There are a few basics to get wrapped around first:
1: when there is a positive arch in the leaf spring, the axle will move back when it moves up. This is just basic leaf geometry, they get longer when they get flatter.
2: Bumpsteer is observed when you hit a 2 wheel bump like a square on speedbump and as the suspension moves through it's travel, the steering wheel whips side to side. Or if you hold the wheel still, the tires will turn and try to steer the truck. This is often seen most dramatically on bad highways with big dips that can cycle the suspension through it's full travel. I've driven trucks with such bad bumpsteer that I would just let go of the wheel so that the thing would stay on the road.
A repeatable way to see bumpsteer is with hard braking. You can often create enough suspension compression with hard braking to see the wheel move if you have bumpsteer. This can bring in other effects like frame and bushing deflection because the frame is essential trying to over run the springs and axle and sometimes spring wrap can change the angle of the axle compared to the chassis and induce some steering input. But it's also a situation where you want to brake confidently so making it all work well here is important.
3: Rollsteer is a different animal. You'll feel rollsteer in twisty asphalt driving when you are constantly having to adjust the steering angle as the roll angle of the body changes through a corner. The extreme end of roll steer is when you're articulating the axle offroad and the tires and steering wheel just don't match up. You can easily get into situations where the steering box is maxed out one way or the other but the tires are pointed straight.
The factory front to back steering system is pretty good at matching the axle's motion in 2 wheel bumps so it's possible for it to be pretty good for bumpsteer. It doesn't really ever work that good for roll steer. And some of this is lift height dependent, there is a huge difference in making the factory steering work with a 3-4" lift vs. 8"+.
So here's the meat of the matter: you need to have some downward angle on the draglink. This goes against the common knowledge idea that the draglink needs to be level but it works and here's why. If your draglink is level at ride height, as the axle compresses the end of the draglink will move forward. We established above that the axle is moving backward as it compresses (assuming some arch in the spring) so something in the steering has to move to keep everything bolted up. The axle is moving back and the end of the draglink is moving forward so either the steering wheel or tires will have to move to keep up. If the draglink has a downward angle, it's axle end will move backward as it compresses so it's going the same direction as the axle. Ideally we'd make the arcs of movement match exactly but there are enough variations that most of the time just keeping the parts from going different directions will let it drive good enough.
What this boils down to in parts selection is that we run the factory steering up to 2" of lift and typically start using a drop pitman arm for 2.5" - 4" of lift. Then at 6" of lift we use the raised steering arm on the knuckle. At 8" of lift you need to start thinking about crossover even for a mostly on-road truck but using the raised arm with the drop pitman will work about as good as possible. Past that, the factory system doesn't do what most guys would call an acceptable job and you really need to convert to crossover.
There are a lot of details involved in how well this works out since things like tire size, wheel backspacing, truck weight, swaybar correction, frame flex and a bunch more start factoring in, but in general, don't think about a flat draglink, think about making it match up closer to what the axle is actually doing.
1: when there is a positive arch in the leaf spring, the axle will move back when it moves up. This is just basic leaf geometry, they get longer when they get flatter.
2: Bumpsteer is observed when you hit a 2 wheel bump like a square on speedbump and as the suspension moves through it's travel, the steering wheel whips side to side. Or if you hold the wheel still, the tires will turn and try to steer the truck. This is often seen most dramatically on bad highways with big dips that can cycle the suspension through it's full travel. I've driven trucks with such bad bumpsteer that I would just let go of the wheel so that the thing would stay on the road.
A repeatable way to see bumpsteer is with hard braking. You can often create enough suspension compression with hard braking to see the wheel move if you have bumpsteer. This can bring in other effects like frame and bushing deflection because the frame is essential trying to over run the springs and axle and sometimes spring wrap can change the angle of the axle compared to the chassis and induce some steering input. But it's also a situation where you want to brake confidently so making it all work well here is important.
3: Rollsteer is a different animal. You'll feel rollsteer in twisty asphalt driving when you are constantly having to adjust the steering angle as the roll angle of the body changes through a corner. The extreme end of roll steer is when you're articulating the axle offroad and the tires and steering wheel just don't match up. You can easily get into situations where the steering box is maxed out one way or the other but the tires are pointed straight.
The factory front to back steering system is pretty good at matching the axle's motion in 2 wheel bumps so it's possible for it to be pretty good for bumpsteer. It doesn't really ever work that good for roll steer. And some of this is lift height dependent, there is a huge difference in making the factory steering work with a 3-4" lift vs. 8"+.
So here's the meat of the matter: you need to have some downward angle on the draglink. This goes against the common knowledge idea that the draglink needs to be level but it works and here's why. If your draglink is level at ride height, as the axle compresses the end of the draglink will move forward. We established above that the axle is moving backward as it compresses (assuming some arch in the spring) so something in the steering has to move to keep everything bolted up. The axle is moving back and the end of the draglink is moving forward so either the steering wheel or tires will have to move to keep up. If the draglink has a downward angle, it's axle end will move backward as it compresses so it's going the same direction as the axle. Ideally we'd make the arcs of movement match exactly but there are enough variations that most of the time just keeping the parts from going different directions will let it drive good enough.
What this boils down to in parts selection is that we run the factory steering up to 2" of lift and typically start using a drop pitman arm for 2.5" - 4" of lift. Then at 6" of lift we use the raised steering arm on the knuckle. At 8" of lift you need to start thinking about crossover even for a mostly on-road truck but using the raised arm with the drop pitman will work about as good as possible. Past that, the factory system doesn't do what most guys would call an acceptable job and you really need to convert to crossover.
There are a lot of details involved in how well this works out since things like tire size, wheel backspacing, truck weight, swaybar correction, frame flex and a bunch more start factoring in, but in general, don't think about a flat draglink, think about making it match up closer to what the axle is actually doing.
