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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.
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).
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