Looking At Front Suspensions
Last month we discussed the basic design characteristics and behaviors as they relate to a double-triangulated four-link rear suspension. We talked about component locations, physical constraints, and discussed a few practical aspects of building one.
This month we move up front where we'll look at some of the considerations of building a linked setup there. For a deep understanding of all the dynamics, far more extensive calculations and physics are involved than will be presented here. We'll again provide a solid introduction and refer you to these books for even greater technical detail: Chassis Engineering by Herb Adams and Race Car Vehicle Dynamics by Milliken and Milliken.
There are several link configurations that can be used up front. These include triangulated four-link styles, trailing arms, and three-link setups with a panhard bar. We'll concentrate mostly on the three-link for several reasons. First, this style offers more geometry options over the trailing arms. Second, since most of our vehicles have the engine in the front, fitting a triangulated four-link around an oil pan while retaining a typical ladder frame often becomes difficult without pushing the ride height up considerably.
Basically, a three-link with a panhard bar (aka track bar) uses a total of four links to confine and control the location and movement of the axle under the vehicle. Typically there are two links that connect each end of the axle tube to points on the frame rails. A third link runs from a higher point above the differential or elevated above the axle tube back to a point inside a frame rail. Finally, a panhard link connects one end of the axle to the chassis framerail on the opposite side of the vehicle to locate the axle from side to side.
Sometimes a wishbone link setup is used on the front, but, it's often difficult to fit the links on a vehicle using a conventional frame due to the need to clear the oil pan and other nearby components. A wishbone front can also end up with greater bump steer (unwanted tire steering input as the suspension moves through its range of travel) as a result of using a single link mount point on the top of the axle.
Designing The Three-Link
As we mentioned last month, it's good to know that designing and building a link suspension setup is not trivial and if not done with at least some forethought and deliberation, the result may leave you with a vehicle that performs worse than before you made the conversion. As with building any such suspension, the end performance will depend on the length and mounting locations of the links, and by changing these variables we can significantly change how the rig behaves.
Building a front link suspension on a front-engine vehicle with a traditional ladder frame can be challenging as you must deal with oil pan, exhaust, and motor mount clearances when fitting all the links onto the axle and frame.
Some physical constraints to address when building a front suspension are: tire-to-body clearance, tire-to-link clearance at full steering lock, differential-to-oil pan clearance, fitting the coils and/or shocks to the axle and frame, and clearances between all the possible suspension and steering links throughout the full travel and articulation range.
Based on link design, the caster angle may change as the axle travels up and down. For a trail-only rig, this change is not largely significant. Also, for small suspension movements on the road, the caster will typically not change a lot. However, if you're running high speeds and using lots of suspension travel, it may be wise to focus on keeping caster angle changes minimized.
Positioning of the steering box on the framerail, along with getting the steering links to clear under all conditions can be tricky. That is one reason many custom rigs swap to full hydraulic steering. Mechanical steering links are eliminated and replaced with flexible fluid hoses that are much easier to route without interference. However, street vehicles are confined to retaining the mechanical linkages for safety reasons.