What is Caster in a Car Steering System?

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How to Make Caster Adjustments....5

Caster is the relationship of a lower ball joint to an upper ball joint, so adjusting caster has wide-reaching effects as our expert mechanic explains in this free auto-repair video.

Part of the Video Series: Wheel Alignment Tips for Custom Cars
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Video Transcript

In this clip, I want to make sure I've explained exactly what caster does and how our adjustments of it are going to have wide-reaching effects here. If you look from above, this joint right here, where the control arm meets the spindle is a ball joint. The relationship of this ball joint to this ball is what we're measure when we measure caster. Ideally, you'd have enough caster, positive, that the lower ball joint is ahead of the upper ball joint enough so that the vehicle wants to go on a straight line down the street. So let's take it to extremes. If I take the lower ball joint and put it here, oh, it's sure going to go straight but you won't be able to turn it, because you're having to lift the whole weight of the car when you do it. Likewise, if I move the lower ball joint back here, it turns very, very easily but it's going to wobble. It's not going to have any stability on the highway. So the reason we put positive caster in a vehicle is two reasons. One is it helps the vehicle track straight as it goes down the highway. It also helps the steering wheel return to zero when you go to come out of a turn. So yeah, if you let go of your steering wheel in your normal, the wheels' kinda want to go straight, that's a factor of caster.

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