Little Help with an Odd Death Wobble issue.

[CBari]

Here is the history of the issue and repairs done to date.

Back in Dec my 03 3500 drw had its first DW issue I had the cruise set at 80 with the truck empty and hit a rough overpass and it started. This was my first time to ever experience it so as you can imagine I pulled off instantly and looked around for some thing that was broken. When I found nothing I went on and never felt it again.

In March I put a CM skirted Flat bed on the truck and about a week later It happened again this time I had 16k goose-neck trailer on and I was doing about 65 and had hit a rough expansion joint on the interstate. When I slowed down below 45 it quit and I speed up and went on. After this I replaced the front tires as they were wore pretty good and I had two pull off Michelin that were in great shape. I also replaced the tie rod and the steering stabilizer. I thoroughly checked everything else and all was good. All was good till May when I had 20k load on my goose-neck when I once again it a rough patch on the interstate that started it slowed to 45 and it quit.

The next time I noticed it was in September truck was empty and I was going 70 and hit a rough spot on interstate and it happened slow down to 45 and it quits and I go on. I then had the truck in the shop for some other work and I had them look at the front end they said all was tight and good, which is what I thought but want some reassurance. They mentioned maybe the tires were bad.

Since then it has happened several times and I got to playing with the air bags if I keep them empty I can almost tell you when it will happen, but if I put about 30 psi in them It basically quits. I know by adding the air I am shifting more weight to the front axle, but what is bad that the weight hides?

I know some will tell me to just keep air in the bags all the time but the ride is a little harsh for that. My wife loves to drive this truck with all the vehicles in the drive way this is the one she gets in with her and my daughter in it all the time I want to fix it right so I don't worry about her having an accident.

 

[Wayne M.]

The question I have is, how did everyone "check" the front end? People do it lots of ways. At my shop, they call me the terminator of ball joints. First let me say what's been going on with my '04. Recently, I have been noticing she's been loosening up a bit. It will follow ruts and generally do a wiggle sometimes. About 3 months ago it happened. DW. BAD. The truck never did it before, and my kids who were in the seats they basically grew up in started screaming because "Big Blue never did THAT before Daddy!" Now I know DW can be tough to diagnose and the shoemakers shoes are the last to get fixed. Anyway, I do a PM, and my semi annual shakedown of the front end. I see a scoch in a lower BJ, but nothing bad. I jack up each wheel and do my thing. Now I'm thinking the tires are getting thin, the Bilstiens from Geno's have 75K on them... NOPE! were NOT throwing parts at it! I drive on, and BOOM it happens again at the same exact ripple in the hiway.
Now I get it back home, and I thought that maybe I should do a class 8 style steering check. I put a kid on the seat and have him saw the wheel with the engine idling. Shazam! There it is. The left inner tie rod (where the stabilizer is attached) is so loose it was jumping, and that whole arm was rotating (I guess that explained the soft clunk I felt going at a certain slow speed). OK The truck is down. I get Moog problem solver left tie's and sleeve and a tenneco stabilizer. I do the deed, gained another grease fitting, and now all my problems are gone. It's fixed.

All I can say is if you set up the harmonics just right, you get DW. Everytime. I'm thinking, by taking out chassis flex by airing up the bags, you're squelching the harmonics. You're only masking the problem. Do a real steering check, tires on the ground, with two people. One to saw the wheel, one to look at everything. My method for the ball joints in particular. I use a 1X1 block of wood and a bar with a bent tip where the bent part is short. Jack the axle so the wheel is approx even with the block, space the block away a couple inches from the sidewall, and put the curve tip of the bar under the center (or just out) of the tread. Have someone push down. The object is to raise the center of the wheel assy straight up- not on an angle, which is what's done if someone just uses a bar without the wood. You're picking up the outer edge of the tire- no good. I promise results.

 

[jelag]

I have owned several of these trucks and most were driven to 350K miles before I sold them... and not one has ever had the DW... and I can honestly say we'd never take the truck in for an alignment unless we had tire wear problems.. once in the life of the truck we'd change out the front end parts.. BUT, one of the angles that the front end can measure for are two imaginary lines... one is through the center of the king pins (ball joints) to where it strikes the road.... the second imaginary line is through the center of the tire, to the pavement... these 2 lines are to intersect the highway at the same point... I've often wondered if the 4WD Dual Rear Wheel trucks hit the same spot in the highway as the 2WD single rear wheel trucks... and if this difference means some of the trucks have a higher potential to this problem... BTW all of my trucks have been 4WD Duallys....

Now that I've said all of the above and stirred the pot... I've always felt that the DW has to be from loose parts plus not enough caster in the alignment.... I used to have an old guy in an alignment shop who'd owned the shop for 30 years... if/when we had tire wear issues... we'd go see him and he'd tell us what parts needed to be changed.. we'd do that, and he'd align the truck and the tire wear would go away... that's the kind of guy I'd find in your town to help you out... my guy has now retired... his by has the shop and frankly the boy is a nice young guy.... but not near the caliber of guy who I trust...

Just my thoughts here.....

 

[SnoKing]

More caster! You can do that yourself.

 

[Wayne M.]

More caster! You can do that yourself.

Hey- we ain't runnin' no shopping cart here! Another mask for a problem. Mr. jelag, I don't know what condition the roads are in your area, but I invite you to hit the blacktop of NYC anytime. 350 on steering components around here just isn't happening.

 

[cerberusiam]

Tires and shocks, 2 biggest contributors to DW there are.

If you haven't put new shocks on in a while get some Bilteins on there, the ones speciffically for these heavy front end trucks.

What tires are you using? If they say BFG throw them away, they are all worthless on these trucks. Make sure they are good E rated tires with the stiff sidewalls and air them up close to the max on the front end.

If all the steering components are tight, check the track bar deflection. Even if it looks good if you get deflection when turning the wheel back and forth sitting stil the potential is there for DW.

 

[brods]

Age old problem with many causes. One guy had DW on three different Chevy trucks. Turned out the common denominator was he had swapped the same rims and tires to each truck.

The quotes below (which someone saved before the Nth Degree website shut down) are attributed to Nth Degree Mobility. Iirc the guy who started Nth Degree (a Jeep suspension parts business) was a Chrysler engineer. Note how he says caster is what supplies the force to shimmy and to reduce caster with larger tires, which is the opposite of the typical forum recommendations. YMMV

What's the deal with death wobble / shimmy?

A doctoral dissertation could be done on shimmy (aka ‘Death Wobble’)…It is not so much a ‘design flaw’ but rather an engineering challenge that has yet to be absolutely overcome in suspension design…and we off-roaders just have to live with it more than the rest due to the fact that we do “all the wrong things” to our rigs and basically invite shimmy to happen as a result. Shimmy continues to defeat even the best full-compliance vehicle dynamics modeling programs (Jeep uses ADAMS). The phenomenon is pretty well researched, but it plagues even the factory at times…Jeep almost had to delay/cancel one model launch because it developed a shimmy problem during the last run of prototype parts – only 3 months before launch. We had to ‘tweak’ four things about the suspension to make it go away: reduced lift height to +0.6” from the traditional 1.0”, change durometer of front LCA bushings, re-valve steering damper, and change nominal caster spec…”traditionally”, the last two are the primary things that affect the risk of shimmy, but another major player is front tire balance…they must be *dynamically* balanced (i.e. weights on both rim edges, so the wheel/tire assy is balanced in all planes, not just one).

So here’s the ‘techno-speak’ dissertation on shimmy: It is an imbalance of the gyroscopic forces generated by the spinning front tire/wheel assys…when the ‘setup’ of the front end is ‘vulnerable’ to starting shimmy, it happens due to an asymmetrical force (i.e. one tire hits a bump that the other doesn’t) or also it can happen when the bump-force is not even side-side (in this case it usually happens at higher speed due to say a ‘swell’ on the highway). The problem is worst with wheel-wheel tie-rods (which is why the TJ and most late Jeeps use the Haltenberger linkage – aka ‘inverted Y’ design) because the tires can ‘crosstalk’ directly to each other. The larger/heavier the tire/wheel assy’s are, the stronger the inertia and thus the worse the shimmy can be if it starts….of course a damper is the most direct defense once shimmy starts, but what you *really* want is for it to not start in the first place. For that, you have to look at the ‘free-body diagram’ of the forces that can influence the stability of a spinning front tire/wheel assy. The key for shimmy issues is the distance between the ‘center of tire contact pressure (CTCP, which is not directly below the hub due to ‘pneumatic trail’ – it’s actually further back.) This is the effective center of downforce of the tire’s contact patch when it’s moving/rolling) from the ‘steering point’ (the point where the steering axis intersects the ground plane). If you leave the caster setting at the factory spec of 6-8 degrees while increasing tire diameter from 27-28” to 33-35”, then you’ve effectively *increased* this distance (and therefore the imbalance force it can generate), while at the same time you’ve increased the mass that drives this force (the larger/heavier tire/wheel assy), so when things get to shimmying, they REALLY shimmy! So…what to do? If you’ve followed this far, the answers are easy to ‘guess’, but should be divided into ‘preventative’ and ‘band-aid’ fixes:

Preventative (i.e. will reduce the likelihood of shimmy, in guessed order of empirical relative effectiveness):
1) Keep wheel/tire mass low (i.e. run stock-sized-or-close tires and/or aluminum rims – both of which are likely unacceptable to a real ‘wheeler)
2) Keep CTCP – to – steering point distance short by running stock-ish diameter tires (also not acceptable)
3) Reduce the CTCP – to – steering point distance by reducing caster (moves steering point back towards CTCP)
4) Run radial tires!
5) Dynamic balance the tire/wheel assys (weights on both rim edges)
6) Avoid direct wheel-to-wheel tie rods, or if so, have deliberate ‘compliance’ in the tie rod (i.e. not too stiff, but steering feel/precision will suffer)
7) Maintain high lateral stiffness via proper trackbar design and bushing rates (i.e. no non-preloaded urethane bushings and hollow-tube t-bars!)
8) Tune front control arm bushings on the ‘stiff side’ (i.e. high-ish durometer…TJ uses stiffer LCA bushings than XJ for example, due to trackbar bracket stiffness differences, etc.)
9) Drive slow and/or don’t hit any bumps!

Band-aid-type fixes:
1) Heavy-valved steering damper (helps a lot if tire/wheel mass is 100# per assy or less. Multiple dampers will help with heavier setups, but too much damping will limit steering response time)
2) Match all compliances together to allow drastic violation of above guidelines (i.e. apply a lot of engineering to the bushings and linkage stiffnesses while screwing up the other engineering parameters…)
…so what’s the “Bottom Line”? It’s this: the stock caster spec is NOT the appropriate spec for your lifted Jeep running bigger tires. For 33” tires, I recommend about 5.0 degrees, and for 35” – about 4.0 degrees.

Ultimately your issues (or not) with shimmy will be determined by how well your rig is ‘set up’ either by you or your shop…so the main thing we’re doing is putting some watered-down version of the above in our product instructions (i.e. a new caster spec based on tire size), but also we’ve made sure that the lower caster setting is in the middle of the adjustment range so you have a chance of setting proper caster (which I’ve found isn’t possible with some lifts), and this will help you understand why we’ve designed the front trackbar as a factory-type one-piece solid forging with high-durometer, rate-plated bushings and a stiff trackbar bracket/brace.

Here is a short list of the key things to check/change when trying to fix the dreaded ‘death wobble’ (properly called ‘shimmy’). These general ‘guidelines’ apply to any vehicle – assuming it’s at least a ‘solid axle’ front suspension (IFS vehicles are a whole different story).

- Caster Angle: The correct caster angle goes DOWN as tire diameter goes UP…so even though late Jeeps call for 6-8 degrees caster in stock form, with 33’s you want caster to be 4-5 degrees maximum…this reduces the available ‘force’ that drives shimmy

- Steering Damper: You MUST use one – and it should be a heavier-duty aftermarket one. If you have late-model Jeeps, I recommend Old Man Emu’s kit.

- Tire Balance: the front tires MUST be ‘dynamically’ balanced…that is, they must have wheel weights on both the inner and outer rim flanges as directed by the balancing machine. If you don’t like hammer-on weights on the outer flange either for appearance reasons or because of frequent rock damage, then use stick-on weights for the outer weights – in this case function must precede form…don’t balance ‘statically’ with weights just on the inner rim edge. If a tire/wheel requires more than 6 ounces of total balance weights (inner and outer combined), I would not use it on the front axle.

- Steering joints – make sure all tie rod ends are tight with no ‘ slop’ – also check pitman nut/splines for slop.

- Axle ball joints: Grab the top of each front tire and push-pull hard on it to make sure the ball joints are tight – if not, replace as needed.

- General Suspension Condition – if your Jeeps see a lot of wear and tear (I assume they do), then the condition of the control arm and trackbar bushings may be a significant contributor to the problem. Urethane bushings (like I think the PC lifts use) tend to wear out quickly if not greased religiously – make sure there is no ‘slop’ in the bushings. Also, if the bushings are smaller in size than the factory rubber parts, their lack of isolation not only makes the ride harsh and jarring, but will also eventually cause fatigue failure of frame and axle brackets that they attach to – this ‘premature aging’ of the chassis could happen very quickly with commercial vehicles like your Jeep fleet – causing you to replace whole vehicles long before it should have been warranted based on mileage. (our upcoming lift systems can help a lot with this issue).

- General Suspension geometry – This is far too complicated to describe what to look for long-distance, so hopefully the items above will solve your issues. If not, then I would suspect that the lift kits you’re using have kinematic (geometry) design flaws that are contributing to the issue…unfortunately this is not at all unlikely since most lifts are not designed with proper geometry in mind…

Try/check these items in decending order – hopefully you will find this helps with your problems. If you get down to the last two points, you’ll find yourself in the market for better suspension components – we can help with that!

 

[RustyJC]

The quotes below (which someone saved before the Nth Degree website shut down) are attributed to Nth Degree Mobility. Iirc the guy who started Nth Degree (a Jeep suspension parts business) was a Chrysler engineer. Note how he says caster is what supplies the force to shimmy and to reduce caster with larger tires, which is the opposite of the typical forum recommendations. YMMV

If you do a Google search for "caster wobble", you'll find that this problem has been around as long as cars & trucks have used solid front axles which are especially prone to the phenomenon. As your reference above pointed out, excessive caster can provide more energy to drive a caster wobble - that's why many trucks are more prone to caster wobble when towing or carrying a heavy load - when the rear of the truck squats, the result is more caster cranked into the front suspension, and more caster can promote caster wobble. That also explains why inflating the airbags (thus raising the rear of the truck) when towing/carrying a heavy load can help to reduce caster wobble - raising the rear reduces the caster in the front suspension.

Rusty

 

[CBari]

When I personally checked the front end I had someone in the seat moving the wheel and also jacked up the front end, However I didn't use the block technique you mentioned. I will personally check again though. The tires are Michliens and 75psi all the way around. I feel like there must be something loose as I think I can almost feel something in the steering when the bags are empty, but it is gone when I add the air. Which in my mind says ball joints if it is truly a caster issue causing the DW.