astock165

I have read nearly everything on this forum about alignment so I hope I'm not

I run a lot of a/x and had a unique experience yesterday and want to make sure I understand fully what I think I learned.

I do my own alignments and run -1.75* camber in front and -1.25* camber in back. I usually run 0 toe up front on the street and dial in a bit of toe out at the events (typically 1/8" to 1/4").

Yesterday was an incredibly tight course (bordering on ridiculously tight) that started with a hard 90* left, a hard 90* right, a 180* left sweeper and then a 180* right sweeper. The course also had a pivot cone a little later. Using my usual setup I had about 1/4" toe out up front to start and although the car changed directions quickly, it began to push harder than it has in a long time (since my alignments, poly bushings & camber brace). I adjusted the toe back to zero and then found that the car pushed significantly less, turn in was a bit slower but the loss was negligible. Out of curiosity I then put in about 1/16” toe IN on the front and found little difference in steady state push.

So, when does “turn in” stop and “steady state” begin in a turn? Is it 20% through the turn? 50%? At the apex? Or is it when you stop turning the wheel and ride that steering input through to unwinding the wheel? Conversely, where does toe out STOP helping you?

Thanks for the inputs as always.

XPC5R

Seems to be something past the 1/4" mark for me, especially on faster courses. I normally run 1/4" out up front all the time.

2000BSME

[newbie on toe adjustments] so how are you guys making your measurements?

Slalom4me

What kinds of speeds?

One benefit of static toe-out is that this generates slip angle
for the inner tire, helping turn-in because the inner tire contributes
to cornering before weight shifts outward. Could Ackermann have played
a part in what you experienced?

With Ackermann, as steering input increases, the inside wheel describes
an diminishing radius, relative to the outside wheel. A tight course
suggests more steering angle input than was customary on prior courses.

A combination of increased slip angle (more Ackermann from more
steering lock) and less weight on the inside tires (more weight shift
under low speed aggressive turns) seems like something that could have
contributed to the higher degree of push/understeer.

Perhaps the slip angle of the inner tire went beyond the ideal range
when set at 1/4" static toe-out and reducing the toe brought the inside
tire back within ideal slip angles.

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astock165

What I have done is set toe to zero and then paint a hash mark on both the tie rod end and the tie rod itself. That way you know where "zero" is and will know how many full revolutions you have rotated.

Then I went 1 rev out and measured toe, went a 2nd rev out and measured toe, etc etc. I painted the results right on the inside wheel well so I always have it nearby.

astock165

low speeds, my guess would be less than 20 mph

Hard to say as I'm still struggling to understand what Ackermann is and how it plays into handling.

True, there was much more steering input than on previous courses. I understand the diminshing radius thing.

That's a great description and I think very acurate to what was going on. It also explains explains why less total toe out helped the push. It also shows me that the amount of toe out I put into the car will probably depend on each different course rather than being a set value.

Thanks for your input, I always find your posts incredibly helpful.

RAFTRACER

Anything more than that will make transition response stellar, but you will pick up a mid to out push.

Slalom4me

These images are from diracdelta science & engineering encyclopedia



I didn't know about these articles until I went searching tonight.
They look like superb discussions of the subject - I've only glanced
at them so far but Part II discusses cases like AutoX where relatively
low speeds and large steering angles make good use of Ackermann.
I am not sure if they mention reverse-Ackermann, but Parallel Steering
is discussed as more desirable for higher speed events.
The educational model displayed in the link below may give some
ideas to people interested in experimenting. A low-buck version
can be made with construction paper in minutes.
I think this is a good conclusion. In fact, my vote is that for a given
course, differing circumstances (tire age, track temp and so on) may
further influence ideal toe settings. But since there is a very finite
limit to track time at AutoX ...

Interesting thread. Keep in mind that I am just a lay person.
Read critically, challange often.

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