Death Wobble - Possible Resolution

[sidetrak06]

Originally Posted by UJOINT

Glad that more + feels better. It may have been brought up but are you still running the ball joint end stock style trac bar end at the axle?

Yes, DW can happen on anything but it's extremely rare on a leaf sprung suspension with x-over steering and a trac bar. I've only come across it once, big lift with no trac bar at all.

Good question, I know the tracking bar is custom so that it is nearly parallel with the drag link to minimize bump steer. I will ask Boulder Offroad about the track bar end at the axle.

[UJOINT]

Originally Posted by sidetrak06

Good question, I know the tracking bar is custom so that it is nearly parallel with the drag link to minimize bump steer. I will ask Boulder Offroad about the track bar end at the axle.

Just looked at your pics (sorry, should have before asking) but your rig doesn't have the factory style trac bar end.

[E350]

sidetrak06: Would you mind hooking up your Gopro back up focused on the frame where one of the radius arms is attached and take a sharp turn and load up the suspension on the outside of the turn? (Be safe, don't roll the van!) Those radius arms are a big lever acting on that thin "C" channel frame and I would like to see if and how the frame flexes when loaded up on a turn. Yes, I understand that the track bar is keeping the axle centered, but I wonder if there is torque supplied by the radius to the frame nonetheless. If you don't have the time or interest no worries.

[sidetrak06]

Originally Posted by E350

sidetrak06: Would you mind hooking up your Gopro back up focused on the frame where one of the radius arms is attached and take a sharp turn and load up the suspension on the outside of the turn? (Be safe, don't roll the van!) Those radius arms are a big lever acting on that thin "C" channel frame and I would like to see if and how the frame flexes when loaded up on a turn. Yes, I understand that the track bar is keeping the axle centered, but I wonder if there is torque supplied by the radius to the frame nonetheless. If you don't have the time or interest no worries.

This is an E350 which has a fully boxed frame. I highly doubt there is flexing but I can probably rig something up. I think i am out of adhesive GoPro mounts so I will have to order some more first.

[mgmetalworks]

Originally Posted by sidetrak06

This is an E350 which has a fully boxed frame. I highly doubt there is flexing but I can probably rig something up. I think i am out of adhesive GoPro mounts so I will have to order some more first.

I'd expect those radius arms to flex a bunch before the frame would. From the pictures they don't look like they have any internal webbing...or at least not much.

[sidetrak06]

What do you mean by internal webbing? If you look at the arms and the frame and consider the junction of these items is a urethane bushing...the movement is not happening in the steel structures.

[E350]

Got it. Sorry for getting off topic.

[BajaSportsmobile]

Pure AWESOME!

Originally Posted by bcaine

AMAZING DISCUSSION... leave it to the internet to find other people who may actually find this kind of thing interesting.

I am an absolute ignoramus when it comes to suspension design and am working to understand these posts to their fullest. Is gyroscopic inertia the same thing as angular momentum?

From the context it seems that may be the case...

If so, am i correct to summarize that:
Since angular momentum is the product of moment of inertia and angular velocity. If we treat a wheel's moment of inertia as that of a disk (1/2mr^2) putting larger tires on substantially (r^2 and >m) increases the moment of inertia. Speed sensitivity is due to the effect of angular velocity (wheel speed) on angular momentum (gyroscopic inertia?). Which I'm not sure how fits into my developing understanding.


Finally if the above is true is there any accuracy to the idea that DW originates when:
1) one tire is jarred,
2) and through the tie rods this jarring is transferred to the other tire.
3) Then (not by the spring like nature of the opposing tire's sidewall) but rather by the fact that the angular momentum of the other wheel will be conserved it will resist being pushed and
4) create a sort of Third Law Force back on the first wheel, which
5) has its own angular momentum to conserve, thus SHM ensues?

If so, does a damper (really wondering if i picked the right one) dissipate some energy from the system thus bringing a more immediate end to the SHM.

[BajaSportsmobile]

Who's radius arms are those and what bushings are in them?

[REF]

^^^^same Q, but I would guess that Boulder off-road either fabbed them or used an after market part and welded them in, looks like new poly bushings. For what it's worth it seems that most sources claim that the track bar/bad bushings along with worn ball joints as the culprit of DW. It looks like your drag link and radius arms are oscillating in the vids, but that may be caused by something else altogether. Did Don replace your track bar or any of its components? Might be a worn bushing on the axle side, anyway, I won't speculate any more, and it's good to see those with much more knowledge on the subject are posting up some good info here as well, but it seems carringB might be on to something with the geometry of the radius arms, I'd look more into that. I will add, you might want to look at dynatrac upper/lower HD ball joints, I preemptively replaced my OEM ball joints with those, while spendy, I believe they took any worry I had with when I would have a ball joint failure but also making sure I was taking care of any potential weak spots that may not be up to the added weight or stresses the rig placed on various components. Here's anther article on the subject, expand the captions under the photos for an explanation. I found this to be informative. There are also replacement HD track bars out there for this application.
http://www.fourwheeler.com/how-to/trans ... bble-cure/
Here's a ready-lift track bar