A car tyre consists of many elements, such as the carcass layer, steel belting tread, sidewalls and sealing layers, making for a very complex product. Put together on a production machine, all of these parts come together to form a finished model. However, deviations and discrepancies may occur where these individual elements are joined, offering irregularities in thickness, mass and rigidity, leading to a non-uniform structure.
This phenomenon can also occur as a result of other stages during the production process, such as the incorrect storage of the tyres prior to vulcanization.
Non-uniformity of tyres may result
from errors in construction, among other things.
What types of non-uniformity are there?
When it comes to car tyres, there are a number of irregularities that can occur. These can all be grouped into three broad categories:
● A non-uniformity of mass
● A non-uniformity of shape and thickness
● A non-uniformity in the rigidity
Each of these will change your driving experience uniquely, offering worse performance due to the irregularities of the tyre. The main symptoms for non-uniform tyres are:
An imbalance, whether static or dynamic.
Conicity. If the conicity of a tyre is unbalanced with the rest of the set, this will cause imbalance while driving, as the vehicle will want 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 perfect circle it is meant to be. This can greatly change your driving experience, as the slouched ‘oval’ shape causes irregularities with the other wheels.
Variations in both radial and lateral rigidity.
Every tyre must undergo tests to check for any defects.
It also helps to remember that tyres are used in conjunction with a rim, which is a very rigid car piece. As such, we have noticed that the rim has an effect (which can be either positive or negative) on the non-uniformity of mass and shape but, in principle, has no influence on the non-uniformity of a tyre’s rigidity.
Generally, it is often noticed that non-uniformity may arise during the manufacturing process. However, many manufacturers employ extensive testing procedures, which includes inspecting the uniformity of the tyres. This is done to a high extent to ensure the quality and repeatability of the production process, which should be able to quickly detect any tyres with parameters outside of the quality limits.
What Are The Reasons For Non-Uniformity?
Non-uniformity in tyres can arise from numerous areas, not necessary the production process. Possible causes for non-uniformity include:
Defects in the rim production
Damage suffered by either the tyre or the rim (and even both) during use. This can be caused 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.
What Are The Possible Effects Of Non-Uniformity?
When above the non-uniformity limits, such a tyre can cause numerous possible effects to your driving experience. These include:
Additional and uncomfortable vibrations
Shaking of the steering wheel
Faster and irregular tyre wear
Increased wear of other vehicle parts
Types And Effects Of Non-Uniformity of Mass
A static imbalance involves an irregular distribution of mass in the tyre. Theoretically, this involves a positive mass 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, 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 additional symptoms may occur. These include the likes of:
Vibration and noise within the vehicle
Unnatural read wear and a shortening of the tyre lifetime
Increased fuel consumption
Faster wear and tear on the suspension system
The Main Reasons For Non-Uniformity Of Mass
There are many reasons why non-uniformity of mass may be occurring. These 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. We also previously covered wheel balancing in a previous article.
The graph below shows the average values of static imbalance and characteristic for tyres of different sizes. These values are expressed in terms of the mass of the weights which would have to be used to compensate for the imbalance, in grams.
Approximate average static imbalance values in grams.
The Effects Of Non-Uniformity Of Shape
Lateral run-out refers to 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. The value of the lateral force is always positive, and it is felt in the passenger cabin in the form of 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 comfort of travel, as well as faster wear of the tyres and suspension elements.
Movement of the wheel’s axis of rotation due to radial run-out.
Usually, these fluctuations in the uniformity of shape, typically resulting from the production process, are relatively small and usually not noticeable to the naked eye. However, it is often occurs during actual use, such as striking an obstacle or creating extra wear during an emergency break, that further irregularities may be produced. These latter changes tend to be felt more strongly and are often clearly visible.
Types And Effects Of Non-Uniformity Of Rigidity
Variations in radial rigidity
Variations in radial rigidity occur when there are places on the circumference of the tyre what are not equally rigid. You could imagine, for the sake of simplification, 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. If a particularly spring was more rigid, the deformation would be smaller, and a less rigid spring would have a greater amount. These variances would cause the wheel’s axis of rotation to move vertically, rather than horizontally with the road. This is why variants in the radial rigidity can disrupt smooth driving.
Under the load of a vehicle, there will be fluctuations in vertical forces, which are felt as vibrations and noise from inside the vehicle, 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 with the radial metaphor, you can imagine a tyre constructed with 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 similar to a cone. Specifically, such an imperfection would cause the tyre to constantly and consistently drift in one direction. The effect can be referred to as negative or positive, depending on the direction it drifts.
When dealing with tyres, conicity can be caused by numerous factors in the production process, including:
An asymmtric application of tread
An asymmetric application of steel belts
Asymmetrically produced tread
When dealing with large amounts of conicity, there are more noticeable effects. These include:
A 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 the lifetime of the tyre
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 to have a negative accumulation of the conicities of 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 have functions to solve problems such as these.
In summary, there are many possible forms of non-uniformity, with many different effects as a result. These all influence your driving experience, so you will often notice if something is wrong.