89FormulaRoth

iTrader: (1)

The following is taken from this link:

http://www.motor.com/article.asp?article_ID=552

Thrust Angle

The thrust angle is the angle between the vehicle's geometric centerline and the direction in which the rear wheels are aimed. If the rear wheels point straight ahead, the thrust line and the geometric centerline are the same, and there is no thrust angle. When a vehicle is being driven straight ahead, the rear wheels steer it along the thrust line, so a zero thrust angle is ideal.

Adjusting rear toe should correct the thrust angle, but the rear suspension design may not allow toe correction. If a thrust angle can't be eliminated, align the front wheels to the thrust line rather than to the vehicle's geometric centerline. Aligning the front wheels to the centerline with the rear wheels driving along a different thrust line can result in any or all of the following: a crooked steering wheel, incorrect front camber and toe while moving, accelerated tire wear or pull.

With that in mind, what would a person need adjustable lower control arms for? I have a set on my car, which is lowered 1.5", but I think in retrospect that maybe I bought them for the wrong reason. I originally bought a set of LCA's with my lowering springs which were stock length, but they wouldn't fit. It seemed that they were too short. Perhaps what happened is that the entire torque arm and rear assembly slipped rearward slightly.

Can anyone shed light on this? What makes this even more confusing for me is that none of the aftermarket companies I've looked at give any real explanation why their adjustable lower control arms are necessary.

Base91

iTrader: (1)

Adjustable lets you lengthen or shorten the wheelbase slightly but most importantly lets you get the axle lined up so it's exactly perpendicular to the body center line. That's how you'd likely always want it so the car drives "straight" and doesn't look like it's going slightly sideways all the time.

89FormulaRoth

iTrader: (1)

Okay, but shouldn't stock arms give me zero thrust angle? Why would I want to lengthen my wheelbase? Does that always happen when you lower an F-body?

Base91

iTrader: (1)

Yes the stock arms should. A longer wheelbase would be more stable at higher speed and a shorter wheelbase would allow quicker cornering response. There may even be cases where you don't want a zero thrust angle either but these are fine tuning details. Almost always you'd want the axle just where the factory put it. It's likely the best all round compromise position.

KWIK84

iTrader: (5)

Should, but there is such a thing in manufacturing called tolerance stack up. Such that just throwing numbers in here..

Say rear arms should be 10.00" center to center and have a +/- .030" tolerance range for production. Rear end brackets have a axial tolerance of .030. Chassis mounting points have a .030" axial tolerance.

This means your new from the factory rear end could come to you being .120" out of square. This is the reason for adjustable control arms. In most cars, this would all be adjusted via the front alignment to compensate and make the car drive straight, but actually the track would not be.

When we start to modify our cars and ask more from them in a performance setting we start to look at this things to "straighten" the car out and improve its performance.