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Leaf springs, also known as carriage springs or cart springs, are among the simplest of suspension types, but dont take that to mean theres anything remotely simple about the way they work. Indeed, the leaf springs design simplicity means that every part of it must perform a number of tasks simultaneously. Changing any one aspect of the suspension geometry can and will have unintended side effects.
Flipping the Axle
Flipping the axle is the quickest and least complicated way to change an over-slung axle to an under-slung configuration. Many vehicles use over-slung leaf springs, meaning that the leaf spring sits on top of the axle. A "flip kit" will allow you to relocate the axle so that it sits on top of the leaf spring. This is the easiest way to get a four- to seven-inch drop without any major modifications or dynamic drawbacks. This assumes that you have the chassis clearance for a drop that extreme and that your vehicle didnt come with under-slung axles from the factory.
Lowering Blocks
The word "best" can mean various things to different people. If you have under-slung axles and equate "best" with "cheapest" or "quickest," then look into a set of lowering blocks. Lowering blocks are spacers that fit between the top of the leaf spring and the bottom of the axle; they install in minutes per side if you have an impact wrench, and they dont require welding or fabrication. The downside is that moving the axles further away from the spring pack can cause the springs to twist into an S-shape instead of bending straight up and down. This can lead to drastic change in axle angle (pinion angle) and axle hop, which is detrimental to traction and handling.
Flatter Springs
Manufacturers produce leaf springs with a certain amount of downward-facing arch built in. This arch allows the leaf spring to absorb bumps without inverting or arching upward. Leaf springs with a bit less arch will lower the vehicle, but youll need to use stiffer springs to reduce the odds of inversion. This might not be a bad thing if youre lowering the vehicle for better handling but it will certainly make the ride a bit rougher.
Relocating the Mounts
Relocating the spring and shackle mounts is probably the best way to lower your vehicle while maintaining or enhancing the stock ride comfort and handling. Depending on your specific chassis and spring configuration, this could entail little more than drilling a new set of holes through the frame for the spring and shackle through-bolts. The worst-case scenario is that you may need to notch the frame to clear the spring or shackles, and you may need to engineer a new set of shackle plates to get the right angle. To determine the shackle angle and proper mounting points, draw an imaginary line from the rear spring eye to the front. The shackle needs to sit at a 90-degree angle to this line when loaded with the weight of the vehicle, and the line itself needs to point directly at the transmission output shaft. One advantage to using this approach is that you may raise the vehicles roll-center a little, reducing the effect of body roll on the vehicles handling.
The Combination Approach
The best approach for your application may be a combination of the above. Dont be afraid to use flatter springs if your vehicle never gets anywhere near inversion under the most extreme circumstances; likewise, lowering blocks are acceptable as long as you keep them under 1-1/2 inches or so. You may only have enough frame clearance to lower your vehicle by an inch using the relocation method but thats an inch more arch you get to keep in the springs. Its a balancing act, so weigh your options carefully and dont rely on any one of them for a massive drop in ride height at the expense of vehicle dynamics or safety. Its far better for you to make four or five minor changes than one or two very large ones.
