Constant Velocity Joints or CV joints allow a rotating shaft to transmit power through a variable angle, at constant rotational speed, without an appreciable increase in friction or play. They are mainly used in full-sized front-wheel-drive and all-wheel-drive automobiles. In radio control applications, miniature CV's are used in much the same manner in lieu of dogbones because of their near-frictionless operation by comparison.
Radio control CV joints are in fact, universal joints (often only at one end rather than both ends while a dogbone remains at the other end) and not the constant velocity joints at all. They are often falsely marketed as CV joints however.
As of yet, the only true constant-velocity joints available for RC (in the strictest sense that they provide ripple-free rotation regardless of input/output shaft angles) are dogbones, but these have sliding parts which suffer increased wear while the CV joints on cars do not. On the "inboard" end of a full-sized automobile, where the shaft only moves up and down with the movement of the suspension, a joint called a "Triax" (also known as "Tripod") joint is used. This has a three-pointed yoke attached to the shaft, which has barrel-shaped rollers on the ends. These fit into a cup with three matching grooves attached to the differential. Since there is only significant movement in one axis, this simple arrangement works well. In RC applications, a two-pointed yoke without rollers on the yoke is used. The yoke simply fits into a slot cut in the drive cup. On the outboard end, the joint must allow for movement both up and down, as well as side-to-side, to allow for steering. The outboard side of the shaft has a drive cup that fits over the ball-shaped axle end. A shaft goes through the ball perpendicular to the axle, and a rod goes through the barrel perpendicularly, and extends through the ball, which has a slot machined to allow the barrel to rotate with the rod protruding though and attaching to the drive cup on the shaft. This scheme unfortunately is equal to universal joint, and has nothing to do with the tripod. See external links for pictures.
Faultfinding and diagnosis Edit
Constant velocity joints are usually reliable and largely trouble-free. The two main failures are wear and partial seizure regardless if the vehicle is a model or full-scale. Both require the use of molybdenum-based grease. Without it, the CV will quickly wear.
Wear in the outer joint usually shows up as vibration at certain speeds, a bit like the vibration caused by an unbalanced wheel. Worn joints will make a rhythmic clicking or cracking noise. Wear in the inner joints shows up as a "clunk" when applying power in a full-sized car; In RC in is usually much easier to just inspect the joint for wear, which can be seen as poor fit between the drive cup and yoke.
- Photo at rc10.com showing the MIP CV upgrade for the RC18T and RC18MT mini-monster trucks
- Diagram of an MIP CV joint This diagram illustrates how the outer joint functions.