Trucks, Jeeps

2018 Jeep Wrangler in depth, Part 2: Suspension choices

Bob Sheaves was responsible for 4x4 suspension design at the Jeep/Truck Engineering's PreProgram Engineering Department from the AMC days until 1993.

Allpar’s owner, David Zatz, asked what approach Jeep is likely to take with the rumored “independent suspension” Wrangler for the 2017 model year, if there is one. This article is based on my engineering evaluation of various public sources, plus private discussions.

Will it really have an independent suspension?

The short answer: Yes, I believe it will have a front independent suspension (“IFS”), adapted from the Ram 4x4, with a live rear axle.

bouncing differential

side viewWhy not Li’l Blue (patent diagrams above and right), which can exceed the capability of current Wrangler designs (due to the floating differential and long-travel design)? The current staff is not capable of tuning and adapting it, and lack the technical expertice to further develop it. There is no incentive for me to give away my own two generations of independent development beyond the original, and they have not got enough money to get Evan Boberg or me back to work on it. It does have a lot of advantages at minimal cost.

I expect a live axle in the rear, since it does not steer. It is cheaper to change the tubes and axle shafts than the front, with the steering issues.

An independent suspension has indirect aerodynamic benefits. One of the most turbulent area on any Wrangler or CJ has always been the gap between the fender leading edge and bumper on either side and the gap between the radiator shell and bumper, creating a swirling mass of air trapped in and around the sheet metal, causing unstable drag. Jack has closed the gaps and improved the airflow by around 10%.

Using a body mounted drive axle would also allow an overall lowering of the body, relative to the ground, for further improvements.

2017 jeep wrangler

Narrowing the body reduces the “hole”  the car has to punch through at speed, further improving the aerodynamics. The cost to narrow a live axle exceeds the cost to narrow an independent suspension, due to retooling the axle tubes, axle shafts, and steering geometry changes for the knuckles.

It would cost less to provide a front suspension and live axle link coil suspension to narrow the track, pulling the tires inside the new narrower body footprint and narrowing the fenders to lower the drag and increasing highway fuel economy.

An independent suspension would also:

  • Reduce unsprung weight, helping on-road handling.
  • Letting wheels react independently, so that one wheel hitting an obstacle does not affect both sides (one cause of wheel shimmy, or “the death wobble”).
    • This would also keep all four of the wheels on the ground, in most on-road and most off-road driving (within the limits of suspension travel) situations, increasing stability and control.
  • Independent suspensions can generally be built as a unit and shipped to the factory, speeding installation and sometimes cutting costs.

Chrysler is not in business to support the aftermarket. This is nothing new. Their only concern is with selling cars and moving iron. How innovation spreads from carline to carline matters. Bigger tires and differential locks don’t.

Disadvantages include higher cost, more difficulty in increasing vertical wheel travel, reduced ground clearance when rebounding from a bump, and the difficulty of increasing ground clearance. Owners who want to modify their vehicles will find the price and/or engineering challenge to be much higher.

Looking at the Jeep Commander vs the Ram, both used a similar front suspension, but with different ball joints, slightly longer upper and lower control arms on the Jeep, different tuning, different frame attachments (changing basic geometry), and different shocks and attachments.

To sum up, I can see the changes to be:

  • 4 inches narrower (track width and fender change)
  • 1.5 inch lower body (suspension up inside the body but the same overall height)
  • A 15% fuel economy improvement highway

A quick review of the Ram 4x4 rear link-coil suspension

The new Ram suspension builds on the success of the original ZJ Jeep (Grand Cherokee) rear link-coil suspension design, but corrects many of the shortcomings of the original ZJ design. It avoids errors of the past Chevy truck indpendent suspension by using five links rather than three.


Dodge Ram rear suspension analysis


The springs are canted and “bent” at the loaded position (two 150-lb passengers, full fluids, and half payload) so reactions to motion of the ground contact patch are efficiently controlled and isolated.

The UCA (Upper Control Arm) links appear to be splayed outward at the frame attachment points, providing lateral stability which, along with the track bar (or “panhard rod”) keeps the lateral shift of the axle, between its maximum travel up and down, to the minimum possible arc. Due to the over-constrained system, the axle will travel laterally in the vehicle, through an arc of around two inches total, left to right. The positioning of the track bar ensures that the travel will be split evenly from jounce to rebound, minimizing the dreaded “head toss” prevalent in the Jeep XJ (Cherokee), MJ (Comanche pickup), and early ZJs (Grand Cherokees).

front suspension

The side view between the upper and lower control arms indicates a long instant center (a theoretical point in space ahead of the axle, that controls the fore and aft arc the axle travels through as it goes from jounce to rebound). This ensures the axle does not change the wheelbase a great deal, affecting braking distances and geometry and upsetting the transient dynamics of handling in an emergency lane change.

ram suspension

The rear stabilizer bar is the only complaint I have about this design. As was shown on the original Dodge Dakota 4x4, the most effective position of a stabilizer bar is to place the end links as close to a rigid body (the center of the frame, where the bend and torsion is minimized) as possible. I do not know all the compromises the engineering staff were forced to make, but I am concerned this location has forced several poor tradeoffs, primarily excess weight needed from larger than desired components to control the loading. (The senior manager of the team, Steve Williams, wrote, “The only room for the bar was behind the axle, so the frame was a clean sheet design to accompany this. The frame is a box section, hydroformed to provide the strength required.”)

The 2013-2015 Ram 1500 suspension: supplemental notes

ram cutaway

Ram trucks built starting in 2009 used a multi-link coil-spring rear suspension, which weighed 40 pounds less than a leaf-spring configuration. The Ram 4x4 had a larger articulation range than its leaf-spring competitors, with less freeway hop. Shock absorbers were forward-facing and positioned on the outside of the frame.

Air suspension

The Ram 1500’s optional air suspension system alters height with air pressure, with five settings:

  • Normal: Stays at 8.7 inches of clearance (from the base of the door sill)
  • Aero: Lowers the Ram by 1.1 inches, increasing gas mileage by up to 1%; activated by highway speeds
  • Off-road 1 and 2: Rises to 9.9 and 10.7 inches of clearance, respectively, to clear obstacles
  • Park: Lowers the truck to a height of 6.7 inches for easy entry/exit and cargo loading

air suspension

The system uses the tanks shown in blue as reservoirs; it does not use outside air.

The four air springs (one in each corner) have spring rates which depend on the ride height. In Off-Road 2, the rate is firmer, while in Aero mode, the rate is softened.

 


How to build it?

Pickups?

Steel or aluminum?
2018 Wrangler engines
Engines

Unit-body, frame...?
2018 Jeep Wrangler
Open or fixed top?
Transmissions
Transmissions
weight
Weight and strength

Related Jeep Wrangler pages

Inside the Wrangler

Years

Background

Variants and related...

The 2018 Jeep Wrangler JL: suspension aluminum vs steelopen or fixed roof pickup
body engineering
weight, strength, and safetytransmissionsengines

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