Why You Don't Box Rear Upper Control Arms

[pontiacgp]

Despite what some in here think, it is a dangerous practice to box the rear upper control arms. Some even suggest not to use polyurethane bushings in the upper control arms. When you look at a 4 link system it's easy to see why you don't box the upper. When the rear end is tilted when your going around a corner the plane for the two ends of the upper control arm are not the same. The stock uppers twist to compensate for this. If the upper cannot twist to compensate the rear end is in a bind and will snap the rear end around, known as oversteer. Mustangs have a 4 link system and it is common problem with the upper even to put in polyurethane bushings because that will tear the box off the car. There are forums regarding this problem if you want to look them up.

here is a picture of a 4 link and it's easy to visualize how the upper would twist when the rear end in on an angle..

and here is the elongated hole from an upper on a g body that was boxed, since the upper could not compensate the bolt through the upper on the rear end side was tearing through the upper when it was binding.

 

[-83MONTESS-]

What about the tubular UMI arms?

 

[307 Regal]

What about the tubular UMI arms?

As long as they have some sort of swivel joint or you mostly just drag race, you're probably fine.

 

[joesregalproject]

What about the tubular UMI arms?

I thought all the tubular UMI UCAs had roto-joints. Maybe I'm mistaken.

 

[pontiacgp]

What about the tubular UMI arms?

I thought all the tubular UMI UCAs had roto-joints. Maybe I'm mistaken.

You can get them with roto joints but the do have a tubular set with a polyurethane bushing and you need to put some kind of swivel joint on the ear to make them work. I have the UMI upper and lowr control arms with a roto joint in each of them

 

[motorheadmike]

You can box the stock arms as long as you have rubber in the ears/arms. It is a question of managing the amount of deflection vs. binding vs. breakage.

Controlling the "roll" of the axle has as much to do with binding up the rear suspension as the bushing-type and arm construction. Using an axle/chassis mounted sway bar in the rear will manage a lot of unnecessary axle movement/articulation and reduce the probability of having a suspension that binds. Live axle cars are a funny beast, especially when you have a front and rear suspension that are speaking two different languages; where a plowing front end instantly turns into snap oversteer when the power comes on and the weight shifts. Good times.

We are putting UMI roto-joint uppers and lowers in the back of the wagon and a Hellwig "pro-touring" axle/chassis mounted sway bar for ride quality as much as handling.

With the UMI roto-joint lowers and heim-joint uppers in my Buick (and the shocks/springs/swaybar left off) I could articulate the differential up and down by hand through the full range of motion (zero potential to bind)... pretty impressive stuff.

 

[pontiacgp]

With the UMI roto-joint lowers and heim-joint uppers in my Buick (and the shocks/springs/swaybar left off) I could articulate the differential up and down by hand through the full range of motion (zero potential to bind)... pretty impressive stuff.

too bad there is no way to correct the rear steer, it turns the back end the wrong way...

 

[Clone TIE Pilot]

I say it is a bad idea to box the factory lower arms too, and Polyurethane and its variations (like poly graphite) have no business being used in any rotational bushing application. This means leaf springs, a-arms, trailing arms, strut rods ect. It should only be used for body mount bushings and swaybar endlink bushings. The rear suspension is designed to flex to prevent binding and frame damage. Something has to twist and when the control arms and bushings no longer can twist, then your frame will!

The factory rubber bushings and control arms are designed to accommodate rotation and twist. The rubber bushings flex or 'deflect' as a normal and required part of their function to prevent binding, they are designed to have multiple axis of rotation. Unfortunately polyurethane bushing replacements sometimes find their way into bushings that require multiple axis of rotation. Nearly incompressible, polyurethane binds along any secondary axis. Polyurethane is an inappropriate choice for such applications. Correct performance replacements for rubber bushings would incorporate spherical bearings to provide incompressibility and freedom of motion on multiple axis simultaneously.

Tubular or boxed control arms - with poly bushings - force the LCA mounting brackets on the chassis and axle to bend. Now that the car relies on twisting metal for suspension, it may at times bind, as the axle cannot move smoothly comply with bumps. The resulting oversteer condition can result in loss of control on high speed corners - not what you want in a high-power rear-drive car. Even for lower speed handling, the poly bushings increase the rear roll stiffness - something you should be tuning with the spring rates and sway bars. Greasing the poly bushings does not help this issue.

 

[motorheadmike]

I say it is a bad idea to box the factory lower arms too, and Polyurethane and its variations (like poly graphite) have no business being used in any rotational bushing application.

You can and I have... you need a high-durometer rubber bushing from the 1LE F-bodies. Works great.

 

[pontiacgp]

the lowers are not on much of an angle and are kept from twisting with the sway bar installed. There is no problem boxing the lowers