Besides traction and power, useable steering is a prime need for a rock crawler. You need the ability to hold a line when crawling and also the capability of turning under some pretty brutal conditions with large tires.
You’ll certainly want to run a beefy servo and servo arm (or firm servo saver), but if you setup poor steering geometry on your axle you’ll be throwing away some performance you could certainly use. What do we mean when we say steering geometry?
We are talking about the length and position of your servo arm and your knuckle arms. We are also talking about the location of these such that they determine the angle at which your draglink and tie rod operate.
You typically have a draglink running from the servo arm to one of the axle knuckles and then a tie rod connecting the two knuckles. Or, you may use a less common alternative with two draglinks and no tie rod as shown above right.
At the knuckle end, the farther you mount the draglink or tie rod from the knuckle pivot point, the greater the leverage force (think how a lever operates) you can apply when steering the tire. However, as you move the link further out you require more steering movement to fully turn the tire. You have to make sure your servo can still turn the knuckle to full lock in each direction.
On the servo end, the opposite effect occurs. When you use a longer servo arm you gain steering rod travel, yet loose steering force. How you say? Take and hold a weight or brick in your hand with your elbow at your side. Do an arm curl and lift the brick. Next, fully extend your arm horizontally and again lift the weight. As your arm (or servo arm) becomes longer it takes more effort to move the weight (steering link).
The angle at which you run your links can also make a difference. On a 1:1 rig where the steering box is mounted on the frame and the steering link runs to the axle, you have to deal with changes in the steering as the suspension moves up and down. Most RC crawlers have the steering servo and all linkages on the axle so they always remain in the same relationship with each other.
Sometimes you’re limited with choice of servo mounting locations and may need to compromise optimum steering geometry. In the case of the 2.2 rig above, the draglink would have worked better at a flatter angle. However, the design of the high clearance axles necessitated mounting the servo up higher which increased the angle. In the case of the Super class rig shown, the draglink is positioned at a very high angle which causes the loss of some of the servo power to turn the knuckles and instead applies that as an up/down force on the knuckle arm.
It’s best if you can keep the draglink running from the servo arm to the knuckle as parallel to the axle as possible for best performance as shown on the 2.2 rig below. This ensures that the force from the servo arm is used to apply a horizontal force to turn the knuckles and not wasting some force in the vertical direction which does nothing for your steering and does place additional twisting strain on your knuckle arms.
When looking from above it also helps to keep the angle of the draglink as close to parallel to the tie rod as well. This reduces the tendency of the servo horn or arm to want to twist or bend, which also wastes servo energy.
While we say that it’s best to keep a relatively flat draglink angle, the shape of the draglink does not affect the performance in any meaningful way. Bending the draglink in an ‘S’ shape (or other shape) only helps if you need some steering rod clearance. The shape of the rod has no affect on the geometry.
When it comes to servo arms, there are several choices and schools of thought. One can use either a spring-loaded servo saver, a plastic arm, or an aluminum arm. The goal is to provide strong, predictable steering while also providing some safety against destroying the servo gears from a hard impact transmitted from a tire back to the steering linkage and servo. Servo savers provide the greatest protection to the servo gears, but their flex also diminishes how well you can steer the tires and hold a line in the rocks under heavy load.
On a rig using 2.2 or smaller size tires, a good bet is to usually run a plastic or aluminum arm and a good quality servo. In most cases, the servo gears and arm will withstand the impact from a crashing a smaller tire rig (not as much so on heavier Super class rigs).
The use of a solid servo arm is a step up in providing a rigid steering setup that can put the maximum torque of the servo to the tires. However, without the safety valve of a saver, the servo is more prone to damage. Running a plastic servo arm is a compromise that provides a solid link with the thought being the plastic will snap in two under an extreme load to the servo. We have seen this effective in action.
