Question about Kingpin Inclination

Why do axle's have Kingpin Inclination? The spindle surface where the wheels mounts is perfectly vertical (for the most part), why not have the kingpins perfectly vertical as well?, does it affect steering or tracking?
I'm basically courious about what the inclination of the Kingpin is actually for
Thanks

 

King Pin inclination influences the scrub radius,also creates
a "self centering effect" when you let go of the wheel.

More noticeable with manual steering.

 

Not to sound really stupid at this point, whats "Scrub Radius"?

 

The idea when it's all stock,as built with OEM rim/tire,is that were the tire is on the ground and that kind of spot the tire makes as it turns,that it is turning near it's spot center vs turning way inside of ground tread spot if rim is off set. If you use a off set rim not like as designed,it makes it harder to steer. That also can add to front wheel shake too. Scrub radius is the spot as it moves ,like how much it moves off center. I left all the big words out so it maybe clear to most. Think of tire movement when your steering ,but not moving the car,just turning the steering wheel.

 

Thanks for the quick education guys, much appreciated

 

Picture a model T axle with no kingpin inclination (vertical kingpin). To steer the wheels, the wheels must roll to allow the spindle to rotate around the kingpin. OK for a car with no front brakes, with brakes it would be almost impossible to turn the wheels and apply the brakes at the same time.

Also, travelling down the road, what would happen if you hit a pothole? The wheel would be pushed rearward, trying to rotate it around the kingpin, and whipping the steering wheel out of the driver's hands.

Every car needs kingpin inclination. On an IFS, a line drawn throught the ball joints is the same thing, it's the spindle pivot axis. That line, when extended, should ideally fall about in the centre of the tire's contact patch. This would be a zero scrub radius. Wider tires or rims with more offset increase the scrub radius, making the front end harder to steer, and hitting a bump in the road is more inclined to yank the steering wheel.

Not to be argumentative, but self-centering is a result of castor, not kingpin inclination. Kingpin inclination is the inward tilt of the top of the kingpin as viewed from the front, positive castor is the rearward tilt of the top of the kingpin as viewed from the side.

 

Simplified..

 

With king pin inclination,and zero caster(for example),
turning the wheels left or right will cause the front of
the vehicle to raise. This influences steering effort.

Adding caster,to improve straight line stability,
also modifies this effect slightly,depending on the values.

Case in point:
Several years ago,before the aftermarket took off,there were two different spindles commonly used on Sprint cars.One was IH,the other Ford (IIRC).The difference was several degrees in king pin inclination.Even with the same caster and camber settings,the difference is king pin inclination changed the way weight was jacked in the middle of a corner. As a result,different chassis set ups were required depending on the spindles used.