Chapter 2 Wheel Geometry

7/28/2019 Chapter 2 Wheel Geometry

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WHEEL AND AXLE GEOMETRY SVOB

Figure 2: Tilted kingpin balt joints and steering

The inclination of the kingpin ensures that the shaft journal no longer moves horizontally, butdownwards so to speak. In figure 2 some points 1, 2 and 3 are shown of the shaft movement.

Because the wheel is held by the road surface, it cannot move downwards (or the car must bejacked up). So, the car wilt be pushed upwards by steering. The car is also lifted when turning tothe right. The weight of the car will, however, work against this movement. The consequence is

that the wheels are pushed back into the straight forward position by the KPI after a bend has beentaken. This also provides steering stability on a straight road.

The purpose of the KPI is:- to ensure the returning of the wheels after a bend, and to keep them in a straight position- together with the camber it provides centre point steering (scrub radius zero or negative).

The KPI angle is expressed in degrees.

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Figure 5: wheel movement with caster

Forgetting the road surface for a moment, when being turned the wheel will move upwardsaccording to A. If the wheel turns in direction B, the wheel moves according to the descending line.The other way around, the tendency will be for the wheel to be pushed in the driving direction.Because this happens with both front wheels and they are linked by the track rod, they keep eachother in balance.

So, the caster and the KPI both provide steering stability. They will also support or work against

each other. The purpose of positive caster is to improve steering stability, so the wheels are keptthe straight forward position.

2.3 Toe-in and toe-outIf the front axie in figure 6 is viewed from above, it is clear that the wheels are a little closertogether at the front. Distances A and D can be measured on the inside of the rim. We call thisposition of the wheels toe-in.

Figure 6: Front axle with toe-in



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The rear wheels can also display toe-in or toe-out. Depending on the suspension, deformation canalso take place here which results in a low degree of turning of the wheels and causes tyre wear.If the wheels are pointing to each other at the front (figure 6) this is toe-in. If the wheels are parallelto each other, we have tracking of 0. If the wheels at the front point away from each other, this istoe-out.

Rule of thumb:a. Rear wheel drive cars have toe-in.b. Front wheel drive cars have toe-out.2.4 Mutual influence of the wheel positions

The different wheel positions are so closely interrelated that with one wrong position the otherpositions also change. In the previous chapters all wheel positions are individually explained. Inpractice these occur at the same time.

To summarise:1. camber influences the KPI;2, camber influences the tracking (by the roll circle);3. caster influences the KPI (by axle journal movement);4. toe-in and toe-out influence the toe-out in a bend (Ackermann principle).

When this is considered properly it is clear that each wheel position influences the others.

2.5 tasks and questions Please answer all the questions.

1. Define Kingpin inclination (KPI).

2. Identify the addition of the positive camber shown in Fig 1a.

3. State the purpose of the inclination of the Kingpin (KPI).

4. In Fig 3b, with the help of an arrow indicate the direction of the castor when the jack isoperated.

5. The angle at which the kingpin ball joints are positioned is called ..............

6. In Fig 4, indicate the direction of positive castor.

7. State the purpose of positive castor.

8. State the rule of thumb.

9. State by which method can castor influences the kingpin inclination.

10. In relation to alignment, define the following.

(a) Centre Line (b) Thrust Line.

11. State what determine the driving direction of a vehicle.

12. State the alignment order of the thrust line method.


Chapter 2 Wheel Geometry

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