Control Arm Clevis

Summary

Background Information 

The front suspension clevises connect the upper and lower control arms to the chassis. 

Requirements

• Safety factor of 3

Loading and Boundary Conditions 

Derivation of Loading Conditions 

The following calculations derive equations for the loading loading conditions as a function of the reaction forces on the tire (from the road). 

Selecting Tire Forces

Devon Copeland's spring rate research project estimates the normal force on the road for hard braking and cornering as well as the lateral tire force required for cornering. Combinations of these values are used to determine the maximum loading conditions on the clevises. In total, 4 different variables were tested resulting in 16 combinations.

1. Braking or no braking
2. Steering or no steering
3. Inside or outside wheel
4. Full rebound or nominal position

The resulting forces on the clevises for each of the 16 combinations is summarized in the following table: 

Notes:

• The distribution of the steering force between the outside and the inside tires was estimated to be directly proportional to the distribution of normal force between the inside and outside tires. 
• Both the full rebound and nominal position was considered to take into account the effect of suspension travel even thought the angle may not make sense in certain combinations (ex. braking, steering, outside whee, nominal position)

Contacts

Friction contacts were applied between the clevis and the fasteners. A coefficient of friction of 0.6 was used for aluminum on steel (Machinery's Handbook). 

Boundary Conditions

The "Summary" section of the above table shows the maximum radial and axial loading conditions for the clevises based on the 16 cases. Since the radial loading can either be in tension or compression. Two loading cases will be tested: maximum radial tension and maximum radial compression. Both these cases will also have the maximum axial force applied as well.

Results

Conclusion & Recommendation 

Stress concentrations played a significant role in causing this part not to pass the requirements. The clevis could be made wider and the fillet radius increase to reduce stress.