Can you describe how this affects the handling? Steering wheel vibration, pulling or something else?
Normally I associate flat spotting with localised rubber wear when you lock up and the tyre slides and it is permanent.
Am I right in thinking your problem may be tyre deformation? I would be interested to see the solution.
My car was in storage for 1.5 year (on concrete floor) and I experienced wheel vibration after that. Had them balanced few times and eventually changed the tyres. Problem is still there but a lot less pronounced. What is strange is that it happened within a range of speed (75~85) and sometimes when accerlerating or decelerating but not both
which make me think maybe the drive trains (4x4)
You are correct that the term "flatspot" is frequently used to describe the tire wear you get when you lock up a brake. Technically, that is called "Spot Wear". (See the Tire Industry Association's manual: Passenger and Light Truck Tire Conditions Manual
But the term also applies to a physical deformation where the part of the tire that was in contact with the ground while the vehicle was parked doesn't completely return to the "round" condition when put back in service. This can be a temporary or permanent condition.
The symptoms are a vibration, that generally goes away after a couple of miles. If it doesn't, the flatspot could be permanent, or the vibration could be coming from something else.
Please note: Wheel end vibrations tend exhibit themselves in 50 to 70 mph range (80 to 115 kph). That's because a wheel end vibration tends to excite the resonant frequency of the suspension. (See Wikipedia: Resonance
) and that generally occurs in the 50 to 70 mph range.
Cause: Most common is the fabric reinforcement's glass transition temperature - nylon being the typical culprit. Please note that polyester and rayon also flat spot, but to a much, much lower degree.
The glass transition temperature is where a solid material (as opposed to a gas, liquid or plasma) undergoes a change but doesn't change it's fundament state (gas, liquid, solid, plasma). In the case of nylon, it's glass transition temperature is at the upper range of a tire's operating temperature.
That's why nylon is no longer used for the ply material, but - unfortunately - it has a property that is very advantageous for use in a cap ply (the fabric overlay of the belts). It shrinks when heated. This shrinkage reduces the growth of the tire due to centrifugal forces and therefore allows a higher speed capability (It also reduces the risk of a belt separation.) So you will see nylon cap plies used in H and higher speed rated tires.
Solution? First, ALL tires will flatspot. It's not just a question of the glass transition temperature as there are other things that affect the speed and severity of the flatspot So, it's a question of how quickly it gets bad. Aggravating factors are high heat, long idle time, low inflation pressure, high loads, etc.. Also, the higher the temperature of the tire when it was idled can trigger a severe flatspot.
Second, not all tires with nylon cap plies have this problem or more precisely stated, they have less of a tendency to develop this problem. The tire manufacturer can reduce the tendency by the choice of nylon type, and the way the nylon gets processed in the manufacturing operation.
What can a consumer do? Allow the tires to cool before putting the car away. Immediately parking a car after driving on the freeway is a good way to generate a flatspot.
Avoid parking on cold surfaces when the weather is warm. An air conditioned garage is not a good place to avoid flatspots.
Avoid H and higher speed rated tires. The problem here is that even some S and T rated tires have nylon cap plies. Plus, the higher the speed rating, the less likely a tire is to fail. In my estimation, unless you have a severe problem with flatspots, you should use the speed rating specified by the vehicle manufacturer on the vehicle tire placard. (many tire shops won't put on tires of a lower speed rating for safety reasons.)