Affects of KPI

[Engineer]

OK I understand King Pin Inclination causes the pivot point of the steering to be at the center of the tire.  Here are my observations and questions.

KPI is also helpful in causing a camber curve.

KPI also makes the tire lean as it is turned.  This raises the car as the wheel is turned.

Does KPI help to center the steering wheel?  Could to much KPI make it feel heavy?

One reason for building in KPI is that you can't get the lower balljoint in to the middle of the tire.  So you move the outer joint out further to compensate.

My question is really, should you get the lower ball joint as close to the center as possible, to reduce the amount of KPI required, or can you build in more KPI to get a desired camber curve, and what are the tradeoffs of doing this.

One tradeoff that I see is KPI negates your camber gain as you turn the tire.  The further you turn the worse it gets.  And when are you hoping to ustilize the camber gain except when your turning?

Can someone define "scrub radius" for me?  I imagine it has something to do with how close you are to having the two front tires turning on the correct radiuses.  But I have also heard it used in referenct to KPI so mabey it is the radius that your tire travels in if you turn the wheels lock to lock while sitting still?

Does anyone know the KPI angle of their ride as built by a pro? 

Would you be better off keeping the KPI down even if it meant not hitting the center of the tire?

[fabr]

OK I understand King Pin Inclination causes the pivot point of the steering to be at the center of the tire.  Here are my observations and questions.

KPI is also helpful in causing a camber curve.

KPI also makes the tire lean as it is turned.  This raises the car as the wheel is turned.

Does KPI help to center the steering wheel?  Could to much KPI make it feel heavy?

One reason for building in KPI is that you can't get the lower balljoint in to the middle of the tire.  So you move the outer joint out further to compensate.

My question is really, should you get the lower ball joint as close to the center as possible, to reduce the amount of KPI required, or can you build in more KPI to get a desired camber curve, and what are the tradeoffs of doing this.

One tradeoff that I see is KPI negates your camber gain as you turn the tire. The further you turn the worse it gets.  And when are you hoping to ustilize the camber gain except when your turning?

Can someone define "scrub radius" for me?  I imagine it has something to do with how close you are to having the two front tires turning on the correct radiuses.  But I have also heard it used in referenct to KPI so mabey it is the radius that your tire travels in if you turn the wheels lock to lock while sitting still?

Does anyone know the KPI angle of their ride as built by a pro? 

Would you be better off keeping the KPI down even if it meant not hitting the center of the tire?

One side raises and the other lowers,firmly planting the outer tire. 
IMO yes.
IMO also yes.
???eh?
 Scrub radius is the distance from the point the KPI axis hits the ground and the actual center point of the tire contact patch.You want the KPI axis to be inside of that point.YOu do not want KPI axis to intersect the tire contact patch centerline as that will make it rather twitchy.Too far inside and it will be heavy steering.

[fabr]

Getting the front tires to track around a common point while turning is an effect of Ackerman not KPI..

[Doug Heim]

You asked if KPI will help center the wheels. I know that the Castor is what  causes the self centering on a steering wheel but not sure what affects KPI has on it. I do agree with master that too muck KPI will cause it to be heavy but so will too much Castor. I also agree that you want the rod ends as close to the wheel as possible but still comfortable clearance! Also try not to keep the lower rod end too low as this may affect ground clearance.

[Engineer]

One tradeoff that I see is KPI negates your camber gain as you turn the tire.  The further you turn the worse it gets.  And when are you hoping to ustilize the camber gain except when your turning?

???eh?
 

Ok,  It is easiest to go to absurd angles to understand how/why it happens, but it shows what is happening at the lesser angles.

Imagine the KPI angle was 45 degrees.  Now imagine that you turned the tire 90 degrees on that axis.  At this point the top of the tire is now leaning out 45 degrees. 

Sure most KPI is probably in the 10-20 degree area, and max turning angle is around 30 degrees, so the top of the tire would only lean out a few degrees, but everyone seems to be crazy about having a few degrees of camber which dissapears as soon as you turn the tire. 



New question.....  I thought the Caster was centering the wheel so you could make KPI theoretically perfect for 0 scrub radius.  If not then what would a "good" scrub radius be?

[artie on edge]

Firstly lets simplify the answer, yes, KPI does make steering feel "heavier", just like castor. KPI's primary role is to help self centre the steering in conjunction with the castor angle.

As a STARTING POINT, scrub radius should be about a half inch INSIDE the tyre centreline for a road car, perhaps a little more for an off road car.

Two things occur from excess scrub, tyre wear (it is called SCRUB radius) when turning the wheel as you are dragging the tyre that little bit and bump leverage on your steering.

Assuming your car is set up with zero scrub radius, you hit a rock with the centre of your tyre, it will try and swing back and up.

If the scrub radius is one inch to the inside of the tyres centre line and you hit that rock the tyre will try and turn out, swing back and up. You have leverage over the tyres steering components now.

The more your scrub radius is away from the centre line, the more leverage 'hits' have on your steering and the more pronounced is the effect you 'feel' at the wheel (pull to one side). this is called tram tracking and the car can seem to steer itself all over the place while it follows imperfections in the road surface (this is why some insurance companies WONT pay up if non standard rims are fitted in an accident)

You need 'some' scrub radius to assist in the 'feel' of the car (loads up the steering a little), again help a little in self centreing the wheel.

How much is good?

Positive scrub (KPI pivot inside the tyres centreline) is dependant on your useage and your tyre width, duning, I would say 0.5 of an inch max, maybe a little more for desert work.

Some Hi Perf road cars use 1.5 inches but with very wide tyre footprint and their KPI set up is designed for it.

King Pin Inclination or KPI is also known as Steering Axis Inclination or SAI.

Im off the lecturn now...

[artie on edge]

One side raises and the other lowers,firmly planting the outer tire.  They BOTH raise (each side of a centreline), If the castor angle is extreme, one a bit less or more than the other, thats how it self centres, gavity tries to make them 'lower' again
IMO yes.
IMO also yes.
???eh?
 Scrub radius is the distance from the point the KPI axis hits the ground and the actual center point of the tire contact patch.You want the KPI axis to be inside of that point.YOu do not want KPI axis to intersect the tire contact patch centerline as that will make it rather twitchy.Too far inside and it will be heavy steering.

[artie on edge]

Just had a brain fade, one thing that is relevant more to onroad useage and scrub radius.

Excess pos scrub can highly load the stub axle. Again its that leverage issue, theoretically , the centreline should be at the inner wheel bearing when viewed from the front, if its significantly out wider it can add leverage to the stub and I know early fords (stangs etc) had a penchant for snapping em off when fitted with wider tyres.

[fabr]

Ok,  It is easiest to go to absurd angles to understand how/why it happens, but it shows what is happening at the lesser angles.

Imagine the KPI angle was 45 degrees.  Now imagine that you turned the tire 90 degrees on that axis.  At this point the top of the tire is now leaning out 45 degrees. 

Sure most KPI is probably in the 10-20 degree area, and max turning angle is around 30 degrees, so the top of the tire would only lean out a few degrees, but everyone seems to be crazy about having a few degrees of camber which dissapears as soon as you turn the tire. 



New question.....  I thought the Caster was centering the wheel so you could make KPI theoretically perfect for 0 scrub radius.  If not then what would a "good" scrub radius be?

I didn't quite follow that but I think you are seeing something I don't.

 Scrub radius is like how much toe in is good.Too many other factors to set a number on it.Just make sure it is inside the patch center. I'd say most would agree on 1/8-1/2" range. Bear in mind the more it is the more force it takes to turn.

[Engineer]

I learned something from reading the site that Nutzforsand posted.  If your KPI were identical to your Caster, then the outside tire would lean out none, while the inside tire would lean out twice as much.


Here is how you can see this principle.  Take a 8-1/2 x 11 piece of paper, and fold it from corner to corner (either).  Now hold it out in front of you with both hands.  the left had represents the A-arm's holding the heims, and the right hand is holding the spindle.  the top of the paper represents the spindle spud, and the crease represents the line that the steering is occuring around.  as you fold the paper either way (towards or away from you) the outer tip of the spindle goes down.....

Now if you lean the whole piece back towards you (at the top), (Caster) then fold it, you will notice that the spindle angle is dropping more as it comes back towards you, and less as it goes away from you.