How do you define Ackermann Percentage?

[fora]

hi
how do i define ackermann percentage ...
i have seen teams writing ackermann percentage of steering system ...is it when the car is at zero steer angle or at maximum steer angle (lock position)

looking forward for ur help

[Brian Evans]

Zero steer angle.

[BenB]

I also had this question before. Can somebody elaborate a little more on this?

Does +6% ackermann mean that the inside tire will turn 6% more than the outside?

I assume thats correct, but I was hoping some of the experts here could confirm thathttp://fsae.com/groupee_common/emoticons/icon_smile.gif

[Doc Ron]

im not sure on this either...

regarding the above: that would mean that parallel steer would be 100% ackermann - which is incorrect, right?

i think its percentage based off its variation from perfect ackermann (i.e. all tyres turning about a common centre point).

[Doc Ron]

oops.. my bad, i read your post wrong.

im lost all over again

i cant believe milliken and milliken doesnt elaborate more on this... or diagonal weight transfer either... pretty much has nothing regarding steering

[STRETCH]

I believe the correct way is as follows...
I have not read this anywhere, but based on the fact that parallel steer is 0%, and 100% is when the steering arms can be projected back to the rear axle at the vehicle centreline, then the range from 0-100% is between this geometry.
Hence, if you find the angle that the steering arms vary from straight ahead, as a percentage of the angle required to intersect the said point on the rear axle, that is your ackermann as a percentage.
Don't ask me about Anti-ackermann, but I would guess to be consistent you stick to the same calculation, just quote the percentage as negative?
Hope this is the 'standard', not that I can find any!

[JHarshbarger]

Originally posted by STRETCH:
I believe the correct way is as follows...
I have not read this anywhere, but based on the fact that parallel steer is 0%, and 100% is when the steering arms can be projected back to the rear axle at the vehicle centreline, then the range from 0-100% is between this geometry.
Hence, if you find the angle that the steering arms vary from straight ahead, as a percentage of the angle required to intersect the said point on the rear axle, that is your ackermann as a percentage.
Don't ask me about Anti-ackermann, but I would guess to be consistent you stick to the same calculation, just quote the percentage as negative?
Hope this is the 'standard', not that I can find any!

The "steering arms intersecting the rear axle" method is only correct when the tie rods are not angled forward or backward. The angle between the tie rod and the steering arm are what decides Ackermann geometry. 100% Ackermann geometry is when both wheels are traveling in concentric circles and parallel steering (0%) is when the wheels are traveling in equal circles.

The amount of Ackermann will change with steering angle, but most teams will simply use the Ackermann percentage at a particular steering angle and corner radius as their Ackermann percentage. Using a corner radius equal to their wheelbase is common. The percentage of Ackermann would be equal to the percentage from 100% Ackermann that your particular steering geometry exhibits. For example, you use an inside wheel steering angle of 15 degrees and the outside wheel is at 12 degrees. If 100% Ackermann is when the outside wheel is at 10 degrees, then you would have 60% Ackermann steering. If in this example your outside wheel is at 17 degrees, then you would have 40% anti-Ackermann geometry. Remember, this is only for this particular corner. The percentage will change at different steering angles and may be progressive or regressive.

This is how I understand it. If someone has another method, please feel free to chime in.

[C.Bradley]

After serveral hours of looking for the "quoted" Ackerman percentage, I discovered this explanaation in the Optimum K software help file, which I've found quite useful as an introduction to basic Kinematics in conjuction with Milliken's RCVD.

% Ackerman = (Angle Inside Wheel - Angle Outside Wheel)/Angle Inside Wheel for 100% Ackerman

Where the inside wheel angle for 100% Ackerman is:

Tan-1(WB/(WB/TAN(Angle outside wheel) - Front Track)) - Angle of outside wheel


I'm not sure if the single figure quoted at events are the static Ackerman angle or full lock angle

Craig

[Edward M. Kasprzak]

I used to wonder about the % Ackermann definition, too, but now I think of it in terms of the K&C "steer-steer" tests. That is, how much steering at each front wheel per degree at the steering wheel. The curves (and their difference) can be plotted against steering wheel angle, giving a very good indication of what's really happening. The curves are rarely linear anyway, so the % Ackermann figure is at best a point linearization.

[SusProg3D]

In the production automobile world they generally refer to "toe out on turn" rather than Ackermann percentages.

The standard measurement seems to be with the inside wheel turned 20 degrees, and then the turn angle of the outside wheel should be as specified.
If the outside wheel is less than 20, then there is some Ackermann; if greater than 20 then there is some anti-Ackermann.

Of course, as Joel mentioned, this single figure tells you nothing about the Ackermann at any other steer angle.

Bob

[flavorPacket]

Our steering is purposely highly nonlinear. Static ackermann is less than 1/2 of what we can 'see' on track.

I think it's time that the spec sheets change to reflect advances in understanding among FSAE teams.

[ben]

Difference in left and right steer angle as a function of outer wheel or average steer angle is more physically meaningful rather than a percentage.

Agreed on the spec sheets. I'd also like to see the kinematic roll centre migration bit taken out as well - it's reinforcing an orthodox view that isn't supported from an engineering point of view.

Ben

[J. Vinella]

Originally posted by ben:
I'd also like to see the kinematic roll centre migration bit taken out as well - it's reinforcing an orthodox view that isn't supported from an engineering point of view.

Ben

Ben, I totally agree with that statement.

As far as ackermann:

The driver has more control over the steering system than any other aspect of the car. The driver cannot call up a burst of torque from the engine but the steering is directly connected to the drivers hands. My feelings are that ackermann is a corner entry phenomena and your toe plays just as much if not more a role in this respect. Therefor "optimizing" ackermann is just one way to reduce the negative effects of toe angle. If you get your settings "wrong" then the driver will adjust.

[flavorPacket]

Well, John, that's true, but you can also use Ackermann to influence steady state slip angles (e.g. in skidpad). We've had great success changing the top of our understeer gradient in this way.

[me.phadke]

I have searched for Static Ackerman(Akermann) on this forum and got about 4 to 5 different methods to calculate it.

What is the correct one among these and what have teams(who qualified the design inspection) written in their design spec sheet?

[flavorPacket]

As long as you understand how you defined it, it shouldn't matter. But the equation used above is what we use and what most professionals use.

[ChristianChalliner]

I've used the formula above and the answer that is thrown out is clearly wrong, would someone mind giving an example calculation as I'm pretty sure I've got a bracketing error occouring somewhere but I can't seem to find out where.

I've attempted different positionings of the brackets but I still get bizaree answers.

(the formula I used was Ai=tan-1(WB/(WB/tanAo-Ftrack))-Ao)