You aren't going to find that information here because we don't deal with modifications, especially those that could land you in court. I know that sounds harsh, but as a suspension and alignment specialist, I know that no mechanic will help you do this because that will make them party to any lawsuits too as well as the shop owner.
At issue are the things that were designed in by the manufacturer. You are going to degrade your braking, handling, steering response, and comfort, and lawyers and insurance investigators know it. They will convince a jury that you were partly at fault for the crash when the other guy ran the red light because you were less able to avoid it, and they will be right.
Reduced handling is easy to understand because the center of gravity will be higher. The truck is going to lean more which means you can't make some steering maneuvers to avoid obstacles. What's less-obvious is loss of braking power. All cars and trucks use a "proportioning" valve in the brake's hydraulic system that limits brake fluid pressure to the rear wheels under hard braking. A skidding tire has no traction. With a higher center of gravity, more weight shifts to the front wheels under hard braking. That will allow the rear tires to skid very easily. To prevent them from locking up and losing traction, you have to let up on the brake pedal a little. The front brakes were not up to their maximum stopping power yet, but you let off the brake pedal. That reduces their stopping ability even more.
Proportioning valves are very carefully-calibrated to your specific truck. Engine weight, options that add weight like air conditioning, ride height, (weight transfer), and spring stiffness are all part of the calibration of that valve. Anything that changes any of those criteria will reduce braking ability.
The comfort comes from a little-known or understood alignment angle called "scrub radius". If you stand in front of the truck and look back at the suspension, draw an imaginary line between the upper and lower ball joints. That line has been designed to intersect the road surface exactly in the middle of the tire. The outer half of the tire tread wants to pull the steering system in that direction, and the inner half of the tread wants to pull the steering system towards the center of the truck. Those two forces counteract each other and the tire wants to go straight down the road, even when it hits a bump.
If all you do is raise the truck body, you will gain exactly nothing for ground clearance because you haven't raised the axles. Most people who do this do it so they can install larger wheels and tires. A tire that's four inches taller will gain you two inches of clearance, and will likely rub when you turn. Using a taller tire, a wider wheel, or a wheel with a deeper offset will change scrub radius. Now you'll have the outer 1/4 of the tread pulling away from the center of the truck, and the inner 3/4 pulling toward the center. The two tires will offset each other on perfectly smooth roads, be every time a tire hits a small bump, it will pull to the other side and tug on the steering wheel and linkage. That becomes very tiring and makes for a miserable truck to drive. You definitely won't want to drive long distances like that.
To relate how much that affects comfort, a popular truck brand in the '90s used a really strong front suspension system but there was no easy method of adjusting "camber" during an alignment. Camber has the biggest effect on pulling to one side. To correct an annoying pull, the manufacturer offered shims that went between a wheel and the hub to space the wheel out a little. A significant pull to the left could be corrected by installing a shim behind the right wheel to change scrub radius on just that one wheel and make it pull more to the right. That shim was less than 1/8" thick but it had a huge affect. That means scrub radius changed less than 1/4" on that tire. Installing mildly larger tires will change scrub radius two to four inches.
You also have to consider how lift kits do their thing. If all you do is raise the body on the frame, you'll have to modify the steering shaft to prevent it from binding and hitting things. The radiator and shroud will move up; the fan will not. You'll need a smaller and ineffective fan, or you'll have to switch to an electric fan. Radiator hoses will be stretched and transmission cooler lines may hit or rub where they didn't before. This is the least damaging way to raise the body but it doesn't gain you any ground clearance.
Most do-it-yourselfers use kits that push the frame up by pushing the suspension down. You will always find these listed as "for off-road use only". That's the manufacturers' attempt to remove them from liability issues. The first thing to consider is the rubber bushings on the control arms. They were installed with the truck raised on a platform and the suspension hanging down, but their bolts were TIGHTENED after the truck was sitting on the tires, at normal ride height. If they were tightened with the suspension hanging down, they would be clamped that way, then they'd be in a permanent twist the rest of the time. If you install taller coil springs or turn up the torsion bars, those control arm bushings will be under constant stress that will greatly shorten their life.
Even if you understand the bushing issue and loosen and retighten the bolts, you also have to look at the arc those control arms go through. Hold your arm straight out to your side, then move it up and down four inches. You'll see your fingertips moved left and right almost nothing, perhaps less than 1/4". Now, to mimic what will happen to your suspension, hold your arm lower at a 45 degree angle. Now move it up and down four inches. You'll see your fingertips move left and right a lot, actually, the same four inches they moved up and down. That's what is going to take place with your tires too. If you have the common "short arm / long arm", (SLA) suspension, you'll see your upper control arm is considerably shorter than the lower one, AND, the lower one is perfectly parallel to the ground while the upper one is angled down. That geometry keeps the bottom of the tire tread from sliding sideways as the truck bounces up and down, and the angled upper arm makes the wheel and tire tilt in and out on top to reduce tire wear and aid in cornering. Weight on the tire makes it want to stand back up straight which helps you turn into a corner. You're attempting to defeat these characteristics that were very carefully designed in.
You're already aware of the drive line vibration inherent in all GM full-size trucks and vans. You're going to be changing that too. The drive shaft angle is designed to be slightly different than that of the slip-shaft in the transmission, and the pinion shaft in the differential. That small difference makes the roller bearings in the u-joint cups roll back and forth just a little with each revolution. If they stayed in one place most of the time, indentations would be hammered into the cups that would set up a horrendous vibration. Raising the frame in relation to the axle increases the drive line angle and makes those bearings roll a lot further. That can increase vibration even more than what you have now, and it will get worse as the bearings wear very quickly. With altered ride height, you can expect to replace universal joints three to four times more often than normal. Those increased angles are why we went to CV joints on a lot of applications.
Ford had a big problem with drive line vibrations on their conversion vans in the mid '90s. The added weight of cabinets and seats, which wasn't much considering vans were built to carry stuff, caused the rear to sag less than two inches. That was enough to result in a lot of complaints when they got to be around six months old. Stiffer springs solved the problem, but it shows how much affect ride height has on these things.
Everything I've mentioned here affects cars that are lowered too. People think they have better handling and braking but that is just an illusion. When I worked for a very nice family-owned new-car dealership, when it came to suspension and alignment issues, I had the owner's blessings and approval to refuse to work on any vehicle with altered ride height, including any that were still in warranty. The manufacturer backed me up too because they are very concerned with anything that could lead to a lawsuit. I strongly urge you to be just as safe. I'm not a fan of GM, the company, but I would rather be driving your truck than ANYTHING newer with all of its unnecessary and unreliable computers. In fact, my daily driver is an '88 model. Of all my '70s muscle cars and all my other vehicles, every one is at the exact specified ride height.
Saturday, November 29th, 2014 AT 8:44 PM