Thursday, March 29, 2007
World's worst mechanic: wheel alignment specialist
When you pick up a Jeep by adding a 2" lift to it, you pull up on the drag link, which in turn pulls up on the tie rod. This causes excessive toe-in. I just checked my Jeep (which hasn't been anywhere since I lifted it except around the block), and the front of the tires are 54 7/16" apart, and the backs are 55" apart, for a total toe-in of 9/16" or over half an inch. Figure that's roughly .25", divide by the 30" tire width, take the asin (boy, you knew that high school trig was gonna come in handy, eh?), and you end up at 0.47 degrees toe-in per wheel. The spec is 0.15 degrees toe-in per wheel.
(hey, so I'm a former math teacher, shoot me!) shows that .15 degrees toe-in, take the sine, multiply by 30, hmm... should be .078" difference between front and back. .075" is a tad more than 1/16 but less than 1/8.
So now I apply some more trigonometry to figure out what the difference *should* be (boy, you knew that high school trig class would come in handy, huh?). The spec is +/-0.06 degrees, so the toe-in should be between 0.09 degrees and 0.21 degrees. Take the sine of each of those, multiply by 30, hmm... that's 0.047" to .101 inches. Multiply by two to add up the toe angle for each wheel, and that's 0.094" to .202". Or, given the limits of my tape measure which don't do decimal,more than 3/32" (0.093"), but less than 7/32 (.21875"). So let's say 1/8" to 3/16" is fine. Definitely *not* 9/16"!
Soooo... tomorrow I get to go under the Jeep and loosen up the tie rod turnbuckles and test out how well my pipe wrench works on the steering tie rod! Now I hear what you're saying. "Should the world's worst mechanic be doing something so... dangerous... as mess with the steering?". Ah, but see, I'm not mechanicing anymore. I'm in the realm of mathematics now... and mathematics doesn't care how incompetent a mechanic I am. As long as I tighten those turnbuckles back up when done, there's nothing I can do wrong here. It's all just measure... adjust... measure... adjust... measure until the measurement is between the limits I mention above.
Oh, how I measured: Take tape measure, and a bit of duct tape. Tape tape measure to first row of tread at the front of the tire. Extend tape measure to other tire. Take measurement at first row of tread. Now do the same thing to the back of the tire. Simple, crude, and ... close enough. This is a Jeep, for cryin' out loud. I could get a bit closer with one of those fancy machines like some real suspension alignment shop would have, but it's not as if the beast really cares that much. You can almost get it close enough just by banging on it with a hammer and duct taping random stuff, which is pretty much how the gnomes in Cleveland put it together in the first place (heh!)
And so a little magic goes out of the world, replaced by mathematics. No incantations needed, just use of the sin and asin buttons on my scientific calculator. That's the world, folks. There is no magic, no matter how much people want to believe in the Invisible Sky Fairy that will do all sortsa neat things for them if they just babble the right incantations. It's all math, in the end. Even mechanicking is all math, in the end. Which is probably why mechanics are the most intuitive mathematicians I've ever run into, even though most of them don't have the slightest damned idea they're doin' it :-).
-- Badtux the Math Penguin
PS - the fun part is going to be re-centering the steering wheel after I do this, since there is only a single adjuster on the tie rod... luckily the drag link has its own adjuster for centering the steering wheel!
Labels: jeep, mathematics
Posted by: BadTux / 3/29/2007 11:51:00 PM
'Tis better to be a slow but careful mechanic than to see a wheel of your car going it's own way in the distance :D
# posted by : 30/3/07 8:39 AM
Indeed. No wheel of mine is going anywhere. I torqued those suckers to 100 ft/lbs with my torque wrench :-). (100 ft/lbs is a lot, for those of you who don't do mechanic'ing stuff).
My problem is that even doing it slow, I don't always do it right. Like putting on one of my rear shocks, I followed the torque specification in the owners' manual rather than the one that came with the shock. Ooops! And I don't always read the directions, though the directions are usually wrong anyhow so ... :-). For example, when I put on the sway bar disconnects/links, the instructions say to put the strap *over* the sway bar. When I put the first disconnect on, I put it *under* the sway bar because that's how the OEM link is connected. Then I moved to the next wheel and glanced at the directions and... over? Okay, so I tried over. Didn't work. Sway bar is too "fat", caused the sides to bulge out. So I shrugged and put it *under*, just like the other side, just like the OEM, completely opposite what the freakin' directions said. Then I went on the Internets and looked up the tensile strength of grade 10 bolts to make sure I was safe. Yep, I'm safe -- those 10mm bolts will hold over 30,000 pounds in tension, thus certainly will hold a 300 pound axle when the front end goes on the lift!
Anyhow, the point is that none of this is magic. It's all math and physics, in the end - force vectors and what not.
- Badtux the Math Penguin
# posted by BadTux : 30/3/07 9:23 AM
math is key stuff. a lot of people don't understand that about music also. i don't see math as ruining the magic so much as explaining what the trick is. pythagoras wrote extensively about the relationship of numbers and sounds. it was required study for his followers. getting into the math involved with sound reproduction is taking me a long way to understanding my new modeling amp and guitar. (i knew if i hung around long enough i would find a use for my computer science degree that didn't involve a cubicle)
# posted by The Minstrel Boy : 31/3/07 8:19 AM
Believe me, one of those fancy machines may NOT do a better job of reading than a tape measure.
You needed a lift kit like you needed another hole in your head for things to leak out of, but maybe that is beside the point.
You didn't mention camber though. Often a lift kit changes camber. If you changed the camber much you need to correct it before you do a toe correction.
I'm not familiar with the suspension on your new jeep but unless it has a strut suspension, or a straight axle, I'm sure that the kit changed the camber.
And I think a 100 ft/lbs of torque on the wheels is a bit much. 7/16 wheel studs are fine with 80 lbs. 1/2 inch studs are fine with 90 lbs.
I tore a wheel off of a race car back when I was green because I used too much torque.
# posted by BBC : 31/3/07 6:13 PM
Jeep Wranglers have straight axles front and rear, so picking it up does nothing to the camber (does reduce the wheelbase slightly due to the higher angle of the control arms that locate the axles, but not significantly so for a 2" lift). Lifting a Jeep does reduce the castor because it tilts the axle a little more, but castor isn't adjustable without spending money on adjustable control arms, so it's not worth worrying about as long as you still have enough castor to re-center the steering wheel (I do). In case you're wondering why Jeep Wranglers have "primitive" solid axles, it's all about wheel travel -- current independent suspension designs do not allow long wheel travel, and long wheel travel ("articulation") is necessary in order to keep all four wheels on the ground on rough terrain (same reason why dirt bikes have long-travel suspension -- it keeps the tires on the ground better). One reason why the Hummer H1 is so wide is because to get the spec'ed wheel travel with independent suspension, they needed very long control arms, thus requiring the H1 to be much wider than you'd expect. And the H1 has less wheel travel than a stock Jeep Wrangler Rubicon...
As for the torque spec on Jeep wheels, the torque spec in the factory manual is 90-110 ft/lbs. I put it in the middle. Good 'nuff.
The reason for the lift kit is to get a little more clearance between the belly scoop and the ground to keep it from getting bashed up so bad. Stock, there was 9 1/2" clearance there, and I have the dings and nicks to prove it. Lifted, I have 11 3/4" clearance. The lift kit also allows fitting 32" tires rather than 30" tires, thus putting another inch of air under both the differentials and the belly pan. The new tires are going to have to wait for finances though (sigh!)...
-Badtux the Greasy-flippered Penguin
# posted by BadTux : 31/3/07 8:07 PM
I've had a Toyota Tercel anywhere in the mountains and deserts that this monkey needed to be.
That jeep is going to hundred dollar you to death.
He, he, he.
Yeah, I won't quibble about ten pounds of torque, just stated what I use is all. They fake it with the torque wrenches they use in tire shops. I always recheck them as soon as I get home.
# posted by BBC : 1/4/07 1:13 PM
All ya need to know to be a successful mechanic is a) don't run 'em out of oil, and b) don't let the wheels fall off.
Put a little anti-seize on the lug stud threads.
# posted by Gordon : 1/4/07 1:45 PM
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