If your truck has a solid front axle, nothing tire-related changes when the ride height is altered. Camber is set by the upper and lower ball joints. That can be adjusted on older Fords and Dodges with offset ball joint stud inserts. For GMs, you might be able to find upper ball joints with offset studs but I don't see them offered real often anymore. Camber will not change with changes in ride height and may only need to be adjusted once in the life of the truck.
If your truck has upper and lower control arms, there will be cam bolts going through the upper control arm bushings. Loosening the nuts and turning the bolts moves the control arm, (and top of the tire), in or out to adjust camber and caster. Unfortunately GM didn't bother to make the holes in the frame slotted to accommodate that adjustment. Many alignment mechanics try to make adjustments not knowing that and get frustrated when very little change takes place. The control arm must be removed so a special knockout tool can be inserted to slot the holes. That only has to be done once, but it's common for the tool to break so some mechanics only slot one hole and leave the other one for the next guy and the next alignment.
The truck can not be raised by simply installing taller coil springs or by winding up the torsion bars. Upper control arms are always shorter than the lower ones. That makes the tire tip in and out on top as the truck goes up and down on bumpy roads. This is extremely bad and easy to see on Ford Bronco IIs and Rangers. You'll never fix the horrendous tire wear on those trucks. It's designed in.
By being different lengths, the two control arms pivot through different diameter arcs. That makes the tire tip one way or the other as the suspension travels up and down. The tire is going to slide left and right as you go down the road. That causes tire wear from scrubbing, but it is lessened by tipping and riding on just the edges. To keep that scrubbing as low as possible, the lower control arm must be perfectly parallel to the ground when the truck is at rest. The lower ball joint will move left and right very little so little scrubbing will occur. The tipping of the tire is caused by the upper control arm. It is shorter and will be angled down a little at the ball joint.
Raising this type of suspension with longer springs puts the control arms into a severely different geometry and there will be severe tire wear. That can not be fixed on the alignment rack. The tire might be able to be set to specs, but the wear comes from the angle changes the tire goes through as the suspension travels up and down.
In an attempt at reducing tire wear, most lift kits involve "drop spindles" for the short-long arm suspension with the two different length control arms. That is a standard spindle that connects to the upper and lower ball joints like normal, but the axle stub shaft, (wheel bearing in your case), is cast lower on it. That moves the tire and wheel down to raise the truck up. The cv joints will go through really big angle changes but they're only under stress when you're in four wheel drive. Drive line vibration is not a real big concern because the half shafts turn only one third as fast as the drive shafts. If you have universal joints, they can set up a vibration due to their increased angles, but you haven't mentioned that.
What is changed is "scrub radius". If you draw an imaginary line through the two ball joints as viewed from the front of the truck, that line must intersect the road in exactly the middle of the tire tread. Manufacturers do a lot of development to achieve that. Springs that are sagged one inch will throw that off and is enough to start causing tire wear. Every good alignment mechanic will measure and correct ride height before trying to align a vehicle. With a 6" lift, proper handling will no longer be possible. Scrub radius makes the left half of the tire try to pull left, and the right half of the tire try to pull right. The two forces balance out when a tire hits a bump in the road. Lowering the suspension to raise the truck, or installing deeper offset wheels, changes scrub radius. That angle has been modified on front-wheel-drive cars that use split-diagonal brake systems to reduce brake pull when half of the hydraulic system has a failure. On Chrysler cars, that brake pull is totally non-existent. It's barely noticeable on other cars as a tiny wiggle in the steering wheel when braking.
Drop spindles raise the ball joints higher off the road so when you look at that line that defines scrub radius, now it intersects the road near the outer edge of the tire. When the left tire hits a bump in the road, most of it wants to tug to the right. The normal rolling resistance from driving down the road also makes the left tire want to pull right. Caster and camber also make a tire want to pull one way. Like them, it is important that scrub radius be equal on both sides of the truck, but unlike caster and camber which can be adjusted, scrub radius is not adjustable except by correcting ride height or by modifying the preferred camber setting. Adjusting camber will now be a trade-off between less miserable handling and less tire wear.
What is common is depending on how the modification was done, the steering linkage may no longer be perfectly parallel to the suspension system. First of all, the center link must be parallel to the ground, and the inner and outer tie rod ends and adjuster assembly must be parallel to the lower control arm. The inner tie rod end stud must be directly in line with the lower control arm pivot bushing. Anything that deviates from that will cause the wheel to turn left or right as the suspension travels up and down. When that affects both tires equally, "total toe" changes. When it only affects one wheel, that's called "bump steer". Any drag racer can tell you all about reducing bump steer and the lengths they go to in correcting it. Manufacturers spend a lot of time designing their steering and suspension systems to reduce this toe change. If you've ever driven a large rear-wheel-drive GM car from the '70s, you'll know what it's like when this isn't designed properly. Turning a sharp corner in a parking lot at low speed caused the inside corner of the car to rise up, then suddenly drop down when the tire slid out. Technically, this was due to improper "toe-out-on-turns. It was the result of one tire not making the correct diameter circle to match the other tire. Think of a line of band members marching around a corner and the guy on the inside doesn't turn sharp enough. He is going to get pushed back in line by the guy next to him. Toe-out-on-turns can be measured with most alignment computers but it doesn't happen automatically. More experienced mechanics will check it and bump steer only when looking deeper for the cause of a problem that isn't corrected with a simple alignment. Many of the same handling complaints are found on vehicles with incorrect ride height so they are often dismissed as "unfixable" until ride height is corrected.
A couple of problems can also occur related to the steering gear box. It is critical to proper handling that the bottom of the pitman arm holds the left end of the center link in the correct position so the left inner tie rod end is in line with the lower control arm bushing. That is accomplished with a pitman arm that drops an additional 6" more than the original one. The idler arm must also have that same drop to keep the center link parallel to the ground. The "poor man's" way of handling this is to do nothing to the steering system. That can work if the steering arms are cast onto the drop spindles in their original location. There are a number of variables to look at. Some will address one problem but create another. In your case, the first thing is to determine if excessive positive camber really is the issue. That can be corrected as long as there is enough adjustment available with the cam bolt slots. If you have a printout from a recent alignment, camber should be close to 0.0 degrees for both front wheels, the left one should be slightly higher to offset road crown, perhaps as much as 0.5 degrees, and GM and Dodge specify around a half degree positive in their specs to reduce the stress on front wheel bearings. That half of a degree will not cause noticeable outer edge wear on the tire tread.
If camber is okay at rest, ask the mechanic to use the jack on the hoist to raise the front end a little, then watch what happens to those readings. Camber can be expected to change a little, but anything under, ... Oh, ... Half of a degree is not significant. Don't try this with older Ford trucks! Anything with the miserable twin I-beam suspension will have around 20 degrees camber change! It's no wonder those trucks eat tires.
If camber seems to remain fairly steady when the front end is raised, look at total toe. Unless you can actually see the excessive camber when standing in front of the truck, I suspect more of your tire wear is coming from total toe that is changing with changes in ride height. Total toe is the last thing that is set on an alignment, but CHANGING total toe is not adjustable as an alignment adjustment. Steering and suspension systems are always carefully designed together so no toe change takes place as the suspension goes up and down. Setup guides for race cars are full of methods for finding and correcting changing toe, and they all revolve around setting the correct ride height as the first step. Maintaining total toe at all ride heights is done by the design of the relationship of the steering linkage to the lower control arms. If that orientation is wrong, it is entirely possible for the wheel to go toe in when the truck moves up AND when it moves down. That will scrub off the outer edges of both front tire equally.
Incorrect total toe will cause the leading edges of both tires to scrub off equally. With excessive total toe in, the fronts of the tires will be closer together than their rears. If you exaggerate that for clarity, think of the two tires turned toward the center of the truck, ... More, ... And more, until they're turned 90 degrees to the truck. Now it is real easy to see that the outer edge is the "leading edge" of each one. The leading edge is what scrubs off. To visualize what total toe causes, hold a pencil upright with the eraser on the table, push down a little, then slowly drag it sideways. You'll see the leading edge scrub off and make eraser crumbs, and the trailing edge lifts up off the table. Only the leading edge wears. That was a severe exaggeration but it's exactly what happens with tires. Both camber and total toe are adjusted as part of any alignment, but you need to be looking at whether either one of them is changing as the suspension moves up and down. Most trucks call for 1/8" total toe in. Road forces pull them back to 0.0" while driving, so the two tires will be perfectly parallel to each other. By the time you get to 1/4" toe in, you will start to see excessive wear on the outer edges of both tires. Your truck spends very little time at the same "at rest" ride height when you're driving, so if total toe is changing with those changes in ride height, no alignment in the world is going to fix that.
Remember too that camber affects wear on only that one tire, although it can be incorrect on both tires at the same time. Total toe always affects both tires equally, however, incorrect camber on only one wheel can aggravate total toe wear on that one tire. That's where a good alignment mechanic is needed to sort it out. That's real easy to do with the numbers on the computer, but modified ride height adds a whole new dimension of complexity and variables.
You may also notice the steering gear box sits at a steep angle on some GM trucks. That means the end of the pitman arm doesn't just move left and right. It moves forward and backward and up and down as you steer from side to side. The idler arm must be in the exact same orientation so it makes the same geometrical movements. To do otherwise is going to make a truck that is lots of work to drive and will be very tiring. You will know about every bump and dip in the road. The lower control arm bushings always stay in the same place on the frame, but the end of the pitman arm goes up and down as you steer to either side, so you can see it is impossible to completely eliminate total toe change and tire wear. Blame that on the design of the steering gear box. Fortunately, that is going to cause more toe wear the further you turn and besides that being a small amount of wear, it's going to occur only during a small percentage of your driving.
If you also installed tires with a larger outside circumference, scrub radius will be changed too. That alone will make for a very poor handling truck even without ride height changes. A lot of people like their trucks to sit higher, and with the competitive nature of the manufacturers, if they could design and build such a truck and make it handle safely, they would surely do it. Whatever modifications you have done, be sure to use commercially available parts. Lawyers and insurance investigators love to find anything that is not original. You can be 100 percent guaranteed that if you are in a crash caused by someone else, you WILL be found partially at fault because any modifications make you less able to avoid that crash. Any mechanic with a few years experience or any advanced training can share all kinds of stories about liability and court cases. Many of them show a huge disregard for common sense, but that's the sue-happy world we made for ourselves.
Monday, January 2nd, 2012 AT 7:57 AM
