Technical bump steer - Level Control arms , but they have a bend

Thanks !
So most bump steer avoidance drawings depicting the lower arms are straight line drawings and depict straight arms. How should a lower control arm that's not straight be leveled?

 

To my way of thinking the bottom sketch is the way to go. That one looks to me like it would keep the rack from fouling the scrub line and at the same time the tie rod ends would be parallel with and follow the arc of the lower arm travel.

 

To me...I would have said the top and 3rd picture down...the object being the least amount of lateral movement thru the arc of suspension travel.

Further, the relation of the top control arm and steering control (rack, tie rods etc.) also needs to be part of the relationship.

 

The pivot point of the bushing and the pivot point of the ball joint should be level. I would think the rack or tie rod should be level or parallel also.

 

They all have merits, but I really can't say which would be right.
There's a big ride height difference that appears when different parts are leveled.
Also I can see no reason to have a bend in the arms other than to alter the ride height.

 

the only thing that matters when figuring arm movement is the point at each end. You can put a pretzel in between the two points, nothing will change. So no matter what the shape of the arms, do the calculations using an imaginary straight line thru the points at each end . . .

 

Well once I figure out where the lower ball joint is going in relation to the ground by leveling the control arms - only then,,,
I can determine the height location of all the other stuff, including the pivots for the lower control arms.
I'm thinking #3 is right but but but but

 

Thanks Pete,
I understand that to be the 3rd sketch correct?

 

Put a compass on each example and draw an arc at the wheel end, note how the angle of the arc differs. The 1st & 3rd are going to have the same arc of travel and will remain closest to perpendicular, which should be the best.

 

Factory control arm are "bent" to provide travel without contacting the frame.

 

I assume you are working on an "A arm" suspension. The arms relationship with the tie rod & each other is what's important. Not their relationship with the ground, ( or "level" )

 

Right!
Everything is in design mode is loose and subject to placement.
I need to start some place, right? My starting point is the relationship of frame to spindle at ride height.

That means I know where my lower ball joint is in elevation but not the LCA pivots. Due to the leveling question.
I know where my steering arm and outer tie rod is but not yet the rack.
I know where my upper ball joint is but not yet the UCA pivots.

Since it's all loose and being level isn't important, what would be a good starting point to backtrack everything else and build in a good ride and handling at the decided and "set in stone" ride height?

 

How level the arm is, or the plane through the pivot points really, doesn't affect bump steer...it's how that plane moves relative to the plane that the tie rod moves, that you have to worry about.

Are you using existing spindles? so you have to live with the set height of the lower ball joint?

Do you have any constraints on the height of the crossmember that connects the two inner lower arm pivots? such as the location of the engine, etc

 

Yes I have spindles, both stock and 2" drop to play with.
if I go high enough with the crossmember it will eventually hit the engine but there's quite a bit of room (1-1/2" give or take a 1/4) before I have to cut the floor and raise the engine.

As far as being "level" goes ,,, every bit of info if found centers around things being level as the starting point. Level lower arm, level tie rod, inner tie rod pivot on intersect line.
Like this or some other artistic interpretations

There's a whole bunch of stuff on correcting bump steer showing un level control arms as a problem and devices to correct the problem by leveling the LCA arms. Like this :

I can put it any way I want, +/- 5* or +/- 45* or at 0* level.
Level is the easiest way to work but if level isn't important what is?

 

Up

 

The usual cause of bump steer (when modifying a car) is moving some part of the steering linkage, so that the tie rod does not follow the same arc as the LCA. It really has nothing to do with being level...although the problems usually start when someone moves something that started out level, to a different position, and it not being level is the first clue that the arcs won't be the same

 

What squirrel said.
The rest, not so much.

Just plain ol high school geometry. Many books on suspension design for the taking.

Mike

 

The people from Fatman Fabrications explained to me that the rack should be mounted so the tie rods run level with the steering arms to avoid bump steer. You can move forward or back in relation to the steering arm but don't let tie rods the run up hill or down.
I know this dose not tell you about how to run your lower a frames, but it did help me when I was mounting my rack on one of my projects.

 

With a rack and pinion, the tie rods want to run parallel with the LCA (as defined by the pivot points, not the arm itself).

The important thing is that when you move the spindle up and down, the tie rod needs to move up and down in a way that makes it so the spindle does not steer.

 

Brand new clean slate Jim.
the world is perfect and you can put it anywhere you want. The frame is set, and only the XYZ coordinates of the spindles's axle are established and set.
Where would Jim put "it" ?
"It" being every other pivot point?

Making pivotal design choices ain't easy.

 

Angle of the dangle, or in this case angle of the tie rods. There are 2 basic trains of thought on that, some say if a rack you should try to keep the tie rods close to level with the rack itself, the other is that the tie rods should follow close to the angle of the lower arm to keep it in relationship with the lower arm as it moves thru travel.

I've seen them set up both ways and both seem to work reasonable well when in resonable angles. But want to see something change quickly? Level the tie rods and then raise left hand outer say 1/2" and lower other side 1/2" and all of a sudden you have one scary clown car!

The most important part is that both tie rods are the same angle side to side.

 

I would put a little bit of angle, running down to the spindle. Maybe an inch. So it doesn't look droopy

 

That would be more towards the last pic and it does look better than 3rd in bare mockup.

 

Actually, you want to start with the front roll center height. As others have said, you can angle the lower control arms to get stuff out of your way. Unless you have some reason not to, I would put the roll center right at ground level. Lower gives you a lot of body roll and higher wants to lift the inside tire. You can adjust the angle of both the upper and lower control arms to achieve this.
The spindle height will help you get in the ball park. The ball joints can only articulate so much. The other thing you might want to consider is the front to rear roll axis. I think the circle track car I crewed on used 6* front to rear. The engineer/driver said that this gives 'feel' to the steering.
Take it for what it cost you.

 

Bump steer can't be corrected with lower control arm location. Lower control arm location (along with upper arm location) determines if camber goes positive or negative on initial spring compression. If the lower ball joint is higher than the inner bushing and the upper control is the same, then camber will go negative. If ball joint is lower on both control arms, camber will go positive until ball joint becomes higher than bushing and this is not what you want. 50's and 60's suspensions created positive camber on spring compression which is not good. Mid 70's GM suspensions corrected this. 31Vicky's illustration shows proper control arm and tie rod location and lengths for zero bump steer.

 

I know there are programs around that will figure all the variables in an IFS. I had one on floppy disk back in the late 1980s - early 1990's. We were building our own late model dirt chassis. Can't recall the name. I do remember that it helped tremendously. As I recall it wasn't very expensive. There has got to be something similar around today. Takes the guess work out. Caster gain and camber gain are very important along with bump steer.

 

What's funny, every kit I've seen available (not that I've seen them all) has at least something or everything wrong. The one I'm taking off had everything wrong and the truck was almost un-drivable. But it sure fit nice.

 

Yes it can.... and it is done in race cars more than you think.
Sometimes the Rack/draglink cannot be raised. So the inner pivots are moved so the instant centre intersects the "tie-rod line"[shown in post# 14]
Sometimes this correction is needed when the ride height is altered.
Zero bump steer [shown in post# 14] is only a compromise in a limited range, altering suspension heights can shift the instant centre to cause bump steer [due to unequal length A -Arms]

 

Understand the suspension control arm is an "imaginary straight line" between the pivot points. It wouldn't matter if the actual arm was Bent or even "S" shaped.

A straight "A" shaped control arm is strongest , but being bent near the ball joint is usually needed. This is to prevent the ball joint binding due to misalignment and bottoming suspension travel. The top control arm is usually bent as well.

Have a good look at your spindles. The tapered holes are not vertical due to "King pin inclination", but they usually line up with each other to provide an axis. The suspension control arms are bent to prevent the ball joints binding because of this King pin inclination.

Most road cars have low frequency suspension of usually 5:1 to 7:1 meaning a 100lbs/in "Wheel Rate" [not spring rate] usually supports 500-700lbs weight. Usually a car needs to be jacked up 5-7" before wheel lifts off the ground unless the shocks limit the travel.

When the wheel is off the ground ,this is maximum "droop" and you don't want the suspension binding in this position. When they did the Shelby mod on Mustangs at the Shelby Facility in the 60's, they lowered the inner/upper A-Arm 1" [1-3/4" is Ideal] But this 1" lowered pivot combined with a lower ride height was the maximum they could go without the suspension binding. Nowadays we add wedge blocks under the ball joint to correct this.[negative wedge kits]



Now onto the subject of bump steer.

These drawings were shown [and attached] are only 1 dimensional. You need to view it 3 dimensionally

Front view as shown.

Top view to allow for Ackermann , and side view to allow for caster [both of these alter the tie-rod placement] .Thisicleveh also depends on whether the vehicle is "front steer" or "rear steer" [ tie-rods in front or behind the spindle] I'll use rear steer as an example

With Ackermann viewed from the top, the steering arms are narrower than the King Pin centreline, so on the drawing [attached] the tie-rod needs to be shifted inward [not shortened] . The inner pivot shifts inward also , requiring a shorter drag-link or rack. [see lower drawing] I have actually seen race cars where they adjusted the toe with a giant tube nut threaded onto a split drag link.

With caster on a front steer, as the spindles are laid back eg:7° the steering arm is also tilted down the same 7° so the drag-link/rack needs to be lowered the same "distance" to correct this.

 

I had a thing called wishbone.exe which worked well, but was a bit of a pain to use because it worked in numerical 3D coordinate inputs. There must be something better out there now.

Link to the wishbone programme here. Scroll down a bit.

This might be worth a look: http://suspensioncalculator.com/index.html