Custom Kart KPI / Caster

Lykkan

Member
Hello, I'm building a kart and I've run into a part where I'm trying to comprehend better before I whip out the welder.
I'm using an L bracket with a heim join at the top, I'll be welding the bracket in a manner that adjusting the top heim joint will be adjusting camber as well as a bit of toe it seems.
My spindle is a 12 degree spindle, so what does that mean in setting the king pin inclination and castor angle when I'm welding it together? Does the camber on the spindle mean that I don't also weld in KPI?
I've never worked with designing front steering system before and it's quite a bit to learn!
Thank you
spindle2.jpg
 

flattop1

Dawg 89
Good Question.
I've often wondered this myself .
My thought is when the spindle axle is zero the king pin needs to be at 12* .
This requires the lower hole and heim to be located accordingly . your L bracket with heim centered , you would need to adjust lower mounting hole to suit that .
 

Lykkan

Member
It looks like the way the previous kart that these spindles came from had 12 degrees or more KPI on the chassis, it's hard to tell what camber they ran, so would a 10 degree spindle actually give more camber than a 12 degree in this setup?
s-l1600 (6).jpg

For better clarification, the way that they're mounted here, the shaft actually slopes towards the ground from the kingpin. I think the spindle having the angle in it helps with "jacking" during turns to help unload the inner rear tire. I just need to fully comprehend all of this so I can weld it the right way
 
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flattop1

Dawg 89
King pin inclanation has nothing to do with caster angle .
A 12 degree spindle will have more jacking effect then a 10 degree .
At the same camber angle .
 

flattop1

Dawg 89
Try this .
KPI greatly affects the amount/timing of camber gain/loss that a spindle can create. It's a little hard to explain without diagrams or mock ups but let me give it a try. Think about this...

If you had a 0* KPI setup with had 0* camber and 0* caster on that spindle. Now you turn that spindle 90* to the rear and now you checked the camber. You would have 0* camber. No camber gain at all with a 0* KPI. Now...

If you had a 13* KPI setup and with 0* camber and 0* caster on that spindle. Now you turn that spindle 90* to the rear and now you check the camber. You would have 13* camber. That's 13* camber gain with a 13* KPI.

You would have the same results with a 2* KPI like we run our super late model as you would have with the 13* KPI in the Triton. KPI = camber gain/loss.

Now something a little deeper to think about. I mentioned gain/loss but with the explanation above starting with 0* camber you only have camber gain when turning the the spindle either direction forward or rearward. If that makes sense???

So now let's start with a 13* KPI setup with 3* negitive camber and 0* caster in the RF spindle. Now you rotate the spindle forward slightly and you now have camber 0* camber. Rotate it slightly forward a little more and you now have 3* positive camber. Rotate that same spindle you started with 3* camber forward the full 90* and 13* positive camber.

I explained all of that with 0* caster because it is easier it explain what is happening with KPI. When you add caster into the formula it becomes a little harder to understand. I guess that is why they call it front end "geometry".


Also. I would like to mention that KPI in none adjustable and really has nothing to do with the inclination of the kingpin. KPI is the angle the spindle is welded to the barrel. If the KPI was 13* and you had 3* negitive camber in that spindle the

Feb 22, 2014
Weddle Racing said:
Can someone school me on the KPI when setting up a chassis? Im wanting to know what the angle of the kingpin does for handling, or what it does in general to adjust the angle of the kingpin. I know changing caster changes the angle of the kingpin, but thats about as far as that knowledge goes. Im trying to learn more about chassis setup to improve a few things. Thanks in advance for any help
lol, I just deleted a full page of stuff and I'll try it this way.

Easiest way to think about what it's doing is to think about if you would extend the kingpin on down, using it to point to a spot on the track. The spot it points to will be either towards the inside or outside of the RF tires contact patch and it will be either pointing ahead or to the rear of the RF tires contact patch.

It's an indicator pointing to where forces going to the RF are being projected. Your RF tire is then hung on a spindle. How the RF then will work with the track is influenced by where the force that is pulling it along the track is aimed. To keep the RF tire stable, it's generally going to be puled along by the kingpin, while it's also being pushed into the track by the kingpin.

Remember your the one who is turning the RF tire with the steering wheel. If you get the forces projected way far ahead of the RF tire, the tire will trail where the force is being projected. The more you let the RF tire trail where the kingpin projects, the more it will want to roll on it's own merry way in a straight direction and you'll have to input more effort with your arms to make it turn. Make it point straight down(KPI) and it'll still want to roll a little on it's own way because your still pushing on the RF tire in a turn, but not so much and maybe not so much that you will not even notice it. Make the kingpin point to the rear(which you'll probably never do) and the RF tire will be ahead of forces pushing on it and it will want to flop hellter skelter every which way. That's just about it either pointing to the front or more towards straight up and down or about how the RF trails where the kingpin projects.

Last is how it points left and right. That's mainly about when the kingpin is pushing the RF tire into the track. It's about how you are aiming the push at the tire contact patch. Do you aim it towards the inside of the RF tire, maybe dead center or possibly even a little towards the outside of the RF tire.

That's the easy part. The hard part is you want all the aiming done as needed, when the wheel is turned. And the forces that are going to the kingpin and being projected to the track can come from different directions.

So, ... you have your king pin pointing somewhere when the wheels are straight. Then out on the track you will need to turn the RF tire and have the contact patch of the RF tire presented to the track the way you want it. Then you have weight being sent to the RF corner in the turn. And you want that weight to be projected to the RF tire so it will work with the track as needed. Where your kingpin points is a reference(not exact but just a reference) for where weight is going to be projected. But how it's angled along with camber, etc., also sets up how the tire gets presented to the track when it's turned.

Complicated? yep. I'll post and read this, it's complicated writing about it too. Maybe I'll delete this too.
Quote mr south 59 and mr kish
 

Lykkan

Member
Try this .
naw don't delete it, I comprehended 100% of what you were saying and I really appreciate your time you put into the post.
Do you have recommendations on how much KPI and caster I should weld this at? 10 degrees caster and 13 degrees kpi?
I need at LEAST 12 degrees to be at zero camber in the front end with these spindles
 

flattop1

Dawg 89
). If you need the amount/timing of the camber gain to be more or quicker you will need more KPI angle. If you need less camber gain you will need less KPI.

Theres several things to consider . Kingpin inclanation will affect scrub radius as well .
What use for the chassis oval or sprint .
Too guess ; set both sides the same caster at 10* and king pin at 12*
 

Lykkan

Member
I'm seeking adjustability, but wouldn't mind changing to pill box setup when I can afford the components
 

JPMKarting

Site Supporter
A good neutral start would be to weld the plates at a 12* angle (matching the spindle KPI)

As far as castor is concerned, if you are going through this amount of work, why not go with an adjustable castor setup? I like your self made L bracket, but it doesnt lend any adjustability to the castor.

I would recommend bolting the spindles back on the original setup (if its still there), and measuring the castor of the spindle barrel, and welding the new setup the same way.
 

Lykkan

Member
Nothing is welded yet, and I can redesign my brackets. I put all this **** in cad for the design, I can change anything, I just need to comprehend how caster is adjusted on karts - outside of requiring all the expensive pill box parts and such. I guess I can look into the prices of all the components I'll need. I could weld on a plate with a groove that allows the spindle plate to rotate, then use 2 bolts to lock it into position... I looked into camber plates, but they're about $90 for a set, and it wasn't something I wanted to work into the budget at the time. Anyone have info on a preference of style on camber / caster design that I can incorporate?
 

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flattop1

Dawg 89
Yes thats how you do it .
You can either use a centering pin or just go with slots .
Weld mounting plate on stub bolt L block too that.
Do you need it is the question .
 

JPMKarting

Site Supporter
Nothing is welded yet, and I can redesign my brackets. I put all this **** in cad for the design, I can change anything, I just need to comprehend how caster is adjusted on karts - outside of requiring all the expensive pill box parts and such. I guess I can look into the prices of all the components I'll need. I could weld on a plate with a groove that allows the spindle plate to rotate, then use 2 bolts to lock it into position... I looked into camber plates, but they're about $90 for a set, and it wasn't something I wanted to work into the budget at the time. Anyone have info on a preference of style on camber / caster design that I can incorporate?

Heres what you need:

https://www.bmikarts.com/Spindle-Plate_p_1332.html

https://www.bmikarts.com/U-Weld-Adjustable-Spindle-Kit-with-Heim-Joints_p_1320.html
 

JPMKarting

Site Supporter
I understand you want to cut costs, but if you are going through all this fab work and effort, might as well make it fully adjustable
 

Lykkan

Member
I understand you want to cut costs, but if you are going through all this fab work and effort, might as well make it fully adjustable
I already have much better spindles than whats offered in the bottom kit tho. I'm gonna study a lot today and see what I come up with for a non ghetto solution
 

XXX#40

2A supporter
naw don't delete it, I comprehended 100% of what you were saying and I really appreciate your time you put into the post.
Do you have recommendations on how much KPI and caster I should weld this at? 10 degrees caster and 13 degrees kpi?
I need at LEAST 12 degrees to be at zero camber in the front end with these spindles
gonna race dirt oval or sprint?
You want to weld the entire assembly so that you can adjust in a range +/- from the basline camber you want, same with caster
12 kpi spindle will have less mechanical weight jacking, and less progressive camber gains, kpi is about how the weight is applied
 
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