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My opinoin is that the only time you will ever notice if a rig has "close" to the right ackerman is when turning really tight (ie parking lots, turning around on a street or on a trail, or turning to full lock on the rocks)

My current arms have no ackerman angle designed into them and I don't notice untill I turn to full lock on a hard surface. Then the tires scrub a bit. Also note that I've got a detriot up front which doesn't help the situation.

Arms that have some typ eof ackerman is probably the best way to go but I don't think it really makes any difference. Another issue I do have about ackerman type arms, is the inside tire wil turn sharper then the outer, so you do have to becareful not to over stress the inside birf when at full lock, BUT if you have your steering stop set correctly thats not an issue.
 

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It is not worth it to design for some particular amount of ackermann whoever it does need to be taken in to account

The easiest way to do that is to design you steering arms so that the line though the bj's on each arm intersect eachother at a line drawn through the center of the rear axel.

Being dead on isn't important, i seriously doubt any of the high steer mfg's take it into account. I asked them once but none of them would respond.

Basically Just Donk It and you will be ok
 

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Discussion Starter #4 (Edited)
ok thanks for clearing that up ... 4RnrRick i would take it you have OTT arms from your previous posts?

TRD what the heck is Donk supposed to mean?
 

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Hick said:
BTW, here is a cool pic on how to illisturate ackerman angle.
Thats not Ackerman angle. That is the toe out in turn which is dictated by the Ackerman angle. Ackerman angle is the imaginary line from the outer tie rod end through the ball joint. these lines should intersect at the same distance as the rear axle center line. We designed Kongs arms based on 106" wheel base.
 

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mobil1syn said:
4RnrRick i would take it you have OTT arms from your previous posts?...
I actually have early generation Sky arms which didn't account for ackerman angle.




Originally posted by zags
Thats not Ackerman angle.
I disagree! My understanding (this is in my own words) is that ackerman angle is the difference in the angle of the front tires you need so the tires won't scrub while turning. If you look at the above picture, this is the angle thats shown.



Originally posted by zags Ackerman angle is the imaginary line from the outer tie rod end through the ball joint. these lines should intersect at the same distance as the rear axle center line.
This is how you therorectically find a starting position for the tie rod hole. But its only that, a starting position.



Myself and Sky went thru this is great detail in 3D CAD to determine if it was possible to get the akerman angle "perfect" on any given vechile. Its impossible to make it perfect from center to locked steering, its only an estimation. If you you do te cad layouts with working components you will see what I mean.




For the average consumer, I feel that its not important if you know how to design ackerman into a steering system, but rather just to understand how it works. I didn't really understand it untill I made a cad model that I could play with.
 

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4RnrRick said:

I disagree! My understanding (this is in my own words) is that ackerman angle is the difference in the angle of the front tires you need so the tires won't scrub while turning. If you look at the above picture, this is the angle thats shown.
This is how you therorectically find a starting position for the tie rod hole. But its only that, a starting position.
.
You have the right idea, you just have your terminology wrong.
The picture above illustrates the resultant toe out in turns that is induced by the Ackerman angle (the angle of the steering arms). The inside tire must turn sharper than the outside because it is moving in a smaller radius. The ideal Ackerman angle would result in zero toe angle change on the inside tire from its turning radius. Although it is never perfect, it is closest when the two angles converge at a point equidistant to the wheel base when the all of the wheels are straight. By increasing the Ackerman angle, you increase the toe out on turns of the inside tire. By decreasing the Ackerman angle, you decrease the toe out. Toe out in turns can be measured on most alignment machines and is done normally after an accident to see if the Ackerman has changed due to a bent steering arm or spindle.
 

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I am not sure if my picture is right or not, i snagged it off this board about 2 years ago.

And zags, in your description of there being no toe in/out in the proper ackerman angle.. then how does it change with wheelbase? My thought would be that if the wheels would be parallel throughout the turning cycle the wheelbase would have no effect on the ackerman angle.

From what i am reading, it sounds to me like your idea of a "perfect" ackerman angle would be one where the tie rod was the same length as the ball joint to ball joint length across the axle... or somewhere real close.

Garrett
 

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Definition of ackerman angle from Autoclassroom.com

Ackerman principle -- The angle of the steering arms causes the inside wheel to turn more sharply than the outer wheel when making a turn. This produces toe-out on turns (TOOT)

Turning radius -- The angle of the steering knuckles that allows the inside wheel to turn at a sharper angle than the outside wheel whenever turning a corner. Also known as toe out on turns (TOOT) or the Ackerman angle.

from http://www.ingallseng.com/performance/primer.html#Ackerman

Ackerman
The Ackerman concept deals with the differing radii followed by the inner and outer steered wheels when turning. By varying the angle between the steering arm (an imaginary line from the tie rod attachment point to the actual steering axis) and the tie rod, the amount of turning of the inner wheel relative to the outer wheel can be varied.  For parking lots and wagons, each steered wheel follow its actual turning radius to minimize tire wear.  For racing there is a theory for any possibility.

My view is that we want to gain as much cornering power as possible from the inside tire.  Tire slip angles under maximum turning allow the actual radii traveled by the tires to differ from each other as well as to deviate from the direction in which each tire points!   The more heavily loaded outer tire pretty much determines the actual cornering line.  Provided the inside tire is not turned so far as to induce it to lose its slip angle and slide (wash), there is worthwhile cornering power to be gained by turning the inside tire even beyond the theoretical Ackerman angle –though the reasons for this are beyond the scope of this discussion.

Optimum Ackerman geometry is best determined by testing.  With rear steer (the steering arms pointing rearward), start with a modest setting and move the tie rod forward and/or increase the inward steering arm angle until the driver notes a fall off in performance, and then back off a bit.  Front steer is a bit more problematic in that the desired steering arm angle often interferes with the tire.  Rearward movement of the tie rods is the easier approach.   Be sure that toe and/or bump steer don’t wander and influence the results.



Garrett
 

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Hick said:
I am not sure if my picture is right or not, i snagged it off this board about 2 years ago.

And zags, in your description of there being no toe in/out in the proper ackerman angle.. then how does it change with wheelbase? My thought would be that if the wheels would be parallel throughout the turning cycle the wheelbase would have no effect on the ackerman angle.

From what i am reading, it sounds to me like your idea of a "perfect" ackerman angle would be one where the tie rod was the same length as the ball joint to ball joint length across the axle... or somewhere real close.

Garrett

The turning radius of any vehicle changes with wheelbase. A bus needs alot more room to turn than a small car.

I am not trying to say that the toe in from one wheel to the other shouldn't change on turns, just the opposite. I am saying that the angle of the the inner wheel needs to stay on its given radius. which is different than the outer wheel in a turn. This difference depends directly on wheelbase.
 

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Hick said:

Turning radius -- The angle of the steering knuckles that allows the inside wheel to turn at a sharper angle than the outside wheel whenever turning a corner. Also known as toe out on turns (TOOT) or the Ackerman angle.

from http://www.ingallseng.com/performance/primer.html#Ackerman

In 20 years of being in the automotive industry and building racing chassis I have never heard of Toe out in turns called the Ackerman angle. But hey, Im no engineer. It has always been what the angle of the steering arms was called. As far as using it as a tuning aid in racing applications, it may be worth playing with on a competition rock crawler. What I found in stock car racing is that going much past 0 (too much toe out on turns) bound the car up in the corners and either created under steer, or just slowed the car down. For a trail rig, it makes absolutely no noticable difference.
 
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