KPI effects on lowered trucks

[XXL]

Since the kingpin inclination on the 67-72 trucks is leaned back a bit, when you lower these trucks drastically, you start to notice that the front wheels no longer center in the fender opening.

For those of you with this problem, what kind of measurements do you have (see pic below) in terms of rearward shift? 1/2", 1", etc??? For those of you who have solved this problem, how did you do it? Pics are verrrrrrrry good

Kenneth

[crazy4chevs]

I also have wondered the same thing.

[Shane]

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.

[XXL]

Originally posted by Shane
I, by no means, am an expert on front suspension geometry,

Good explanation of suspension geometry, and I (though I did front end alignments as a co-op/flunkie back in high school) am not an expert on it either... but I believe my reference to KPI is correct. I am referring to the imaginary line that connects the two ball joints together. Though, as you mention, there is no longer a kingpin in modern suspensions (some Ford trucks excepted), the concept still exists, and is affected by caster and camber adjustments. The point of all this is to say that, as we lower our trucks, this imaginary line is the axis which becomes more pronounced, and to solve this issue, it is the axis which must be altered. Here's a 2-second drawing to clarify the point.

Kenneth

[Shane]

I am referring to the imaginary line that connects the two ball joints together.

I have always been taught that the particular imaginary line you are referring to is known as CASTER ...

CASTER is controlled by the A-Arms ... and the A-Arms are what are moving UP and BACK when the suspension is at its LOWEST. That's why I suggested what I did for the "fix."

Let us know what you come up with ... I am interested in a solution to this as well ... Good Luck.

[jay-dawg]

wholly sh!t...taking the game to a whole new level. how about moving the front crossmember forward, then adding a rack and pinion to minimize the pain of having to fix all the steering etc.

just using the brain of a minitrucker !!!

[Slammed67]

I'm currently working on a "poor man's" solution to this problem. I should have it all put back together this week sometime. I've been documenting the whole project so I can show everyone what I did (assuming it works okay!).

I thought about moving the whole cross member forward, but they you run into all kinds of steering linkage problems, as well as motor mount issues. The way I'm doing it is kind of primative, but I think it'll work (and work on a budget!)

What's the matter XXL? Don't like my "cut and paste" idea??

[XXL]

Originally posted by Slammed67
The way I'm doing it is kind of primative, but I think it'll work

Hey, duct tape is NOT primative!

Originally posted by Slammed67
What's the matter XXL? Don't like my "cut and paste" idea??

Eh??

Here's a mockup of what I had in mind for the lower arm. I stole one of slammed67's control arm pics and Pshopped it up. Aside from the obvious removal of spring pocket, note the ball joint location. I believe that, after removing the forwardmost part of the curled lip at the end of the arm, I can move the ball joint position forward about 1". For me, I think this will work out as the shift I'm dealing with (see first pic) is about 7/8" or so. I believe slammed67 is dealing with a significantly larger shift on his so my solution might not have been enough. However, I think making a pie cut from the front of the arm in between the ball joint and the spring pocket (in that flat space) running parallel with the bushings would allow you to kick the ball joint forward a little farther forward and actually help with camber issues in the process. We'll see as soon as I can round up some control arms and get time to mess with it.

Kenneth

[XXL]

Here's a WAG at the pie cut I was describing...

[jay-dawg]

good, now all you have to do is cut and paste with duct tape!!! j/k anyway, that looks good (from a non-engineer point of view). does turner enterprise have anything to tackle the problem?

[racindego]

I also noticed this problem on my truck and it is quite severe. I had a quick idea that I was considering testing out before I build a custom front suspension. If you notice, the upper a-arms on our trucks generally offset the upper ball joint toward the rear of the truck. If you were to switch the a-arms to the opposite sides, it should theoretically move the top ball joint forward about an inch and a half, and the wheel forward 3/4 of an inch. Like I said, I havent tried this yet, just an idea.

michael

[Slammed67]

racindego: I thought about that also, but don't think it'll work. Plus you'll probably need more than 3/4" to center it (depending on how low you are). I should have mine done this week, then I'll post some pics of how I did it.

Here are some pics of mine before the modification.... Hmmmmmm? The rear end is off a little too........

[brad2]

Since mine got a new frame we stretched the wheelbase an inch in the front. Now that it is done I think it could have even been stretched a little more.

[Slammed67]

I'm moving the spindles 1.5" forward on mine which I think should center the wheels pretty well. I hope to have some pictures soon!

[jayssunm]

Slammed67, on the second pic it that with all the air out? Can u drive like that? If not what hangs down the lowest (A-arms, exhaust?).

My mini-truck was low like that but I decided my 67 Chevy would not be. I changed out way too many A-arms and a crossmember once >.

Jay

[Slammed67]

Jay,

Yes, it is aired out in the FIRST picture. I can drive it like that at slow speeds, like in parking lots (at car shows), but I wouldn't drive it like that on roads. The front end could actually go lower, but I have it limited to keep the tires off the inner fenders and the a-arms off the ground.

[AccuAir]

I wouldn't suggest moving your lower balljoint, unless you plan to also move the upper ball joint. If you move the lower only, you will have so much caster, that you'll need a bigger steering wheel to turn the thing. Especially if you are talking inches. You need to keep the castor angle from upper to lower ball joint somewhere near the factory 4-6 degrees. Check that number, but I know it isn't very much. If you are really concerned about the look of the wheel in the fender well, I would move the cross member.

[Slammed67]

I'm pretty sure he's going to alter the upper a-arm also. He was just showing his ideas for the lower one.

[Slammed67]

Also, you can't just move the cross member forward without running into steering linkage and steering shaft interferences. Plus there are the motor mount issues. I considered that option but felt that the way I did it would accomplish what I needed without having to alter the steering system. Much easier.