Comparing Alignment Specs to Measured Toe In

[DirtDonk]

I believe they are inextricably entwined with all aspects of "geometry" no matter what aspect you're talking about.
Vehicle height changes this relationship. Spring rate, tire size, shock absorber effectiveness and compliance (stiffness, length and angle), length of the radius arms, stiffness of the radius arm bushings, how level the radius arms are compared to the ground, the overall angle of the trackbar, draglink, length of the pitman arm, AND the relationship of the draglink and trackbar to each other in all planes and dimensions.
It's all inter-related.

So I'm pretty sure what Yeller was saying when talking about "plane" is not just the angle. But in fact the plane in this case would be as you are looking at the bars from the front (angle of the picture basically) the bars in inline so that for example the trackbar, being behind the draglink, would be invisible to the eye for being hidden behind the draglink in front.
That might still make them out of the same plane when viewed from the side down their length, but with minimal separation front to back they can be in the same plane for having their pivot points at the same height from the ground. Or in relationship to the frame if you prefer.

When looking at them from the front or the back as they sit in the picture, they proscribe a parallelogram. The farther apart they are the larger the parallelogram and the more effect they would have.
I've personally never looked into the aspect of oversteer vs understeer of the changes to their pivot points in relation to each other, but it makes a certain amount of sense given what all else can change those performance perameters.
Just stiffening up one end or the other, or using anti-swaybars of varying ratings, mounting locations, etc. can have a drastic effect on over/under steer.

Study a book on 4-link geometry characteristics sometime if you want your brain to explode (or implode as the case may be) while learning about anti-dive, anti-squat, torque vectoring (not sure it has anything to do with it, but it sounded cool so I added it!) and roll and yaw and pitch!

Remember how our radius arms act as torsion bars? These effect those handling characteristics too. Hence mentioning them above with regard to length, angle and compliance.

Lot of words, but hopefully I got it all at least mostly correct.
And if not, hopefully someone with more geometry under their belts will clarify and correct anything I got wrong.

Paul

 

[Yeller]

Thanks Paul, you are correct in the explanation. When the drag link and track bar match in angle and height from the ground it is truly optimal. The relationship that doesn’t include is the inclination angle, flatter is better. The flatter they are the less leverage they have to crest pitch and yaw. Sitting still if when turning the wheel back and forth if you front end raises up and down your angles are too steep. Traditionally built, lifted broncos are notorious for having a drag link angle that is too steep.

This example is picture angle challenged. They are the same length and parallel. The drag link is higher so it does have a touch of under steer, it is mounted as even on the same plane as packaging wound allow, oil pan has to fit somewhere lol.

 

[toddz69]

Interesting detail, thanks for sharing it. I can't visualize how the relationship of the height of the track bar to drag link affects under or oversteer. Could you elaborate?

Do you mean the height of the bottom end, the top end, the whole thing?

Sorry - I had my original post backward. I corrected it - drag link above the trac bar = oversteer. drag link below the trac bar (like stock) = understeer. Unfortunately I haven't delved into the engineering texts enough to learn all about the scenarios yet.

I learned this many years ago from reading the following from a really smart solid axle suspension engineer on a J##p forum: (applies to Broncos due to the same type of suspension configuration)

"If the draglink doesn’t pass through the front suspension roll center, there will be roll-steer effects (draglink higher causes roll oversteer, under generates understeer; any oversteer is bad, and too much understeer makes the car feel ‘dull’ with slow turn-in response)."

As for the height, I mean the whole thing, but I'm specifically interested in the height of the center of the trac bar as that's what determines the roll center.

Todd Z.

 

[ksagis]

Sorry - I had my original post backward. I corrected it - drag link above the trac bar = oversteer. drag link below the trac bar (like stock) = understeer. Unfortunately I haven't delved into the engineering texts enough to learn all about the scenarios yet.

I learned this many years ago from reading the following from a really smart solid axle suspension engineer on a J##p forum: (applies to Broncos due to the same type of suspension configuration)

"If the draglink doesn’t pass through the front suspension roll center, there will be roll-steer effects (draglink higher causes roll oversteer, under generates understeer; any oversteer is bad, and too much understeer makes the car feel ‘dull’ with slow turn-in response)."

As for the height, I mean the whole thing, but I'm specifically interested in the height of the center of the trac bar as that's what determines the roll center.

Todd Z.

Thanks for clarifying, do you think trac bar determines the roll center or the center of gravity sets the roll center?

 

[toddz69]

Thanks for clarifying, do you think trac bar determines the roll center or the center of gravity sets the roll center?

In a suspension system with a trac bar/panhard bar, the trac bar determines the roll center. The rear suspension also has a roll center. Draw an imaginary line between those two roll centers and you have the roll axis. How far the Cg is away from that roll axis plays a part in how the vehicle handles. You want to keep that distance as small as possible, or at least close to what the factory-spec vehicle had.

That's another reason why using a drop pitman arm and a trac bar lowering bracket to "correct" your geometry is a sub-optimal solution to the problem, and a TRO and a raised trac bar are a good idea.

OK smarty-pants, you say, why did Ford lower the trac bar (longer frame drop) on the 76-77 Broncos? Well, I have an idea. They wanted to make the front end handling more neutral (have the trac bar and the drag link in the same plane as Yeller explained in an earlier post), so they lowered the trac bar mount. Now you have a great distance between the roll center and the Cg (larger roll moment couple), which causes more body roll. So they added an anti-sway bar to the front end to counteract that tendency. As Paul I believed noted earlier - all this stuff is tied together and interacts together.

Todd Z.

 

[DirtDonk]

And to add to the equation, Ford also changed the height, angle and length of the steering arms where the tie-rod ends attached. Along with changing out the old direct link "Inverted-T" style steering linkage to the completely different "Inverted-Y" linkage with the pivot on the draglink.
Changing the arms would have changed the Ackerman angle, but perhaps other factors were involved as well, that were not so easily seen.

And it was literally a '76 and '77 thing ONLY which included the '76 and '77 F150 trucks. It all disappeared in '78 for F150's and the big Broncos when they went back to the old "Inverted-T" style steering linkage with a full tie-rod link.
Never looked closely at the location of the trackbar on those models though. Perhaps even though they kept the anti-sway bars, they also relocated the trackbar to accomplish some different geometry?

Paul

 

[ksagis]

Man, you guys have so much knowledge, thank you for being willing to share it and post. You likely have no idea how much the rest of us appreciate it.

 

[Yeller]

Higher mounted track bars definitely have advantages. Not a great pic but you can clearly see the track bar on this buggy, it has a pivot 36” off the ground. The center of gravity on this car is 30” above the ground. It is incredibly stable, doesn’t hurt that 55% of its weight is axles, tires and wheels. But it is proof of concept higher is better, the best way I can describe it is to compare it to a hammock, the control points are higher than the CG so it swings like a hammock. The opposite is like a bronco with lifted suspension and drop bracket and dropped pitman arm, your sitting on the back of the horse, a manageable position but if you could sit in the belly of the horse how much more stable is that? That is the comparison for why a few of us preach TRO and track bar riser.

This design is not very practical but for a purpose built machine without many constraints it works and makes a good example.

PS bonus points if you can name the guys in the background

 

[toddz69]

Higher mounted track bars definitely have advantages. Not a great pic but you can clearly see the track bar on this buggy, it has a pivot 36” off the ground. The center of gravity on this car is 30” above the ground. It is incredibly stable, doesn’t hurt that 55% of its weight is axles, tires and wheels. But it is proof of concept higher is better, the best way I can describe it is to compare it to a hammock, the control points are higher than the CG so it swings like a hammock. The opposite is like a bronco with lifted suspension and drop bracket and dropped pitman arm, your sitting on the back of the horse, a manageable position but if you could sit in the belly of the horse how much more stable is that? That is the comparison for why a few of us preach TRO and track bar riser.

This design is not very practical but for a purpose built machine without many constraints it works and makes a good example.

PS bonus points if you can name the guys in the background

I think my first memory of those super high track bars was on Jon Nelson's 'Tiny' buggy, but maybe I'm not remembering things correctly.

I see a blue Bronco, so maybe Jake Good?

Todd Z.

 

[Yeller]

I think my first memory of those super high track bars was on Jon Nelson's 'Tiny' buggy, but maybe I'm not remembering things correctly.

I see a blue Bronco, so maybe Jake Good?

Todd Z.

My recollection is the same, may have been a few before but they did not make near the splash that Tiny made. Tiny turned 20 in 2022 and is still out there competing and being competitive. Not many competition vehicles in any circuit can make that claim, especially ones that were so far out there in design that everyone said "that'll never work" LOL

The buggy in that pic was built in 2004 and in the rec world is still phenomenal.

Not Jake, one of them does own an orange bronco though LOL