The construction of car tyres is quite complex considering that every tyre consists of many elements. All tyre parts, such as the carcass layer, steel belting tread, sidewalls and sealing layers, are put together on manufacturers’ production lines creating the final product.
However, sometimes deviations and discrepancies may occur while machines join tyre elements. This may result in irregularities in tyre thickness, mass and rigidity, which, in turn, lead to a non-uniform structure of a tyre. It can also happen as a result of other stages of the tyre production process, for instance incorrect tyre storage prior to vulcanization.
Non-uniformity of tyres may result from errors in construction, among other things.
Types of tyre non-uniformity
When it comes to car tyres, factory faults or irregularities can be found in various aspects within three broad categories:
- Non-uniformity of tyre mass
- Non-uniformity of tyre shape and thickness
- Non-uniformity in tyre rigidity
Any of these types of tyre non-uniformity will affect your driving experience in a slightly different way. Generally, they will all worsen the performance of your tyres due to the irregularities. The main consequences of non-uniform tyres are:
- Imbalance, whether static or dynamic.
- Conicity: If the conicity of a tyre is unbalanced with the rest of the set, this will make the vehicle tend to lean into the direction of the over-powered conicity forces.
- Lateral run-out: This represents the deterioration of the sidewall and lateral run-out can most commonly be found in the upper sidewall, where it meets the treads.
- Radial run-out: This refers to the tyres inability to remain in shape, moving away from the circle they are meant to be, e.g. towards a slouched ‘oval’ shape.
- Variations in both radial and lateral rigidity.
Every tyre undergoes tests to be checked for any possible defects.
Remember that tyres are used in conjunction with a rim, which is a very rigid car part. As such, a rim may influence (either positive or negative) the non-uniformity of tyre mass and shape but, in principle, it cannot affect the non-uniformity of tyre rigidity.
Generally, various tyre non-uniformity issues may from time to time come up during the manufacturing process. This is why many manufacturers employ extensive testing procedures which include inspecting tyre uniformity. Tyre producers run a lot of such tests to be able to quickly detect any tyres with parameters outside of the quality limits and therefore ensure the quality and efficiency of manufacturing.
Reasons for non-uniformity of tyres
Flaws in the production happen but they are not the only possible reason for non-uniformity of tyres. Other possible causes include:
- Defects in rim production
- Damage suffered by either the tyre or the rim (and even both) during use. This can be caused e.g. by hitting an obstacle or braking on locked wheels.
- An error in fitting the tyre on the rim, or not optimising the tyre’s position. While both the tyre and the rim might be within quality limits, their interactive accumulation of negative effects may nonetheless cause non-uniformity.
Non-uniformity of a tyre may lead to faster tread wear.
Possible effects of tyre non-uniformity
When a tyre reaches or exceeds the non-uniformity limits, it may cause numerous possible effects. The most important ones include:
- Extra noise
- Additional uncomfortable vibrations
- Shaking of the steering wheel
- Faster and irregular tyre wear
- Increased wear of other vehicle parts
Non-uniformity of tyre mass
Types and results of non-uniformity of mass
Static imbalance involves an irregular distribution of mass in the tyre. Theoretically, positive mass is placed precisely on the axis of symmetry on the tyre tread. This would result in radial vibrations.
The other type of non-uniformity of mass is dynamic moment, where it is assumed that a mass is additionally placed ideally symmetrically around the tyre’s centre of gravity, causing lateral vibrations. In reality, however, the dynamic moment and static imbalance often occur together. This produces dynamic imbalance which can often be compensated for by balancing out the wheels.
If this imbalance is not compensated, or the non-uniformity of tyres and rims means the imbalance is too large to handle this way, then further consequences may appear, including symptoms similar to:
Reasons for non-uniformity of mass
There are many reasons why non-uniformity of mass may happen. The main ones include:
- An incorrect distribution of the overlaps of particular tyre elements. These need to be positioned in different places on the circumference, to avoid building up particularly heavier areas.
- The tread may often overlap other elements incorrectly, usually by being too large or too small.
- Incorrect tensioning of the tread.
- The tyre elements may vary in thickness, rather than being consistent in their dimensions.
The recommended maximum balancing value for car tyres is 60 g per side or 100 g per wheel. Manufacturers sometimes apply a repair technique where the inside of the tyre is painted with a special fluid in a place where the mass is too low, increasing it by as much as 100 g.
The graph below shows the average values of static imbalance and the characteristics for tyres of different sizes. These values are expressed in terms of mass of weight which would have to be used to compensate for the imbalance, in grams.
Approximate average static imbalance values in grams.
Non-uniformity of tyre shape
Lateral run-out means the tendency of a tyre to move left and right during one rotation. This continues cyclically every time the wheel turns. The existence of this force results from irregularity in the distance between the outer part of the side of the tyre and its axis of symmetry. As the lateral force value is always positive, it is felt in the passenger cabin as sideways vibration.
Radial run-out results from changes in the radius of the face of an inflated tyre as it rotates. An ideal tyre should have a perfectly round shape. However, as a result of even small irregularities in the thickness of structural elements of the tyre, in practice it may have local bumps on its circumference or have a slightly oval shape.
On the road, this leads to a situation where the wheel’s axis of rotation does not travel in a straight line, but moves in a vertical direction, which – under a load – causes fluctuations in the radial force. These fluctuations may be felt by passengers as vertical vibrations, which may be accompanied by noise and general worsening of the travel comfort as well as faster wear of the tyres and suspension elements.
Usually, these fluctuations in the uniformity of shape, typically resulting from a flaw in the production, are relatively small and often not even noticeable. However, it may happen during the actual use of the car, for instance when striking an obstacle or creating extra wear during an emergency break. Such situations may trigger further irregularities which will be felt more strongly and be clearly visible.
Non-uniformity of tyre rigidity
Variations in radial rigidity
Variations in radial rigidity occur when there are places on the circumference of the tyre that are not equally rigid. For the sake of simplification, imagine that the sides of the tyre are made of springs with different rigidities. As the tyre rolls, these springs successively make contact with the road surface. A more rigid spring will cause less deformation and a less rigid spring could affect the tyre to a higher extent. These variances would cause the wheel’s axis of rotation to move vertically, rather than horizontally with the road. This is how variants of radial rigidity can disrupt smooth driving.
Under the load of a car, there will be fluctuations in vertical forces, which are felt from inside the vehicle as vibrations and car noises, especially when travelling at higher speeds. The causes for these problems include a variation in the radius of the carcass layer, changes in the thickness of tyre elements and problems with joints and tensioning.
Variations in lateral rigidity
These lateral variations occur when the tyre doesn’t have the same lateral rigidity on its whole circumference. Just as in the case of radial rigidity, you can imagine a tyre constructed of springs with different rigidities. This time, they are placed laterally, between the sides of the tyre. The occurrence of variation in lateral rigidity causes sideways vibrations similar to those produced by lateral run-out.
Non-uniformity of tyres may noticeably reduce the comfort of travel.
Conicity refers to the tendency of a tyre to roll similarly to a cone. Such a fault makes the tyre constantly and consistently drift in one direction. The effect can be referred to as negative or positive, depending on the direction it drifts.
Tyre conicity can be caused by numerous factors in the production process, including:
- Asymmetric application of tread
- Asymmetric application of steel belts
- Asymmetrically produced tread
A large amount of conicity may have even more noticeable effects including:
- Constant pulling of the wheels, consequently moving the entire vehicle to one sire
- Poor steering, especially when trying to steer away from the angle of conicity
- Increased tread wear, reducing tyre lifespan
While a tyre affected by conicity can be returned to the store of purchase, it is not the only reason why a vehicle may pull to the right or left. This pull may be the result of other actions (or inactions), including incorrect wheel geometry or the asymmetric factors of the vehicle itself.
It’s also possible that negative accumulation of the conicities will form within individual tyres, in which case you may be able to swap tyres around to rebalance the conicity. For instance, if your car’s front tyres have a negative conicity, pulling one direction and your rear tyres have a positive conicity, pulling in the other direction, you could solve this by changing the position of the tyres. While this wouldn’t be a perfect match, it would greatly reduce the conicity felt while driving. Similarly, there are many high-range balancers available that may help solving similar problems.
In summary, tyre non-uniformity comes in various forms and may show different consequences. All of them influence your driving experience and, consequently, may affect safety, so if you notice that something is wrong, verify the condition of your tyres or have them checked.