It’s not uncommon for some of us to leave our cars parked for long stretches of time- sometimes spanning days, weeks or even months. If you’re one of these car owners, you may notice a slight ride disturbance or vibration the first time you drive your car, but it will usually disappear after a few miles of driving. This phenomenon is known as flat-spotting.
Flat-spotting occurs when the weight of a car presses down on the same section of an immobile tire for long enough, and under the right conditions, to change the tire's circumference from circular to less circular. It's more of a problem in cold weather, when rubber gets less pliable, and it's more likely to affect low-profile tires, and those with high-performance (cold-sensitive) compounds, like those on Challengers.
Tires are meant to be round, so obviously flat spots damage the tires' functionality. Tire flat spots are dangerous because one or more flat spotted tires can cause a shimmy or harmonic vibration that makes it difficult to steer your car after a few miles of driving. While short-term flat spots can work themselves out after they are warmed up, sometimes tires can develop a memory and won't return to shape.
From Rolling Tires to Flat Spots
As you drive your car, each tire rapidly rotates on the wheel from a “relaxed state” to a “loaded state.” This constant motion and friction with the road surface generate heat, which makes the rubber in the tires more supple – this is standard behavior for your tires.
Once you’ve stopped and parked your car, however, the contact patch of the tire can flatten slightly where it is in contact with the ground, as the tires cool down. It’s especially likely to occur when the tires experience a dramatic swing in ambient temperatures, the car is parked overnight in cold temperatures, or the vehicle is in storage for a long time. Another factor is tire pressure, and whether the tires are underinflated or overloaded.
There are two types of flat-spotting: temporary and semi-permanent. The severity of a tire flat spot will depend on factors like size, load, internal structure, the ambient temperature and the amount of time it is stationary.
Temporary vs. Semi-Permanent Flat-Spotting
In the majority of cases, flat-spotting is temporary. If a vehicle has been stationary for a few days or weeks drivers may experience some slight vibrations during the first few miles of their trip. The flat spots will disappear once the tires have reached their operational temperature and regained their normal shape.
Semi-permanent flat-spotting will occur if the tires have been standing still under vehicle load for a month or longer. A typical situation where this can happen is when you are storing your vehicle away for the winter months. The long standing times, especially in tandem with high temperatures and low tire pressure, can result in more severe flat-spotting that normal driving won’t remedy.
If you suspect that your tires have semi-permanent flat-spotting, you should contact your local tire specialist for options on how to fix it.
Tips to Prevent Temporary Flat-Spotting
By far, the most common cause of flat spots on tires is storage. If a car is left too long in the same place, the contact patch- the area of tire touching the ground- can become rigid. This is worse in cold weather and with under-inflated tires. Once you start driving again, if you have flat spots, you will likely notice a shimmy or vibration. Then, as the tires warm and the rubber "relaxes," the shimmy might be subdued or disappear altogether, but the tires are still unsafe, especially if they are not properly inflated.
How you ward off flat spots depends on what kind of tires you have, the space you have to store them and your car and how much effort you feel like putting forward.
First of all, tires should be stored clean. You might not see it, but dirt and brake dust on the surface can mess with the compound at a molecular level. A soapy bath and a good brush will get the rubber nice and clean.
Be sure to fill the tires to their correct pressures (Note- The proper psi. can be found on a sticker on the driver’s side door jam). Maintaining the correct tire pressure is an effective strategy to mitigate the chances of flat-spotting. Tires that are underinflated are more susceptible to a flat spot, and it’s generally advisable not to operate your tires when they’re underinflated or overloaded. Check them periodically over the winter, since pressure drops with temperature (i.e., a tire will lose 1 psi. for every 10 degree change in temperature).
If you’re about to undertake a long-distance trip, at high speeds with heavy loads, you should increase the inflation pressure of your tires in line with the vehicle manufacturer’s recommendations. (Tip- A slight increase of +0.2 bar in tire inflation pressure results in lower tire running temperatures. This decreases the likelihood of flat-spotting).
Similarly, if you’re about to park your car for a long time without driving, then slightly increasing the inflation pressure of the tires by +0.2 bar before parking will help to reduce the chances of flat-spotting. (Note- It is common practice to find new cars, on dealer lots, with 50 psi. in their tires).
Keep the tires out of direct sunlight to avoid damage. If your garage has windows, consider covering them.
If you want to be extra-careful, remove the wheels and tires and store them in a temperature-controlled area- basements are great. This is an extra step for most tires, but it's crucial for R-compound rubber, which is extremely sensitive to temperature change. The wheel-less car can either be set back down on a dedicated set of inexpensive storage wheels and tires, or you can put it up on jack stands for motionless safekeeping.
Jack stands also work if you don't feel like removing the wheels or don't need to because you live somewhere temperate or have a heated garage. With stands, there's no load on the tires, so no fear of flat-spotting. Be sure to place the stands securely under a solid mounting point, and use good judgement while lifting the vehicle.
If you do not want to use jack stands, or want to be able to move the car during storage, a good option is to use concave tire race ramps, like Flatstoppers. Once tires are set on Flatstoppers (on a level surface), they settle into the ramps’ depression, which helps support more of the tire. This evenly disperses the weight of the car across the increased contact patch and prevent flat spots. The pads measure 22.37" long x 3.5"high x 14" wide, and will accommodate tires up to 12" wide.
Flatstoppers are pricey, starting at $189, but for those interested, here is the company’s website:
Another solution that is better than nothing, is to just move the car occasionally, provided you have the room. Roll the car back or forth every couple of weeks so it's not sitting on the same patch all the time.
Locking Your Brakes
Locking your brakes up can also grind a flat spot in one or more tires. Of course, this happens much more often with race cars than street cars, but it happens on any car if you lock the brakes at speed. The instant your braking exceeds the traction limit of a tire and it begins to slide, it’s like running nutmeg over a grate. The instant the wheel begins spinning again, it stops. The difference is, with this kind of flat spotting, you haven't only changed the shape or your tire, but you've removed material from it. In that case, it might have to be replaced.
Like locking your breaks, you can flat-spot a tire by sliding your car sideways- not drifting the back end sideways with the wheels spinning, but actually sliding it sideways. It's essentially the same thing as locking your brakes. The tires are not spinning and they're grinding across pavement as if it was sandpaper. When racing or on the street, it's important to be aware of this kind of flat-spotting if you've just had a near-disaster and slid your car. If you're on the street, inspect your tires to make sure the flat spot didn't penetrate the tread. Proceed cautiously. If you're unsure, have them checked out at your local tire shop.