Broken frame

dhackney

Expedition Leader
FWIW any new frame needs more Section Modulus. Trying to do that through simply increased metal thickness is the least efficient method, though it will help with any pull-through or tear-out problems. Some thickening (mostly to reduce the stress riser that holes create) combined with an increase in the rail height will probably show itself to work the best.

I do not have the skills, abilities or experience to know how to create a new frame with increased Section Modulus.

Do you, or anyone else here, have the ability to advise me on how to accomplish this goal?

For instance, I have the following basic questions:
1) What type of steel should the new frame be created from, specifically?
2) What increase in rail (web) height will yield what increase in SM, e.g., for every 10mm there is X increase in SM?
3) Do I need to increase the width of the flanges along with the increase in web height?

Any knowledgeable input or advice would be greatly appreciated.

PM me at my email address on our web site if this needs to be a commercial transaction.
 

dhackney

Expedition Leader
Here's the stock section modulus of the 2007 Fuso FG Frame

http://www.hackneys.com/mitsu/docs/fusoframesectionmodulus.pdf

We extended the frame at the 'C' cross member.

We have one crack and one break in the section aft of that crossmember that is rated 72.3 (4.41). We had bending on both sides in the area above the rear axle line that is rated ~50.

There may be other issues that are currently unknown and we will only learn about once we get the truck stripped down to the frame.
 

kerry

Expedition Leader
Just throwing out some ideas here partly based on how my own service body is attached to the frame and partly on an article about garbage trucks I found when googling the question. It said garbage truck bodies are fixed to the frame in the rear but spring loaded at the front to allow for frame movement on uneven terrain.
If the very back section of the pivot frame were firmly attached to the truck frame, the original rear pivot point moved forward directly over the axle and the front pivot points left where they are, would this improve the situation? I think the pivot point above the axle would cut down on frame flex by reducing the distance between pivot points but would a fixed mount at the back of the frame (like a garbage truck?) still allow adequate frame flex?
 

mog

Kodiak Buckaroo
I do not have the skills, abilities or experience to know how to create a new frame with increased Section Modulus.

No expert here, but what about just duplicating the thickness and cross section from a Mitsubishi FM series. That way the 'factory' has already figured out the higher rating for you, 32,900 GVWR (FM) vs. 14,050 GVWR (FG).
 
The strength with respect to bending or section modulus of a frame follows the same rules as a leaf spring. However, they are expressed in different units. Section modulus is in-lb and is calculated like a moment of inertia. Spring constant is lbs/in. But a frame works like a leaf spring, and general rules like power laws are approximately correct.
Spring constant of a leaf spring is proportional to the 1st power of the width, 2nd power of the thickness and inverse 3rd power of the length. In other words, if you go up from 4.5mm to 8mm in metal thickness (the "width" of the frame as a spring operating up and down), the strength or resistance to bending increases 78%. If you make the frame 1.78 times taller it will get 3.17 times stiffer. If you make the frame 1.78 times longer (which maybe you did) it will get 5.64 times floppier. This does not take into account the effect of the horizontal flanges, and it incorrectly assumes the weight is at the ends like a leaf spring in a suspension rather than distributed along the length.
For more accurate information look in Machinery's Handbook. But it gives you an idea of the power laws.
Probably in real life they twist a bit in response to transient inputs.
Boxing a frame may be the most weight-effective strengthening measure, but creates problems as far as access to wiring and tubing and buildup of corrosion.
I'm no expert in metallurgy but it seems mfgs advertise high manganese steel for frame. Tempered to some extent after bending and drilling? There are outfits that make "custom" frames for certain vehicles like older Jeeps but having a true custom frame built that would match yours could be really really expensive.

Charlie
 
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Lynn

Expedition Leader
In sympathy, and desiring to help, but feeling pretty worthless to this situation, I have to offer up the only info I can come up with.

The only other talk I have heard regarding custom frames was in this post from this thread.
 

ntsqd

Heretic Car Camper
I think Mog's suggestion should be investigated as the first option.

As Charlie has pointed out, building a frame from scratch is not a venture to be undertaken lightly, nor will it be inexpensive. Experienced design and fabrication people should be sought out and retained for such a venture. I can make guesses, you need to know.
 

Robthebrit

Explorer
I have some notes somewhere regarding 3point vs 4point mounts from when Eddie built my camper mog.

A mog typically has a spider frame which 3 point mounts to the frame and gives a rigid platform which you can use for whatever. Initially my box was going to be put on this spider frame but the platform is about 8 inches higher than it needs to be. Eddie was a mechanical engineer and decided to mount camper shell directly to the frame to get the center of gravity that much lower, this is pretty much what unicat and those folks do but I don't know how they decide where to put their mounts.

He ultimately decided on a 4 point mount which is rigid in the center and pivoted at front and back. He did a load of tests figuring out where to put the pivots, weather to use 3 or 4 points and where to place the center bar. His reason for moving the pivots around was to find the place that had the least movement in the pivots so he never exceeded them and stressed the box. This is very specific to how a mog frame, specifically a long wheel base 416 and the info may not be applicable to anything else, regardless I'll dig it out and share it.

When emailing with him at the weekend I as how much thought he put into the reverse problem of the load stressing the frame/truck. He generally put no thought in to because he was under gross weight and the mogs GVW rating is for an absolute worst case scenario, something 20 degrees of frame twist, off camber, 45 degree hill, locked diffs with a single traction wheel. The only thing he did to the frame was to modify the front cross member as its a common place for a 416 frame to break due to the engine, the suspension mount and the power steering all being in the same area.

Eddie never had frame problems but he had a number of drive line problems. He bust two sets of portals before Mercedes realized what the problem was. It was related to the disk brake trucks having the same GVW as the drum brake trucks but the disk brake trucks have about a 5 inch spacer to clear the caliper and rotors. At gross weight in bad conditions this puts a huge amount of stress on the final drive bearing in the hub. The final solution was to use 1300L bearings, the 1300 uses the same hub casting as the 416 but they use 13mm roller bearings instead of the 9mm in the 416. The 1300 also has a higher gross weight than the 416. You put a bigger bearing in the same sized hole and you then need smaller portal gears and the 1300 gears are a different ratio to the 416 so he had to replace all 4 hubs. Fortunately Mercedes paid for it as long as he returned all the failed parts to them, that would have probably been a $15k repair. Since this repair Eddie did another 100,000 miles over many years and I have put 25000 miles on it and never had a problem.

I don't think huge mechanical failures are uncommon, even the mighty unimog can fail in some spectacular way that has never seen before.

One thing I would ask Mitsubishi is how they derive the gross weight of the frame? Is it based on loading, braking, twisting etc? Does it factor in 4wd situations or was the fuso 4wd more of an after thought, is the 4wd made for what its being used for or was it more intended for making deliveries in snow. Without knowing this Doug's truck could be way way over gross weight for the scenario.

I'll dig out and scan all the notes and experiments and post them up.

Rob
 
, this is pretty much what unicat and those folks do but I don't know how they decide where to put their mounts.

Rob

Unicat uses a transverse pivot in the front and rear and in the center a trunnion that allows fore/aft pitching with a cylindrical rubber bushing. It obviously can't pitch when the frame is in the rest position because the pivots only allow transverse motion. In other words, the frame can twist below the camper but the camper stays steady. These pivots/trunnion(s) are mounted directly to the main frame, there is no subframe.

Charlie
 

btggraphix

Observer
To echo a lot of other sentiment already expressed, I also really appreciate you continuing to teach us all the lessons you've been learning, and allow us to come along for the ride. We'd all love to teleport down there are share a beer (or shot of that tequilla?) with you.

I guess one thing I wondered about but didn't see any comments on the incident page of your blog anyway...was any comments about the removability of the camper. Being a truck camper guy, I've always been a fan on being able to remove the camper from the truck, in particular for repair work and to provide a living quarters seperate from the truck. Did you remove the camper to live during the work? Do you feel that gives a substantial advantage for preparedness? It certainly limits some of the choices for systems not having them connected at all times...

This is not to say that I haven't spent a few hours here and there listenting to Dylan's Blood on the Tracks... :)

I hear you there, that album got me through a tough time or two. Just don't start wandering down around the docks without your wife....

Good luck!
 

Robthebrit

Explorer
Unicat uses a transverse pivot in the front and rear and in the center a trunnion that allows fore/aft pitching with a cylindrical rubber bushing. It obviously can't pitch when the frame is in the rest position because the pivots only allow transverse motion. In other words, the frame can twist below the camper but the camper stays steady. These pivots/trunnion(s) are mounted directly to the main frame, there is no subframe.

Charlie

Yeah, that is pretty much exactly what Eddie built for mine and its much better than using the platform, it works exactly as you describe - its done 500,000 miles and still works to this day!

The downside is you have to make wheel arches for the camper which most home made camper mogs don't have because if you sit the camper on the platform you don't need them.

Rob
 

kerry

Expedition Leader
I don't think those kinds of mounting systems could be used with commercial campers like Bigfoots or the Texson I have because the camper itself is not built strongly enough to be anchored directly to the truck frame. Some kind of secondary bed system would be necessary. There's a guy in upstate NY with an FG and a standard delivery truck box converted to a camper. I wonder how his has been holding up. I doubt it has any kind of pivot system.
 

Robthebrit

Explorer
That's the trick, a stress free mount does not require the box to be strong. My camper is made of fiberglass and foam panels. The three/four point mounts themselves are quite big but between them they provide a torsion free platform (3 points make a plane) on which you can much mount the more fragile shell.

Doug's Fuso is a great example, its big foot camper that is properly mounted and camper has not suffered any stress. By itself a big foot is not that strong, if that camper was mounted directly to the fuso frame is would have broken apart. The flex is even worse for a unimog (on a mog the engine, trans, cab and everything else is 3 point mounted to allow the frame to flex), my camper has no signs of stress but again if my shell was mounted to the frame it would get torn apart.

The only way to mount to the frame is to have a stiff frame to begin with (I think the eco roamer is going down this path) or to have a box that is so strong it will stop the frame from flexing, ultimately forcing a flexible frame to be rigid with a box will cause to something to fail/warp/crack.

Rob
 

kerry

Expedition Leader
I understand that the box does not need to be that strong. All I'm saying is that the floors of commercial truck campers are not designed to have 3 or 4 mounts directly attached to them as the sole weight carrying points. They need some kind of frame or bed underneath.
 

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