Found via the intergoogle on
http://www.automotiveforums.com/t936476-jeep_tj_alignment_bible.html
I've posted this write-up I made on a few various forums I am on, hopefully someone finds this helpful.
This thread is to talk about the
alignment terms and measurements and how they relate to TJs and other vehicles in general. I’ll try to explain why and how our Jeeps act how they do to the best of my ability. I’ve been doing 10-12 alignments a day on everything from Civics and Malibus to TJs and Cherokees to Corvettes and high dollar Mercedes for the last few months straight so I’d like to think I’m fairly qualified to do this write-up.
Let’s start with the basics: Camber, caster, and toe.
http://img.photobucket.com/albums/v294/flatlander757/Misc/camber.gif
Camber – Camber is the angle that the tires are at from top to bottom when looking at it from the front. It is generally slightly negative (top of tires tilted inwards). I don’t know of any vehicles that have the actual spec in the positive (top of tires tilted outwards), but some allow enough tolerance to allow a small amount of positive camber though it’s not recommended.
Whatever side has more positive camber is the side that will want to generate a pull. This is because whatever is closer to 0 will have more of the tire touching the ground causing more rolling resistance on that side.
Example: -0.30 camber on left and +0.10 camber on right, should pull to the right
One question I’ve been asked was what about -0.30 on the left and +0.30 on the right, shouldn’t it drive straight because equal amounts of the tire are touching the ground? Common sense would initially say yes, but since the right side’s contact patch is further away from the ball joints in relation to the left(steering knuckle pivot), there is more leverage on the right side making it easier for the smallest imperfections in the road to make it want to turn that way.
Also know that camber is NOT adjustable on TJs without special offset ball joints. Chances are though that if you need those ball joints, you very well could have a bent axle housing. Ensure it is straight before buying expensive ball joints!
http://img.photobucket.com/albums/v294/flatlander757/Misc/caster.gif
Caster – As far as caster goes, it can give the feeling of pulling. When you turn your wheels left and right, they do not rotate on a 100% vertical axis. This primarily affects your “return to center” effect. The upper
ball joint
should be further back than the lower ball joint resulting in a 4-8 degree tilted axis that the wheels rotate on (viewed from the side). Under no circumstances am I aware of should you have negative caster (tilted forward). You should have positive caster. The more caster you have (say 8 degrees); the more quickly the
steering wheel will return to center when coming out of a turn. With less caster (say 4 degrees) it will not return to center as quickly. Also note that caster will very slightly affect how “hard” it is to turn, as with less caster you aren’t fighting as much of that return to center tendency that the vehicle has. Really low overall caster will cause a “wandering” feeling and will more likely pull whatever way it wants with the smallest road imperfections/dips.
If you have driven a BMW/Mercedes/Audi/etc, you’ve probably noticed how “stiff” the steering feels, this is mainly because they have a lot of caster (some between 9-10 degrees!) built in for more high speed Autobahn stability.
Generally when I do alignments, I will set the caster on the right side to whatever the left side is, +0.3 to +0.5 degrees.
Example: +6.5 on left side and +6.8 to +7.0 on right side
What this does is account for road crown.
Road crown is the term for how most public roads are tilted towards the right side for water to run off and keep it from pooling where you're driving. Without caster being set a bit higher on the right side, in the right lane on the road the vehicle tends to wander to the right.
Either way it's non adjustable on Jeeps, all you can adjust (with adjustable
upper control arms or cam-bolts) is the overall caster. Both steering knuckles are attached to each other with the solid axle, you tilt one backwards, you tilt the other backwards. Not adjustable in relation to each other. You could cut the weld on a knuckle and rotate it, but that's rarely needed.
http://img.photobucket.com/albums/v294/flatlander757/Misc/toe.gif
Toe – Let me start by saying TOE DOES NOT CAUSE A VEHICLE TO PULL! You just set the toe to spec and then center the steering wheel with the drag link(the drag link does not affect the alignment of the tires, only centers the steering wheel). On a solid front axle vehicle (like TJs), you can only set the TOTAL toe, since both steering knuckles are connected. It’s impossible to adjust individual toe as there is not a separate tie rod on each side of the steering gear.
Now that you’ve had a basic crash course in alignments, let’s get onto some more good stuff!
Here are a few terms to describe alignment problems because customers misuse the crap out of these all the time. I’m sure every tech that does alignments can appreciate this:
Steering off-center – If your vehicle drives straight, doesn’t wander, but the steering wheel isn’t centered. Pretty easy to correct, just adjust the drag-link until it is centered.
Pulling – If your vehicle wanders off a perfectly level and straight road to the left or right on it’s own with no or hardly any steering input. Let me say this: IGNORE WHAT YOUR STEERING WHEEL DOES! IT’S ALL ABOUT WHAT THE VEHICLE IS DOING; YOUR STEERING WHEEL NOT BEING CENTERED IS NOT AN ALIGNMENT ISSUE!
Unusual tire wear – It could be from alignments, but in all honesty, with Jeeps with aggressive tires and lifts, you can have a perfect alignment and still get bad tire wear. Keep your tires rotated and balanced often! Every 3000-6000 miles; I do mine every 3000 miles as I have Swampers. For this I HIGHLY suggest Sears as they have a lifetime balance deal for like $50-60, you can take your vehicle back there and get the tires rotated and balanced every other week if you want to. It’s great to do after some wheeling when you’ve knocked off 8 oz. of weights.
For the record, excessive toe usually causes tires “cupping” and excessive camber usually causes the inside edge of the tire to wear quicker.
Steering shimmy – At certain speeds the steering wheel will rock back and forth kind of gently to mildly, this is usually due to unbalanced tires. Could be from a slightly less than ideal caster angle, but 8 times out of 10 it’s the tires.
Bump-steer – This is solely caused by having poor track bar and drag link angles/lengths. In all stock applications, both the track bar and the drag link are the same length and at the same angle. So because of this, when you hit a bump, they both travel in the same arc. If you lift the
Jeep and you install a drop pitman arm without relocating the track bar, you basically have the drag link traveling in a different arc in relation to the track bar. What this will do is jolt the steering wheel left or right when hitting a bump.
Death wobble – There is no way in hell you can drive the vehicle on the highway safely. The wheel will shake so violently that you think the entire front end is about to rip out from under you and send you to your death into a Jersey wall. You need to come to a complete or nearly complete stop to get it to go away. You’ll also need new underpants. If you have any question about whether you had death-wobble or not, you didn’t have it. Aside from terrible caster angles, it is more commonly caused by loose or worn steering components or front track bar components. Check all tie-rod ends for play and most importantly ensure that the axle-side of your track bar is torqued to spec (OEM is 55ft-lbs). If you have an aftermarket track bar make sure you know it’s torque spec.
Let me say this: A STEERING STABILIZER WILL NOT FIX DEATH WOBBLE. It will only mask it if anything. It’s equivalent to putting a band-aid over a gun shot wound. Death wobble will shortly kill the stabilizer and then rear it’s ugly head again. So it’s both not fixed AND you’re out $40-70 for a stabilizer. A well-engineered system should not need a steering stabilizer at all. It’s sole purpose is to dampen the steering so it doesn’t feel so “sharp” and harsh.
Now that all that stuff is out of the way, let’s go into a couple scenarios.
YOUR TJ IS PULLING LEFT OR RIGHT:
Knowing some of what was learned earlier, we can say that taking a TJ in for an alignment because of pulling is pretty much useless since camber and caster are non-adjustable.
The reasonable steps I would take are the following:
1. Check tires pressures, ensure all are correct. Vehicles will pull toward the side with less pressure in it. The front tires will cause the most pulling; rears won’t do a whole lot.
2. Check to make sure that one of your brake calipers isn’t dragging when released. The vehicle will pull toward the side that it is dragging on. You’ll probably notice that the center of the wheel on one side will be hotter than the other after a short drive as well if this is the case.
3. Swap your front tires left to right. Sometimes tires will internally wear the belts differently (or slip the belts entirely, noticeable as a bulge in the sidewall, in this case stop driving on it!) causing more rolling resistance than another comparable tire. If rotating them causes the pull to switch directions, try moving tires around front to back and left to right until you find a combo that drives straight. Or just live with the pull.
Don’t forget, check for pulling on a flat, straight, level road. Roads with a crown to them will make it want to pull right to some degree, so keep that in mind.
If it still pulls, I would recommend going in for an alignment. Even though it is non-adjustable stock, alignments are a great tool to aid in finding bent or warped parts. Read further below for what to look out for when you get your results from your alignment.
YOUR TJ HAS DEATH WOBBLE:
Here is a good action plan IMO to go about should you have full-on death wobble:
1. Check under your Jeep, look at and inspect ALL steering joints, bushings, bolts, etc. Make sure all tie-rod-ends are tight, make sure your track bar bushing at the axle end isn’t worn, make sure the axle end of the track bar is torqued to 55ft-lbs (or whatever your aftermarket track bar calls for), make sure that the axle end track bar mount’s hole is not wallowed out, make sure the ball joints and unit bearings are tight with no play, etc. If anything has play in it, it’s just an invitation for death wobble!
2. Get your tires rotated and balanced. Take it to a shop that will balance the tires out to a perfect zero. 0.25 oz off is not good enough. It is very critical to have a perfect balance. If any joints have even the slightest bit of play, unbalanced tires will trigger it.
3. Get an alignment. With bigger tires, you actually need to get a lower caster angle. 7 degrees is the factory spec for a stock TJ w/ 29-31” tires. Draw an imaginary line through both ball joints down to the road. It should land just slightly in front of the center of the tire (“Lead point” marked in caster photo further above”). With bigger tires on the same amount of lift, this imaginary line will hit the ground much further ahead of the tire than it should be.
*I believe* this causes instability/death wobble because big tires(33”+) have a lot of sidewall play as there is a lot more of it, so basically the entire rim is trying to wobble left and right inside the tire. German cars have a ton of caster without problems, but they have 17-19in wheels, skinny and stiff tires, and less play in the steering systems (rack and pinion vs. steering box and linkages).
That is my take on caster angles vs. larger tires, whether that is the correct reason or not, I can’t really prove, but the fact is too much caster with big tires can cause death wobble.
It should also be noted that as you lift a TJ, because of the front control arm lengths and angles, caster decreases so it actually works out in our favor alignment-wise. I predict the problem could be more pronounced in low COG builds (example: 35” tires on 1.5” lift). In this case adjustable front upper control arms should be installed for optimum caster angles.
I’ve seen it posted around that 5 to 5.5 degrees of caster is ideal for 35” tires.
Now onto the last part:
GETTING YOUR ALIGNMENT RESULTS BACK:
First off, according to my 03 TJ Factory Service Manual, here are the specs and tolerances:
Camber: -0.25 degrees +/- 0.63 degrees
Cross camber: +/- 1.0 degrees
Caster: 7.0 degrees +/- 1.0 degree
Cross caster: +/- 0.65 degrees
Total toe: +0.30 degrees +/- 0.06 degrees
Thrust angle: +/- 0.25 degrees
I haven’t mentioned it yet, but “cross” camber or caster is the allowable difference between left and right. It’s a pretty easy concept.
http://img.photobucket.com/albums/v294/flatlander757/Misc/thrustangle.gif
Thrust angle is the angle at which the rear axle wants to push the vehicle. If it is out of spec then you should be suspect that either your adjustable control arms are not the right lengths (if you have all 4 rears adjustable) or something may be bent.
VERY IMPORTANT: Look at the alignment sheet that the shop gave you, in the alignment machine it is possible to change the specs just so that they show up in the green. It's a sneaky way that some people will use to make the customer think there is no problem with caster or camber. Usually it's done because on some vehicles it is a PITA to do, and for mechanics on flat rate time is money.
If you don’t know what the factory specs are, and you get a printout with everything appearing in the green, you would never know they were altered at the alignment shop. This can however be used to your advantage in the case of wanting less caster for larger tires (you can have them target 6.0 degrees for 33” tires for instance).
Those are pretty much the basic points and things you need to know when looking at the alignment sheet that you get back. There are many other measurements that factor into how your TJ will drive as well.
Here is some more knowledge to absorb that delves a bit further into knowing why your TJ does what it does:
Going back to caster and camber, another measurement that the alignment machine may give you is steering axis inclination.
http://img.photobucket.com/albums/v294/flatlander757/Misc/steering_axis.jpg
Steering axis inclination - This is the angle that runs through the centers of your ball joints down to the ground when viewing it from the front of the vehicle(same way you view camber). Do NOT get this confused with camber, it is the angle of the wheels in relation to each other, SAI is through the ball joints to the ground. While the angle itself isn't necessarily important, where it lands on the ground is. It ideally will end up right in the center of or slightly inside of the contact patch of the tire, resulting in a small scrub radius.
http://img.photobucket.com/albums/v294/flatlander757/Misc/sai-scrub.gif
The scrub radius is measured from the tire centerline to the point on the ground that intersects the SAI angle. This should be an inch or two wide ideally (from what I have gathered). Any more would cause excessive steering effort/feedback and too close to zero kills nearly all feeling of the road causing a very unstable feeling. It is the single point of SAI on the ground that the tire actually wants to pivot on (or the pivot radius).
http://img.photobucket.com/albums/v294/flatlander757/Misc/scrubradius.gif
The scrub radius and pivot radius are affected by larger tires and aftermarket wheels with [numerically] less backspacing. I'm not necessarily sure as to whether TJs have a positive or negative SAI in stock form, I would have to guess negative (SAI intersects ground OUTSIDE of center of tire contact patch). Mainly because that is pretty much the norm for every vehicle I've seen. Reason that the pivot radius is generally on the OUTSIDE of the tire contact patch center line is because the road will not have as much leverage on the wheel. Also should a brake caliper seize on one side, a positive scrub radius/pivot radius will transmit less force back through the steering wheel. In general both positive and negative have the same effects on steering stability and feel, except negative SAI will result in more feedback and will be more likely to follow imperfections in the road.
It is possible that with small tires(30") and aftermarket wheels(say less than 4.5" of backspacing) that you would be putting steering stability in jeopardy; reason being as you are moving the wheel centerline outwards, the SAI stays the same, and keeping tires the same or similar diameter, causes a smaller scrub radius. The scrub radius should remain fine provided you have larger tires AND less backspacing to keep the steering pivot point similar in relation to the tire centerline.
Now that I think about it, this also explains why my TJ on 33" tires w/ stock offset (5.5") wheels has a relatively strong return to center feeling in comparison to stock TJs.
For all intents and purposes, the SAI is not adjustable. Here is an excerpt from my 03 TJ FSM:
STEERING AXIS INCLINATION ANGLE is
measured in degrees and is the angle that the steering
knuckles are tilted. The inclination angle has a
fixed relationship with the camber angle. It will not
change except when a spindle or ball stud is damaged
or bent. The angle is not adjustable, damaged
component(s) must be replaced to correct the steering
axis inclination angle
Why the Caster line/SAI points where it does:
Technically SAI and caster are different angles, but they are also one in the same(SAI is viewed from the front of the vehicle, caster is viewed from the side).
These angles are each laid out in two separate dimensions (think of SAI being measured along the X and Y axis, and caster on the Y and Z axis in 3 dimensions).
While I can't explain the scientific reasoning with numbers and formulas and stuff (I'm sure you know though), it works the exact opposite way that shopping carts' front wheels are set back.
The shopping cart has it's wheels set behind the pivot's centerline as it is the cart that determines where the wheels go.
In vehicles, it is the wheels that we need to determine where the vehicle goes(ie, the vehicle shouldn't control what the wheels do). Because of this, the caster is set to put the wheels' pivot radius in front of the axle and tire contact patch centerline.
Ever ride a bicycle with the handlebars turned backwards? It is much less stable and it makes it far more difficult to ride with no hands.
I see it like this: the weight of the vehicle is supported by the front and rear tires. With the caster pointed behind the contact patch centerline, the vehicle weight makes the front wheels want to pivot about the steering axis causing instability.
With the caster pointed in front of the contact patch centerline(like it is supposed to be), the vehicle weight keeps pressure on the steering axis, and with the caster behind, it is naturally going to want to self-correct steering itself to some degree.
Kind of hard to explain, but from the side, draw a triangle from the upper ball-joint, to the tire contact patch centerline, and to the point on the ground at which caster intersects. The weight is supported by the ball joints, and thus is transferred to the wheels through them, so with the weight being pushed from the upper ball joint to the caster/ground point, you can see why a super low caster angle would want the steering axis to rotate about itself.
maybe water
. Try disconnecting the battery and then turn your lights on. Reconnect the battery and see if it goes away.
