Steering question? Rack travel for the experts.

[Bugpac]

How long do i need to make my control arm? I posted a drawing of the top of my spindle, i want to have 28 degrees each direction of steering, my rack i believe is 4.25 lock to lock? i want 28 because the mis are good for 30, i want the rack to stop first.

[KWiKSand]

It is really a trig question, but I am better at cad then I am at trig.

It depends on a few things. In the attached picture The length of D2(your tierod) and the d0 dimension(the location front to back of the inner tierod attachment) affect the value along with the arm length. (d1)

[Bugpac]

ok, i probbaly could go off what you have there, i can get about the 3" on the inside, the tie rod length probbaly wont effect to much will it? as if i just need to have a starting point. and the 4" +/- i cant see right now, on the outside will have to have 1/2 the width of my spindle deducted from that right? 8)

[KWiKSand]

Yes, the 4.5 is to the center of the spindle.... basically the pivot point inline with the tierod arm.

I just used some random numbers, .... it might be better if the 3.5 was a little longer to make it so the tierod tube is straight across half way through the travel?? Good luck

[Bugpac]

cool, i am gonna put the steer on hold for now, i am gonna buy a diffrent rack. the one i have will not work without extensions on it, i have found one that is 16.25 long eye to eye my frame is 17 so should be perfect, or almost, thanks for the input and when i get the correct measures i will hit you up for some help, also your stuff is shipping in am, they are picking up at my house wont have tr # till tommorow night, cause i am out of waybills and they have to bring me one to get the number!! dhl to , 8)

[K-fab]

Make sure that you have the rack positioned closer to the spindle (front to rear in the car) than you have drawn or as shown in the cad drawings.

If you get the rack positioned too far back in the chassis in relation to the spindles, the spindles can over rotate and end up locking the wheel past where you wanted it to stop.

I'll try to draw some things up here a bit later to explain what I'm trying to say...
--------------------------
(later that evening)

Okay, where to start...

How about with the hard facts - you have a rack that has 4.25" of travel from lock to lock and you want to have 28 degrees of deflection - and I am ASSUMING that you mean that the heim actually has 56 total degrees of deflection - 28 degrees off center both left and right.

That would equal 2.125" of travel from center to full lock to get 28 degrees of rotation on your spindle in each direction.

Steering motion and motion ratios are some of the tricky things.

Let's start with the spindle and where you mount the tie rod end to the spindle:
We'll work in a "perfect world" scenario first. - a parallel system.

Let's draw it first. Draw a circle that's about 9.05" in diameter. The center is the king pin axis.

Draw two lines through it - one vertically, one horizontally.

The horizontal line is your axle, the verticle line is your control arm. Where the verticle line intersects the circle is where the tie rod is attached.

Now draw two more verticle lines - 2.125" to each side of the verticle line. Where these intersect the arc is where the tie rod will pull/push the control arm. If you draw a line from each intersection of these verticle lines to the center of the circle, they should be 28 degrees from vert. - there's your turning of your spindle about the king pin axis.

This setup says that when the spindle's control axis (line drawn from the middle of the axis that the spindle pivots around (king pin) to the middle of the end of the tie rod's heim) is perpendicular to the chassis's axis.

As you move the tie rod left and right, it's based on a zero point on the rotation around the axis of the spindle. 2.125" of movement left will give 28 degrees of rotation to the left and vice versa. - in other words, you get equal movement about the spindle's axis.

The length of the control arm, which is measured from the center of the spindle's pivot axis (king pin) to the center of the tie rod end's pivot axis ends up being 4.5264 inches - so there is the number you were looking for.

Now, let's play a bit and get some ackerman included into your motion.
To make it really obvious, we'll throw out some pretty big angles.

Go back and draw your circle with a horizontal line.
Instead of drawing a verticle line, this time draw a line that's 28 degrees off verticle, counter clockwise. - in other words, copy the line you found at full left turn. - we'll call this line the center point of the steering now.

So far, so good?

Okay, back to our tie rod motion - moves 2.125 inches each way from the center point of the steering.

If you push the 28 degree from verticle line to verticle, you'll get a turn angle of your axle of 28 degrees. - the same as you had earlier.

But, try this - draw a horizontal line that's 2.125" long to the left from the point where your 28 degree line hits the circle. From here, draw a line down vertically until it hits the circle again. From that point, draw another line back to the center of the circle. - the angle you've just made is 42 degrees instead of the 28 of previous movement. - 1.5 times the angular movement for the same amount of tie rod motion.

It all has to do with the horizontal travel aspect on the curve of the arc that the tie rods follow around the king pin.

By playing with the location of the tie rod mount, in relation to the axis of the chassis and on the arc, you can induce more arc in the rotation of the spindle. This is how you put ackerman into your steering - set it up so that you have a few degrees more rotation on the inside wheel than you do the outside.

If you want to get really fancy, draw up your chassis (at least the contact points). Draw a line that goes through both rear wheels and then out from them about 10 - 15 feet to the left. Turn your front wheels (at least the outside {right} wheel) to full lock - 28 degrees. Draw a line that represents the axle on this wheel and extend it so that it meets the line you drew through the rear wheels.

Turn the left wheel (independently of the right) until the line that runs through the axle meets the right hand wheel's line where it meets the rear wheel's line. - this would be the amount of ackerman that you want. (in general).

You can see how it gets pretty deep.

I hope this post, long as it is, helps. I have a bunch of it drawn up in ACAD, but I can't figure out how to post the pix. I tried converting it to a bmp file, but it ended up being a huge file...

[ProtoDie]

K FAB,
Getting pics out of CAD can be a pain.
Below is a link to capture express software. I use it all the time to take screen pics of CAD , it works great and is so easy to use.
I think it is only $15



http://www.captureexpress.com/

[KWiKSand]

I got this program back before they started adding a watermark after 30 days. http://www.wisepixel.com/ The watermark might be worth putting up with.

Also a easy way to get a screen capture is to simply press PrintScreen and paste it into a graphics program such as the free Irfanview. www.irfanview.com You can save it to all kinds of formats from there.