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Author Topic: The Ackerman Angle is not what you think.  (Read 85 times)

zorproducts

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The Ackerman Angle is not what you think.
« on: June 08, 2018, 12:06:18 AM »

Recently a friend who was on Facebook asking questions about an Astro Van steering system had some replies that were not detailed enough to answer his questions.


Ackerman Geometry is math based on lever ratios. Being a factory trained certified Scale Technician I have experience in the math of levers. I have reposted here my explanation to my friend.


In answer to your question it’s called the internet. It’s main feature is SEARCH

So when you do a search on the internet “Ackerman formula, Ackerman steering”  you will see the explanations are about how to get the toe out to go more inward on a circle on one tire than another. Or basically how to make the inner tire when turning in a circle to lean more into the circle.  It is a formula conceptualized  for race cars to hug corners better. Unfortunealty, It has often been referred to as, “a method to prevent bump steer” which is not entirely false. Just often explained on the wrong plane. So many confuse panhard bar parallel with steering parallel and steering geometry angles can be confusing when applied to a pivoting steering arm with fixed location pivot points whether ball joints or heims.

A draglink that is higher on the pitman than on the steering arm on passenger side makes for a bad angle for “bump steer” but has nothing to do with Ackerman Formula which occurs from the steering arm geometry. It's a lever principle. The knuckle pivots on the trunions, the tie rod pivot is farther out the lever than the knuckle pivot point and the drag link pivot point even farther thus you have various but defined arc movements.

Or another way to say the same thing is

On a Over The Top steering arm system whether a stage 1 or a complete OTT system including the driver tire steering arm and tie rod called stage 2 by previous manufacturers. The Ackerman is related  to the tie rod arc movement in relationship to the steering arm arc movements.

The tie rod ball joint is behind the ball joint from the drag link so the movement of the draglink will pivot the knuckle further because it's farther out the lever. The drag link  ball joint doesn't matter which angle it is being pushed on, the Ackerman is in the steering arms.

What matters for bump steer is how parallel to tie rod the drag link is and a lower pitman arm that is sturdier with a fast turning ratio is a plus.

A forward steer arm allows more clearance for moved forward axles. In comparison what are the disadvantages to using a rearward steer pitman arm that is higher.
A) possible contact of drag link and tie rod in a articulating situation
B) some rear steer arm boxes have to be mounted farther forward along frame.
C) running a pitman arm between frame and springs without making contact can be a little tricky.
D) higher non parallel angle to tie rod increases bump steer.

It's just math.



So are you clear on that Ackerman is how to make more toe out on a tire in a turn?
That math is in the steering arm design or where the ball joint positions are.
The steering arm gets pushed and pulled by drag link the more parallel it is to tie rod on on tie rod plane (looking at it from front) the less bump steer you have. Pushing and pulling on steering arm isn’t changing Ackerman angle.

 
« Last Edit: June 08, 2018, 06:04:49 AM by zorproducts »
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BamaRob

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Re: The Ackerman Angle is not what you think.
« Reply #1 on: June 08, 2018, 04:59:52 AM »

Excellent explanation!  On an editorial note, I think when you are using "plain", you should be using "plane."

plane
plān
noun
1.
a flat surface on which a straight line joining any two points on it would wholly lie.
"the horizontal plane"
synonyms:   flat surface, level surface; horizontal
"a horizontal plane"
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zorproducts

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Re: The Ackerman Angle is not what you think.
« Reply #2 on: June 08, 2018, 05:58:54 AM »

You are correct, thank you. Those  midnight explanations and auto correct can get me in trouble, right?
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