The third link can refer to either the upper link in a traditional three link rear suspension, or it could refer to a lift arm, pull bar, or torque arm. Those too make up a third attachment that restricts rear end rotation from the torque forces of engine acceleration.
When you change the angle of the third link, or possibly the spring rate of a pull-bar or lift arm, there are certain things you need to pay attention to. Also, there are reasons to make changes to the links we mentioned and we will discuss those too.
Drive Shaft -Pinion Angle – The driveshaft to pinion angle is one of those critical settings we have talked about in the past. If you have made sure your angles are correct, and we sure hope you have, then when you make changes to the third link assembly, whatever that may be, we need to think about how the change might affect our driveline angles.
If you move a traditional third link front mount down to increase the angle, you will be rotation the rear end and the driveline angles will change, not only at the pinion, but at the transmission also. That is if the mount is vertical and not radiused. And the change won’t be the same at each end. So, we end up with a miss-aligned driveline.
If we install a different spring in a pull-bar, or lift/torque arm, it will compress more or less depending on whether you installed a softer or stiffer spring. What you should have done in the original installation is take into consideration the compression of the spring and where the rear end will be positioned under full throttle acceleration. That is where we need the driveline angle to be correct, not when it is at rest.
There is an easy way to bring back your driveline angles. If the link were solid, then just check the rear end angle before the change and reset it after the change by adjusting the length of the third link. That was easy.
If you have a link with a spring, then you need to figure out how much differently the link moves with the new spring. We can do that with a small amount of math. If our spring compressed 2.0 inches with a 600 ppi (pounds per inch) rate and we install an 800 ppi spring, it will move less with the stiffer spring.
To find out how much less, we do a simple division. If, say, the 600 ppi spring compressed 2.0 inches, then the new stiffer spring will compress 600 ÷ 800 x 2.0-inch = 1.5-inch, and that equals the new travel. Just subtract the new travel from the old travel (2.0-inch – 1.5-inch = 0.5-inch) to get how much you need to move the end of the link at the spring to adjust the link angle.
Force Adjustment – To adjust the force magnitude the link will produce, we make a different adjustment than changing the springs. Changing the spring won’t alter the force of anti-squat, just the speed and amount of movement of the link.
To adjust the force, we need to do one of these things: 1) change the angle in a traditional solid, or sprung, three link, 2) change the height off the rear end (keeping the same angle) of the traditional solid or sprung three link, 3) change the length of the lift or torque arm from the center of the axle to the front mount.
An increase in traditional third link angle will create more anti-squat force. A shorter distance from the traditional third link rear mount to the axle will increase the anti-squat force too. A shorter distance from the axle to the front mount on a lift or torque arm will increase the anti-squat force.
Force Placement – We can adjust the placement of the force created by our third link, whatever the design, by moving it left or right in relation to the rear tires. These links put not only a lifting force on the chassis, but also a downward force on the rear end and ultimately the tires that replaces some of the load on the rear ride springs.
Since we are taking load off the springs and onto the tires through the rear end, the placement left and right dictates how much of the displaced load goes onto each rear tire. If the link is placed midway between the two rear tires, then 50% of the loading goes onto the left tire and 50% goes onto the right tire.
If the link were closer to the left rear tire, say 40% of the rear track width, then 60% of the displaced load would go onto the LR tire. If we had a magic calculator that would tell us how much load ended up on each rear tire, and the LR was 10% less than the RR tire, we could move the third link 10% closer to the LR tire and cause a more equally loaded rear tire scenario for greater traction.
To cause this redistribution of load, we have to physically move the third link position. That’s not easy to do in some cases. And, we don’t want to move it too far either. If the LR needed 10%, more loading and we move the link 20% closer to the LR tire, we are back where we started with unequally loaded tires, only in reverse of where we were to begin with.
To give you some kind of idea how far we are talking about, 10% of a track width of 65 inches is 6.5 inches. So, based on that, if you want to make a 10% change in the force distribution, you would move your third link 3.25 inches towards the LR tire if that is where you need more force.
Conclusion – As with the other changes we talk about in this issue, we need to think out what we are changing that isn’t in our original thoughts. Step back and take a good look at what you are doing and try to imagine what else is going to be affected by your changes.
And as always, try to do only one change at a time. Either change the angle of your third link, or move it laterally, not both at the same time. Or, change the spring rate or the length of the lift/torque arm, not both.
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