Death Wobble - Possible Resolution - Page 4

BajaSportsmobile · 04-10-2015, 10:25 AM

You are not alone - everyone does it, even injuneers.

I totally agree. This is what that engineer Turbostew said in the Quadvan Death Wobble thread:

"I don't believe you will ever get rid of the DW without changing some mass some where or dampener some where on the 05-0? Superduty setups. Lowering caster to 0 will do it but it will drive like $#&@. Sometimes changing backspacing on the wheels will change the "mass" adding more rational inertia about the kinkpin axis. Tires might do it. Vehicles with more angle from horizontal on the track bar or more bump steer (like the 05+ Fords) seem to be more prone to it. Just add more dampening and be done with it. Thats the way it works, you think you got it fixed, then at 2:00 AM on some road in BFE while you half asleep and there is a 500 ft drop-off the DW will hit you, screw that, if I learned anything in 15 years of engineering, it is dampening in mechanical systems is good thing!"

shenrie · 04-10-2015, 10:31 AM

^^^ lol, well played ramsey

Quote:

Originally Posted by E350

I am embarrassed for sure, but happy to learn now (albiet after looking stupid in a number of long posts I made on this issue on various threads over the years... So, uh, thanks!).

ha! man, dont even sweat it. ive had that explained to me in the past and still make the mistake...often actually.

good info going on in this thread...keep it up gents!

mgmetalworks · 04-10-2015, 10:51 AM

I don't have much to add that hasn't already been covered but I thought that since I have the CAD models of the '05-up Super Duty axle, I'd give y'all some reference from the source...

As seen in the side view, the radius of the arc that the axle swings through is 38.64".

With the Radius arm sitting parallel to the ground, the steering axis is 4.5 deg from vertical.

The radius arms are parallel to each other and they are 35.11" apart.

The front holes of the radius arm are the same distance above and below the rear hole.

sidetrak06 · 04-10-2015, 11:19 AM

Quote:

Originally Posted by mgmetalworks

I don't have much to add that hasn't already been covered but I thought that since I have the CAD models of the '05-up Super Duty axle, I'd give y'all some reference from the source...

As seen in the side view, the radius of the arc that the axle swings through is 38.64".

With the Radius arm sitting parallel to the ground, the steering axis is 4.5 deg from vertical.

The radius arms are parallel to each other and they are 35.11" apart.

The front holes of the radius arm are the same distance above and below the rear hole.

Thanks this is great info. Where is the 35.11" distance measured, center-center?

mgmetalworks · 04-10-2015, 11:24 AM

Quote:

Originally Posted by sidetrak06

Thanks this is great info. Where is the 35.11" distance measured, center-center?

Yes, center to center.

bcaine · 04-10-2015, 01:52 PM

Quote:

Originally Posted by BajaSportsmobile

Look to gyroscopic inertia around an ever changing spin axis for an understanding of DW's - that is why DW's are rotational speed sensitive (speeding up or slowing down the vehicle will stop DW's) and steering angle sensitive (turning will stop DW's).

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.

E350 · 04-10-2015, 02:01 PM

Well Duh!

(Ok, I thought about posting just that wise-ass remark in response to your post which is way over my head.)

But I guess you are saying that instead of air filled rubber sidewalls pushing against each other through the tie rod as I surmised, it is two spinning gyroscopes pushing against each other through the tie rod?

Hmmm... I actually think I remember that if I push a spinning top sideways it immediately pushes back, thereby righting itself.

Your theory would explain my experience that DW does not exhibit itself until at least 40 mph.

sidetrak06 · 04-10-2015, 03:50 PM

Well I just picked up the van and it is now maxed out at 5.1 degrees of caster. It handles better than it ever has and tracks very well. These 1.2 degrees made an enormous difference in driveability, which honestly surprises me.

The moment we've all been waiting for...yes I can still get it to go into a wobble. But now it has to be 40 mph or higher and it eventually damps itself out and stops wobbling without me coming to a stop. As I promised I took a few more videos with the new caster...enjoy! I recommend watching them in 720p so you can really see the movement.

Video 4:

https://www.youtube.com/watch?v=f0ZC...ature=youtu.be

Video 5:

https://www.youtube.com/watch?v=SbAW...ature=youtu.be

Video 6:

https://www.youtube.com/watch?v=Xi8h...ature=youtu.be

Video 7:

https://www.youtube.com/watch?v=IEd2...ature=youtu.be

Video 8:

https://www.youtube.com/watch?v=l-YQ...ature=youtu.be

Video 9:

https://www.youtube.com/watch?v=6KgY...ature=youtu.be

To get the rear video angle I had to use quite a few GoPro camera adapters in series which causes it to shake from the DW as well, sorry about that. The last Video #9 shows movement of the radius arms forward and backward relative to the frame. This just might be the culprit!

Thoughts anyone?

E350 · 04-10-2015, 04:11 PM

Dude your videos rock! You are definitely earning the participants' comments in your thread. I hope more knowledgeable people respond. Until then here are my thoughts:

Radius arms. Now you know why the SuperDuties originally had leaf spring suspensions and it took Ford quite a few years to suspend them with coils. And why I believe Ujoint still uses a leaf spring suspension.

It takes an incredible balance of multiple components to handle the mass of a 4x4 front axle to make a radius armed coil suspension not DW.

But even a leaf spring suspension can DW, and IMHO you would still have DW even if the radius arms were as solid as a leaf spring can be.

UJOINT · 04-10-2015, 07:22 PM

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.