These pivots are where the body attaches. The body is controlled in the pitch direction by a set of links that form a differential, keeping the body at an average angle between the two rockers. The system has no springs or intentionally elastic members, which improves Sojourner's traction. When a wheel is suspended with an elastic system, the downward force increases as the wheel is raised according to the spring rate of the system. The greater force makes it more difficult to raise the wheel as it takes downward force away from the remaining wheels, reducing their traction.
Stability is key to the vehicle's overall effectiveness.
This includes the ability to stop at any point and proceed or reverse, even while climbing a vertical wall. For this reason, the first analyses were done to optimize the ability to climb vertical bumps that are axle-high or larger. Bumps are obstacles that fit between the wheels, so that all wheels not climbing are on a single plane. Steps, which carry the vehicle from one level to another, represent another type of obstacle. Bumps, on the other hand, cause the forward wheels to be dragged backwards as the rear climbs.