A Comprehensive Methodology to Design and Develop Suspension System Bolted Joints using Vehicle Test Loads and CAE Simulation

The bolted joints in suspension systems are subjected to severe external service loads during vehicle operation. To prevent the loaded joint from loosening and allowing it to retain its potential energy stored during assembly, a holistic design approach is needed. This paper explains the methodology to design and optimize bolted joints for the suspension systems of a modern 7-seater sports utility vehicle. The optimization technique consists of -

1. 1
   Extensive benchmarking of global benchmark vehicles with similar suspension architecture and gross vehicle weights to derive preliminary torque and joint preloads.
2. 2
   Measuring external loads acting in x, y and z directions using a wheel force transducer on various durability tracks.
3. 3
   Performing Multi body dynamic simulations to obtain the loads at various bolted joint locations.
4. 4
   Taking the input of the external loads acting on the individual joints and perform a simulation to evaluate slip at joinery for a given preload.
5. 5
   Bolt characterization measurements to establish the torque to be applied to achieve the required preload at the joint.

Based on the results, joinery design approaches are discussed which focus on increasing the resistance against slip and utilizing the optimum preload applied during assembly. For critical joints experiencing high service loads, the torque to yield (torque plus angle) method is prescribed instead of the elastic tightening method along with reduction of the axial gap. For double shear joints having steel sleeve and sheet metal interface, improvements seen with the addition of serrations/knurling on the contact surface of the sleeve are discussed. The stiffness of mating parts is increased for proper bolt load transfer. With the incorporation of these design modifications in the joinery, extensive vehicle durability tests are performed in the vehicle, and no incidences of joinery loosening/slipping are observed. The efficacy of the design improvement was reflected in the residual torque values which showed no reduction after the test compared to the start of the test.