Steering Paradox

[Edward M. Kasprzak]

Going back to the IP: Davis and Ackermann steering have nothing to do with where the rack is located. "Davis steering" uses sliding connections between the links while "Ackermann steering" uses the vastly more common rotating joints and fixed-length links.

Crispy's post makes a lot of sense. I'll take a plot of outside wheel steer angle divided by inside wheel steer angle vs. steering wheel angle anyday. That's what matters, right? The simple geometric approximations and associated terms ("percent Ackermann") can give a reasonable starting point and a general idea of the behvaior, but cannot describe the full range of operation due to their assumptions.

[DougMilliken]

Originally posted by Edward M. Kasprzak:
Going back to the IP: Davis and Ackermann steering have nothing to do with where the rack is located. ...

Ackermann's name is used in at least three different ways in automotive engineering --

* He gets credit (historians can debate if it is deserved) for individual front wheel steering (two pivots, one for each wheel), as opposed to horse & carriage steering with a single steering pivot in the center of the axle.

* Ackermann correction -- the topic of this thread. Steering all the wheels so they turn about a common turn center, in the absence of tire slip angles.

* Ackermann Steer Angle -- "the angle whose tangent is the wheelbase divided by the radius of turn". Quoted from SAE Vehicle Dynamics Terminology, J670e. In other words, the "theoretically correct" geometric steer angle required for a turn, often referenced to the vehicle centerline.

More history in RCVD p.713-14. Also Wm. (Bill) Mitchell (WinGeo author) wrote an SAE paper on the accuracy of various linkages that generate approximate Ackermann corrections.

-- Doug Milliken

[axe27]

hi guys...!! i m in a bit confusion abt antiackerman geometry..!! our vehicle suits anti ackerman. i m a bit cnfused abt location of icr for antiackerman geometry. where it must be located above front wheel axle unlike ackerman..???

[Marvel]

Originally posted by axe27:
hi guys...!! i m in a bit confusion abt antiackerman geometry..!! our vehicle suits anti ackerman. i m a bit cnfused abt location of icr for antiackerman geometry. where it must be located above front wheel axle unlike ackerman..???

You were able to justify that "our vehicle suits anti ackerman" but cannot locate "ICR" for anti-ackerman???

ICR for anti-ackerman must be somewhere ahead of front wheels centerline...

(method for ackerman and anti-ackerman is same..just rotate yourself 180degree upside down and proceed to get ICR!!!!)

[axe27]

marvel..i know that actually , but in a way i want to ask how i can constraint it..?? for ackerman it lies on extended line drawn through rear axle...likewise for ant-iackerman...???

[Claude Rouelle]

I gave up looking at the exact definition of Ackerman: I have see so many...

At the end I simply try to look for an answer to the 2 following questions: a) which one of the inside or outside wheel steers the most for a given steering wheel input and b) by how much.

You can find out the answer with a simple 2D model in Excel or a more sophisticated kinematics software like OptimumKinematics. I suggest you start with a simple 2D model (make it useful before you make it complicated)

5 quick remarks

1. As mentioned by other guys in this forum you can have the same Ackerman number whether the longitudinal position of the steering axis is ahead or behind of the front wheel axis. The issue will be more about packaging when you design your steering system and the front upright; if you want a pro-ackerman and the steering rack is ahead of the front axis you could have the toe link outboard rod end right in the brake disc! Every car part is linked

2. Think about the the influence that the steering rack position will have about the steering column and the steering wheel angle and how much universal joint or CV you will need. Again, every part of the car is linked.

3. Judges often ask where your front and rear roll center are. Or how much camber do you run. The question is oversimplified because it is about roll centers position and camber at static ride heights. The roll center can move laterally and vertically. Also you camber will change in heave and roll and steering. I would prefer to see CURVES of roll center and camber movement in roll and heave and steering. Similarly Ackerman number won't necessarily be a fixed number; the ration inside / outside wheel steering angle will most probably change. Better come with a curve than a fixed number

4. No need to be accurate bu 0/1 % Ackerman, when your compliance will be taken into account, this accuracy will not be relevant.

5. At the end how much Ackerman do you need, or which wheel need to be steered the most and by how much?
2 answers:
A) Use you tire models and see for a given vertical load and given camber how much slip angle each wheel needs to get the most lateral grip. The inside outside delta needed slip angle will give you a pretty good idea of the delta steering angle you need. It will give you a very theoretical answer but at least the study will help you to understand how to get for the ideal steering system. You will see that the ideal / theoretical Ackerman for tire A is not the one for tire B. Be careful; if you did a good job it could be the front grip will be much improved ad the driver will start to complain about oversteer.
B) The practical answer; TEST! Try different Ackerman solutions and ask your driver how he feels, look at the lap time and your tire temperatures (ideally with IR temperature sensors). In Nebraska I saw the Brazilian team EFI having a bracket (which hold the outside toe link rod end) bolted on the upright via shims (as used in camber adjustment) between the bracket and the upright. Not perfect in terms of compliance but so nice, cheap, useful to test different Ackerman. The student told me the initial Ackerman made the car balance very bad but by changing the Ackerman with shims they got the car better and better.

Have fun!

Claude

[Z]

Originally posted by Claude Rouelle:
A) Use you tire models and see for a given vertical load and given camber how much slip angle each wheel needs to get the most lateral grip. The inside outside delta needed slip angle will give you a pretty good idea of the delta steering angle you need.
...
... the study will help you to understand how to get for the ideal steering system.

Claude,

Clarification please.

Are you suggesting that if the more heavily loaded outer tyre reaches peak lateral force (peak Fy) at, say, 1 degree greater slip-angle than the inner tyre, then the steering should be anti-Ackermann (ie. the wheels should toe-in when steered away from straight ahead)???

Z

[Mike Cook]

Really Z,.....
Even if the heavier loaded outside tire desired more slip angle than the lower loaded inside tire, it does not mean the tires would be toe-in when steered. I consider 100% ackerman to be the geometrically correct steer angles for low speed, no slip conditions. For our skid pad this might be about 12deg on the outside tire and 14deg on the inside. This is clearly a toe out condition. As we increase the speed of the vehicle the slip will increase equally on the front two tires. If at peak lateral acceleration we want a total of 8 degree slip on the inside tire and 9 degree slip on the outside tire, we would need to change the steering geometry so that the outside tire is steered 13 degrees and the inside is steered 14 degrees (approximates only). At this point, we would still have a toe-out condition, and more slip on the outside tire than the inside.

And looking at the tire data this is usually the case. This is why most teams run less than 100% ackerman (i.e. more slip on the outside than inside tire).

[Z]

Originally posted by Mike Cook:
Really Z,.....
...
This is why most teams run less than 100% ackerman (i.e. more slip on the outside than inside tire).

Mike,

I will wait for Claude's response before going into more detail. But I reckon any team running, say, 50% Ackermann, based on arguments similar to above, is losing a lot of performance.

Z

[Claude Rouelle]

Even if I have a feel for it, I honestly do not know what the ideal Ackerman is for a FSAE. It depends on so many factors starting by the tires, the car, the circuit, the driving style etc...

A few hints

1. You do not per say "decide" the front (and rear) tire slip angle, the car (think for example about the contribution of the rear tire to the whole car slip and yaw acceleration) and the driver do.

2. You cannot look at slip angle only on the kinematic way; you need to involve forces and moments generated by the tires. That is what the whole method of the yaw moment Vs lateral G created by Milliken is about

3. On a given car (given tracks and wheelbase, steering geometry etc...) besides the initial toe in or to out setup, bumpsteer and steer by compliance, there are 3 causes for tire slip angle
- CG slip angle Beta
- Yaw velocity r (or speed V and radius r)
- And steering delta

4. There are 13 causes for the yaw moment
- 4 tire Fy
- 4 tire Fx
- 4 tire Mz
- The aero yaw moment (If you have big wings and big side plates it will not be insignificant)

5. I have seen several cars (including very successful FSAE) winning races with anti-Ackerman. Some pictures of this website are very revealing. I can also tell you that some F1 do not use the same Ackerman at a quick track like Monza and at twisty circuit like Monaco. Best prove that ideal Ackerman is circuit dependent.

6. There only 3 way to decide what is the ideal Ackerman and each is more and more useful but you need to go though each step.

A. Analyze your tire data. Make a grid of vertical load and camber and see what the ideal slip angle (the peak slip angle) is for each condition. That should give you at least a qualitative appreciation of what your Ackerman should be. I say qualitative because there no in-lab tire testing which is spot on with the grip and moment numbers compared to a race track (well which race track, which asphalt is the closest for the in-lab Calspan test..?)

B. Create a graph Yaw moment vs lateral acceleration. That is THE method which was created by Millikan and his book described it very well. That is THE method which will tell you the influence that weight distributions, camber variation in roll and heave and steering, spring and their motion ratio, ARB and their motion ration, downforce and downforce distribution.... you name it... and of course Ackerman will have on Grip, Balance, Stability and Control. That is the method that the best FSAE teams use and that is one of the methods that makes our OptimumG customer winning races. Try different Ackerman and you will see how grip, balance, stability and control change.

C. TEST!! That is the ultimate solution. You need to manufacture a front upright where you can change the Ackerman. However, even if you find the perfect Ackerman and the quickest car, the 2 previous steps are still indispensable: in FSAE / FS the goal is not only to be quick but to know WHY you are quick.

Claude