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shreyashbalpande View Drop Down
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    Posted: Jun/02/2015 at 6:43am
What does ackerman percentage above 100% means ?

Is it like -
0% - Parallel steering
below 100% - Understeer
At 100% - true ackerman
Above 100% - oversteer
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richie_Dagger Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2015 at 12:06pm
It means your inside wheel is turning too sharp relative to the outside wheel. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SOM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2015 at 12:58pm
% ackerman = (angle inside wheel - angle outside wheel)  / angle inside wheel for 100% ackerman

and for 100% ackerman angle inside wheel u can get from ackerman condition

cot(outside wheel angle) - cot (inside wheel angle ) = track width / wheelbase
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Goost Quote  Post ReplyReply Direct Link To This Post Posted: Jun/02/2015 at 1:05pm
I saw many students miss this in Design comp at Auburn this Spring - in addition to answering your question I will try to give some direction on why it matters.

Simple response:
There are often bad definitions of this 'percentage' concept, confused by two useful meanings of 'Ackerman steer angle'. You are mixing the two - a reasonable error I think because few sources clarify which they are using or That there are two!

~~~

Definition 1) regarding steering angle of the Vehicle relative to the Turn. The steering angle required to turn with turn radius [measured from turn-center to vehicle-centerline-at-rear-axle] (call it 'R') is a function of wheelbase (call it 'l') and that turn radius. This steer angle is called the 'Ackerman steer angle' (call it 'd_ack') and can be found using trig:

d_ack=atan(l/R)

This is an important concept in oversteer under steer - it defines the steering angle of the not-slipping vehicle at low speed, and is in a sense the 'idealized' steering response. However, we know real vehicle Do slip and not in equal amounts front and rear (based on tire choice and CG location and speed! and etc.). So we have this concept of
A) 'over-steer' steering required to maintain turn is less than d_ack
B) 'Neutral-steer' steering required ... Is exactly d_ack
C) 'Under-steer' steering required ... Is more than d_ack

This is Not a Constant for all conditions, so we look at the slope at any point - the gradient or ratio. It is particularly useful to see the effect of Speed - the concept can be tested on a constant radius (ie constant l/R) skidpad by starting at very low speed and slowly speeding up until the limit of grip throws you in/out of the turn.
This limit behavior leads to a joke among some racing enthusiasts that
Under-steer is when you hit the wal with the nose of the car
over-steer is when you hit the wall with the tail of the car
[pop quiz - is this description close to correct?]

So this concept of Ackerman is typically Not presented as a percent because it actually has units! if you like it can be presented as deg/G (degrees of steering - d_ack)/(lateral acceleration) for instance. And even this slope/gradient/ratio Not constant! [pop quiz - is it constant with linear tire grip vs slip characteristics?]

Ok so enough on that one except to say - 'X% Ackerman is Not useful in this context'. Moving along

~~~

Definition 2) regarding steer angle of the Inside tire relative to the Outside tire. This is probably the important one for Baja suspension design. Wikipedia covers this one well.

When going around our turn of radius R, it is quickly apparent that the inside tire and outside tire Don't Follow the Same Radius! They are different by the vehicle track width. Note that the diffence is very small for race cars on fast circuits, and very large for Baja cars and some slower road racing (autocross for instance). So back to the slow speed no slip argument - we now look at the ideal path of inside and outside tires, and see that they are different. It is possible to make both tire almost exactly achieve their respective turn radius such that neither tire slips/slides in the corner. Supposedly this originated as an invention for keeping the gravel in fine driveways from being disturbed by a carriage. Anyway, we can find the exact angles again for any given turn radius - however to Plot the response of steer angle vs turning radius, we would have two (seemingly very similar) curves for inside and outside tires. If we realize that Actual turn radius is ~proportional to Outside tire turn radius, we can plot (inside steer angle) vs (outside steer angle) and actually see what's going on! So if you have parallel steering both tires turn exactly the same amount and so the plot is a straight line with slope 1. If you have perfect Ackerman steering there is a curved line - the exact curve depends on your choice (aw...) of the Ackerman angle from definition 1, above. [I mean choice of outside tire, average tire steer, CG, or whatever in choosing R]. Regardless, the plot should show the inside tire turns more than the outside tire, because it follows a smaller turn radius! Now to answer the question of what is the 'percentage' Ackerman: the % is Usually defined as
(Steer angle of inside tire relative to outside tire)/(Ackerman suggested steer angle of inside tire relative to outside tire)*100%

So, considering that the outside tire is 'on a rail' or Not Slipping, this implies:

A) <100% Ackerman : inside tire is trying to push/roll/slip Out of the turn
B) 100% Ackerman : inside tire rolls smoothly along turn
C) >100% Ackerman: inside tire is trying to pull/roll/slip Into the turn.

the Only possible reason to have <100% is because of some knowledge about tire load characteristics and peak slip vs load (or you Want to be slow).
[popquiz - there is a reason to want >100% Ackerman on very tight courses - why?]

So shoot for 100% Ackerman (or more?) in your design!

And that's enough for a day and was probably far more than you wanted as an answer. Answers to the rhetorical popquiz questions tomorrow and sorry for typos, I'm on a phone.






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Post Options Post Options   Thanks (0) Thanks(0)   Quote njgedr Quote  Post ReplyReply Direct Link To This Post Posted: Jun/03/2015 at 8:51am
Goost,

My understanding has always been that less than 100% Ackerman is best because of the Lateral Load Transfer. Basically, because of the added load on the outside tire it can handle more slip angle than the inside, before peak lateral grip is reached (Source: Tune to Win). I am interested to hear why more than 100% would be an advantage. Only thing I can come up with, is a yaw effect from a higher longitudinal drag on the inside of the car. I don't think tire data ever covers longitudinal force based on slip angle... so tough to verify on paper.

For FSAE I setup at Parallel steer after looking at tire data to match peak lateral slip angles inside to outside at a median radius and speed turn. However, I had a design judge inform me that most the teams that are winning have over 100% Ackerman. Obviously, they know something I don't. 

Oddly enough, Even though Tune to Win describes what I mentioned above and recommends parallel steer and a general insensitivity to Ackerman. I saw a youtube video of Carrol Smith giving a design review to a FSAE team, where he claims "The more Ackerman the better!". That is the problem with the internet, Now we can keep these racer folk honest! I felt betrayed haha.

For Baja, we made plates to experiment with Ackerman, but since we were rear steer (rear of upright), it reduced our toggle (Tie Rod went straight). We found that more toggle made the car much better than more Ackerman and ended the study there. 

I don't think you could over do toggle for baja. Maneuverability courses are based on lawnmower requirements at most events. Probably has to do with Briggs and Deere being so involved. Market research for next gen zero turns. 

   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/03/2015 at 10:34am
An interesting discussion point: How does Ackerman work in highly cambered tires?  Baja/Atv tires are typically much more rounded and have less shear ability in their tread, so we often run them at high camber angles.  Ackerman is typically presented in a top down format with tires with little to no camber.  Which plays directly into the high speed / low speed turning concept, since turning at high speed results in high load transfer and typically large rotations of the baja car.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote krowx Quote  Post ReplyReply Direct Link To This Post Posted: Jun/03/2015 at 2:52pm
The best read i have found is this one here--http://www.me.ua.edu/me364/PDF/Steering_Ackerman.pdf

It really helped me understand and design my steering system. It also determine that other factors will play a larger role in steering than the actual steering angle. Such as Camber, Caster and bump steer and the slip angle.

Happy Designing
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jun/03/2015 at 6:00pm
An example of machine for slip angle measurement:

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pedro UFPBaja Quote  Post ReplyReply Direct Link To This Post Posted: Jun/03/2015 at 6:21pm
and other


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Post Options Post Options   Thanks (0) Thanks(0)   Quote njgedr Quote  Post ReplyReply Direct Link To This Post Posted: Jun/04/2015 at 8:32am
Karson,

Thank you, this is excellent. The last page in particular spells it out nicely.

Regards, Noah
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/04/2015 at 10:51am
Investigation of Lateral Performance of an ATV Tire on Natural, Deformable Surfaces
Krueger, Darrell 2007

http://hdl.handle.net/10415/1350
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Goost Quote  Post ReplyReply Direct Link To This Post Posted: Jun/04/2015 at 11:48am
krowx, that pdf is very interesting, especially the last page.

There is still a trap in the definition:
A) if "anti-ackerman" means "% Ackerman < 100%" then this all makes sense.
B) if "anti-ackerman" means "% Ackerman < 000%" ...

Unfortunately the word implies definition B, and many people design based on that...
Think about it - if the peak slip angles are within ~2 degrees for loaded and unloaded tires, how could you ever want them farther than 2 degrees from pointing at the turn center - i.e. a feww percent away from 100% ackerman??

~~~

Well there is one way - you might want More than 100% ackerman: The comments about inducing a yawing moment are crucial here.

Can anyone whose team uses a cutting brake who has played with this tell us about their steering geometry? I would bet that a car with >100% Ackerman could eliminate the cutting brake, saving weight and time and training...
Any experience? Ackerman can also be used to offset the anti-yawing moment that a spool creates, compensate for bad caster/kingpin choices, etc.









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