AH40.10-P-1030-01A Notes on Tire Flat Spots

AH40.10-P-1030-01A Notes on tire flat spots

Flat spot (flat spot/bald spot)

Because of the fact that time and again wheel assemblies (wheel and tire) when in new condition have to have unnecessary work performed on them due to "flat spots", we would like to offer our support here in the form of corresponding references with regard to subjective assessment.

How are flat spots caused?
When tires heat up during a journey (e.g. when the vehicle is driven at high speeds) and/or are heated by external temperature influences, plastic deformation at the tire contact patch occurs during the subsequent cooling phase (when the vehicle is stationary).

This deformation can lead to more or less powerful vibrations during the first few kilometers of a journey.

In principle, the size of a flat spot is dependent upon the vehicle weight, temperature and idle time. The tire make plays only a subordinate role among modern OE (Original Equipment) tires.

How does one assess a flat spot?
The term "flat spot" refers to localized flattening at the point on the tire where it makes contact with the road surface. This flattened area usually disappears again after the vehicle has been driven at speed over a few kilometers.

Depending on the severity of the flat spot, a distance of up to 15 km may be required in extremely unfavorable cases.

A permanent (irreversible) flat spot may be caused by the effect of heated chambers (e.g. painting booth) or by the vehicle being parked up for a long period of time under extreme climatic conditions. These types of deformation are only diminished to a certain degree.

Flat spots cannot be rectified by balancing the wheels, as this refers basically to increased radial-force variations and only to a small extent to any actual imbalance!

Paintwork
In the case of forced paint drying in drying booths, flat spots may occur on the tires at object temperatures > 40 degrees C as a result of weight, temperature and drying time.

Such flat spots are the result of permanent, irreparable deformations of the tire sidewall and bead reinforcement. This permanent distortion leads to wheel load variations and vibrations which cannot be rectified either by balancing or by rematching the wheels.

In order to prevent flat spots, both conventional and special protection vehicles must have "dummy wheels" for painting mounted when brought into the painting/drying facility.

While inflating the standard tires to 4 bar does not provide adequate protection on standard tires, it is not necessary when "dummy wheels" are mounted. Covering the original tires with protective foils and protective plates is likewise inadequate.

Idle time
Flat spots resulting from long idle times (>1 month) can reform again under certain conditions.

To prevent flat spots caused by long idle times, the vehicle should be moved by 0.5 to 1 m (wheels rotated by 120 degrees) at least once per month.

Increasing the air pressure during the idle time reduces the reformability of flat spots.