ackerman

I decided to focus in the cheapest two front suspension options.
1)  Move the rack far enough forward to get out of the way of the stuff, but still rear steer
2)  Swap the left and right knuckles

The issue that makes or breaks either of these is options is ackerman.  When driving in a circle, the inside tire is going around a smaller circle than the outside circle.  Ackerman steering refers to the system of linkages in the car that corrects for this (sends the inside wheel on a smaller circle).  This is done with the steering arm shown in dark brown below.

For perfect ackerman steering the lines extended from the axis of rotation of each wheel intersect at the line coming from the rear axle (so that all wheels are turning about the same instant center).  When going straight ahead. this means that a line extended from the arms intersects at the rear axle.

We don't want perfect ackerman though.  When a wheel turns the rubber in the contact patch is getting twisted such that the car is travelling straighter than the tire is pointed (called slip angle).  This exact angle depends on lots of things, but since we know that the outside tire is doing most of the turning, it will have a bigger slip angle.  This means that while both tires are turning more than the car is turning, the outside tire has to do this more than the inside tire.  So we need a little less ackerman than perfect ackerman (we will call it anti-ackerman)

Carroll Smith writes in engineer to win that while what I described above is the prevailing wisdom up to the time he wrote the book, he found that using more ackerman than perfect (pro-ackerman) actually led to some really nice handling characteristics.  Particularly that the car was looser (more prone to oversteer).  I would like to speculate on why:

With anti ackerman, if you hit a bump or temporarily lose traction with the outside wheel, the inside wheel suddenly gets more load and has more influence on the direction of the car.  With anti-ackerman, the inside wheel is trying to send the car into a larger radius turn (understeer).  With pro-ackerman sudden loading of the inside wheel will pull the front of the car into the turn and initiate oversteer.  This achieves a similar effect as a static toe out setup without some of the other flakiness that goes with it.

So next we need to describe a way of measuring this.  Then we can look at how ackerman changes when the rack moves forward or backward, or if the steering arms points in a different direction.  This is already getting long, so I think I will break this here and look at that in another post.

Here is a pdf document that describes some basic suspension terminology.

Here is a great article about ackerman that references a few articles and texts.