rack's Recommended Installers are your installation solution. You can drop-ship to their location, too!

Sidewall Separations/Bubbles

Pneumatic tires are made of specialized rubber compounds reinforced by plies of fabric cords and metal wires. While most rubber compounds can be stretched easily, the underlying fabric cords and steel wires actually define the tire's shape by limiting stretching. In order to bond these dissimilar materials, the cords and wires are coated with adhesives and/or rubber before the other components are bonded to them during curing.

A strong bond between these various components is necessary to provide the desired durability. However the strength of the bond can be reduced if 1) any of the components are contaminated during manufacturing (resulting in incomplete bonding), or 2) components are damaged in service due to use while overloaded/underinflated, or by impact with potholes, curbs or other road hazards that pinch the tire between the rim and the road, or simply stretch the rubber beyond the elastic limit of the underlying cords and wires.

 

 

Past experience indicates that a sidewall separation/bubble caused by component contamination or incomplete bonding during manufacturing will appear within the first six months of service. Fortunately these separations/bubbles typically appear when they are small in size and before the tire's strength is substantially reduced. However since typical tires roll about 800 times every mile and the air pressure inside the tire is greater than outside, tire separations/bubbles that are unseen or ignored will continue to grow in size, further reduce strength, often generate noise and vibration, and ultimately lead to tire failure as the tire stretches under load (similar to the way that continuously bending a paperclip back and forth will cause it to weaken and break). However there is one last thing to remember; while a separation/bubble early in a tire's life is usually associated with a manufacturing condition, even a single, significant impact with a deep pothole or sharp curb can cause a new tire and wheel to be damaged.

If the sidewall separation/bubble appears after six months of on-vehicle service, prolonged driving on overloaded/underinflated tires or a road hazard are the most likely causes. However it may take weeks or months after an impact for a separation/bubble to appear as the damaged or bruised area continues to weaken. Unfortunately the time differential between the impact that caused the initial damage and the delayed appearance of visible evidence often means that the driver has forgotten about the impact that damaged the tire in the first place.



The varieties of possible causes make it necessary to inspect the tire while mounted on the wheel (sometimes the wheel will show impact damage adjacent to the separation/bubble), as well as to dismount the tire and inspect the condition of its innerliner thoroughly. Sometimes it is necessary to return a tire to the manufacturer's inspection center where it can be dissected before the actual cause can be determined.



While taller profile tires can be damaged by the more severe impacts with deeper potholes and sharper curbs, low profile tires mounted on large diameter wheels are the most susceptible to this type of damage. The driver of vehicles equipped with low profile tires should make special efforts to avoid potholes, curbs or other road hazards.

Tire Rack's Recommended Installers are your installation solution. You can drop-ship to their location, too!

Determining the Age of a Tire

When it comes to determining the age of a tire, it is easy to identify when a tire was manufactured by reading its Tire Identification Number (often referred to as the tire’s serial number). Unlike vehicle identification numbers (VINs) and the serial numbers used on many other consumer goods (which identify one specific item), Tire Identification Numbers are really batch codes that identify the week and year the tire was produced.



The U.S. Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) requires that Tire Identification Numbers be a combination of the letters DOT, followed by ten, eleven or twelve letters and/or numbers that identify the manufacturing location, tire size and manufacturer's code, along with the week and year the tire was manufactured.



Tires Manufactured Since 2000



Since 2000, the week and year the tire was produced has been provided by the last four digits of the Tire Identification Number with the 2 digits being used to identify the week immediately preceding the 2 digits used to identify the year.



Example of a tire manufactured since 2000 with the current Tire Identification Number format:



DOT U2 LL MMLR 5107

DOT U2 LL MMLR 51XX Manufactured during the 51st week of the year

DOT U2 LL MMLR XX07 Manufactured during 2007



While the entire Tire Identification Number is required to be branded onto one sidewall of every tire, current regulations also require that DOT and the first digits of the Tire Identification Number must also be branded onto the opposite sidewall. Therefore, it is possible to see a Tire Identification Number that appears incomplete and requires looking at the tire’s other sidewall to find the entire Tire Identification Number.



The use of a partial Tire Identification Number on the one sidewall reduces the risk of injury to the mold technician that would have to install the weekly date code on the top sidewall portion of a hot tire mold.



Tires Manufactured Before 2000



The Tire Identification Number for tires produced prior to 2000 was based on the assumption that tires would not be in service for ten years. While they were required to provided the same information as today’s tires, the week and year the tire was produced was contained in the last three digits. The 2 digits used to identify the week a tire was manufactured immediately preceded a single digit used to identify the year.



Example of a tire manufactured before 2000 with the earlier Tire Identification Number format:



DOT EJ 8J FFM 408

DOT EJ 8J FFM 40X Manufactured during the 40th week of the year

DOT EJ 8J FFM XX8 Manufactured during the 8th year of the decade



While the previous Tire Identification Number format identified that a tire was built in the 8th year of a decade, there was no universal identifier that confirmed which decade (tires produced in the 1990s may have a small triangle following the Tire Identification Number to identify the decade).



And finally, hold on to your sales receipt. Most tire manufacturer's warranties cover their tires for four years from the date of purchase or five years from the week the tires were manufactured. So if you purchase new tires that were manufactured exactly two years ago they will be covered for a total of six years (four years from the date of purchase) as long as you have your receipt. If you lose your receipt, your tires' warranty coverage will end five years from the week the tire was produced (resulting in the tire manufacturer's warranty coverage ending only three years from the date of purchase in this example).

Tire Rack's Recommended Installers are your installation solution. You can drop-ship to their location, too!