Steering arms, ackerman angle

[pvfjr]

So I've heard the term "ackerman angle" dropped around here a lot lately, so I researched it and figured out what it was. Now that I know what it is, I'm trying to take it into consideration for my buildup. I've been looking around for the best price on arms, and I can probably get it at sky-manufacturing. But I can tell from the pics, it almost seems like their TRE holes are centered on the kingpin, meaning NO ackerman angle. WMS and others also look like this. Am I seeing things wrong? It also looks like NWF's arms would provide a really extreme ackerman angle, being angled in quite a bit. The setup I'm going for is just the histeer/rear steer with the double ended arm on the pass side and the tie rod behind the axle. Any thoughts on different companies and their arms as far as ackerman is concerned?

 

[sled_dog]

ORD and Sky arms are more of a negative Ackerman angle I believe. Unless you go custom one off you won't get an Ackerman angle you'll like. So many people run different things with no consideration for Ackerman that I wouldn't worry about it. Some places claim to have spent time designing Ackerman into their stuff but I don't particularly believe it. I was taught your steering arms should point towards the center of your rear axle. No ones arms do that. Opposite actually.

 

[ntsqd]

Ackerman has a huge effect on "turn-in" in road racing. For a 4x4, parallel steering is probably accetable IF high speed desert type stuff or DD use isn't on your list of things to normally do.

With 100% Ackerman (defined as both steering arm Lines of Action (LoA) meeting at the intersection of the rear axle centerline (CL) and the chassis CL) you won't notice either tire scrubbing in a corner. With more or less Ackerman one or the other tire will scrub in a corner. How much depends on how far off the Ackerman is. Most ideal is if the inside tire scrubs and that requires that the LoA - Chassis CL intersection happen at or fwds of the rear axle CL. I doubt that it will result in significant tire wear in the dirt but pavement could be a different story.

 

[pvfjr]

Well I don't have the money for a trailor, and I might have to sell my 1-ton soon anyway. So my stepside might have to be more of a multipurpose vehicle. So a good portion of it's miles will be on pavement, which is why I'd like to take ackerman into consideration.

If I'm looking at things right, then a behind the axle tie rod has to have the TRE's mount inside of the kingpin, and a front of the axle tie rod needs to have them mount outside of the kingpin. If I'm right, I would see why a lot of the regular histeer arms don't do too much for ackerman angle. Seems like there isn't much room to move towards the outside or you'd be hitting the rim. But since the rear steer arms can be angled inward, I don't see why they can't drill the TRE holes with enough offset to give us a decent universal ackerman angle.

 

[hack500]

just curious but why the concern with ackerman?
iirc, you'd either have to run some outrageous toe-out or not use a single tierod - i.e some type of bellcrank system mounted to the frontend in order to get the right angles.

 

[sled_dog]

I don't see why they can't drill the TRE holes with enough offset to give us a decent universal ackerman angle.

You are actually onto something there. You will almost certainly have to have something custom made though. I guess you could try and run from your kingpins to the center of your rear with a couple pieces and string, figure it out and have someone make them. That is the big reason I see that companies don't do it, everyone would be different for 100%. Wheelbase is a large factor. With so many of us changing out wheelbase 1" here, 2" there, it would be a custom piece every time they make an arm, you know?

 

[Greg72]

You are actually onto something there. You will almost certainly have to have something custom made though. I guess you could try and run from your kingpins to the center of your rear with a couple pieces and string, figure it out and have someone make them. That is the big reason I see that companies don't do it, everyone would be different for 100%. Wheelbase is a large factor. With so many of us changing out wheelbase 1" here, 2" there, it would be a custom piece every time they make an arm, you know?

So what are the units in the Ackermann Formula? I've seen:

Angle C = Tan^-1(b/a)

Where:

C is the Angle of the imaginary line to each steering arm hole vs. vehicle centerline at the rear axle.
A is the Wheelbase (presumably in inches?)
B is the Center of front axle to Kingpin axis measurement (basically where the imaginary line intersects over the axle housing)

(I have a .jpg of this, but I am sure others have seen the same diagram floating around the boards and can link to it)

My suspicion is that a change in wheelbase of a few inches either way will not adversely affect the "number"... and what is the Ackerman value represented in....percent? Percent of what, then.??

 

[sled_dog]

My suspicion is that a change in wheelbase of a few inches either way will not adversely affect the "number"... and what is the Ackerman value represented in....percent? Percent of what, then.??

I am often left wondering such things. I have asked a few times "so this is 14 right?" "yeah" ".... 14 what?" "..."

Let me see if I can find my book quickly.

 

[pvfjr]

Well as it is now, the arms are basically designed for a truck with a infinite wheelbase. I don't think it would be to hard to design a couple different arms, maybe one for subs and crewcabs, and one for k5's and shortbeds. Even if they were for trucks that varied up to a foot in wheelbase, it would be a heck of a lot closer than what they got going now. Shoot, even the factory didn't really get accurate with things, not if a suburban and a K5 can roll off the lot with the same steering arm.