The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

*Shane* · 05-24-2003, 11:46 PM

I, by no means, am an expert on front suspension geometry, but do have some experience with race kart chassis design and fabrication and the various "fixed" geometries that are dictacted by NOT having a suspension. Maybe I can relate some things that have been explained to me ...

I think when you say King Pin Inclination you are referring to Caster ... Kingpins are what were used in the days before A-arm/wishbone suspensions ... they went thru the top and the bottom of the spindle, much like our ball joints do today. In the days of straight axles, the wheel had no need to have motion related to "up and inward" of the chassis centerline. When one hit a bump, the entire axle went UP and not in the larger arc motion that a modern A-arm/wishbone suspensions travels, hence the need for ball joints in today's suspensions.

King Pin Inclination is referred to as the angular difference of the upper and lower mount points when viewed from the front. With the top mount being closer to the centerline of the chassis and lower being further away. This is a fixed position, but it DOES change the geometry once the tires are pointed either left of right ... it changes the camber. But that is an entirely different conversation.

Check out this pic for a much better explaination ... remember that this is in its most basic form ... a non-suspended kart frame.

King-pin inclination (KPI) is the inward lean of the king-pins (up, towards the centerline of the chassis). KPI causes some of the self centering action of the steering. It also modifies the amount of camber change caused by the caster angle when the steering is turned, lessening negative camber gain on the outside front wheel and increasing positive camber gain on the inside front wheel.
Caster angle is the rearward lean of the king-pins. Caster angle is responsible for most of the self-centering action of the steering.

Now on to the original converstion ... CASTER does change the position of the wheel centerline when you place the upper and lower A-arms in either a raised or lowered position. If you notice, the upper and lower A-arm mounting points are kinda "leaned back" with the front of the A-arm being mounted "higher" than the rear. (- think of a bicycle/motorcycle and how the front forks are mounted on an angle. This "leaned back angle" helps to curb some of the "high speed wiggle" felt on a front-end that has the caster angle that is closer to perpendicular to the road surface.) When you lower the truck, the A-arms not only travel UP but also BACK in an arc, which places the tire and wheel centerline further back in the fenderwell.

As far as how to fix this, the only way I can figure is to move the mounting point of the upper and lower A-arms, which is the crossmember. You would need to move the cross member towards the front of the vehicle.

05-24-2003, 11:46 PM	#3
Shane Account Suspended Join Date: Jun 2000 Location: over yonder Posts: 14,270	I, by no means, am an expert on front suspension geometry, but do have some experience with race kart chassis design and fabrication and the various "fixed" geometries that are dictacted by NOT having a suspension. Maybe I can relate some things that have been explained to me ... I think when you say King Pin Inclination you are referring to Caster ... Kingpins are what were used in the days before A-arm/wishbone suspensions ... they went thru the top and the bottom of the spindle, much like our ball joints do today. In the days of straight axles, the wheel had no need to have motion related to "up and inward" of the chassis centerline. When one hit a bump, the entire axle went UP and not in the larger arc motion that a modern A-arm/wishbone suspensions travels, hence the need for ball joints in today's suspensions. King Pin Inclination is referred to as the angular difference of the upper and lower mount points when viewed from the front. With the top mount being closer to the centerline of the chassis and lower being further away. This is a fixed position, but it DOES change the geometry once the tires are pointed either left of right ... it changes the camber. But that is an entirely different conversation. Check out this pic for a much better explaination ... remember that this is in its most basic form ... a non-suspended kart frame. King-pin inclination (KPI) is the inward lean of the king-pins (up, towards the centerline of the chassis). KPI causes some of the self centering action of the steering. It also modifies the amount of camber change caused by the caster angle when the steering is turned, lessening negative camber gain on the outside front wheel and increasing positive camber gain on the inside front wheel. Caster angle is the rearward lean of the king-pins. Caster angle is responsible for most of the self-centering action of the steering. Now on to the original converstion ... CASTER does change the position of the wheel centerline when you place the upper and lower A-arms in either a raised or lowered position. If you notice, the upper and lower A-arm mounting points are kinda "leaned back" with the front of the A-arm being mounted "higher" than the rear. (- think of a bicycle/motorcycle and how the front forks are mounted on an angle. This "leaned back angle" helps to curb some of the "high speed wiggle" felt on a front-end that has the caster angle that is closer to perpendicular to the road surface.) When you lower the truck, the A-arms not only travel UP but also BACK in an arc, which places the tire and wheel centerline further back in the fenderwell. As far as how to fix this, the only way I can figure is to move the mounting point of the upper and lower A-arms, which is the crossmember. You would need to move the cross member towards the front of the vehicle. Last edited by Shane; 05-25-2003 at 12:44 AM.