What's kingpin angle & it's effect on handling?
 

I was looking at some alignment specs for my TME and wanted to know what kingpin inclination angle is and how it effects handling. I also noticed that the TME has the most castor and Kingpin angle stock. I assume these are both positives, true?

Its the angle created by the upper and lower pivoting joints from vertical. The angle causes the tires rotation to create a jacking effect on both tires where caster only causes the jacking on the inside tire.

Its a compromise in production cars to get the scrub radius as close to zero as possible and a minimal kingpin inclination. You'll notice that a lot of double wishbone cars have a pivot point up very high, far above the tire, helping to reduce it and mac strut cars pivot is at the top mount.

There is a relationship between kingpin and camber gain/loss but off the top of my head I cant remember. Its been a long time since I was doing FSAE suspension. I mostly remember trying to get it as close to zero as possible with the scrub radius zero while keeping 30* of steering angle. Not as easy as it sounds when you're the only guy doing it...

Kingpin angle is a necessary evil in suspension geometry. As Dallas said, generally a higher kingpin angle is not a good thing because it adds a jacking force on the front wheel as you turn and it also takes away camber. Caster however adds camber as you turn the wheel, compensating (or overcompensating) for the negative effect of the kingpin angle. Looking at that chart you can see that though the TME has the highest kingpin angle, it also has the highest caster.

In general caster acts to straighten the steering in a turn and give feedback to the driver. More caster makes a car stable at high speeds. Less caster makes a car nimble at low speeds and the steering lighter. Negative caster can make the steering go to full lock if you let go of the steering wheel.

Tires also have an effect that mimics positive caster. Tires don't like to turn - this is caused by the elasticity of the rubber.

This thread hads a little bit on caster and steering geometry:
https://www.evolutionm.net/forums/ev...vs-caster.html

It has a link to this excellent thread on IWSTI:
http://www.iwsti.com/forums/gd-suspe...-analysis.html

- Andrew

Two great threads I cant believe I haven't seen before.

Funny though, Andrew commented on one of the threads "I've tried to get people to rotate/cant their camber plates so they can add caster too but so far I don't know of anyone that has done it.".

I just installed some el-cheapo' tein camber plates (hey, they only need to last one season!) and playing with how I wanted to install them ended up rotated with them set diagonally to max (+)caster and (-)camber. With the stock camber bolts set to full max (-)camber I was able to eek out -2.5* on both sides. Though I'm now wishing I went with Ciro places for more camber, but its good to see my intuition on initial setup was along the right lines :)

^ Nicely done! How much caster did you get?

- Andrew

I have no clue since I do my own alignments but dont have a way to measure caster :(. I guess its time for a real camber/caster gauge.

I just knew Evo's didnt have enough. Its too bad perrin's PSRS isnt legal for BSP.

On another note, only a week and a half before I can get a real review of your springs with the rest of my setup on the ZZ214's. First auto-x weekend is almost here!

I will be doing something similiar, I got Evo 7-9 camber plates which will sit at a slightly cocked angle when installed installed on the earlier cars, so I will be able to get some camber and caster at the same time. I'm just not sure how much of each I will get from them yet.

Just curious, has anybody gone through and measured the stock suspension mounting points and arm lengths, etc? I've been really tempted to while I have everything tore apart. I have so many other car projects going on that I can't justify the time spent on it right now though. I think studying the actual suspension geometry might shine some light on this subject though and make things a little more clear.

As for Kingpin angle, I think a lot of this ties into your wheel selection. With +22mm offset 18x9.5 wheels, I can put the wheel about 1/4" from the coilover and leave the camber plate set to 0 and it gives me 2.5 degrees of negative camber. With my other wheels being +15mm 18x9.5, I can add more camber to get back to that 1/4" clearance using just the strut lower mount. Idea being that the kingpin angle stays the same but I'm getting more camber. The problem though, the scrub radius gets larger.

I could go the opposite way and max out the camber on the camber plates and then set my final camber with the camber bolts. This would give a larger kingpin angle, but a lower scrub radius.

Which is better, lower kingpin angle or lower scrub radius???

I plan on trying to get around 6-7 degrees of caster and about -3.5 camber this year. Here are the parts I'll be using to get there.

Attachment 313903

Attachment 313904

The scrub radius should have a greater effect on the steering feel than on the overall grip whereas the kingpin angle will effect camber during cornering. The loss of camber will certainly have a negative effect on grip. I'm sure it's not that simple but it's the best conclusion I can come to.

OK, I'm really considering measuring out the front suspension. Anybody got any tips on leveling the car to a measuring surface? I don't know if my garage floor is all that flat nor level.

Search for water level or manometer. You can make a cheap one with clear tubing and a bucket. Then find the relative heights of all the points on your floor. Add 1/8" floor tiles to level it. You can also put grease between the top plates to get cheap turn plates.

Realized that I really don't need to bother with leveling the car as I've got solidworks I can just toss all the dimensions into and get my reference planes. I'll use it to rough model the suspension components as well. With COSMOS motion, it should be pretty easy to get all kinds of fun stuff out of the model.

Got all the heights ("Z") on the front suspension mounting points using a water level. I did it a fairly easy way I think. Just taped a yard stick to the wall and the one side of the tube next to it. Pulled the other end of the tube with water in it to the mounting points and started measuring. I don't have a zero point but I have all the relative heights. The unibody was within about an 1/8" of level anyway so no biggie...

I also had taken a bunch of measurements before I tore my car apart so I'll have a relationship between these measurements and then my previous ride height adjustment.


I gotta clean the floor up and get some butcher paper or something now to mark out "X" and "Y" positions. Just planned on a plumb bob to make it happen unless somebody else has a better idea. I don't really want to spend the money for a laser plumb/level right now so that's out.