I sent Wheelin98TJ a PM to add you to the FAQ
Originally Posted by Jerry Bransford
Nicely done writeup and I agree it belongs in the FAQ!
Borderline spectacular !!
Def should be included in FAQ.
Glad you like it!
Thrust angle should have nothing to do with torque steer as far as I know, an alignment machine compensates for it by setting the front toe-in centered in relation to the thrust angle. Or in other words you set the steering wheel straight in relation to thrust angle. The only real affect from a high thrust angle is your Jeep would be "crab-walking" down the road(body/chassis appears sideways going down the road, but all the wheels pointed straight).
I noticed in your thread you had no mention of swapping tires left to right. Try doing that with the front first, then if that doesn't do anything try it with the rear too. Aside from worn control arm bushings(could be any 16 of them), I don't see anything else that could cause it.
This theory is sort of "out there," but I suppose if your rear axle was bent on one side, the camber would be out of whack. Based on your left side tire wearing even and the right side wearing on the outside, it sounds like the camber is in the positive on the right side(how that would happen I do not know). Since the side that has more positive will cause the pull in the front, in the rear it would have more "leverage" in relation to the left(contact patch is further to the outside of the axle housing) and also the left side would have more drag(larger overall contact patch). It wouldn't be detectable just cruising or in decel because the front primarily accounts for the vehicles habits in those conditions, but under accelleration the rear axle sees weight transfer. This is just speculation of course. You can buy an angle finder real cheap, jack up the rear axle and remove wheels. With the drums/rotors FLAT against the axle shaft, you can check the angles in relation to each other. You can even check the angles of your axle tubes in relation to each wheel-mounting-surface angles. This is the cheap way, but an alignment wouldn't be a bad idea at this point.
Wow, this is such a useful tool. Nice job!
Fantastic post, kudos man.
I guess I wasn't chillin' in my jammies eating Cocoa Puffs for nothing this morning
I'd like to see you expand on the kingpin axis inclination and it's relationship to the contact patch and why it needs to point where it does.
Great work though.
Thanks Blaine, I guess I'll write something up here, if it gets your approval I'll add it to the main write-up
Going back to caster and camber, another measurement that the alignment machine may give you is steering axis inclination.
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
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 few unstable feeling. It is the single point of SAI on the ground that the tire actually wants to pivot on. Or the pivot radius
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 positive(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 sieze 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