Have you ever noticed how the front wheels on your car turn? Look closely and you’ll see that they do not pivot around a vertical axis. In simpler words, they are never perfectly straight when they turn — they’ll always tilt slightly.
This happens because of a suspension angle called king pin inclination.
To see this in action, park your car on a smooth surface, and have someone else operate the steering wheel while you watch your tires turn.
You’ll notice that your wheels gain a slight amount of camber when you turn the steering wheel to full lock in either direction. To be more precise, turning in either direction will result in:
- Positive camber on the outside tire, and
- Negative camber on the inside tire.
But wait, isn’t that a bad thing? Ideally, to ensure a flat contact patch, you want negative camber on the outside tire when you’re cornering. We’ve explained this in our positive and negative wheel camber article.
Why then do cars do this? And does it have any detrimental effects? Read on as we explain king pin inclination — what it is, what it does, and why you should know about it.
Steering Axis Explained
A quick note before we start; we’ve used double wishbone or double A-arm suspension type as the main example throughout this guide.
Suspension concepts are easier to explain in the context of double-wishbone suspensions. The core principles remain the same when these concepts are applied to other suspension types like the MacPherson strut.
The steering axis and king pin angle axis are exactly the same — it is an imaginary line about which your entire wheel pivots as it turns. To visualize this, picture a line that passes through the two ball joints where your wheel mounts to your steering knuckle.
Fun fact: The term “king pin axis” gets its name from older cars that used to have an actual vertical physical pin about which the entire wheel would turn. This pin was called the king pin.
Modern-day suspension systems have done away with real king pins and use dual ball joints instead. Why do they do that? Because modern cars are required to pack a lot more unsprung components around the inner-wheel area.
It is virtually impossible to have a king pin that can allow for a perfectly vertical steering axis. This means that the steering axis inevitably has to be inclined. This brings us to the next part.
King Pin Inclination Explained
Before we get into what king pin inclination is, you need to know what the vertical axis is — it is an imaginary line that passes through the center of the tire vertically.
Visualize your car from either the front or rear view.
Line A passes through the upper and lower ball joint on your control arm. Line B is the vertical axis.
See the angle between line A and line B? That’s the steering axis inclination or suspension king pin inclination. In other words, king pin inclination refers to how diagonal the steering axis is in comparison to the vertical axis.
Why King Pin Inclination Exists
At first glance, KPI seems rather redundant, but it exists for a reason:
- Due to a structural limitation, there’s no way to cram your control arms, steering knuckle, and brakes into your wheel, there’s just not enough space. Hypothetically if we were able to do that, we could achieve a perfectly vertical steering axis and zero scrub radius at the same time.
- Since we cannot do this, we have to rely on an inclined steering axis.
Besides that, KPI fulfills other important purposes such as:
- Producing a lifting force that has a self centering effect on the steering wheel just like the caster angle does.
- Generating scrub radius.
The reason why KPI produces a lifting force is that this angle makes the wheel axle trajectory travel in an upside-down arc, as opposed to traveling in a perfectly horizontal plane. So when the wheels turn, they dig into the ground and actually lift the car up slightly.
This lifting force has a self-centering effect on the steering wheel at slow speeds which adds to straight-line stability. At high speeds, the self-centering effect is more caster-dominant.
What Does KPI Do?
There are no universal rules around what KPI is supposed to do. It all depends on the width of the tire, stiffness of the tire, all the other suspension angles working together create the final handling characteristics of the vehicle.
It’s going to be there, whether we like it or not. What matters is what we do with it and how we can use it to our advantage.
As mentioned earlier, KPI is mostly a consequence of not being able to fit all the unsprung components in a structurally limited space.
It acts as a self-centering mechanism that uses the weight of the car to bring the wheel back to the central point. This ensures straight-line stability and ease of steering at slow speeds.
Caster angle does something similar — it creates a self-centering effect as a result of momentum and inertia. But KPI creates a self-centering force as an effect of lifting force.
KPI does add unwanted camber angles on the outside wheel, however, suspension engineers typically offset this effect by adding additional caster angles. This helps to make the best of both worlds — slow-speed steering stability and wheel centering + high-speed cornering stability.
KPI is very closely interlinked with scrub radius and caster. These three steering geometry angles interact with each other and have a significant effect on the overall handling of the car. It also allows us to alter the scrub radius in a way that suits the car.
So why should you care about KPI, caster, and scrub radius?
If you’re a hellaflush advocate or if you’re someone who digs the stance look but wants a functional car, understanding these suspension angles and implementing what you learn is the key.
Before you do anything that’s going to affect your suspension geometry, it’s important to consider the implications and choose your modifications wisely. If you’re taking your suspension set up seriously, be sure to read our guide to corner balancing, too.
What are the biggest challenges you face when it comes to balancing out form vs function? Let us know in the comments below!