Im finally pulling the trigger on adding power steering to my '49 Ford coupe. A little background on the steering that was in the car. I used a stock steering box with the Fat Man Fabrications trick tie rod/center link setup and AMC idler arm. The stock steering box had gotten really loose and upon disassembly I found that the sector shaft was broken. I had purchased a power steering kit from Crown Automotive Engineering a year and a half ago which works in conjunction with the Fat Man center link and AMC idler. Looks like Crown has since either moved or gone out of business. With all the parts on the bench I noticed that the pitman arm and idler arms are different lengths. The wheels started turning... I have a basic working understanding of the ackerman principle on a solid axle front end, but am curious how these different length arms will affect an IFS? Should they be closer to the same length? The pitman arm on the new p/s box is 4 1/2" center to center (stock Ford arm was 4 7/8") and the AMC idler arm is 5 1/4 center to center. (Stock idler arm is long gone, so I cant compare its length to the stock pitman arm). The pitman and idler arms on this setup point forward (with the stock box both arms point to the rear), so when the AMC idler arm is flipped around the mounting points will be above the frame rail (frame sweeps down towards the rear) see Fat Man sketch. Since I will be looking for an idler arm that better fits the new configuration, Id like to get one with a length that works best with the new setup.
You have a good point but i'm not sure how critical it is. This is the best and simplest explanation of steering ackerman that i have found so far and it is pretty well self explanatory. I'd have to think you are correct in thinking that when the inside wheel is turned to X degrees the outside wheel is not going to be at Y degrees but at Y.5 or another slightly off perfect point. It may or may not be noticeable to a large degree in actual driving. At worse you might notice a slight tire scrubbing on tight radius turns. most likely you won't know how much it's actually off unless you put it on a high zoot front end machine with sensors that can compare one side to the other and give a readout on the difference.
I agree with Mr48chev.........it's not quite "right".....but just how "wrong" is it? Probably not enough to make a consequential difference in everyday driving and even then, only as turn radius tightens. Ackermann is primarily a function of the angle of the steering arms, at the spindle, not the tie rods.
I believe the difference in Ackerman caused by the different radius of the box.idler lengths should be inconsequential put her together and try it I bet you'll like it!
I could be wrong here, but the steering geometry from the idler out is what sets Ackerman. The pitman arm simply pushes or pulls the idler arm. Seems to me the only difference the length of the pitman arm makes is the quickness of the steering.
OK, I'm no steering expert, but I remember most of my early geometry and for that reason I'd be concerned with unequal-length pitman and idler arms. The steering mechanism is a parallelogram [I wish I could draw a picture here] . The drag link basically moves in an arc, as you can see that the distance between the drag link and the oil pan changes through its range of motion. This arc changes the turning rate of the wheels--- the more you turn the steering wheel the quicker the wheels turn at the spindles. If the pitman and ldler arms are unequal length, you no longer have a parallelogram. You will be upsetting the arc that the drag link travels in, potentially causing the turning rate of each wheel to be different than the other.
55 dude: none of the box specs were in the company's literature, but by comparison to other kits Ive seen, Im fairly certain it is an Isuzu box. I was thinking I might be over analyzing the whole thing and was leaning towards King fords "try it, Ill bet you like it" and Hnstray's "just how wrong is it?"comments, when Mike51Merc had to deal the geometry card which is what got me going on this in the first place. In my mind I keep hearing this Model A sedan that rolled into our show (I beam axle with the tie rod out front) squalling the front tires while parking... I found an "unknown" idler arm in my parts stash with the configuration I need. It is 1/4" shorter in length than the AMC arm...I guess Ill try it and see if I like it!
Mr48chev: I have saved your ackerman diagram. My solid axle ackerman theory is basically that a line drawn rearward from the kingpin thru the steering arm/tie rod joint on the left side should intersect the same line on the right side at about the center of the cars rear differential. Sadly many of the high zoot alignment shops around here cannot, or will not do an alignment on a car if they cant attach sensors to the rear wheels. Its 4 wheel alignment or nothing...
Mike51merc, In my opinion you are correct about the parallelogram movement of the drag link....and that it is distorted by the unequal lengths of the pitman vs the idler arm. However, the difference in the length has minimal effect on the parallelogram geometry and more importantly, total movement of the drag link/tie rods. In any case, the Ackermann Effect is a function of the STEERING ARMS angle, and the arc they travel. In the course of their movement they effectively become longer or shorter, each doing the opposite of the other. It is THAT arc and effective length that turns the inside wheel a greater number of degrees than the outside wheel. Again, in my opinion, that geometric effect is very minimally changed, if at all, by the small variance in the idler arm length in the OP's case.
You may be right, but the thing that troubles me is that wheel alignment specs are pretty close tolerances with measurements in single degrees and 1/16" increments. If it was my car I'd probably be satisfied by a test drive. If it feels right and doesn't scrub the tires in tight turns I'd be OK with it. Perhaps the bigger question is how the hell that sector broke?
Thinking about it, the length of the idler arm probably does not matter as it is just a slave of the pitman arm. The eye of the idler arm always travels the same distance as the pitman arm since they are linked together no matter the length of the idler arm itself. So unless the rest of the geometry is somehow messed up, it should still work the same with your AMC idler arm.
Think about this: The center link doesn't move in a straight line like a rack-and-pinion linkage does. Idler arm length will effect the distance travelled because it changes the angle of attack of the tie rod on that side (actually changes the angle on both sides making one more acute and the other more obtuse). It may not matter so much if the difference is small, but if the difference in length is enough it will matter.
I've been meaning to post on this but keep forgetting...the older I get, the worse it becomes! While converting the car to power steering I made the decision to ditch the front airbags and go with Aerostar coils. After finding an idler arm that was a little shorter in length (still 1/2" longer than the pitman arm) and a better shape to fit the installation we did a basic garage alignment and added a new set of 6.70-15 Firestone bias plies. I've driven the car in town, on the highway, sharp slow turns, turns at speed, etc... It steers like a champ with no squalling of tires and no odd marking on the tire tread. Other than a little wandering on straight sections of road (which hopefully will be made better with a professional front end alignment) I'm very happy with the way the steering works. Thanks to everybody for their input, it was all very appreciated and I learned a bit from you all.
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