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.
Unlike the rear axle, a front axle differential is offset to one side to match up with the front t-case output. When building a three-link with a panhard bar, the location of the steering box will determine how the panhard mounts. For the common driver side steering box (without double crossover steering), the draglink and panhard will run [parallel] from the frame rail on the driver side to a point on the axle on the passenger side.
When building a linked suspension on a factory frame using a body, you often have the option of adjusting where the front axle will end up and can work on gaining tire clearance at the fenders or firewall. On a leaf spring suspension with rear shackles, the axle and tires move backwards as the suspension compresses and often sends large tires rubbing on the rear fender portion or firewall. On a front linked suspension, this will not be the case. As the suspension compresses, the axle will now move forward and the tires may end up rubbing the front portions of the fenders. How much the axle moves fore/aft will depend on the link angles, lengths, and mounting locations of the links.
Last month we discussed anti-squat in relation to the rear suspension. Up front, we have concern for a similar characteristic called anti-dive as the weight of the vehicle transfers towards the front during braking. One hundred percent means the vehicle will not dive on level terrain. You'll see from the calculator that the upper link geometry largely determines how your vehicle reacts to the forces of acceleration and braking. The geometry can cause the front end to rise or drop under acceleration. The greater the anti-dive, the less front end dive you'll experience during braking. This characteristic can also affect the manner in which the vehicle climbs hills as weight is unloaded from front to rear. With too much anti-dive, the axle may want to droop out, rather than allowing the tires to climb an obstacle. Common design targets seem to often be in the 50- to 100-percent range.
If you draw a line through the upper and lower links (viewed from the side) and extended them rearward, the point at which they would intersect in space is the instant center. This is the point about which the suspension linkage will act. Imagine you draw a line from the front tire contact patch to a point where the height of the COG and the rear axle centerline meet. If the instant center lies below this drawn line, you have less than 100-percent anti-dive. If it lies above the line, you have greater than 100-percent anti-dive.
Last month we talked about roll center height being the height at which the body/chassis "pivots" with respect to the axle and how it helps determine the amount of body lean you experience. In the case of a three-link setup, the roll center height is the height of the center point of the panhard bar. Again, if this point is fairly low, then you will get increased body roll, or lean. If this point is higher, then body roll will be less.
One thing to watch for with a linked system is the need to limit droop. Allowing the coilovers or shocks to stop the downward travel can cause them to become damaged. Use of limiting straps at the axle ends is often a good idea for vehicles that often see conditions (jumping) where the front axle drops all the way out. For those concerned more with articulation and less about full droop travel, a center mounted limit strap may suffice. In either case, the limit of your driveshaft angular travel should be checked against how far the axle is allowed to fall.
Some good points to remember when setting up a front 3-link are:
(1) Make the front links as reasonably long as you can (frame constraints and ground clearance will dictate here)
(2) Try to keep the top link near parallel to the ground
(3) Try to maximize vertical link separation at the front axle (within reason; 8 to 12 inches is a good target)