The case of the mysterious shimmy

Jeep Wranglers of my Jeep's vintage have a tendency towards Death Wobble. This is a shimmy that kicks in at around 45mph and makes the front end wobble back and forth.

To a certain extent this is inherent in the inverted-Y type steering that my Jeep has. Modern vehicles have rack-and-pinion steering, where a cog on the end of the steering column moves a toothed rack to the left and right to steer the vehicle left and right. The problem is that rack-and-pinion steering doesn't do too well with long-travel suspension used for offroad purposes. With rack-and-pinion steering, hitting a rock or bump will cause the suspension to thump upwards, which in turn pushes on the rack as the steering rod pivots upwards on its ball joints and shortens the effective distance between the wheel and the steering column, which then makes your steering wheel twist violently. Inverted-Y type steering instead diverts much of the force of the bump into a toe change -- rather than the steering wheel twisting violently, the other tire toes out. Basically, the more you compress the front end of a Jeep, the more the front tires toe out (point slightly outwards rather than straight ahead). If you lift a Jeep, the wheels toe in (point slightly inwards). Adjusting via the driver's side tie rod is necessary whenever you lift a Jeep, and you want a slight amount of toe-in at rest because aerodynamic forces push the front of a Jeep down at speed and cause the front tires to toe-out.

So why does this cause death wobble on lifted Jeeps? Well, first of all, lifting the Jeep increases the angle of the Y. This means that an upward movement of the suspension causes toe-out to increase by a larger amount than with an unlifted Jeep. So if your passenger tire hits a bump, the driver's tire toes out and tries to steer the Jeep towards the driver's side. This then causes the Jeep to lean towards the driver's side, which loads the driver's side and unloads the passenger's side, which in turn causes the passenger's side to toe out and try to steer the Jeep towards the passenger's side. And so on. This can set up a resonance -- the "death wobble" -- when the natural wiggle overcomes the natural damping of caster, shocks, and steering damper (yes, my Jeep has a steering damper -- a shock absorber for the steering -- to help dampen any such motions).

So my Jeep started doing this at 45mph after my last tire rotation. So what did I do? First thing I did was make sure all my steering components were tight. Death wobble often happens when the track bar is loose, for example. But nothing was loose.

Next thing I did was check all ball joints and tie rod ends for play. They were all tight and play-free.

That having failed, I took my Jeep to the tire store and had them balance my tires. They reported that my back tires were fine and they'd removed some weight from the front tires.

That didn't affect things at all, so the next thing I did was check my alignment. I did this by duct-taping two pieces of angle iron to my front tires, one on each of my front tires, with the front and back of each piece of angle iron exactly 10 inches above the ground (this is necessary due to the fact that my Jeep has exactly 10 inches of clearance under the front control arms at the back of the front tires). I then grabbed a pair of $1.99 Cheap Chinese Tool Store tape measures. I duct-taped the end of the first one to the angle iron on the driver's side in front of the tires and laid the other end on top of the angle iron in front of the passenger side, then did the same with the other tape measure *behind* the wheels. I then pulled both tape measures taut, and found that there was 1/8" difference in measurement between front and back -- I was toed in by 1/8".

I checked my service manual and did a bit of trigonometry and that's well within specs for my Jeep. Still, one thing that helps with death wobble is less toe-in, because if the wheels start out pointing outwards more, they don't turn in more when you hit a bump. So I fiddled with the tie rod and reduced toe-in to 1/16", then headed out to test. I was worried that this would make my Jeep more skittish on the highway, but it wasn't significantly more skittish. But what I *did* find was that my Jeep no longer understeered -- when I cut the wheel to make a turn, instead of fighting me and trying to lift the inside tire, my Jeep now dives into turns with alacrity. Much driving since then has verified this: My Jeep now handles *much* better than with the stock suspension geometry, because a slight bit less toe-in has reduced the understeer that Jeep's engineers built in because, well, that's how American automotive engineers think cars are supposed to handle (i.e., with lots of understeer). Although to be fair, the wider tires on my Jeep, and the lift, also contributed to the understeer effect.

Anyhow, my test verified that the reduction of toe-in didn't make my Jeep wander on the highway (what I was worried about) and made it handle better in the curves. But: It did not eliminate the "death wobble" entirely. It made the death wobble less severe, it showed up as a pulsing in the steering rather than a violent shaking, but it was still there.

At this point I decided I must have a broken belt in a tire. That happens when you drive a Jeep offroad and hit a rock too hard with too much air pressure in your tires. I even know where it probably happened -- on the old toll road up Darwin Wash to Darwin, where it goes over the mountain, where I didn't bother airing down because that's a fairly mild road as far as 4x4 roads go (I could do it in a 2wd pickup truck). So I started taking tires off and replacing them with the spare. I started with the right rear tire, because it has a dent in the rim from whacking into a rock (advantage of steel over alloy -- an alloy wheel would have shattered). No change. Next, I put that tire back on the rear and rolled the spare over to the front right and replaced that tire. And...

Wobble gone.

So it appears that my new spare has a broken belt that causes a vibration at around 45mph. I'm not sure what I'm going to do about that. Probably just leave it as a spare for now... I only have about 15,000 miles of wear left in this set of tires, and it works well enough to serve as a spare, so if I get a blowout I'll just put it on in place of whatever tire, then head over to the tire store and get some new tires (bigger meats, 33" ones, this time).

So lessons learned?

1. The stock settings for toe-in are bunk once you put bigger slower-steering tires and a suspension lift on the beast. You want less toe-in then, because the bigger tires will steer slower than the OEM tires (meaning they don't need as much toe in order to keep the Jeep from wandering on the highway) and the taller suspension will make the inverted-Y add more toe-in when you lean over to do a turn, thus causing more understeer than with a stock Jeep.
2. And don't eliminate tires as the culprit even if they are balanced, because there's far more than being out of balance that can cause a tire to add a "thump" that can set up a shimmy. This tire isn't even out-of-round when you spin it with no load on the tire. But once you put weight on the tire, the broken belt shifts and lets the rubber stretch, making it effectively out of round as you roll it down the highway.

And thus ends this session of Jeeping Suspension 101...

-- Badtux the Wrenchin' Penguin

Baby loves her Kraut plugs

Drove the Silver Demon to Point Reyes National Seashore last weekend. She got close to 17mpg. Baby apparently loves her Kraut plugs (the Bosch Platinum+4 ones), because I never got that even when she was new and lower to the ground/skinnier smaller tires. Jeep Wranglers aren't known for their fuel economy :}.

I also took the Weestrom shopping last Saturday. She's a nice pleasant bike, but I don't ride her enough, especially since I now live only 4 miles from work and could even bicycle it if I wanted to do so. Still conflicted over whether to sell her or not. Probably will hang on to her until next spring now, given that we're already in the second week of June (heh!). Guess that means I need to plan a trip with the Wee just to let her spread her wings a little... So tonight I'll go ahead and install the new GPS wiring to my fuse panel, because the old wiring was pretty much fail. Might also go see if I can find that handlebar-mounted windshield in my storage, which might give calmer air behind my fairing... I don't *really* need to see my speedometer (cowling-mounted) if I can glance at the GPS and see how fast I'm going, right? :-).

-- Badtux the Wrenchin' Penguin

Where's my coolant going?

I'm baffled. My coolant tank was down to the "Min" mark. I'd filled it up to the "Max" mark maybe 10,000 miles ago. My oil has no water in it -- I'd notice when I dumped it. I don't see any leaks anywhere. I don't smell anything inside the Jeep. Should I take it to the dealer under warranty and have them do that black light thing?

Now, my brake fluid and my clutch fluid, I can see why they went down. My front brake pads are about half worn out (at 30,000 miles!), assuming they started out at the same thickness as my rear brake pads (which look barely worn). They're about a quarter inch thick now, which is a fair amount of fluid. I assume my clutch has worn some too. So I poured a bit of DOT3 fluid into both from a fresh container of brake fluid (unopened until I did this), and they should be good for a while. But there ain't no wear parts in my cooling system!

Oh yeah, finished greasing my zerks, and rotating my tires, and changed out the air filter while I was at it too. I'm not going to change the oil right now because it only has 2,000 miles on it, so I'll check the rear diff fluid (#$!@ diff seeps fluid despite my best attempts to seal it with black RTV after the last diff oil change, I'm going to have to go to the aftermarket gasket to see if that works), put the (new, formerly passenger front) spare back on the back of the Jeep, and then I'm done for the 30,000 mile service. The next "big" service will be at 36,000 miles, when I change the diff fluid again, 33K will just be an oil change and zerk puffer.

Last thing: I put anti-seize on the wheel studs. The threads were feeling mighty sticky, which is a bad thing, but this is the first time I've rotated the tires myself since the 6,000 mile rotation (which was the first -- and last -- with the original tires). I suspect the tire shop of not using anti-seize on these things and now they're a bit galled from the rotations every 5-6K miles. At least the tire shop didn't use an air wrench to put the lug nuts back on, I watched'em torque them down right, but I didn't think to check behind them on stud wear caused by galling. Oh well, when I do the brakes in 20,000 miles or so (based on current wear patterns) I'll see about whacking some new studs in. It's really easy to do on a Jeep, just take the caliper off, knock the disk off with the rubber mallet, and then you can whack the studs right out and knock new ones without having to take the axle end off and put it in a hydraulic press. We've done it on the trail before, when a guy who had his studs overstretched by morons with an air hammer had the studs break on the trail and we had to round up some studs from people's spare tire carriers and whack'em in to get him on four wheels again... crap, you can just about fix a pre-2007 Jeep Wrangler with a rock tied to a stick, when it comes down to it :-).

-- Badtux the Wrenchin' Penguin

The downsides of wrenching late at night

I got the new spark plugs into the Silver Demon tonight. I was going to ride down to the nearest NAPA auto parts and get some NAPA goodness, but they went out of business :-(. So I kept riding down El Camino Real until I passed a Kragen's on the left, at which point I got over into the left lane and did a U-turn. I asked the parts guy to give me the best plugs he had, and he started scrolling down the list on his computer. I stopped him on the Bosch plugs and got the best ones he had. Six of them for $6 apiece isn't bad at all for plugs you don't have to gap and the Krauts know how to make good shit, hell, I got a German transmission and German rear brakes (Bosch rear brakes, believe it or not!) so those plugs ought to feel right at home in my Jeep.

So the next problem was getting the coil-over-plug rail out of the way. I had to figure out how to get that $#%! connector off the end of it without being able to, like, actually *see* the bloody thing because the tabs are facing the firewall. But I figured it out eventually, and figured out a way to wiggle the coil-over-plug rail so that I could get it out of there. Six teats in a row, remember? That reliable old 4.0L I-6 engine might date to 1964 in many of its details, but that coil-over-plug rail is state of the art as of 2003. o that's a long, long coil-over rail, and a PITA to wiggle outta there.

The OEM plugs were semi-frozen in the heads (30K miles on'em), but a big 1/2" ratchet worked. The one on the end, in the indentation of the firewall, I had to grab my cheater pipe and a metal bar to get it out. But I got it out, and anti-seized the new plugs and put'em in and torqued'em to 260 inch-pounds (comes out to something like 29.7 n/m, the spec says to put'em to 30 n/m, close 'nuff already), wiggled the rail back in and hooked it back up, and then headed out to test-drive the beast. It test drove fine. Seemed to be a bit smoother, but that may have just been my imagination.

So anyhow, it was only 9:30PM by this time, so I started the next task on my list -- rotate the tires and check the brakes. I do a five-tire rotation, that way I only need one jack, besides it helps keep wear down on the locker up front to have all the tires be the same general wear level. I inspected the brakes while doing so this time because the brake fluid is looking a little low. So the spare went onto the rear driver's side. While it was off, I looked at the brake pads (rear disks). Damned things looked new -- they were freakin' half an inch thick. So I put the spare on and torqued it down to 100 ft/lbs (in stages), rolled the former rear tire to the front of the Jeep, and jacked that wheel up and took it off. That brake pad looked considerably more worn, and I found another oddity: My factory service manual has no minimum spec listed for the friggin' brake pad thickness! However, I do have a Chilton manual around and it said 0.125 inches. Well, I grabbed my micrometer and the brake pads are considerably thicker than that, probably half-worn, so that was that.

So next thing I realize is that, with my wheel off, all my zerks are right there in front of me instead of me having to crawl under the car. So I grab the grease gun and start pumping grease into zerks. One... two... three... four -- oops, what happened to my grease pressure?! Turns out I managed to run out of grease. Darn, and I just put this grease cartridge in there a year or so ago, wonder what happened to the grease given that I only use it to pump grease into my Jeep?

So now it's 10:30PM, and I'm out of grease, and I go look for grease on my lubricants shelf. I find a vat of grease, but no friggin' tube. Oh come on now, I know I had a spare tube of grease! Oh yeah, that's right, that's the empty tube of grease that I just took out of the grease gun because I did not buy another tube of grease when I put my spare into the grease gun! GAH! And it's 10:30PM. Nothing open. I wish I lived in a real city that didn't close down at 9PM!

So I put the rear tire onto the front, and roll the former front tire to the back for tomorrow, and wrap everything up for the night. Tomorrow I'll ride the Nightstrom to Kragen's again and buy a couple of tubes of Mobil 1 grease for my grease gun (*gonna buy that spare this time*!) and a new air filter because my air filter looked dirty too and I've already knocked sand out of it enough times that I'm starting to get dubious about its integrity (you can whack an airfilter on your fender only so many times before it starts getting a bit, well, mushy). That's what happens when ya drive around in the desert a lot eating the dust of other Jeeps, heh!

So g'nite, all. And the continued saga of the Silver Demon's 30,000 mile service shall, well, continue, 'cause I still got the other side of the tire rotatin' to do, as well as the air filter, add some brake fluid (checked and yeppers I still have a sealed DOT3 container), and finally, change the oil... not to mention finish greasing all those damned zerks!

Oh yah: Before taking tires off:

1. Put yer emergency brake on.
2. Put the transmission in first gear, or park if it's an auto. (Or if your reverse is lower than first, put it in reverse, but my 1st is lower).
3. Put the transfer case into 4-Hi (if you have 4wd of the part-time type)
4. Block the wheels with wheel chocks that are opposite the side you're working on.

Safety counts. I'm using the OEM jack which fits up under the axle and is mechanical in nature (it's a screw-type jack) so it's pretty much bullet-proof, but you're darn tootin' that I ain't gettin' under that Heep without jackstands even so. I ain't interested in being a flat penguin, nosirree...

-- Badtux the Wrenchin' Penguin

Piece of shit spark plugs

I've been working on cars for over 30 years now. I thought I'd seen every fucking piece of shit car that had ever come down the pike, every crappy half-assed aftermarket part that was possible to see. But this is the first time I've ever seen a brand name that was so fucking cheap ass I wouldn't put it in the car of my goddamned worst enemy.

The culprit: Champion "Platinum Power Premium Spark Plug, part number 3034 RC12PEC5". Look. I've put spark plugs in everything from a 1971 Ford Torino to a 2008 Suzuki V-Strom. I didn't expect a problem putting spark plugs into my Jeep. I mean, fuck. That goddamned I-6 engine dates back to before I was fucking born. So I took the spark plug coil rail off, and grabbed one of these spark plugs out of the box and... what the fuck? The goddamned electrode was set at like 0.60, when a Jeep wants 0.35. Well, fuck, that's not a problem, you just rap the plug on the front bumper with your feeler gage at .35 between the electrodes and... what the fuck? The goddamned electrode won't fucking bend straight it's so over-long for the gap required for the Jeep 4.0L I6, it's like all half-ass and shit? Yeppers, I tried it on two different plugs, same goddamned result. There's just too much motherfucking electrode for a Jeep 4.0L I6.

Fuck this shit. I threw these goddamned piece of shit spark plugs in the fucking trash. I'm going back to the parts house tomorrow and get some better plugs. Sue me for fucking cheaping out on spark plugs, I shoulda known better. "Platinum" or no, these are fucking bottom feeder piece of shit spark plugs, not fit for my worst enemy's car, much less my own.

-- Badtux the Rude Penguin

Gears!

Okay, so here's the deal. The Silver Demon (my Jeep LJ) came with 30 inch tires (that's 30 inches from top to bottom, on 15 inch wheels) and 3.73 gearing in the differentials (remember, I have two differentials). So what happens if I go to 35 inch tires?

Well, here's what happens. The 30 inch tires are 86% smaller than the 35 inch tires. What that means is that the effective gear ratio becomes 3.73 * 0.86, or 3.21. Which is very tall gearing for a manual-transmission Jeep LJ. Frankly, the stock gearing was already a little tall in 6th gear, I had to downshift every time I hit a hill, this would make it horrible indeed. How horrible? 6th gear in a Chrysler NSG370 manual transmission is a 0.83 overdrive. Using this gear calculator, at 75mph I'd be at 2256 rpm, rendering 6th gear useless for pretty much anything slower or on any slope steeper than my bedroom floor. Whereas with the stock 30 inch tires, I'd be at 2632 rpm, which is much closer to the torque peak of the venerable old 4.0L AMC I-6. 235 lb-ft @ 3,200 rpm. But more important, my crawl speed at idle (1000 rpm) with 2.71 low range transfer case and 4.46 1st gear ratio goes from 1.98mph to 2.31mph -- which is significant when you're rock-crawling, where slower is better. And the 1st gear mph at 1000rpm in 2hi (1:0 transfer case ratio) is 6.26mph, which would make it hard to start off without slipping the clutch as vs. the 5.37mph of the stock tires. Not Good.

So I can upgrade to 35 inch tires, but to do that, I have to go to lower gearing (higher multiplication ratio). My Jeep LJ is equipped with the Dana 30 axle. This is a relatively small axle that has only three gearsets available that are lower than the 3.73 in my Jeep: 4.10, 4.56, and 4.88. Because of the small size of the axle housing, the pinion for 4.88 is *very* small -- Dana only recommends 4.56 as the maximum. So I plug 4.56 into the gear calculator to compute 1st gear mph at 1000 rpm and... 1.88mph crawl speed in 1st at 1000 in 4-lo, or slightly lower than OEM. At 75mph, I'd be going at 2758rpm as vs. 2632rpm with the stock gearing. Not a big deal, it'd make 6th gear more usable in less-than-flat terrain though I'd still have to downshift for big hills.

Now, why am I talking about gears? Because I'm thinking about putting a ARB Air Locker in my rear end for better traction off-road. The stock limited slip has very little friction material and is slowly losing its capability to provide any traction benefit at all in offroad situations. Thing is, this replaces the stock spider gear carrier with the ARB spider carrier, and requires complete gear setup as a result. Gear setup on a Dana 44 rear axle is a PITA, requiring you to set pinion depth with a crush sleeve and a 250 ft/lb torque wrench, set pinion bearing preload with a 15 inch-pound torque wrench and spacers prior to setting pinion depth, spread the case with a case spreader and use shims on the insides of the bearings to properly preload the carrier bearings, and adjust the shims on the inside of the carrier bearings side to side to properly adjust the side-to-side gear lash. It can take a couple of hours and a lot of cursing to get all the depths and preloads and everything right (and remember that the shims are on the *insides* of the bearings and the bearings are pressed on and pulled off with a bearing puller), and people justifiably charge an arm and a leg to do it. So when I have someone put in the ARB, it makes sense to do the gear setup for the 4.56 gears at the same time. That way they only have to do the gear setup *once*, not *twice* (once for the 3.73, once for the 4.56), which would save a lot of time and hassle. And remember, when you're talking about wrenching, time and hassle is $$$.

So anyhow, that leaves one last problem. Right now I have 32" tires on my Jeep. They still have at least 20,000 miles of wear left on them before I am ready to replace them (i.e., until the tread depth gets low enough to start affecting offroad traction). What happens if I put 4.56 gears on the Jeep before I put the 35" tires? Well... rpm at 75mph goes to 3016 rpm. As vs. 2937 rpm if I am running 5th gear (1:0 ration) with 32 inch tires at 75mph. So I tested 5th gear at 75mph, and it's a bit busier than 2600 rpm was with the stock tires and gearing, but still acceptable. Note that I never -- ever -- go faster than 75mph in my Jeep. It just isn't what a Jeep is made for. So I could go ahead and get my Jeep re-geared at the same time I get the locker put into the rear, and I'll still be fine, it's just that my Jeep will be a bit buzzy (but not excessively so -- 3000 rpm isn't all that high).

Now comes the final issue: Should I pay to have someone do the locker install and gear setup, or should I do it myself? Well, here's the deal. To do it myself, I'd need the following spendy stuff: A 0-30 in/lb torque wrench. A hydraulic press. A 500 ft/lb torque wrench. A case spreader. Probably a couple other things I've forgotten, while adding up the above $500+ worth of tools (all by itself) which is more than I'd pay for the gear setup. Given that we're talking about a job that'd take me several days, I'm taking names of any competent axle people you know in the San Francisco Bay area. Heck, anywhere else within reasonable driving distance, for that matter... I'd rather pay to have it done right but have to drive 250 miles to get there, than to pay someone local who screws it up and then I end up with the axle tearing apart and needing to buy a new axle. Bummer, big time, dude!

-- Badtux the Jeepin' Penguin

It'll buff out

A few things here:

1. The driver was *not* hurt, she was wearing her seat belt and Jeep Wranglers have a very effective roll bar.
2. The Jeep itself wasn't significantly hurt. Her hood needed banging back into shape but bungied right back down with the stock bungies, and there were a couple of holes in the top that needed duct tape to make it watertight again, and the rocks left a few dents in the sheetmetal here and there, but that was pretty much the extent of the damage. Jeeps are *tough*, and the sheetmetal is just that -- sheetmetal, with no structural role. So dented sheetmetal is nothing to a Jeep.
3. The driver claims that the reason she didn't follow her spotter's direction to cut hard right to follow the seam was because the sun was in her eyes.

Which brings up the fatal flaw in this driver's excuse. If I'm heading up a seam like this and my left front starts heading up into the air and I can't see my spotter, I *stop*. I then wait either for my spotter to come running to my window to tell me what I should be doing, or for him to tell me over my CB radio what I should be doing, or for him to send a runner to tell me what I should be doing. It may be that the best thing to do is to back up and take another cut, it may be that I'm in a position where I can cut hard right where I am and still make it, it may be that the best thing to do is to put the hook (winch cable) on me to make sure I don't flip backwards and just keep bulling forward, but thing is, I'm inside the Jeep where I can't see clearly, and my spotter is outside where he can see everything.

In short, I don't buy this driver's excuse. She was an idiot, plain and simple. Bulling forward after she lost her spotter was just *stupid*. If she was a guy I'd accuse her of testosterone poisoning, wanting to prove how macho he was. Since she's not a guy, well, not much to say there.

So remember, boys and girls, if you're in a tricky spot like this where it's possible to roll over, and you can't see your spotter, and your wheel starts heading up, *stop*. Maybe slowly back up a little to get your wheel back down, but at the very least, stop! You might need the hook to make it up this seam, or you may need to back up and take another cut, or a hard cut to the right might get you out of it, but simply bulling straight forward when you can't see your spotter is just, well, DUMB.

-- Badtux the Jeepin' Penguin