Project update Feb 2010

[Bajaroad]

Brent - Welcome back. Congratulations on your completed project (are they ever really "completed"?)
Your White Rim Trail video was the "last straw" that pushed me over the edge...so I started my own FG build... So thanks.Peter

I stole your independent of the pivot, linked airbags idea...thank you again...

Thanks Peter. The last year or so I've been too busy and other things to keep up with Forum. I just checked out some pictures of your build - very cool. More than ever I really appreciate the work and creativity it took to build your camper.

We'll have to get the West Coast Band of FG-ers together for a beer in the desert or one of Joaquin Suave's gigs.
Brent

 

[DontPanic42]

The ones I used are rated at 1330lbs and you can buy them at McMaster Carr for $10. I have three at the rear pivot and two on each side in the front. I used custom plates to hold and capture the mounts, which adds a lot side load capacity. I hope they reduce the shock/vibe stress to both the subframe and the truck frame.

Interesting setup. May consider it if I decide to replace my airbags. Do you have the catalogue number for McMaster-Carr?

 

[justduck]

If I had to do it over, I'd probably put the pivot in the front.

I'm curious about why you feel the pivot would be better in the front. It seems like the rear of the frame twists more than the front which would cause the camper to move more than if the pivot was in the rear. I haven't driven one, so this is just a guess on my part.

Thanks, Earl

 

[Bajaroad]

Re:

I'm curious about why you feel the pivot would be better in the front. It seems like the rear of the frame twists more than the front which would cause the camper to move more than if the pivot was in the rear. I haven't driven one, so this is just a guess on my part.

Thanks, Earl

Earl,
I'm not sure where the frame twists the most.
The first reason I put my fixed points in the front is I wanted to minimize the mismatch between the cab and the camper because I was going to make a permanent pass-thru. I will still have a pass-thru but through two independent windows, allowing me to easily use the truck without the camper. The second reason is it seemed easier from a design standpoint to put the pivot in the rear.

I think an advantage of placing the fixed points in the rear is less body roll. Basically with the engine, cab, and camper all anchored near the front, the front suspension must carry a larger load in resisting body roll because the frame can twist between where that weight is mounted and the rear suspension. The rear suspension carries more weight but the pivot can't transfer a torsional load to the truck frame going around a curve. I can confirm the affect (twist) if I rock my truck/camper back and forth on level ground.

My suspension is completely stock. I think an ideal system on the Fuso would engage a front sway bar or lock the rear pivot while on the road. I'm going to experiment with my pivot air springs. Currently the pressure is equalized between them so they always apply the same upward force, but I am going to pressurize them independently which will resist the pivot movement.

If I was going to start from scratch with a 3pt system I would try to place the pivot as close to the cab as possible and the fixed points over the rear axle.

 

[Jacob7963]

Great camper Brent! Really impressed by how everything turned out.

Note that the mounting you describe is similar to the Unimog 4 point (diamond) mounting system - the fixed mount is right above the rear axle, and there is a forward pivot near the cab and a rear pivot near the end of the frame. That way the torsional forces are almost directly supported by the rear axle, rather than twisting the frame.

 

[justduck]

Brent, thanks for the reply. Can you post pics of your airbag setup and mounts?

Earl

 

[gait]

I think an advantage of placing the fixed points in the rear is less body roll. Basically with the engine, cab, and camper all anchored near the front, the front suspension must carry a larger load in resisting body roll because the frame can twist between where that weight is mounted and the rear suspension. The rear suspension carries more weight but the pivot can't transfer a torsional load to the truck frame going around a curve. I can confirm the affect (twist) if I rock my truck/camper back and forth on level ground.

I'm having a little trouble visualising that. I suspect that using the road as the starting point if a single rear wheel is raised then the most chassis movement will occur at the rearmost point of the chassis on the chassis rail on the same side as the wheel that's raised. That's the static situation just considering chassis twist. It just gets more complicated from there as the dynamics of twist, roll and flex, and the relative stiffness of suspension, chassis and body, along with mass, are considered.

 

[Bajaroad]

I'm having a little trouble visualising that. I suspect that using the road as the starting point if a single rear wheel is raised then the most chassis movement will occur at the rearmost point of the chassis on the chassis rail on the same side as the wheel that's raised. That's the static situation just considering chassis twist. It just gets more complicated from there as the dynamics of twist, roll and flex, and the relative stiffness of suspension, chassis and body, along with mass, are considered.

I totally agree vehicle dynamics are complicated and lots of things are going with the vehicle while going around a curve. I'm pointing out that the frame can flex while all four wheels are on level ground going around a curve and this might be reduced by rigidly attaching the camper over the rear axle w/ pivot in front - might be a better option.

In my case both the truck/camper are rigidly mounted at the front of the frame, camper pivot in the rear. I'm suggesting the front suspension (less stiff) is burden more in this setup with resisting body roll.

My experiment: with the truck/camper sitting on level ground on the street and the rig rocking side-to-side (manually pushed) the front suspension compresses more, the rear suspension compresses less than the front, and the difference is frame flex - and this is an approximation of what happens during in a curve.
If you look at the rear of my rig there is a clear measure of frame flex - the angle between my bumper and the aluminum understorage (the bumper is attached to the frame at the rear of the truck frame but the aluminum is attached to the sub-frame which is attached to the front of the truck frame). An angle between these is frame flex.

My next experiement is locking my pivot. I believe this will engage the rear suspension more to help resist body roll more.

 

[DzlToy]

I was advised by a truck body builder a few years ago that small trucks (under 150-160" wheelbase and 10-15k GVW) should be built to resist frame flex as much as possible and let the suspension do the work.

You dont want a frame/chassis that weighs 2000 pounds but its much easier IMO to let links and springs and shocks and bushings do what they are supposed to do than to take a piece of box or channel steel and make it do that which it does not want to do (twist, flex, bend, etc)

I would not advocate a completely rigid mounting of a body on an FG, but you dont want your box flopping around and bending all over the place or causing the frame to bend or twist more due to those movements either. This is especially prominent at the ends of the frame rails, where the most flex will occur due to its second moment of inertia.

 

[ntsqd]

Spotted: Bajaroad on the road

:sombrero:
On this evening's drive home.

Thanks again for the tour and the demo ride!

 

[ntsqd]

I've been thinking on the rear vs. front cross-chassis mounts. I think that by placing the cross-chassis mounts at the rear and putting the single mount at the front that then the weight of the camper assembly more directly works the rear suspension and applies less twisting to the frame rails. All of the torsion applied by the box then occurs very near the rear suspension. As Bajaroad suggested to me a couple Sundays ago, if those box mounts are placed between the mounting points for the rear suspension then the total length of frame under box-induced torsion is the distance between the rear suspension mounts.

In this case the front mount can't apply any torsion to the frame rails, so all torsion possible in the frame forward of the front mount of the rear suspension is solely due to the engine, cab, and front suspension loads. This would be true up to the point where the frame torsion of a raw truck* would cause a rear tire to lift. It is possible that the weight of the box applied to the rear suspension will 'anchor' it and allow the engine/cab/front suspension mass to put more torsion in the chassis, but purely box-induced torsion will always be confined to only spanning to the nearest suspension mounting points.

*"Raw truck" meaning one that had nothing attached behind the cab, an as-delivered cab & chassis.

 

[MotoDave]

Hah, I was asking Thom about your truck the other day, and here is the thread! I walk my dog past your truck almost every day, I'll stop by for a tour some day. I'm over on Emma Ave.

 

[Bajaroad]

Pic in Mammoth

Hah, I was asking Thom about your truck the other day, and here is the thread! I walk my dog past your truck almost every day, I'll stop by for a tour some day. I'm over on Emma Ave.

Hey that's great - stop by anytime. It's still a work in progress, but at least it's at a point I can enjoy it.

Went to Mammoth last weekend. We stayed in the RV park there for the first time (a bit expensive), but we explored offroad out near the hot springs area in the valley. The snow is really wet, deep, and difficult to plow through. Our destination was one called "crab cooker". We got to the end of the tracks where several trucks had got stuck. We got further than anyone else, but didn't want to push our luck so we planned to walk the rest of the distance. As we were getting ready for the hike some guys in a 4x4 PU followed our tracks to the bottom of a slope where we had given up and backed up. They immediately got stuck when they tried to back up. Tire chains, two shoves, tow strap, and two hours, we got him unstuck.

 

[Overland Hadley]

Hi Brent,

Good to see the progress on your FG!