VW Golf manuals

Volkswagen Golf Service & Repair Manual: Conicity

Conicity is caused by a slight offset of the tread and/or the belt (amounting to a few tenths of a millimetre) relative to the geometric centre of the tyre. Taper is not visible and cannot be measured with equipment available in the workshop.
   
Parts of a tyre
1 -  Bead
2 -  Shoulder
3 -  Tread
4 -  Steel cord belt
A -  Geometrical centre of tyre
B -  Actual centre of belt. It can be offset to inside or outside.
 

Conicity

Exaggerated for clarity.
1 -  Offset of belt and tread
F1 -  Unequal vertical wheel forces
F2 -  Unequal vertical wheel forces
Fk -  Conicity force
The offset produces differences in stiffness at the inner and outer shoulders of the tyre, resulting in differing vertical wheel forces. Consequently the belt or tread will not be pressed onto the road surface with the same force (F1, F2). A conical, or tapered, shape develops. The resulting force (conicity force Fk) can, depending on the speed, become so great that the vehicle then pulls to one side.
If the force Fk on one wheel of the axle is, for example, 50 Newton, and also 50 Newton on the other wheel, and both forces are exerted in the same direction, the forces are cumulated. Reversing a tyre on the rim can compensate for the lateral pull because the forces then act in opposite directions.
Because the direction in which the force of taper is exerted is not visible, only road tests and strategic rotation of wheels and tyres can establish which tyres cause the pulling.
The tyre consists of numerous components and materials which are vulcanised to form a single part at the end of a complicated manufacturing process. The result is differing production tolerances which make themselves noticeable through more or less strong lateral forces (conicity forces). These forces can also occur in new tyres.
Pulling to one side on front axle
Pulling to one side can be caused by the running gear. However, experience shows that in 90% of all complaints, the tyres cause pulling to one side.
Pulling to one side during normal driving
On a straight, level road surface, the vehicle wants to pull to one side at a constant speed or with moderate acceleration. Force can be felt at the steering wheel.
Pulling to one side during fast acceleration
Pulling to one side during fast acceleration is, in part, due to the basic design of vehicles with front wheel drive. Different friction levels at the left and right wheels or possible irregularities in the road surface (potholes) and consequently varying road adhesion have a substantial influence on the handling characteristics. This does not constitute a complaint which is covered by the warranty.
 

Conicity

General
Perform a road test to determine whether a vehicle is pulling to one side and if so, which side. If the vehicle pulls to one side → Chapter. ...

Remedies when vehicle pulls to one side
Test conditions before and during the road test: –  Check all suspension components on the front and rear axles for damage. ...

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