Blue1989RS

Has anyone changed the steering box and idler mounting positions to change the ackermann angles at full lock? I'm prepared to change this to make my ligher nosed V6 3rd gen turn better for the upcoming autocross season.

In my opinion, this needs to change depending on weight and roll/suspension stiffness. I am more than unimpressed at how the front of the car does not hook when turning a sharp corner. I attribute this to the steering system designed to operate with the typical 3rd gen (SBC engine, small sway, weak springs, and daily driver).

If you have any information that would be great. If not, I will post what I find and do, so that others may follow.

Dewey316

When I auto-x, I run a bit of tow out, it doesn't change the ackerman angle, but in the end, gives you a touch more ackerman effect. You don't want to go to extreme, and it causes for some twitching handling at speed, but is great for turn-in in auto-x.

sofakingdom

iTrader: (1)

I think you're barking up the wrong tree. Maybe that tree isn't even in the right forest.

Put softer front springs and stiffer rears in it; softer front sway bar and stiffer rear; set your camber to as far negative as it will go, while maintaining ½° split if it's a daily driver, or no split if it's not, and caster as far positive as it will go.

How much of the basic stuff have you done so far?

Blue1989RS

While determining my turning radius, I noticed again that the tires are pointed at such different angles that they fight between them. This would clearly be too much ackermann. Since the autocross courses up here are tight little miata courses, I need to get better control at full lock. I don't need more ackermann, I need less.

Blue1989RS

Initial measurements have shown the front steering alignment may be giving the car toe-in (or inadequete Ackermann) on full lock steering. My calculations show that the left and right tires need to have an 8* difference in angles to make the car turn a 24' diameter circle (full lock).

More to come.

I'm going to load the steering setup in Solidworks and give the angles. Looks like the system may function better if the idler and box were located further aft towards the k-member. This will give more ackermann angle and possibly more turning angle. The result would be a front end that hooks strongly.

Blue1989RS

#1 As you can see, the suspension looks fine going straight.

#2 Slight steering, the wheels will turn, but still look fairly parallel

#3 Hard over (and it may be shown too hard) and the right tire tries to pull a harder corner than the left. I would imagine that adding toe-out would help a bunch, but still won't solve this problem.

Now that I have it modeled in Solidworks, I'm going to play with what is modify-able and see if I can make it work correctly. It sure seems that GM did not intend this to have any performance at full lock.

smartman__007

iTrader: (1)

Keep us posted on your developments. I've noticed the same thing and I never knew the name for it or even what to do. With full lock the tire is just leaning too far into the turn. Even with the stock springs and ESPECIALLY with performance Eibach's.

Dale

iTrader: (12)

dodge dakota from the factory is bad about this. Keep up the tech stuff T!

Blue1989RS

I'm all over it. I will lay waste to miata's this year!

Al Miles

The amount of Ackermann is built into the spindle and outer tie rod mounting position. I think you are really trying to correct bump steer which is the change in toe (in or out) as the front wheels move up and down and can very from side to side. Moving the steering box and idler arm mounting points in any combienation forward/aft or up/down changes the length of the tie rods to get the same toe measurement. The inner front wheel geometry is designed to make it a few degrees farther than the outer wheel in a turn as it must turn harder. This is most common on rear steer (steering components mounted behind the C/L of the front spindle) cars. To have correct rear steer car geometry a straight line is drawn from the center of the lower front ball joint to the center of the rear housing (side to side at the verticle C/L of the axle). The mounting point for the outer tie rod will fall at a point on this line. This is the Ackermann theory. On a front steer (steering components mounted ahead of C/L of the spindle) car to have Ackermann the line is continued through the ball joint and the outer tie rod mounting point falls on the extended line at a point that is wider than the position of the ball joint. This is not usually done on front steer cars, usually they have anti Ackermann. This is when the mounting points for the ball joint and the outer tie rod end are equal distance from the front to rear C/L of the car.

As you can see it is difficult to change these points, but if the computer program can produce bump steer information by adding verticle measurements (distance from the ground to the suspension/steering mounting points) I think you will find that some correction to your bump steer is needed and should improve your handling. Also be sure that the program can handle MacStruts as front end geometry calculation points can be different than calculation points for double A frame suspensions.

Al Miles

Blue1989RS

That would make sense with what I've observed. The outer tie-rod end is equi-distant from the C/L as the ball joint. This should produce anti-Ackermann, but even more so due to the main tie-bar swinging away from the tie-rods. If the steering box was replaced with a rack-pinion setup, it would produce less anti-ackermann, but since the outer tie-rod end is equi-distant it would still produce anti-ackermann. But not as much.

Would you agree Al? (Thank you very much for your input on this topic!)

Blue1989RS

This screen shot provides a little more depth to my modeling setup. My main goal was to represent the suspension on my car as it sits, no bump steer. I could make a better one that included wheel position but then I would have to model more of the chassis.

Blue1989RS

Al,

If you Ackermann tie-rod alignment is correct, then the tie-rod end will need to be moved outboard by 1.48". this will put it in line with the line that connects the C/L of the rear axle and front ball joint. I'll put this new measurement in Solidworks and see how it behaves.

Al Miles

Thank you for a screen shot of your model, it will help to clarify our discussion. As I look at your Solidworks model I am having some difficulty as it appears to me that your model does not have the arcs created by the steering arm and idler arm moving in three dimentional arcs. The outer tie rod mounting point and the lower ball joint also moves in three dimentional arc with this type of steering linkage (a rack and pinion setup may reduce this to a two dimentional arc). The lower ball joint may move in either a two or three dimentional arc depending on the mounting points of the lower A frame and the top of the strut. I think you understand the arc concept, but allow me to clarify it to others who may not understand. Draw an arc on a piece of paper and it covers two dimentions, length and width. Lift only the side of the paper at the top of arc. You have now added heigth to the arc making it a three dimentional arc. I think you have the right idea with your model, but it needs a lot more refinement in the details. All of the suspension and steering mounting points must be done in three dimensions and exactly how the relate to each other. An example is the center link, the inner tie rods mount in a different plane than the mounts for the steering arm and idler arm. I hope this does not discourage you from building a correct model that will work with Solidworks. Programs designed just for vehicle suspensions cost $100's

Another important thing to remember is that what you see the tires do when turned on a stationary car is not the same thing that happens when the vehicle is in motion as the suspension moves up and down in relation to the ground.

Al Miles

Al Miles

We have not answered the original question "How much farther should the inner wheel turn in relation to the outer wheel with the steering wheel turned to the stop position?" To determine this we must establish some parameters. If the turning radius is 24 feet for the inner wheel and the tread width is 5 feet center to center, then the following happens;

The distance the inner tire travels for a complete circle is 2R x Pi or 2x24x3.14 which equals about 150 feet. The outer wheel R = 29 feet (the inner radius, 24' plus the tread width 5'). 2x29x3.14 which equals about 182 feet of outer tire travel. The difference between the inner and outer tire travel 182/150 is 1.21%. So in theory the inner wheel needs to turn 1.21 degrees for each 1 degree of turn of the outer wheel. If the outer wheel turns 20 degrees then the inner wheel must turn 24.2 degrees. This is only theory as other things in the real world such as the design of the suspension, surface friction, tire design and vehicle speed can mandate a larger difference. The only thing we can positively conclude is the minimum number of degrees that it must have for a given turning radius. Too bad our Algebra and Geometry teachers didn't tell us we can really use this stuff.

Al Miles

Blue1989RS

This is how I determined what angles the wheels should be. (view attached)

If it should be different thats no biggie. My diagram was just a stab at something I've never researched before.

The solidworks model is valid with 0 suspension deflection. I will work on making a real model, but it takes some time to make all the parts in the computer. For now, what I've created represents what my car currently looks like. Center link in line with the tie rod ends, spindle locations and attachments. The steering arm could be at the correct angle tho, something to fix with the 3D model I'll create.

So, can we establish that the stock steering is setup to induce Anti-Ackermann? If we can, then I can push forward with how to correct it.

Al Miles

Agreed, the stock suspension is anti Ackermann. Now the question is "Will a move toward Ackermann improve handling?" This depends on a complete study of the current suspension. As stated there many variables in the real world that have a direct effect on the relationship between the angles of the front tires in a turn. A biggie that I have not seen in discussions is the scrub angle of the front tires. Scrub angle directly affects the function of the grip the tires can apply to the road. I think the scrub angle on struts is determined by drawing a line from the upper mounting point through the ball joint to the ground. Then a line is drawn vertically from the ground through the center of the tire. The ideal situation has the two lines meet at the ground in the center of the tire. This means the steering pivots on the center of the tire contact patch and makes the tire tread 100% effective. In the real world this seldom occurs, usually the two lines meet below or above ground level and area between the two lines at ground level is the scrub area. This portion of the tread scrubs on the road and does not provide the same traction as the rest of the tread. It is possible that there are bigger gains to be made by correcting the scrub angle than by trying to remake the steering linkage without having all of the necessary information in a complete 3D working model.
If I understand correctly you have no bump steer, is this on the model or on your car? My car is at the point where I can soon work on the front suspension and I would be very interested in seeing some pictures. I am also willing to share pictures of my project. Currently it is a shell with a 10 point cage, mini tubs, rear seats elimimated, front seats moved rearward, chromemoly LCA's, short tork arm and panhard bar, all with adjustable mounting points and four corner adjustable coil spring mounts.

Al Miles

Dewey316

I think you are confusing 2 diffrent terms. Slip angle, and scrub radius. Your scrub radius has more of an effect on steering feel, the slip angle, is the actualy angle fo the wheels compared to the direction of travel.

(not jumping in on what you said, but scrub angle is not something I have ever heard of.) What you are making makes sense, but they way i see this interaction happening, is that if hte ackermann is wrong, one of the front tires, has a much higher slip angle, and much less grip. The scrub radius, has a lot to do with steering feel, it is the line you describe, it has to do with how the reactions to changing grip feel thru the wheel, and the natrual reaction of the wheel.

Blue1989RS

I do have some sort of bump steer. The car has been lowered enough that that the a-arms are flat, as are the tie-rods.

Funny you should mention scrub angle. I've been thinking about that as well. I've been playing with the notion to hog out the strut/spindle bolt holes so to change the strut angle. It looks like the scrub is about 5" away from the king pin drop point (see attached).

My current dilemma is to make the car handle better at full lock. I autocross this car, so I'd like it to hook and go! Would moving the spindle/tie-rod attachment point really effect scrub? Or is scrub another thing to be aware of?

You can view some of the things I've done to my car by clicking on the links in my Signature below.

smartman__007

iTrader: (1)

I'm just kind of jumping in and reading as I go. (Isn't this what the forum was founded for???) I found some information on the little known facts, at least to me, of scrub radius and other wheel alignment terminology. Thanks for all the wonderful info everybody. It helps to know how and why if something ever breaks or you want to improve. http://www.familycar.com/Alignment.h...ination%20(SAI)

Al Miles

Dewey, I think you are right about terms, its been 30+ years since I had my last race car. As I think about it scrub radius is the correct term, but I think it determines the effectiveness of the tire. It is the slip angle that is felt in the steering, the greater the slip angle the looser the front of the car becomes.
King pin inclination is built into the front spindle. I'm not sure how it is determined on a strut spindle, maybe the lower mounting hole acts as the upper ball joint mount. On a spindle with an upper and lower ball joint looking from the front of the car a line is drawn through the center of the ball joint mounts and line is drawn perpendicular to the center line of the spindle through the lower ball joint mount. The angle is measured where the two lines meet. This like Ackermann is built into the spindleand can only be changed by filling the existing holes and redrilling them. If I remember correctly it is king pin inclination that is responsible for helping the steering to return easily to the center position after a turn. I can see where scrub on the tires could aid with returning the wheels to center, a desirable trait for a daily driver, but not wanted for maximum cornering traction.
The outer tie rod mount relates to scrub radius only if there is a lack of clearance when the proper wheel rim needed to reduce scrub radius interfears with it.
My thoughts are that minimum scrub radius and zero bump steer will provide better results than trying to change Ackerman at this time.It may be possible that changing KPI will eliminate the need to rework Ackermann.
I can't beleive what I get into when its too cold to work in the garage.

Al Miles

Al Miles

Many thanks Smartman, your link has some good drawings and explainations for some of the things we have been discussing here. Some of the street alignment information is a compromise when it comes to performance driving, but the theory applies to both types of driving.

Al Miles

Al Miles

I have done some additional research and I have found that there are two current meanings attached to Ackermann. The first is when a vehicle's front suspension is designed, it is either refered to as Ackermann or Anti Ackermann. This refers to Ackermann Theory. Ackermann's theory, developed nearly 200 years ago, has two major components. It has a relationship between the front axle steering design and the rear axle. In a modern vehicle this relationship is when a straight line passes through the lower ball joint mounting point, outer tie rod mounting point and the center, by width, of the rear axle. If this relationship exists, the vehicle has Ackermann. If it does not, then the vehicle has anti Ackermann. The theory also says the inside wheel in a turn will turn harder than the outside wheel. The difference in degrees between the two wheels is the Ackermann Angle. All modern vehicles have an Ackermann Angle even if they are anti Ackermann. That is they do not have the relationship between ball joint, outer tie rod and rear axle. The Ackermann Angle is built into the spindle and cannot be ajusted without making modifications to the steering geometry.

Blue 1989RS really wants to change the Ackermann Angle of his front suspension. He does not need to go from anti Ackermann to Ackermann to get the results he wants. He needs to make suspension geometry changes. Ackermann Theory when applied to modern vehicles has the rear axle applying steering input into the front suspension. This is not considered desirable in modern rear wheel drive vehicles and is intentionally engineered out of modern front suspensions (anti Ackermann).

Al Miles

Blue1989RS

After digging around under the car this weekend, I've discovered that there is no way to move the mounting position on the spindle without hitting the rim of the wheel. I am running 18x8 with 245/45/ rubber and there simply isn't any room to move it. The wheels almost poke out the side of the fender anyway, so spacing the wheel out really isn't an option. I've attached a picture to help clarify what I'm talking about. Its not my car, but it gets the point across.

So it looks like I'll be taking Al's advice after all. If I end up doing something interesting, I'll make sure to post. Thanks all!