Ackermann Calculation

[LegendaryPuma]

I've seen teams post that their car have, let's say, 110% Ackermann. I know how the Ackermann, Anti-Ackermann and Parallel Steering System works but I have no idea how to calculate the % of Ackermann. I've searched the forums and I get about 5 different formulas.

So basically I am asking what is the formula to calculate Ackermann percentage.

Thanks

[Claude Rouelle]

That is an academic question. The right Ackermann definition is the one which makes sense to you, that you choose and that makes the car quick, your driver and your tire happy!

[Kevin Hayward]

I agree with Claude. It is much better to think of an inside and an outside tyre angle during steering. It is much closer to what you are interested in which is the slip angles of each tyre in different operating conditions.

Unfortunately the spec sheet still calls for you to present a figure in % ackermann. This should really be updated.

My immediate questions would be at what steering angle? It is possible to have more or less ackermann affect at differing steering angles.

Check out this post:

Post Covering Exactly what you want (http://fsae.com/eve/forums/a/tpc/f/125607348/m/31310782341)

Kev

[Tim.Wright]

Yes, to have a better understanding of the steering geometry you need two values.

%Ackerman and a given steer angle would do the job. I find a more intuitive way is to give:
1. %Ackerman (calculated as below but with zereo static toe)
2. Static toe angle

This way you can visualise the effect better because the toe out gives you the starting point, and the %Ackerman gives you the tendency.

I like the definition of %Ackerman that Adams/Car uses because its simple and it doesn't care what steering system you use. The formula (from memory) is basically something like:

100 x dynamic_toe_out / dyn_toe_out_for_100%_Ack

"dynamic_toe_out" is simply the difference between the inside and outside steer angle
"dyn_toe_out_for_100%_Ack" is the amount of toe out required to give you kinematically perfect steering (zero slip angles left and right)

Using such a calculation would also help stop people thinking that Ackerman is defined as the distance to the top view steering arm intersection point divided by the wheelbase.

[Claude Rouelle]

Legendary Puma,

As a design judge I do not care what Ackermann definition you use. If you tell me if the outside wheel turn more or less than the inside and by how much I am fine.

If you just show me a graph of (outside wheel steering angle - outside wheel steering angle) Vs steering wheel angle I will be even better.

But I will be much more happy if you explain WHY you prefer a pro or anti - Ackermann and HOW you use your understanding of the steering kinematics and tire force and moments and tire models to design and tune your car steering system. I am not interested in just number, I am interested in understanding, skills in problem solving, leadership and pro-activity.

By the way who the heck are you? Would you have the courtesy to introduce your self? If we would meet in person you probably would first tell me who you are, and what team you work with, correct? So... what is the difference in a forum? Because it is on the internet you can hide behind a pseudonym? Is that fair to the people you solicit help from and even more to the ones who answer you? Didn't you have parents or teachers who told you why and how you needed to introduce yourself?

[LegendaryPuma]

Hello. Thank you for your answers. I work them out to see if it works.

Claude Rouelle, I know that I have to explain why I chose "X" over "Y" but my captain demands me numbers so I have to satisfy both him the the judges when the time comes.

Also, I am very sorry I did not formally presented myself. College has been stressful lately.

My name is Wilberto Agosto, a 2nd year mechanical engineering student from the University of Puerto Rico: Mayaguez Campus. I am part of the Suspension and Steering Department from our UPRM Formula SAE team. I am in charge of steering as you may have deduced.

Well, anymore you wish to learn about me feel free to ask.


Wilberto Agosto
U. of Puerto Rico: Mayaguez Campus - Student
2nd year Mechanical Engineering
FSAE 2012-present

[Claude Rouelle]

Welcome to this forum Wilberto!

Which competition does your team intend to compete in?

[LegendaryPuma]

Michigan 2014

[prateek11]

can anyone tell me which is better to have positive or negative ackerman??? why??

[Claude Rouelle]

Can anyone tell me who prateek11 is?