Mopar_Mudder

You here all kinds of things but personally first thing I would do it have them replace the steering stabalizer, or get a better aftermarket one. I have read of a few or them being bad, and it easy to put a new one on.

Oh ya you really haven't experienced death wobble until you have done it in a truck with 8" of lift and 40" Boggers on it when the sterring stabilizer breaks needles to say that truck has dual stabilizers on it now.

1sttruck

Look at the book on tire pressure that comes with the truck, as 45 psi at 75mph seems low. I think that you're supposed to add 10 psi if doing over 65 mph.

stutz

JamesP,

I have 25,000 miles on the truck. The tires still have better than half the tread left. I was thinking about changing to a set of Hummer take-offs but not now until I get this problem fixed.

GregBoardman,
I will give KORE a call and see what they suggest.

As for those folks who said I was going too fast - That is the speed the traffic moves through that area and I didn't buy this truck to cower in the slow lane. If I wanted to do that I would have bought one of the other brand of diesel trucks.

Thanks to everybody that provided useful dialog.

PS: I was not aware of how many English teachers we have on this site. I promise I will try harder next time.

Stutz

1sttruck

'Too fast' will probably be limited by the tires as most truck tires aren't rated for speeds over 85mph as I recall. This is provided they're at the right air pressure.

blownaway

Why are you guys running such low tire pressure? run them at max.

Sam Strano

First, you have to make sure the tires are up to proper inflation levels. 45 psi is too low for an LT tire. Second the truck has pretty stiff springs and solid axles at both ends. These things individually are not great for bumps, and together make things worse.

I do not think that swaybars will help you in this situation, and over any bumps that wheels hit individually, can make the truck want to skip worse since the bars transmit more reaction to the other wheel. Anyway, the dip causing the problem indicates a damping problem (shocks). When the suspension compressed with a cornering load, she had a larger amount of energy stored in the springs than most situations. Shock absorbers (or more correctly dampers) have the task of damping out the energy that is released by the spring when it extends, so you don't get oscillation or "wobble". And make no mistake, this oscillation if bad enough will greatly effect the tracking of the vehicle.

Now, I'm not saying you stock shocks are worn out. I'm saying that it's typical for OEM shocks (and the Dodge shocks are WAY better than say GM's shocks) to be somewhat lacking in certain areas. A good point is that you never experienced it. Your situation took a number of varibles to happen, and very welll might not have been experienced by engineers when testing. Also (and more likely knowing a number of Big 3 engineers), the bean counters were the problem. Good shocks are much more expensive (relatively) than not-so-good shocks. Add the cost to the fact the engineers could have missed something and I think you can see there is a lot of room for the shocks to be improved. Think about this, we're talking about trucks that don't come with a block heater cord anymore, whic is much less than 4 shocks.

Shocks are one of the most critical parts of any suspension. They are the one part of the setup that is always in use. When you hit a bump, dip or the axle otherwise moves up, the compression damping of the shock controls the speed of the upsrung weight, which is anything not supported by the springs, like the axles, brakes, wheels and tires. Which incidentially is why when folks put bigger wheels and tires on the trucks with OEM shocks you see much more of the wobble, the shocks aren't valved to deal with it. I digress...... The rebound function of the shock is in play anytime the body and axle/wheels want to more apart, like coming out of a bump or dip. But also, the rebound is in play with just plain old body roll as well (since the inside shocks have to extend...).

Anyway, sorry for the lesson. In short, I think you have two problems. The lack of tire pressure which is letting the sidewalls of the tire flex too much, and you could use better shock damping for the reasons stated above.

And in case you are wondering (since I'm newer here), I'm no stranger to this stuff. Aside from having my parts business, I have 3 SCCA National Championships too, which is why I have some insight on the shocks. Suspension is an everyday business with me.

Mark Thomas

WHat he said!

stutz

Thanks for all the info Sam.

GregBoardman

I really believe that tire pressure or the speed you are going is not an issue. Death Wobble is generated by caster, sidewall strength, not weight rating, and loose front end parts. Shocks on a stock truck cannot even control the bounce of a speed bump or an expansion joint. I know that DW does not occure under 70 mph. That does not mean drive your truck at a slower speed to accomodate the limitations the dealer cannot find. I run 25 PSI at all times. I also run an E rated tire that is 37"s tall. I drive at speeds that are fast most of the time and can go from on road to off road at the flick of the wheel at any speed. My DW was the worst with BFD 315 75 17 D Hummer take offs. Even with the 2.5" Fox racing shocks and stupid race valving, with the crummy BFG tire, I had DW. I changed to the 37" GY MT/R's and it cured all problems. Now what does this have to do with you? The information you read here is good, current, and opinionated. Some of us have different theories on how DW occurs. Some of us have expierenced it. Some haven't. I went through a new truck completly to cure DW. The only cure was a better tire. Air pressure was no difference, traveling at slower speeds didn't matter, Track bar, tie rods, steering stabilizer, ball joints, shocks, springs, control arms,and a steering box didn't help. Same issue, all parts replaced by Dealer. The cure is the caster, and a better tire.

JMHO
Greg

RustyJC

Try a Google search on "caster wobble" if you want a few hours of interesting reading. Solid axle suspensions are especially prone to caster wobble - that's one of the reasons the car manufacturers finally went to IFS in the 1930s and (in Ford's case) the late 1940s.

4x2 trucks such as mine with IFS generally don't have the Death Wobble problem.

Rusty

Sam Strano

It'll be like oil I guess. Everyone has an opinion.

From my perspective it all goes back to the dampers. In Greg's case, shocks didn't cure it and changing tires did. Here is why: The "race" shocks are likely to be very soft in compression damping, being off road units. That's not what you want, because it's not damping the unsprung weight well. So I should clarify that excellent shocks not suitable for the specific use can be as poor as bad shocks. The change of tires helped. Well, tires have different constructions, and those constructions alter the spring rate of the tire (as does air pressure). It's again very likely that the GoodYear's different construction allow the tire so suck up bumps and ripples that the BFG's don't and get transmitted to the axle, where the shock then have to try and control the movement.

This is why suspensions are complicated, EVERYTHING works together, and has to do so properly.

If you think about what's changed from the 40's and 50's to now, it's mostly shock technology. The dampers then had no prayer of controlling unsprung weight effectively, as we can now. And independent suspensions don't have a wobble problem because they don't have near the unsprung weight to begin with, and of course what one wheel does doesn't effect the other. Where on a solid axle, you have a ton of upsrung weight, and any bump you hit with say the RF will make the whole axle quiver (including the LF) and need controlled by dampers.

That's my take on it. Again, YMMV and not trying to argue, but I think it's pretty apparent and is very similar to axle hop you can get with a solid axle out back, which shocks can greatly help.

GregBoardman

Well,
For the sake of not arguing,
I just want to clear up that the valving in the shocks on my truck has to be very stiff to control the front axle at speed off road. The compression is not as stiff as the rebound but still extremely agressive. The bleed in a shock is a whole different story and that is what will let a DW condition seem to appear. Bleed is just that, the amount of hydrolic fluid that can get past the valve stack and not affect the dampening. It is how a shock has a floating feeling off road. Then when you hit the larger stuff, the valvestack comes into play. In the Kore trucks, we can hit 12 inch whoops at 60 mph and not feel it at all. Now go ten mph over the same bump and it almost feels stock. The valving controls the speed the shaft is allowed to travel per second. The bleed only helps with slow speed. With a dw at 75-80 mph, in my expierence, is caused by a bump as small as 1". Maybe the shocks on my truck don't even see that bump. The energy is distributed throughout the sidewall and carcas of the tire. Being that the BFG has less of a sidewall, could it be possible that it didn't absorb the hit the same way? Therein causing a DW? I am still unsure about that. I know that installing a 2.5" spacer or spring is not the cause of DW. Otherwise every leveled truck would have DW.

Greg

1sttruck

My experience is that 'speed wobble', or more dramatically 'DEATH WOBBLE', is caused by an undampened 'excited' wobble frequency. Hitting a bump a speed is the initiator, and for what is sometimes one reason, more often multiple reasons, the wobble frequency is not adequately dampened and the only way to to get it to stop in many cases is to get the speed down so that the frequency changes. With my stock truck running at what appears to be higher pressures than most, when I hit a bump at speed, especially on a turn, the truck pitches a bit and then gets back on track, a characteristic of a solid axle. With any assembly you have a 'tolerance stack', and the tolerances that we're probably most interested in on the front suspension are play, dampening, and steering geometry attributes like caster, camber, and such.

So far most of the reports about DW seem to involve people running larger tires, Hummer takeoffs are typical, some are lifted, and many seem to run lower tire pressure. Some reports are from stock trucks, most seem to on trucks with a few miles, and so far most seem to involve defective steering stabilizers or low tire presure. It seems reasonable that a defective steering stabilizer would cause problems, and since the large BFGs seem to be a common contributor it'd be interesting to know if people with stock setups that have DW are running the OEM BFGs as compared to the Michelins. Tire construction and pressure are a couple of other parts of the tolerance stack, where I'll guess the stock Michelins at the correct pressure for the load and speed will produce fewer problems than the BFGs, but it's just a guess. In any case it seems that there isn't much margin available in preventing DW, so the design is implicated. We get good reviews for how easy the truck handles, so the designers may have eaten into the margin for DW.

Suspension tweakers will be able to provide better suggestions, but I'd start with reducing unsprung mass (alloy rims, stock tires), making sure that the steering damper is in good shape, of good quality, and perhaps adding another one, making sure that the shocks are in good shape and of good quality, running the correct tire pressure for load and speed, and making sure that there is minimal play in the steering.

Comstock

I personally haven't experienced any wobble-yet. After reading all the posts in this thread I recall taking delivery of my truck 3 months ago and how specific the salesman was on tire pressures and not exceeding 75 mph. I have the Michelins by the way.
It would seem Dodge is very aware of the problem but cover themselves by suggesting ridiculous recommendations- who doesn't exceed 75 mph every now and then or everyday on the freeway for that matter.
The printout in addition to the manual delivered to me by the dealer recommends 50 psi front and 40 psi rear light load and 60 psi front and 70 psi rear heavy load cold inflation pressure. For sustained speeds above 65 mph they recommend an additional 10 psi.
In any case I will be checking my tire pressures more diligently now.

1sttruck

http://www.naxja.org/forum/archive/i...hp/t-9164.html

I agree that when you reduce castor you will feel more of the out of balance shimmy condition, but it will not grow into DW----- the link has been broken. I have adjustable JKS CAs and have experimented with the Castor adjustment. At 6 to 7 degrees I would have terrible DW. Yes my tires are old and out of balance but a lot of trucks and cars would be tolerant of tires in this condition. When I go to 3 degrees, no more DW. I can feel the shimmy at 55 and drive there all day and it never goes into DW. I think true death wobble is more prevalent in those XJs that are using stock length CAs and are lifted in the 3.5 inch range. At this lift, most don’t pay much attention to their pinion angle and the castor will be on the high side of stock settings. As we lift higher we start rotating the axle to align the pinion with the drive shaft and the castor is reduced.

My next test is to increase scrub radius. The one- inch wheel spacers are in the mail. I will install the spacers with the exiting rims and old tires to see if I can feel a stability increase. My theory is that at stock back space, the tire patch is close to the center of the steering axis. Because the steering axis is close to the CG of the tire, it can more freely oscillate. Decreasing back space (increasing positive scrub) will move the tire out and away form the steering axis and make it more difficult to shimmy. The added scrub will also add some damping---after this test I will install the new 32 BFG ATs the real reason for the alloy spacers.


http://www.millikenresearch.com/olleyfn.html

As the owner of the LM 14-40 once owned by Brian Frank, I was particularly interested in wheel wobble. In correspondence with "Flutenews" about twelve years ago, Mr Frank suggested reducing the camber angle (!) and increasing the caster angle, (perhaps the intuitive thing to do, to increase the self-centring), to cure wheel wobble, but finished up fitting a steering damper between the track-rod and the axle.

In the book Olley states that, on the Chevrolet, increasing the caster increased the violence of the wobble, but had little effect on the frequency, and that caster wobble can be damped out by decreasing the caster or by a small increase in king-pin friction - friction applied elsewhere in the steering linkage is less effective and more liable to spoil the handling. On my car, the damper certainly made the steering feel dead, but when I removed it, the first bump in the road hit at about 15mph set off a violent wobble. I then fitted some wedges, tapering from 3/16ins to 1/16ins over a 4ins length, between the springs and the axle, so as to reduce the caster angle (as described on page 29a of Supplement No 2 to the 14-40 HP Vauxhall Chassis Shop Manual, kindly sent to me by fellow VOC member and 14-40 owner Alan Livingstone).