Determining Ackerman Steering Geometries 3

[motoboy]

How do you determine steering arm angles for Ackerman steering on a front steer axle (ie. steering linkages in front of axle). I've seen the "draw a line through the kingpin axis to the rear axle approach" for rear steer axles but applying the determined steering arm angles to a front steer setup doesn't yield as good an approximation of Ackerman. Any insight into the reasoning behind the aforementioned method would also be appreciated.

 

[GregLocock]

I model it in a suspension program and it spit out the answer.

Realistically my main interest in Ackerman is turning circle, and to a much lesser extent, parking efforts.

So far as I know the construction you suggest is just as (in) accurate for steering ahead of the axle as behind.

Cheers

Greg Locock

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[NormPeterson]

Any insight into the reasoning behind the aforementioned method would also be appreciated.

It's an attempt to have all four tires turn about a single point so that all four tires roll freely without any lateral scrubbing while cornering. Legend has it that "to avoid disturbing the gravel in the driveways of the well-to-do" may have provided additional early incentive for development of this concept. Understand that the idea of steering the front wheels by different amounts dates back to the 1870s, with all that implies with respect to road, vehicle, wheel/(tire?) development and cornering speeds/lateral accelerations.

The method itself appears at least in part to make use of the nonlinearity of the sine function and the angle the tierods make with the steering arms rather than just the angle of the steering arms themselves. You can get ~parallel steer, >100% Ackermann, or anti-Ackermann just by moving the rack longitudinally in the vehicle and affecting this angle.

When you apply that angle from the kingpin axis to the rear axle centerline to the steering arm, are you using that for the physical angle of the arm itself or as an angle to locate the coordinates of the outer tierod pivot relative to the kingpin axis at the steering arm elevation and then building a steering arm to fit? The presence of any steering arm lateral offset means that the virtual steering arm doesn't coincide with the physical one. If you're working with the physical arm orientation I think that any given offset would push the intersection of the construction for a front-steer arrangement back faster than it would pull the intersection for a rear-steer arrangement with the same length steering arms forward, though I don't know which would represent the greater effect on %Ackermann.

Anyway, and as Greg suggests, it's at best an approximation thats most applicable at low levels of lateral acceleration. Lots of things are being assumed in the basic construction that's typically illustrated, including zero static toe at the front and zero static toe/zero toe change at the rear (none of which is necessarily true and usually none are).

Understand that the relation between where the steering arms are pointed and the directions in which the tires are actually traveling is not mechanically fixed either. Once you have much of anything going on in the way of slip angles and lateral load transfer, by definition you don't have pure rolling any more, contact patches move around, and the construction itself will be "off". Hence the use of computers (or lots of instrumented test-and-tune).

Unless you're designing primarily for low speed on unpaved roads (golf cart?) and/or really short-radius turns (fork lift?), there isn't anything particularly sacred about 100% Ackermann correction.

Norm

 

[mloew]

motoboy,

I have done this in a CAD program capable of handling mechanisms. I am using the optimization capabilities to minimize the maximum error throughout the steering range. My procedure allows for the optimization to be automatically regenerated when other changes are made (track, wheelbase, etc.) You need to figure out what your objective function is then control the influences to optimize the performance.

Best regards,

Matthew Ian Loew


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[GregLocock]

Incidentally I might as well record here how one person sets up Ackerman for race cars:

Set front axle toe to 'normal', and Ackerman to zero

drive round track

add x degrees of toe in

drive round track

iteratively adjust x (somehow) in order to optimise whichever corners (the fast ones or the slow ones) feel better with toe in.

set y degrees toe out

drive round track

iteratively adjust y (somehow) in order to optimise the other corners which should feel better with toe out.

Now use Ackermann to give you x in the slow corners and y in the fast ones, or vice versa.

Cheers

Greg Locock

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[NormPeterson]

Star for that, Greg. Ought to be useful somehow, sometime . . .

Norm

 

[JakubMech]

Am I right to say that theoretically akerman should be adjusted to match the tyre lateral friction data. Pro or anti akerman, depending on if the peak lateral forces generated by the tyre happen at increasing or decreases slip angles as normal load increases?

JMech

 

[mloew]

JakubMech,

If you are trying to maximize lateral acceleration, you would be correct. If you are trying to minimize the turning radius at parking speeds, you would not be. This is why I said that "you need to figure out what your objective function is then control the influences to optimize the performance." This problem is just like all others: it depends on what you want to do.

Best regards,

Matthew Ian Loew


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[evelrod]

"...it depends on what you want to do."

A lot of "cut and try" in setting up a race car suspension as Greg indicates, only a LOT more involved. Point of fact, ackerman, in most race car setups, is irrelevant to cornering speed as the inside tire is generally lightly loaded (indeed, often unloaded). Bump steer is much more critical---but that's another question.
Chapman dialed out all the ackerman in my Lotus but, what the heck do I do with the ackerman in my vintage racing mini? If you have not lived with a mini, this will be a dumb question.

Rod

 

[mloew]

Rod,

The original poster did not specify the type of vehicle in question. I would guess that many more engineers posting on this forum for engineering professional are working on non-racing vehicles as part of their profession. Professionally employed engineers asking these types of questions are unlikely to be employed by professional race teams.

Best regards,

Matthew Ian Loew


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[evelrod]

Actually, Matthew, I thought at first it was a homework question when I first saw it a few weeks ago. Perhaps it was the "front steer" termonology that pointed me in the direction of "race car".
Sorry, I am predisposed toward that venue.

Rod

 

[mloew]

Rod,

No harm done.



Best regards,

Matthew Ian Loew


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[motoboy]

The application is a 6 wheeled military trailer with tandem rear axles and tongue/tie rod steering system on the front axle.

 

[mloew]

Very interesting. I am working on a similar project.

Best regards,

Matthew Ian Loew


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