Steering is very "twitchy"....... - Ford Mustang Forums : Corral.net Mustang Forum
 
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Old 02-24-2017, 06:46 PM Thread Starter
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Steering is very "twitchy".......

Please keep the flames to min.

By twitchy, I mean that while driving down a straight road, it doesn't take much input from the steering wheel to make the truck quickly jerk to one side or the other.

This is a home built setup on my 90 Ranger min truck.

I converted it to manual rack/pinion steering using an old Fox Mustang rack.

Its a double wishbone front suspension with Ridtech air adjustable shocks, and an aftermarket 1" sway bar with urethane bushings/end links. 1 deg neg camber, 1 deg pos caster.

The rear is a 9" Currie with a four link, and watts link.
The rack sits level with the tie rod ends.

BUT due to the 363 swap the rack sits forward of the tie rod ends in that the tie rod ends are swept back at about a 35 deg angle going to the spindles.

Since fabbing all of that I have seen vehicles with the rack placed forward of the spindles, so I guess it can work.

I did have the front end professionally aligned, but I can't remember if they set toe in, or toe out.

Can, I get the steering corrected by changing the toe in or out?

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Old 02-24-2017, 07:49 PM
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I'm still learning as I go, so take my advice with a grain of salt, BUT:

I bet the 2 deg positive caster is the culprit.

Try increasing the caster to somewhere between 4-6 degrees positive. I bet the symptoms improve.

Toe could also be to blame if its significantly toe'd out.

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Old 02-25-2017, 08:29 PM
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I would say the backwards sweep of the tie rod ends from the rack to the spindle is the real issue. Because they're not even close to being on the same vertical plane as the spindles' travel is, they're drawing the steering arms inward when the suspension moves. That's making them toe-in whenever the suspension is not at ride height. In other words, bumpsteer. Probably huge. You're going to have to find a way to extend the steering arms forward IMO.
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Old 02-25-2017, 08:38 PM
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Having the steering rack in front of the end of the steering arms makes the steering have antiAckermann. This means that has you turn the steering wheel, the outside wheel turns at a faster rate than the inside wheel. This generates lots of understeer as the front tires are fighting each other to make the car go straight. It is making the truck less twitchy.

This rack placement has almost no affect on bumpsteer.

The first thing I would do is to find out what the toe setting is. More toe in, will make the truck more stable. This is the first thing I would look at.

Isometric photos of the front suspension would be helpful. Side, front and bottom with wheel off.

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Old 02-25-2017, 10:28 PM Thread Starter
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Quote:
Originally Posted by Jack Hidley View Post
Having the steering rack in front of the end of the steering arms makes the steering have antiAckermann. This means that has you turn the steering wheel, the outside wheel turns at a faster rate than the inside wheel. This generates lots of understeer as the front tires are fighting each other to make the car go straight. It is making the truck less twitchy.

This rack placement has almost no affect on bumpsteer.

The first thing I would do is to find out what the toe setting is. More toe in, will make the truck more stable. This is the first thing I would look at.

Isometric photos of the front suspension would be helpful. Side, front and bottom with wheel off.
Ok, Jack I will send you some pictures tomorrow via a PM. Thanks.

Also, the rack wasn't as far forward as I remember it.

I think the issue with the rack placement was, that if I had put the rack out a bit further it would have made the angles of the steering linkage [u-joints] a little less severe.

Of course the steering linkage isn't binding so there is no need to move the rack forward.

But, back to correct what I said earlier the rack isn't that far forward from the spindles.
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Old 02-26-2017, 05:39 PM
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Quote:
Originally Posted by Jack Hidley View Post
This rack placement has almost no affect on bumpsteer.
Why is that? When I "model" it, analog not computer, it seems like it very much would cause bumpsteer unless the tie rod can change length as it articulates.
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Old 02-26-2017, 06:53 PM
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Fraser,

This is a bit difficult to visualize in words.

Imagine that the steering rack were moved backwards so that it and the tie rods formed a straight line in the top view. To align the car with zero toe, each tie rod needs to be 10 units long. If the car has the FCA geometry similar to an S197 Mustang where the control arm pivots are parallel to the centerline of the car, and the car has zero caster, then as the suspension moves up and down, the end of the steering arm is going to stay in the same fore/aft location. Below is a video of that suspension moving up and down.


If you then move the steering rack back in front of the steering arms, then the tie rods are going to have to get longer to still align the car with zero toe. Say 13 units long. When you look at this new suspension from the front, the effective length of the tie rod is still going to be 10 units long, because the steering rack width has not changed and the steering arm is in the same location. In this type of suspension, the length of the tie rod in the the front view is what determines the bumpsteer behavior. It doesn't matter what the length is in the side view.

If you take the different tie rod locations from the first and second suspensions I've described here and draw a line from the inner pivot of one tie rod to the second tie rod, they form a triangle. The line you've just drawn in going to be parallel to the centerline of the car. In both suspensions, the outer tip of this triangle follows the exact same path in space, therefore both suspensions will have the same bumpsteer behavior. It doesn't matter where the steering rack is placed fore/aft. As long as the rack is the same width, the tip of this triangle must follow the same path as the suspension rises and falls.

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Old 03-02-2017, 07:24 PM Thread Starter
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Well for the update: I turned both tie rods IN two turns.

Today I took the truck out for a drive, and its night/day difference in the way it drives.

The steering is a lot less white knuckle driving vs the previous setting.

Thanks for the help.
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Old 03-02-2017, 07:41 PM Thread Starter
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Here are the pictures of the front suspension I fabbed up. No its not pretty, but there are NO KITs available for this vehicle.

I didn't have time to take a pic of the rack under the car while it was on jack stands.








Steering linkage


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