Front suspension design questions

[ONE37TUDOR]

I am in the process of making radical changes to the front suspension of my car. It is a 61 Galaxy and I am adding rack and pinion to it. I will be attempting to locate the inner mounting points to bring the tie rods more in line with the lower control arms to minimize the amount of bump steer. I will be trying to miss some of the minor obstacles like the headers and the motor but should have the space to get the length right if not the height.

Now for the dumb question, Will the amount of ackerman change if the tie rod length changes, meaning the inner mounting point will have to move inboard some? Or is the amount of ackerman entirely a function of the relationship between the outer tie rod end and the lower ball joint?

I have worked this out on circle track cars in the past but there I was needing to eliminate the ackerman where here I need it to stay about the same as factory specs.

Can anyone enlighten me a bit on this?

Scott...

[enjenjo]

The Ackerman can change by moving the rack forward or back from the line the outer tie rod ends are on. Look at a Cavalier rack, you can make an adapter to put the inner tie rod ends just about any where you want.

[unklian]

The Ackerman Effect creates Toe Out while turning.
Like Enjenjo says,this can be done several ways.

I think relocating the inner tie rod in or out,from the centerline,
will only effect bumpsteer.

[Bob Paulin]

I am in the process of making radical changes to the front suspension of my car. It is a 61 Galaxy and I am adding rack and pinion to it. I will be attempting to locate the inner mounting points to bring the tie rods more in line with the lower control arms to minimize the amount of bump steer. I will be trying to miss some of the minor obstacles like the headers and the motor but should have the space to get the length right if not the height....

You are oh-so-close, but if you're trying to eliminate ALL bump steer, it is just a tad more complicated than that.

Just aligning your inner tie rod ends to the lower control arm pivots will, actually, increase bump steer - since the tie rod will now be working on a different plane - most likely pointing uphill from the rack -  from the other suspension points.

Geometrically correct would dictate that lines projected through the inner and outer pivots of the upper control arm, the tie rod and the lower control arm of a typical short-arm/long-arm suspension should all converge at a point towards the outside of the car.

If you have the inner tie rod in line with the lower inner control arm pivot point going uphill to the steering arm, there is no way that the line projected through the inner and outer tie rod pivots will converge at the point where the lines through the upper and lower control arm pivots converge. It will overshoot them by quite a bit.

Chances are that the OEM setup is pretty close, so you should be shooting for an inner tie rod pivot location pretty close to the OEM point.


Take a piece of graph paper and plot a couple of things out.

Locate - in scale - the inner and outer pivot points for the upper and lower control arms with the vehicle and suspension sitting at normal ride height.

Project (draw) a vertical line through the inner pivot points for the upper and lower control arms and another through the outer pivot points. Call the outer line "Line O" and the inner line "Line I".

Project horizontal lines through the upper control arm pivot points and the lower control arm pivot points to where they converge. This is known as the Instantaneous Center...or "IC".

Call the upper line "Line U" and the lower line "Line L"

Pinpoint the steering arm pivot point height on the outer pivot point vertical line.

Now, project a line - "Line T" - from the IC through the steering arm pivot height - on the outer vertical line - "Line O" - to the inner vertical line - "Line I" - projected through the line projected through the inner pivot points.

The height where the line crosses the inner vertical line will be CLOSE to the correct location for the inner tie rod pivot.

The length of "Line T" between "Line O" and "Line I" will be CLOSE to the correct length for the tie rod from pivot-to-pivot.

But, another problem comes from the fact that the upper and lower control arm pivot lines - when viewed from above - are usually NOT parallel, causing the correct projected planes to skew a bit.

IOW - a line projected through the two UPPER pivot points MAY come out just a bit ahead of the nearest pivot while the line projected through the two lower pivots MAY come out just a bit behind the nearest lower pivot - when projected forward or backward to the vertical plane of the tie rod pivot.

That's why we can only get things CLOSE on the drawing board, and then use the bump steer guage to "fine tune" by moving things around a few thousandths at a time.

Now for the dumb question, Will the amount of ackerman change if the tie rod length changes, .....

It is quite possible, since a tie rod that is not operating on the correct plane will be creating more or less Ackerman as a result of bump or rebound as it traverses its incorrect arc while the car is rolling.

......meaning the inner mounting point will have to move inboard some? ?

The tie rod will be of an incorrect length for the geometry involved. Changing the inner pivot point will create bump steer problems.

Or is the amount of ackerman entirely a function of the relationship between the outer tie rod end and the lower ball joint?

Ackerman is the result steering arm angle. It is plotted based upon the location of steering and suspension components. Changing any of these WILL change the *dynamic* Ackerman - even though *static* Ackerman will remain unchanged.


B.P.

[ONE37TUDOR]

Thanks for the help - and the refesher course brought up some steps I had forgotten over the past few years.
My goal (at least until I read the replies) was to just get rid of as much toe change as possible by just moving the inner tie rod location around a bit. On the stock suspension the amount of toe change from bump to rebound is quiet a lot. As I am changing to a Chrysler 300M rack (Much like the Cavalier rack) I can place the inner points anywhere they will clear all the important parts, motor, headers, crossmember, ground, etc.

I felt like I could not remove all the toe change doing this but hoped I could reduce it somewhat but I did not wish to remove all the toe change necessary to make both wheels steer in the correct arc while cornering.

I will graph out the suspension points and see what it show me before I get to creative.
Again thanks for the help.

Scott...

[river1]

this might help you out also

http://www.longacreracing.com/articles/art.asp?ARTID=13#Q6

later jim