ARTICLE:
INITIAL DESIGN REVIEW
We were approached by a customer looking to upgrade his existing long travel suspension on a Ford Expedition. The customer was having issues with his multi-link suspension impacting objects in the ground. With the threat of catastrophic failure given a hard enough impact, he came to us looking for a solution. Upon initial inspection, we noticed the rear lower control arm hung down so low, it was impressive that it had not yet been torn off of the vehicle on any moderately rocky trail. As is, the control arm was extremely vulnerable to rocks and debris that could create life threatening situations if catastrophic damage were to occur at high speeds.
Unfortunately this multi-link suspension required the lower rear control arm to be placed very low in relation to the vehicle.
PEN TO PAPER
After inspection of the vehicle and communication of goals with the customer, our team went to work to keep or increase the amount of travel while focusing on raising the lowest point of the suspension to prevent any potential damage. We considered a few viable options, ranging from most expensive and labor intensive, to a more simple and cost effective approach.
The first approach (most costly and labor intensive) was to remove all existing suspension and mounting locations and devise a suspension system more to the liking of high performance off-road. While utilizing the factory differential, and without changing the main chassis structure, this option involves a trailing/radius arm setup similar to how the current high performance UTV's are. This would require all new arms to be fabricated or machined, along with new chassis side pivots and shock towers. To get the most performance out of this setup, the shock tower would likely protrude into the cab of the vehicle. While this option would be the best for shear performance, it was also excessive for the customers requirements and would be much more labor and cost intensive.
The next approach, and ultimately the option we decided to go with, was to redesign the existing control arm for more ground clearance with the addition of designing a skid plate wing in front of the arm. This will allow the vehicle to slide off protruding rocks or obstacles rather than getting caught up on them with a full stop and causing irreparable damage.
LOGIC BEHIND THE DESIGN APPROACH
We feel the skid plate wing was a unique approach to solve this problem. This new setup will be the most cost/labor effective by keeping the stock arms and mounting locations. While also allowing the same amount of suspension travel (even slightly more with minor modifications to the chassis). As shown below you can see the new arm design is higher off the ground than the existing configuration with an added benefit of the skid plate wing for maximum protection.
We opted for UHMW plastic on the bottom of the control arm and skid plate for serviceability and the low friction coefficient when an impact occurs. The plastic will be machined and formed, then bolted through the sheet metal with welded on tabs and clip nuts. See below for the UHMW plastic covering the bottom of both parts.
DESIGN CHALLENGES
One of the obstacles of this design was ensuring that when the skid plate gets hit, it won't allow the force to be transferred into the control arm in manner that wants to tear it off the vehicle. To prevent this negative interaction, we decided to mount the front pivot of the skid plate wing to the frame rail and then dynamically mount it to the control arm with a double ended (male-female) heim joint. This allows the control arm to cycle freely as needed with the multi-link suspension style.
Now in the case where a rock impacts skid, the majority of the impact drag force will be translated to the mount on the frame rail rather than the control arm.
It was crucial that at any point in the suspension cycle, the skid plate remained lower than the control arm to prevent the arm from being the main point of impact. Using the power of advanced engineering software and 3D scanning, we were able to take our scans and the newly drawn control arm assembly to dynamically view the suspension cycle and verify clearances on all components. While doing this we can also monitor if the control arm ever falls below the skid plate during the cycle. In the images below, you can see the rear suspension through its cycle from droop to bump.
RIDE HEIGHT
Once we were confident there were no interferences in the assembly we moved onto manufacturing and will continue to work with the customer until this design is out tearing up trails! Check out the video below showing the dynamic suspension assembly.
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