Tuesday Tech Tip #2 - Master your Caster

Bronco picture of the day:

Caster is the difference in angles between the center-line of the steering ball joints and plumb (0 degrees). Negative caster (or not enough positive caster) will result in twitchy steering and wandering all over the road.
Broncos are designed to have about 6-7 degrees of positive caster (axle steering center line canted rearward 6 degrees off of plumb). Anywhere from 4 to 9 degrees is acceptable. When a Bronco is lifted the axle rotates forward, this is because its is being moved on an arc with the pivot point being the end of the radius arm. This results in a loss of positive caster.

photo courtesy of Wild Horses Four Wheel Drive

The idea is similar to tongue weight on a trailer, when you have no tongue weight, the trailer tires attempt to "steer themselves" rather than being towed. When you have no or too little positive caster the axle attempts to "steer itself" rather than being steered...if that makes any sense.

Most companies that make aftermarket C bushings sell them with 2, 4 & 6 or 7 degree caster adjustment built in. Like these ones from Jeff's Bronco Graveyard

Sometimes that is not enough to bring the caster back in line. In those cases you can add a radius arm drop bracket like this one from James Duff

Or these ones from Tom's Bronco Parts

The obvious downside to these brackets is the loss of ground clearance.

Another option is to install longer radius arms. By increasing the length of the arm, you decrease the amount of swing in the arc, thus the amount of degrees off of plumb are reduced. The arm on top is a James Duff unit that is 10.75" longer than stock and have 4.25 degrees of positive caster off set built in. The bottom radius arm is a stock unit. I'll be covering extended radius arms in another installment later on.

Some of these come with caster adjustment like these ones from BC Broncos. You can adjust the caster from 0 to 15 degrees positive. They come in stock length and extended 11".

  Getting your caster angle right is imperative. While there is no chart to tell you how much you need to adjust for each inch of lift, here is guideline:

•  2 degrees for stock suspension with bigger tires
•  4-5 degrees for 2-3" lift
•  6-7 degrees for 3.5-4" lift
•  8-9 degrees for 5.5" lift

This of course depends on where your caster was when you started. It is best to take it to a front end alignment shop and have them measure it for you.