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Old 04-09-2012, 11:14 AM
Brian Martin,Freelance adviser
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Originally Posted by enjenjo
Here is more info that may be useful.

The first thing the length of the outer tie rod is determined by the distance between the king pin axis, a line through the upper and lower ball joint centers, and a line drawn through the upper and lower control arm inner pivot shafts, at the height above the lower arm pivot that the steering arm tie rod end center line is above the horizontal center line of the lower ball joint. So if the steering arm height is near the lower control arm height at the ball joint center, then yes, the outer tie rod would be near the same length. But if the steering arm is much higher, as it is on some spindles, the tie rod will be shorter.

You have to then consider the Ackerman angle, because on a rear steer car, the outer tie rod vertical center in as much as an inch inside the king pin axis. This will move the ideal position of the inner tie rod end in by the same distance, which would make the center link shorter. On a front steer, the opposite takes place, and the center link becomes longer with the same tie rod length.

One other thing that is not real critical on a street car, but a big consideration on a race car is the instant center. To be ideal, the outer tie rod should point to the instant center of the car to minimise bump steer caused by body roll. This can cause the tie rods to angle up or down depending on the roll center. In most cases, it will angle down toward the lower control arm pivot a bit, but for most street cars that are not driven to the edge, it is not critical.

All of this is for a perfect world, and in the real world we have to compromise. but if you can take all this into consideration, you should be pretty close to what is needed.

I have not covered everything here, there are other angles that can figure into this too. steering arm length vs center link centerline can cause problems, as can the angles between the steering arm, and the tie rod. But in most cases these are not critical, as long as you are close to correct.
Very well thought out and explained, thanks.

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