T244 Load Bed Builds (Pete)

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T244 Load Bed Builds (Pete)

Postby crinklystarfish » Thu Apr 02, 2020 8:08 am

PeteB wrote:Logic for leaving the stripped load bed was 1) simpler and quicker build, 2) suspect it must contribute to chassis strength, 3) will be strong and stiff enough to support a composite cabin.


Re recent post on Ivan's Tramp Build thread.

1) +1 with Ivan - definitely.
2) It does, but it's a moot point. The chassis doesn't need this augmentation. It'll perform just fine fully naked - or when incorporated with a (well designed and built) pivoting subframe (which allows full chassis deflection).
3) A qualified 'yes'. The bare load bed does NOT deflect very much at all under full cross-axle loads. I measured it crudely, Beep-Beep Tom measured it, well... like Tom does. Hopefully he'll pop up and take due credit, otherwise he did at one point post the work on his Beep-Beep blog (it may still be there).

With the completely standard chassis-to-loadbed mounting arrangement, it's just plain sensible to build a very sturdy box - which is why I specified my own panels with peripheral frames and (partially) interlocking rebates.

NB To relieve the torsional stress passed from the chassis to the load bed, I've also slightly tweaked the front chassis-to-loadbed mount. Not strictly necessary, perhaps, but it makes me feel better.

From what I've seen of my own and other builds, I suspect (but obviously wouldn't be so reckless as to guarantee) that a somewhat flimsier box might still be OK with the perfectly standard chassis-to-loadbed mounting system.

If required, I'd be reasonably confident in suggesting that the front chassis-to-loadbed mount could be further tweaked to allow still more articulation if combined with a further mod. That mod being: the second chassis-to-loadbed mount (a static nut and bolt) being removed and replaced with a (very conservative) opposing-spring arrangement - perhaps allowing a max of 10mm of separation at that point.

This combination would further isolate the standard loadbed's torsional stress.

All this said, my box is heavier than it strictly needs to be and we can still hit 7.5t. NB part of the solution is to keep wife-space to a minimum.

To qualify: wife-space is literally any space clumsily left available by the builder, into which a wife is genetically predisposed to quite literally cram utterly useless and wholly unnecessary items to be transported thousands of miles for absolutely zero logical purpose: actual or imagined.

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Re: T244 Load Bed Builds (Pete)

Postby PeteB » Thu Apr 02, 2020 9:26 am

Steve,
Thanks for the comprehensive response - more food for thought as ever. Picking up on some of your comments.

Strength/twist of load bed: I saw Tom's measurement approach (rigorous) and was influenced by your points & decision to build directly onto the bed in thinking this is a good option. I agree there will be some flex and this comes with risks when you use a pure composite cabin. Telling that you & Tom both added strength to the cabin via wood inserts or steel, even if on a 'just in case' basis.
What cabin strength is required: Would I like to know the cabin can survive seriously rough conditions? Of course. Am I likely to drive in such conditions? Doubtful. Accepting there are no guarantees, a composite cabin built onto the load bed is probably fine.
Weight implications: I estimate the bare T244 weight with wood stripped from load bed is around 5.3 - 5.5t, which makes for a tight build weight, although you show it's doable. However pull off the steel load bed and replace with a rail on rail subframe and I should save 200+kg? Extra work but useful extra load margin and more capacity to handle rough terrain.
Conclusions: More thinking to be done! Stuck at home though so time to think.

Finally, wife-space: You are a braver man than I putting that in writing (but I agree!).

Pete

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Re: T244 Load Bed Builds (Pete)

Postby Tom_DAF » Thu Apr 02, 2020 4:35 pm

Hi Pete,

Uh oh, sounds like I'm developing a reputation haha. Unfortunately I can't bring any hard numbers to the table on this one. Predicting the torsional forces applied through the load bed is surprisingly complicated.

Luckily, between Steve's build and mine we know two things:
1. Steve's box on the standard bed + longer spring bolts on the mounts works great.
2. Welding an over-engineered steel frame to the bed and using the standard rubber mounts (no spring bolts) also works.

So somewhere between our two builds should also be fine. Without a metal frame I'd opt for spring bolts as Steve has done, they can't hurt at least. I started modelling the T244 chassis in cad to do FEA on it but with no way to validate I'm worried it would build false confidence (or fears).

I think the torsion often looks worse than it is because people compare the top corner of the hab box (the farthest point from the axis of torsion) with the cab which is mounted at the very front of the chassis. This exaggerates the effect quite a bit. The chassis rails are quite close together so you only need to allow 50mm of displacement on the front and 25mm on the second mounts with springs or rubbers you'll probably accommodate all the torsion with minimal forces applied to the hab (but these are napkin calculations).

On my build I left the mounts factory standard and I drove up Sani Pass, in and out of countless dry riverbeds in Turkana, and lots of full torsion situation without issue. The T244 is a pretty tough truck.

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Re: T244 Load Bed Builds (Pete)

Postby Ivan C » Thu Apr 02, 2020 4:48 pm

That's a good point about the appearance Tom.
On those figures the Daf has a stiffer chassis than the MAN.
From memory when I lifted one wheel about 450mm the opposite wheel was starting to lift.
The rail/subframe gap at 4 metres of subframe length was 75mm.
I think Phil got similar figures.
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Re: T244 Load Bed Builds (Pete)

Postby PeteB » Thu Apr 02, 2020 5:52 pm

Cheers guys, food for thoughts anyway. I can continue to argue with myself on the merits of building on load bed or going for the rail on rail solution.

In occasional flights of fancy I do wonder about linking each corner of box and bed with a seismic spring mount...
https://www.mason-uk.co.uk/restrained-spring-mounts/

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Re: T244 Load Bed Builds (Pete)

Postby Tom_DAF » Fri Apr 03, 2020 8:07 am

I probably wouldn't go for seismic mounts because they aren't designed to withstand X and Y axis forces so you'd need to add yet another device to resist forces in those directions. The farther away from the chassis you go, the more movement needs to be accommodated.

Another option is to increase the stiffness of the bed (e.g. by adding a tall steel profile all the way around) but I don't think it's necessary if you have spring bolts like Steve does.

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Re: T244 Load Bed Builds (Pete)

Postby PeteB » Fri Apr 03, 2020 8:22 am

Tom, agreed - these things are great in compression but that's what they are designed for. Would need to deal with lateral forces or hab box may fly into back of cab when braking fast! As I said, 'flight of fancy'.
Did think about an L-frame steel 'box' to wrap around the composite panels. Would give benefits of helping lock the panelse together and providing protection against impacts on corners/edges but I can't see the L frame giving and significant structural / torsional strength.
Will do some pro's and con's for load bed vs rail on rail and see if I can make a decision.

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Re: T244 Load Bed Builds (Pete)

Postby crinklystarfish » Fri Apr 03, 2020 9:11 am

The T244's chassis does indeed seem to be quite 'stiff' compared to similar trucks.

Another of my crude early experiments was to completely remove the front opposing rubber-bobbin mounts and cross axle the (fully built and loaded) truck; the total separation at this point was surprisingly minimal. I added springs to the bobbins to accommodate the total separation observed and kept a close eye on the next mounts along to watch for signs of stress. All seems OK and, like Tom's, the truck's done a good bit of rough work now.

I might still have a picture of the stripped mount somewhere and I'll post it later - actually, it might even be in the book - I forget.

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Re: T244 Load Bed Builds (Pete)

Postby crinklystarfish » Sat Apr 04, 2020 7:13 am

Can't find the full cross-axle scenario but found this.

This is as far as the chassis / load bed part when front bobbins are removed completely.

Truck at 7.5t and subject to stress shown. As discussed, my box is pretty stiff so will be adding to rigidity of loadbed. Between them they will conspire to restrict separation to some degree but - as can be seen - we're apparently not talking Unimog-scale torsional stress here.
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Re: T244 Load Bed Builds (Pete)

Postby Ivan C » Sat Apr 04, 2020 7:35 am

I'd guess that's about 1/3 of the equivalent opening on the MAN
Looking at the first picture, would using a rail-on-rail subframe allow a reduction in overall height?
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