[mrblaine]

Originally Posted by Jerry Bransford

Nicely done writeup and I agree it belongs in the FAQ!

We need to tune it up slightly before it goes there.

[mrblaine]

Originally Posted by flatlander757

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 theory 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:

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.

[flatlander757]

Originally Posted by never monday

I sent Wheelin98TJ a PM to add you to the FAQ

Awesome, thanks!

Originally Posted by Jerry Bransford

Nicely done writeup and I agree it belongs in the FAQ!

Thanks!

Originally Posted by 95BadBoy

flatfender757

Borderline spectacular !! Def should be included in FAQ.

Glad you like it!

Originally Posted by dz1087

So is thrust angle synonymous with torque steer? i.e. would that be causing my left pull, or would that be a symptom?

http://www.jeepforum.com/forum/f9/torque-steer-651790/

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.

Originally Posted by imped4now

Wow, this is such a useful tool. Nice job!

Thanks!

Originally Posted by spokanistan

Fantastic post, kudos man.

I guess I wasn't chillin' in my jammies eating Cocoa Puffs for nothing this morning

Originally Posted by mrblaine

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

[dz1087]

Originally Posted by flatlander757

Awesome, thanks!






Torque steer 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.

I've pulled and checked all the rear CA's, all of them are tight and not worn. The only loose component I have found is the sway bar.

http://www.jeepforum.com/forum/f9/rear-sway-bar-bushings-655559/#post6255658

The bushings look a little loose to me, and the pass side is lower by about an inch. Think that could be contributing?

[saharaTJ7638]

So i have really weird tire wear on my 32's so they are 32x1150 on stock caynon wheels with 3" of lift. The outer edge is wearing alot faster than the rest of the tire is. no pull or any thing involved. I did purchase these tires used and they were worn like this when i got them but not nearly as bad.

[flatlander757]

dz1087: The swaybar isn't an "essential" part of the suspension, but it being bent will preload one side vs the other. Try pulling your rear swaybar entirely and drive it.

If you've done that and tried swapping tires left to right, and still nothing, I need to see some alignment specs for the whole vehicle to see if something is bent.

saharaTJ7638: Which tires are wearing fast? It's possible you have your toe set too far in causing feathering and accellerated wearing of the outter edges. Follow the write-up on www.stu-offroad.com for just setting toe in your driveway.

[mrcleanstj]

Going in my jeep tj bible ----- Thanks for the info!

[mrblaine]

Originally Posted by flatlander757

Awesome, thanks!



Thanks!



Glad you like it!



Torque steer 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.



Thanks!



I guess I wasn't chillin' in my jammies eating Cocoa Puffs for nothing this morning



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:

Why does the SAI also point out in front of the contact patch?

[dz1087]

Originally Posted by flatlander757

dz1087: The swaybar isn't an "essential" part of the suspension, but it being bent will preload one side vs the other. Try pulling your rear swaybar entirely and drive it.

If you've done that and tried swapping tires left to right, and still nothing, I need to see some alignment specs for the whole vehicle to see if something is bent.

BTW - thanks for the help Flatlander.

I took off the sway bar - it rode a little smoother, but still pulled.

Swapped the tires out, still pulled.

I measured the wheel base about 3 weeks ago and both sides came out to 93.5", so I think it has to be dynamic loading of some sort. About the only thing I haven't swapped to the other side are the UCA's. I had an alignment done in September, but did not get a print out.

[dz1087]

I just swapped my UCA's. Still pulling to the left.

[blackcat471]

This page has been bookmarked.

[flatlander757]

Originally Posted by mrblaine

Why does the SAI also point out in front of the contact patch?

Technically it's not the SAI that points in front of the tire, it's the caster angle(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 on running 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 balljoint to the caster/ground point, you can see why a super low caster angle would want the steering axis to rotate about itself.

Originally Posted by dz1087

BTW - thanks for the help Flatlander.

I took off the sway bar - it rode a little smoother, but still pulled.

Swapped the tires out, still pulled.

I measured the wheel base about 3 weeks ago and both sides came out to 93.5", so I think it has to be dynamic loading of some sort. About the only thing I haven't swapped to the other side are the UCA's. I had an alignment done in September, but did not get a print out.

Originally Posted by dz1087

I just swapped my UCA's. Still pulling to the left.

Yeah you need an alignment print out in order to see what may be going on. If any of the control arm bushings were worn, they SHOULD have produced the pull in the opposite direction when switched. With that said, track-bar bushings are pretty cheap and relatively easy to replace. You could try replacing those first(even if they look good). I fail to see how the trackbar bushings would produce a pull though, as it's only job is to keep the suspension under the vehicle and should have no real effect on thrust angle. I'd like to see what your rear camber and thrust angles are in particular.

[Skyryder]

Super write up, Thanks Much !

Mods, any chance for a stickie?
Should be.

Again, thanks.

[jeepmare]

Thanks for taking the time to do this write up. It now makes sense.

[Scotch740]

Great write up! Belongs in the FAQ in case it hasn't been said enough.

Question about toe in.

When you say TOTAL toe and give the factory spec of +0.30 degrees +/- 0.06 degrees. Is that saying one wheel is straight and the other is +.30 degrees inwards? Or are both wheels angled 0.30 degrees inwards.

I'm relating this to larger tires and setting the toe with a tape measure. It appears larger tires hardly effect what you would measure.
For a 30" tire I got 0.157" of toe in. For a 35" I got 0.183". So a little less than 1/32" difference for a 5" tire size increase. Sound correct?