Basic Link Arrangement and Servo Mounts

[summerof95]

There seems to be a bit of confusion when it comes to various link arrangements, bump-steer, and pan-hard bars.

Maybe this is will clear a few things up.
If I missed anything important, or messed anything up, just let me know.



An Introduction to Linked Suspension


Suspensions systems are designed to allow an axle to move relative to the chassis, in a predictable and consistent fashion; controlling any unwanted front-back or literal movement.


Common types of linked suspension on a solid axle RC are:

• Triangulated 4 link
• Wishbone or Y link
• Parallel 4 link
• 3 link

Triangulated 4 Link

On of the most common link set-ups found on crawlers is the triangulated 4-link.
This style consists of 4 links connecting the axle to the chassis. Viewed from above, the links form two opposing triangles. Usually, the lower links connect the center of the chassis to the outside of the axle, while the upper links connect the center of the axle to a wider mount on the chassis. These two opposing triangles keep the axle centered under the chassis, preventing lateral movement.



Wishbone or Y-Link

Seen on stock Axial scx10s, this setup is very close to a 4 link, but shifts the rotational axis slightly, keeping the top of the axle more inline with chassis' center during its articulation
it is composed of two lower links and one Y-shaped upper link, with the two arms of the Y attached to the chassis, and the base of the Y attached to the center of the axle. This forms two opposing triangles, preventing lateral movement.



Parallel 4 Link

A parallel 4 link is less common on RC vehicles. In this set-up, when viewed from above, the upper and lower connecting links are parallel to each other. This arrangement allows the axle unpredictable lateral movement under the chassis.



3 Link

A 3-link set up is composed of two triangulated lower links, and one diagonal upper link. Similar to a parallel 4-link, this setup also allows a large amount of unpredictable axle movement.


Pan-Hard: The Missing Link

The last two arrangements allow for unpredictable, and thereby unwanted, movement of the axle relative to the chassis. In order to utilize these set-up, the movement must be made constant and predicable. This is accomplished through use of a track bar, also known as a “pan-hard” link. This link runs from the front left of the axle, to a mount on the right of the chassis, or vise-versa.
The pan-hard link forces the axle to move in a arc as it cycles, controlling the lateral motion. This does not eliminate the lateral movement, but keeps the it consistent and predicable.
For this reason, a pan-hard link must not be used on a set-up that has no lateral movement, such as the Triangulated 4 Link or the Wishbone.

[summerof95]

Servo Placement, and Its Relation to Suspension.




The servo steers the wheels by pushing or pulling on the steering knuckles through a fixed link called a “drag-link”. The servo and the suspension must work together to provide steering to the axle. When using solid axles, there are two common methods of mounting a steering servo.


Axle Mounted

The simplest option is to mount the servo to the axle itself. This has the advantage of maintaining a set steering geometry, regardless of the orientation of the axle, since the servo will move along with it. This set up requires additional chassis clearance, however, as the servo will need to be kept from contacting the chassis during the axles articulation.


Chassis Mounted

The second option is to mount the servo to the chassis.
The this allows the servo and the axle to move independently of each other.
As the axle move closer to the chassis, the distance between the servo and the steering knuckles lessens. As the axle moves away from the chassis, the distance grows. The drag-link is always a fixed length, and as the distance between the servo and the knuckle changes, the drag-link pushes or pulls the knuckle, causing the wheels to steer left or right.
This is known as “bump-steer”.


In order to eliminate bump-steer, the distance between the servo and knuckle must be kept constant. The only way to keep the distance consistent as the axle moves vertically, is to also move it laterally. In other words: to eliminate bump-steer the axle must move laterally in a controlled and predicable fashion.
This happens to be exactly the effect of a pan-hard link.


If viewed from the front of the chassis, a pan-hard bar mounted on points identical to the servo and knuckle mounts of the drag-link, on both the X and Y axis, will enforce a constant distance between the two points, and bump-steer will be eliminated entirely.
Simply put, a perfect pan-hard will be the same length as the drag-link, and when viewed from the front, be completely hidden behind it.


Because this is not always feasible, compromises may be made. A pan-hard link which is a slightly different length, or is mounted to the left or right of the drag-link, will still reduce bump-steer, but will not eliminate it entirely.