High steer and Ackermann geometry

[Broncobowsher]

So I was thinking about all the little nuances of a front axle and was noticing that due to steering axis inclination (kingpin inclination) that you could change the Ackermann with nothing more than raising or lowering the steering.

OK, if you don't know Ackermann, try this https://en.wikipedia.org/wiki/Ackermann_steering_geometry

The examples all show a pivot point, but not an axis. From what I see a tie rod can be the exact same length. But mounting high over low you change the effective axle width, making it act narrower. That will get you more Ackerman.

I'm guessing nobody has played with this yet?

This is what happens when I have too much time and start thinking about stuff.

 

[Jebus]

I think I remember that from diesel school. Brakes, suspension, and steering.
For a semi truck and setting wheelbase, it is an important aspect of a trucks ability to turn without scrubbing the tires off. IIRC, the wheelbase determins the values for the ackerman geometry and steering arm, pittman arm, lengths. Change the wheelbase and the values need to change for the steering linkages.

 

[Broncobowsher]

Correct. Although with light trucks it is often just a compromise. The front steering is the same regardless if a short bed, long bed, king cab. If you ever watch a full lock turn it is common to see one of the front tires scrubbing like mad.

If you ever want to see really good Ackermann check out a forklift. The steering angle is insane but the tires don't scrub.

 

[Steve83]

...you could change the Ackermann with nothing more than raising or lowering the steering.

I assume you mean raising/lowering the point at which the tie rod connects to the steering knuckle. Yes, it would change the Ackerman if you raise & lower along a vertical axis. But if your change is along any axis PARALLEL to the kingpin angle, Ackerman wouldn't change.

 

[Broncobowsher]

I assume you mean raising/lowering the point at which the tie rod connects to the steering knuckle. Yes, it would change the Ackerman if you raise & lower along a vertical axis. But if your change is along any axis PARALLEL to the kingpin angle, Ackerman wouldn't change.

Yes. Since the kingpin angles point toward each other at the top.
I was just thinking of all of us who have moved the steering from under the steering arm on the knuckle to above it. And to those who have raised it even further putting it on top of the knuckle. Just moving the same length tie rod but higher or lower on the axle.

Now going to a different knuckle (like a '76/77) that has the arm reshaped and requires a different length tie rod, that adds a whole new level to figuring the changes. Now there are 2 variables that interact with each other.

 

[toddz69]

Yes. Since the kingpin angles point toward each other at the top.
I was just thinking of all of us who have moved the steering from under the steering arm on the knuckle to above it. And to those who have raised it even further putting it on top of the knuckle. Just moving the same length tie rod but higher or lower on the axle.

Now going to a different knuckle (like a '76/77) that has the arm reshaped and requires a different length tie rod, that adds a whole new level to figuring the changes. Now there are 2 variables that interact with each other.

I obsessed about this somewhat when I put the 78-79 knuckles on my truck and then put the tie rod on top of them since the knuckles were used on vehicles with a much longer wheelbase. Then I realized Ford used them on both short and long wheelbase trucks and figured it might not matter that much.

Todd Z.

 

[DirtDonk]

But they were both longer wheelbase than the EB, so it was probably not without merit that you obsessed over it.
Isn't this why Ford changed the '76/'77 EB knuckles anyway? Not simply to accommodate the Y steering linkage, but to improve Ackerman as much as possible in such a short package.
Moving the arms both higher up AND angled outward a bit was part of the improved steering feel I thought.

Paul

 

[toddz69]

But they were both longer wheelbase than the EB, so it was probably not without merit that you obsessed over it.
Isn't this why Ford changed the '76/'77 EB knuckles anyway? Not simply to accommodate the Y steering linkage, but to improve Ackerman as much as possible in such a short package.
Moving the arms both higher up AND angled outward a bit was part of the improved steering feel I thought.

Paul

Yes, they did improve the Ackerman with the 76-77 knuckles.

Todd Z.

 

[Yeller]

From my studies and experience proper Ackerman is achieved when the plane from the tie rod hole through the kingpin (or ball joint) axis point converge at the center of the rear differential. In theory there is no scrub at that point. For a lot of the dedicated off-road axles I build I set them with no Ackerman angle so that we can get maximum steering angle both directions equally. Works great for drifting or doing front digs, in those scenarios the tires don’t fight each other

 

[Steve83]

From my studies and experience proper Ackerman is achieved when the plane from the tie rod hole through the kingpin (or ball joint) axis point converge at the center of the rear differential.

Yes, that's almost the definition of Ackermann angle.
https://en.wikipedia.org/wiki/Ackermann_steering_geometry

 

[suckerpunched]

If you move them up and reshape the steering arm so that the tie rod was as close to the wheel as possible, you could improve it. But I have never seen any vehicle with a steering box in front of the kingpin that had correct ackermann. the tie rod ends hit wheel before you can achieve it. I have read that with invention of radial tires, there was enough sidewall flex that correct ackermann was not necessary. That coincides with the time many automakers got away from the steering box behind the kingpin. I'm not sure I really buy it though, If that was the case, why is alignment so critical?
The kingpin inclination angle affects "scrub radius" which was probably done for stability / control improvements but you can google that read about the pros and cons.

 

[68ford]

If you simply raise the the steer arm along the same axis of the ball joints, ackermann will not change. You only created the need for a shorter tierod

 

[Yeller]

If you simply raise the the steer arm along the same axis of the ball joints, ackermann will not change. You only created the need for a shorter tierod

Correct.

King pin inclination also helps with return to center as well.

 

[Steve83]

...tie rod was as close to the wheel as possible, you could improve it.

Improve what? The distance from the tie rod end to the wheel isn't specifically relevant to any steering or suspension characteristic. A longer steering arm (from the kingpin axis to the outer tie rod end) reduces the force that the steering linkage has to apply to steer the wheel. But moving that point close to the wheel ruins the Ackermann angle (for a typically-sized wheel with typical camber).

...steering box in front of the kingpin that had correct ackermann.

Fore or aft doesn't impact Ackermann - the angle is the same for steering linkage forward of the axle as rearward - it's just mirrored along the axle's axis. (NO THAT'S RONG!!!) But for truly-correct geometry, the steering box (pitman arm pivot) needs to be on the same plane as the kingpins/BJs.

the tie rod ends hit wheel before you can achieve it.

No, they're always angled AWAY from the wheels.

...enough sidewall flex that correct ackermann was not necessary.

Sidewall flex has no effect on actual Ackerman, or the tire wear that always results from scrub.

If that was the case, why is alignment so critical?

Even if sidewall flex obviated Ackermann, alignment would still be necessary for many OTHER steering & handling characteristics.

The kingpin inclination angle affects "scrub radius"...

No, it doesn't. Only the location at which that line intersects the ground, relative to the center of the tire contact patch. The kingpin angle can be virtually anything, as long as the tire radius & offset are correct for it. IOW: you can use tire size & wheel offset to eliminate scrub for any kingpin angle.

If you simply raise the the steer arm... You only created the need for a shorter tierod

Not necessarily, and not exclusively. If the steering arm is raised drastically, the tie rod will get longer. It's shortest when the line from the inner TRE to the outer is perpendicular to the kingpin/BJ angle. But raising (or lowering) the steering arm also affects steering linkage geometry, which impacts bumpsteer (toe change through suspension travel). If you want to raise the steering arm, you also need to raise the pitman arm, so the tie rod travel mimics the suspension travel (much like keeping the track bar parallel to the drag link).

Steering & suspension geometry (even on a solid axle) is a lot more complicated than most people grasp. I understand a lot of it, but still not all.

 

[toddz69]

INo, it doesn't. Only the location at which that line intersects the ground, relative to the center of the tire contact patch. The kingpin angle can be virtually anything, as long as the tire radius & offset are correct for it. IOW: you can use tire size & wheel offset to eliminate scrub for any kingpin angle.Not necessarily, and not exclusively.

Steering & suspension geometry (even on a solid axle) is a lot more complicated than most people grasp. I understand a lot of it, but still not all.

You state that king pin inclination or SAI doesn't effect scrub radius and and then you give the definition of what we call scrub radius (" ....the location at which that line intersects the ground, relative to the center of the tire contact patch"). Which is it? Maybe I'm missing something...

I don't disagree that you can use tire size and wheel offset to eliminate it or change it.

Todd Z.

 

[ntsqd]

Fore or aft doesn't impact Ackermann - the angle is the same for steering linkage forward of the axle as rearward - it's just mirrored along the axle's axis.

Actually, No. It is not mirrored about the front axle. The angle is defined as being "100%" when the steering input joint lies on a line drawn from the intersection of the rear axle centerline and the chassis centerline, and is extended to infinity while passing thru steering axis.

So if it is a rear-steer tie-rod (AKA FJ80) then the joint location will be inboard of the SA; and if it is a front-steer tie-rod then the joint location will be outboard of the SA.

Right or wrong, in my considerations of Ackerman I have always set the elevation of interest to be the axle centerline. The 100% Ackerman line would then intersect the SA at the height of the front axle centerline and the vertical axis of the steering joint would be translated up or down to the horizontal plane that the Ackerman Line lays in. I'm not saying this is the correct geometry, just that it is how I lay it out and think about it when pondering adjustments to Ackerman.
Which would negate the OP's observation.

Something to consider is if you move the point where the Ackerman Line intersects the chassis centerline forward of the rear axle centerline then the vehicle will drive much like it would with toe-out (i.e. REALLY likes to turn-in to a turn), but without the attendant tire wear and excessive road drag. This is commonly employed in road racing, some cars run as much as 50% Ackerman (meaning the Ackerman Line intersects the chassis centerline at 50% of the wheel base).
How this might be employed in a crawler, or even if it should be, I've no idea.

 

[Broncobowsher]

Not that this is the crowd for it, but the hot ticket in the drifting world is "parallel steer". No Ackermann. It allows for control when doing 90MPH sideways with the tires light up in smoke. There is a lot of trick stuff they can do to the front end of those cars, the steering angle is insane.

 

[Steve83]

...you give the definition of what we call scrub radius...

Yes, that's what I was saying. The only thing that affects scrub radius is where that line intersects the ground relative to the contact patch - NOT the angle at which it intersects. Scrub radius can be 0 with 1° SAI, or with 5°.

...extended to infinity...

I was taught only to the kingpin/BJ axis. On the other side of the front axle, the angle is mirrored (RONG!!!!); IOW, if the steering linkage is rear of the axle, the TRE at the knuckle will be rear, and lie on that line (to the center of the non-steering axle). But if the steering linkage is forward, it's set up on exactly the same angles as if the non-steering axle was the same WB forward.

But I don't remember actually checking, so now you've got me wondering... If what you're saying is true, you could have both the short rear drag link & the long front drag link connected to both knuckles, and they'd move exactly as freely as if only either one was connected. Intuitively, it seems that having a different-length drag link (or whatever you want to call the one from one knuckle to the other) on the other side would result in VERY-different motion of the steered knuckles; so having both connected would cause binding (inability for the knuckles to steer). But having a mirror-image of the shorter rear drag link attached exactly the same way on the front would allow steering because they should move identically (always parallel).

I'm going to draw it out & check...

Neither of my tests came out right - either because I drew them both wrong, or because we're both wrong.


(phone app link)

In the main drawing & the center inset, I started with the tires on a 92" wheelbase & 57" track width (early Bronco typical dimensions), and added the big Ackermann "V" to their centers (contact patches). The short line from the contact patch inward represents the steering axis, but I had to guess how far inboard of the tire the upper BJ is, based on looking at the '74 I have outside. I drew the circle representing a rear-facing steering arm's TRE arc, centered about as far inboard as it appeared on that truck. The light blue line is a copy of the Ackermann angle, passing through the center of, and extended to both sides of the TRE circle (ntsqd's description of the geometry), representing the steering knuckle's arms. The blue line is copied FOR THE UPPER (driver's) TIRE ONLY in gray, rotated 10 degrees at a time. The first red line is a rear-axle drag link connecting the rears of the original blue lines; magenta is a forward-axle drag link.
Each got copied to the ends of the driver's gray lines; then I struck an arc showing how the drag links would rotate to follow the passenger (lower) TRE circle, and rotated them to meet it. Last, I added the passenger's gray lines joining the ends of the drag links. The fact that they don't remain centered on the TRE circle says that they don't steer the knuckles the same way, so I either misinterpreted some step of the process, or I just goofed the drawing somehow.

Then I erased everything back to the blue lines, and re-drew the forward section of the driver's as a mirror-image of the rearward one (my understanding of the geometry), and repeated the other steps. Except for the passenger blue lines, I copied & rotated them in gray to follow the passenger end of the red drag links, and only inset that close-up at the bottom of the pic. Their forward ends don't follow the magenta drag links, so that version didn't work, either. :-[



BTW
When I was looking at the '74 to gauge the BJ & outer TRE positions, I noticed that the line from the TRE horizontally through the steering axis doesn't come anywhere close to the center of the rear axle - it's probably 3x the wheelbase back (300% Ackermann?).

 

[suckerpunched]

"Improve what? The distance from the tie rod end to the wheel isn't specifically relevant to any steering or suspension characteristic. A longer steering arm (from the kingpin axis to the outer tie rod end) reduces the force that the steering linkage has to apply to steer the wheel. But moving that point close to the wheel ruins the Ackermann angle (for a typically-sized wheel with typical camber)."

When the tie rod is located in front of the kingpins, or ball joints. the center of tie rod ends will need to be set wider than the kingpins or ball joints. Your own picture shows what I an talking about. draw lines from center of rear end through the kingpin, If the ackermann is correct it will also pass through the center of the tie rod end. problem is the steering arms will hit the wheels before you can bend them outward far enough get the geometry correct. I never mentioned longer steering arms, that would further complicate the issue. I guess you could run a 3" wheel spacer or a 22" wheel to make room to get the geometry right. not worth the trade off to me.


"Fore or aft doesn't impact Ackermann - the angle is the same for steering linkage forward of the axle as rearward - it's just mirrored along the axle's axis. But for truly-correct geometry, the steering box (pitman arm pivot) needs to be on the same plane as the kingpins/BJs."

simply locating in fore or aft does not impact it, you are correct, the angle is the same. but when it is fore, like I said, good luck getting the distance between tie rod ends wide enough to get the geometry right.


"No, they're always angled AWAY from the wheels."

that's why the ackermann is wrong

"Sidewall flex has no effect on actual Ackerman,"

never said it did, but may help compensate for the ackerman being wrong.

"No, it doesn't. Only the location at which that line intersects the ground, relative to the center of the tire contact patch. The kingpin angle can be virtually anything, as long as the tire radius & offset are correct for it. IOW: you can use tire size & wheel offset to eliminate scrub for any kingpin angle."

I think if you look at simple picture describing scrub radius you will see that the kingpin angle does affect it. like I said, ford may have increased the kingpin angle to improve scrub radius, I actually have no idea of what early scrub radius is or whether there actually was a change made to the later dana 44's but like you said, yes tire width, diameter and wheel offset will affect it too.

"Steering & suspension geometry (even on a solid axle) is a lot more complicated than most people grasp. I understand a lot of it, but still not all."

I've got quite a few years of experience in stock car suspension design, fabricating and track setup. a few track records and championships to back it up. So I think at this point I have the basics covered. But go ahead and get the ackermann right on your Bronco, take pics and let us know how you did it.

 

[Steve83]

...get the ackermann right on your Bronco, take pics and let us know how you did it.

That was Bowsher - not me. I'm happy to drive it essentially the way Ford built it.