Death Wobble - Possible Resolution - Page 7

sidetrak06 · 04-13-2015, 10:02 AM

Quote:

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.

E350, after looking under the van today I realized just the front of the frame is boxed and the everything rear of the radius arm is C-channel. Sorry, I thought the whole thing was boxed!

mgmetalworks · 04-13-2015, 10:43 AM

Here are some screen shots of the axle/radius arms/frame in relation to each other. I've placed the axle in approximately the same location as my Quadvan has it in right now. You can see that the radius arms are inboard slightly.

sidetrak06 · 04-13-2015, 11:09 AM

Quote:

Originally Posted by mgmetalworks

Here are some screen shots of the axle/radius arms/frame in relation to each other. I've placed the axle in approximately the same location as my Quadvan has it in right now. You can see that the radius arms are inboard slightly.

Nice models, did you download those parts or create them from scratch? What do you plan on using that for? I would love to have a solidworks model of my van, but I can't justify the time to do it myself. I particularly want a model of the interior so I can start planning mine out.

If my arm angles end up being an issue I will have to modify the rear brackets to sit them slightly inside the frame rails as your model shows.

mgmetalworks · 04-13-2015, 12:13 PM

Quote:

Originally Posted by sidetrak06

Nice models, did you download those parts or create them from scratch? What do you plan on using that for? I would love to have a solidworks model of my van, but I can't justify the time to do it myself. I particularly want a model of the interior so I can start planning mine out.

If my arm angles end up being an issue I will have to modify the rear brackets to sit them slightly inside the frame rails as your model shows.

I purchased the models through a design house with close ties to Ford. There are probably easier ways to obtain these...the easiest is becoming a member of SEMA then you have access to the tech transfer program with models of all sorts of vehicle parts.

I have lots of plans for these models. The Cummins engine mounts and trans cross member are first up on the project list.

carringb · 04-13-2015, 02:46 PM

Quote:

Originally Posted by sidetrak06

I also checked the geometry of the radius arms and they are NOT parallel. Center-center in the rear measured about 39.5" while up near the axle it was closer to 35.25", which means they narrow up by about 4.25".
...
I am not sure if/how this has an effect on my DW but it is something I never realized before. Can anyone chime in on this? I would imagine that the urethane bushings allow for the misalignment. I can't see how this alone would cause the axle to move forwards/backwards vs a perfectly parallel setup.

This is exactly what I expected you to find, based on the videos showing the fore-aft oscillation. Basically, your suspension is a trapezoid. Any lateral motion on the front axle axle, due to the arc motion created by the drag link, causes the ends of the axle top move fore and aft in opposite directions. That induces a steering moment, which pulls the axle to other extreme, inducing an opposite steering moment. The up and down motion is just a side affect from the axle pushing and pulling on the drag link and steering.

If might make more sense if you built a lego trapezoid.... In fact, if you added wheels to your lego trapezoid, you'll find you can steer without even having steer knuckles installed.

Anyways, that's why QuadVan builds their own off-set frame bracket. Those radius arms MUST be parallel when you use radius arms on a solid axle. The stock Ford bracket don't work for this application due to the wider frames on the vans.

You might be able to reduce the affects by lowering the drag link to it's arc doesn't move the axle as much laterally. However this will then introduce bump-steeer, unless you also use a longer pitman arm.

REF · 04-13-2015, 03:43 PM

Quote:

Originally Posted by carringb

Definitely some bad geometry there. Since the front of those radius arms mount to the stock axle mounts, the trailing end should be mounted a little inboard of the frame rails. Instead, they just built some off-set into those arms.

I think to really fix your problem, you will need to replace the current radius arms with OEM Ford F-series radius arms (the dog bone style, i.e. 2005_+) and some E-series specific brackets. Colorado Camper Van is the only outfit I'm aware of that sells them, since QuadVan will only install parts, not sell their parts ala cart.

out of curiosity, would it be possible to replace the radius arms with torque arms, like the adj. ones from CCV? would that set up be better?

sidetrak06 · 04-13-2015, 03:53 PM

Quote:

Originally Posted by carringb

This is exactly what I expected you to find, based on the videos showing the fore-aft oscillation. Basically, your suspension is a trapezoid. Any lateral motion on the front axle axle, due to the arc motion created by the drag link, causes the ends of the axle top move fore and aft in opposite directions. That induces a steering moment, which pulls the axle to other extreme, inducing an opposite steering moment. The up and down motion is just a side affect from the axle pushing and pulling on the drag link and steering.

If might make more sense if you built a lego trapezoid.... In fact, if you added wheels to your lego trapezoid, you'll find you can steer without even having steer knuckles installed.

Anyways, that's why QuadVan builds their own off-set frame bracket. Those radius arms MUST be parallel when you use radius arms on a solid axle. The stock Ford bracket don't work for this application due to the wider frames on the vans.

You might be able to reduce the affects by lowering the drag link to it's arc doesn't move the axle as much laterally. However this will then introduce bump-steeer, unless you also use a longer pitman arm.

I am still struggling to see this one, sorry. Looking at the system over head:

Regardless of the angle of the radius arm, you still have the effective radius (R) that the axle can rotate about. This will be slightly shorter than the length of the radius arm due to the angle relative to the frame. I don't see how this translates into for/aft movement of the axle? Do you mind drawing this for me?

mgmetalworks · 04-13-2015, 04:48 PM

I know this is crude but I only had a few minutes... This might help with visualization.

The radius arms in your set up are moving along paths that want to move closer to or away from each other as they swing. Even though the distances aren't a lot, you still have to take up that distance somewhere. The bushings perhaps...maybe some deflection of the radius arms... You're not going to move the mounting points on the axle, that's for sure. I don't think you're moving the frame side mounts either. Like I said, we're not talking about a lot of change but it is probably enough to load up and unload bushings.

rallypanam · 04-13-2015, 05:03 PM

Sorry for the interruption to a great discussion... But this just showed up on my faceblog feed and I immediately thought of his thread:

sidetrak06 · 04-13-2015, 06:00 PM

Quote:

Originally Posted by mgmetalworks

I know this is crude but I only had a few minutes... This might help with visualization.

The radius arms in your set up are moving along paths that want to move closer to or away from each other as they swing. Even though the distances aren't a lot, you still have to take up that distance somewhere. The bushings perhaps...maybe some deflection of the radius arms... You're not going to move the mounting points on the axle, that's for sure. I don't think you're moving the frame side mounts either. Like I said, we're not talking about a lot of change but it is probably enough to load up and unload bushings.

This makes perfect sense that it would want to follow those angular paths if the axis of rotation were fixed. Given that there are bushings at either end, the bushings will accept the angle ? by distortion or movement in the heim joint at the frame end of the radius arm. This will effectively give a radius of rotation R for the system.

Sorry for the elementary analogy, but if you think about this like a swing on a playground:

The swing has no issues with an angle in its' support (radius arm) as long as the angle ? can occur at the 2 connection points. If we were to replace the eyelets on the swing with a bearing that has only 1 axis of rotation, the swing would bind up and not be able to move.

My question is, how would any of these scenarios result in movement of the axle fore and aft? In what case would the kid on the swing have his right butt cheek get closer or further to the ground when he is not even swinging...