The majority of vehicle simulations are for vehicles in optimal or close to optimal conditions. However, evaluating the vehicle which is in a worn condition can severely inhibit its performance and greatly increase peak loads. This can accelerate the degradation of the bushes further, and also cause dangerous handling characteristics.
For this blog post I wanted to investigate the effect of degraded bushes on both peak forces and the performance of a vehicle compared to one with non-degraded bushes.
Creating Degraded Bushes
Degraded bushes can result in a range of undesired effects, in some cases bushes could degrade and get harder or get softer. But the most common result of a degraded bush or joint that I’ve come across is lash being introduced.
To model this effect, a new bush model needed to be created, as all pre-existing bush models produce damping forces in both directions along each axis, regardless of position. This is correct for bushes that haven’t separated from the bush material. But if the bush has separated and can move freely between two surfaces then the damping will only react against the compression of the side it’s contacting. This effect was key to make sure that the motion was as realistic as possible.
To implement this I duplicated an existing bush model (Claytex.Mechanics.MultiBody.Bushes.Components.NonLinearBush) which relies on 1D spring damper models for each degree of freedom. This made it simple to replace the 1D spring dampers with another simple lash model (Claytex.Mechanics.Translational.SpringsAndDampers.LinearDeadZone).
Figure 1: Bush model with Lash 1D spring dampers.
This resulted in a linear spring-damper effect with prescribed stiffness, damping and gap.
Applying Degraded Bushes to Suspension
Given I created the degraded bush model by extending the bush interface (Claytex.Mechanics.MultiBody.Bushes.BaseClasses.Bush), this allows me to redeclare them into any joint. Duplicating Suspensions.QuarterCar.Front.DoubleWishboneBushed allows me to replace all bushes and apply applicable stiffness, damping and lash to each joint.
All inner wishbone mounts and the outer tie/track rod ends were given lash and constant stiffness and damping.
Figure 2: Front Quarter car model with bushes with lash.
I used the RoadsterSportMTRT as a base, which uses double wishbone suspension front and rear. This allowed a direct back to back comparison between the performance with or without a degraded bush.
The first test was a check of the kinematics and load difference between the two linkages. There were significant changes resulting in the degraded bushes, in both regards.
Figure 3: Toe angle comparison (Linear bushes vs Degraded linear bushes)
Figure 4: Camber angle comparison (Linear bushes vs Degraded linear bushes)
The toe and camber angles both had a zero offset at zero bump and for a majority of the test. During the steering sweep the force balance shifts and between 6 and 7.5 seconds the geometry dramatically changes with a different offset.
Figure 5: Bush force comparison (Linear bushes vs Degraded linear bushes)
The force difference between the tests was significant with up to a 12x force increase when the degraded bushes were used during the steering sweep; primarily due to the altered geometry as a result of the bushes being out of alignment.
Full Vehicle Test
Both the front and rear quarter car suspension models were replaced in the respective half car models and slalom tests were run back to back, with and without the degraded bushes.
The obvious difference is that the car with the degraded bushes spins due to the reduced alignment control. But there are many more differences, including significant changes in peak loads, for example through the steering rack.
Figure 6: Track rod inner force comparison during slalom
There was a load spike as the steering position and lateral force transitions across zero with the degraded track rod end that was over 3x the normal lateral load. There was also an increased amount of load variation throughout the test. Both effects would rapidly decrease the lifespan of mounts and components.
Final Thoughts
While the result trends were not surprising, what was surprising was the magnitude of the changes from comparatively small differences; with a 12x force increase being very significant, demonstrating the change in load paths as a result of the altered alignments.
It is also worth noting that these examples demonstrate one specific type of failure; But alongside the different types of bushes, there are many different types of failures. Quantifying and measuring a specific type and grade of failure would greatly influence the results.
Written by: David Briant – Project Engineer
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