Possible to add a 2nd fuel tank to Isuzu in frame?

[Jfet]

Our 2006 Isuzu NRR 170 inch wheelbase 20 foot flatbed is getting a bit heavy on the front axle. Mostly I think things will be ok when we have a full load on the back portion of the exposed flatbed (motorcycles, snowmobiles, gear) but if we are traveling empty the front axle is close to being overloaded even with just the driver. We could have different positions for the camper based on rear cargo but I like having it snug against the headache rack.

Our diesel tank is only 30 gallons, in frame right behind the rear axle. It looks like there is room behind that for another tank, which would give us a 600 mile range (roughly). This would also take about 100 pounds off the front axle assuming the full tank was around 300 pounds.

Is this a suitable location or are there issues I do not know about? Easy DIY or take to dealer?

 

[Gatsma]

The one thing you might have to be careful of is how much leverage you are applying to that rear frame, which, as you know, is of smaller dimension that that over and ahead of the rear axle. Especially if you load those rear-most cargo boxes very much. Also take into account how rough a roads you may travel over, as much bouncing WILL get that rear frame oscillating, considering how much it has been extended. Bottom line: Not sure I would be putting any sizable amount of weight that far back.

 

[Jfet]

Gatsma, that is a good point. I did notice that the main frame of the Isuzu stops just past the rear axle and a smaller frame has been attached. The steel of the flatbed itself however is on top of that frame and are pretty beefy members (5" c-channel IIRC).

I really hope you are being overly cautious because we really planned on loading up this rear portion of the flatbed with toys.

Here is a picture of the underside showing the existing fuel tank, mainframe, rear frame extension and flatbed c-channel.

 

[Gatsma]

The way you added to the OEM frame looks beefy, but the main concern is how much leverage is caused hanging that much weight so far out back. I would be worried about actually bending the frame over the rear axle, as that is where the OEM frame is already decreasing in dimension, and would probably be the "weakest link" in the whole frame setup. 5" channel looks tough, but is still smaller than the OEM frame rail. I would have used something comparable to the actual frame size or section modulus to extend the frame, but you would still be dealing with the leverage issue, and that is the main concern here.

 

[Jfet]

I didn't do the flatbed.

I am curious about Isuzu and Fuso box trucks that have 20 foot boxes with my wheelbase and also have a 400? pound liftgate at the very back of the box. I should crawl under one of those and see what their frame extension looks like.

 

[Gatsma]

If a frame extension is used, USUALLY (not always) it will be the same dimensions and thickness as the truck frame it's welded to. That way the channel webs and flanges meet and can be completely welded.
Not to be critical, but your frame extensions are somewhat smaller, and only welded along the top flange and down the web only as far as the added channel goes, which is not an ideal situation.
If a liftgate is used, they are only supporting additional weight while the truck is stationary. The weight of the liftgate IS a concern, but if the frame extension is the same spec as the truck frame, it is minor. On your frame extensions there is more to worry about. The part of the frame where the bottom of the extension channel meets the middle of the frame web could collapse under too much leverage.
Don't mean to be a doomsayer here, but be aware of the limitations of your frame configuration.

 

[Jfet]

It is curious they didn't go with the same size channel considering the flatbed was built to haul concrete blocks.

We don't plan on rock climbing with this vehicle...it is more of a hauler than an expedition vehicle but I don't want to be super limited on what I can put behind the rear axle.

I guess I do not understand the limitations of the frame/flatbed I have yet. You expressed concern over a 300 pound fuel tank while I am considering putting equipment back there of about 3000 pounds.

 

[SkiFreak]

You expressed concern over a 300 pound fuel tank while I am considering putting equipment back there of about 3000 pounds.

There is a significant difference between hanging stuff off the chassis compared to putting things on top of a tray.
Lots of trucks running around out there that carry pretty big loads, so I would not be too concerned about the weight on your tray, as long as the loads are within the carrying capacity of the individual axles.

 

[Gatsma]

OK- I did NOT know what size tank you planned on; 30 gal. might not be a problem IF you don't go too far back. 3000# of gear might be manageable if you can concentrate most of that weight as close to the camper as possible.
Considering the weight of concrete blocks, I doubt a load of them ended anywhere past the axle. I'm thinking they made the bed that long so they could haul lighter, bulkier items. That way the truck had more uses than just blocks.
On edit SkiFreak (his post came in while I was writing my previous one) makes a point. Your 3000# gear load WILL be supported by the bed(or tray as the Aussies call them) which DOES have its own frame, which will "bridge" the load over the whole truck frame. So my fears seem less-well founded. Sorry about that!
About the fuel tank, I would try to mount it as close to the present one as practical. No sense going too far back, as the cantilever effect would increase the further you go. So maybe after a day of fright stories, it's looking more do-able! I would still ask a frame guy if what we've said here holds water, or 3000# of gear! ;-)

 

[Jfet]

You do have me considering bigger trucks now lol.

I don't know what is currently available in the USA that is cab forward and larger than the NRR. It does look like the front axle capacity on the new NRR has been increased from 6830 to 7250 pounds.

I can't seem to find if Isuzu still makes the F series or GMC makes the similar T-6500 series for sale in the USA. They have front axle capacities of 10,000+ pounds and 26k GVWR. The frames are also much much beefier.

Still, I have the NRR right now and have done some work on it (adding the storage boxes).

But having a 26k GVWR vehicle with a beefier front axle and maybe a 22 foot flatbed instead of 20 feet...

 

[Gatsma]

You can always get a different truck, but for now I think this one will get the job done.
Question- are the storage boxes attached to the flatbed, or the truck frame?

 

[Jfet]

They are attached to the flatbed.

 

[Gatsma]

They are attached to the flatbed.

That would make the whole thing easier to transfer over IF you ever decide to get a different truck.
Use this one for a while, then decide. You never know, this one might work out pretty good!

 

[Jfet]

Yes, I think I will use this one for a bit, at least until I get loaded up and see how things are balancing out. The T7500 are just too expensive (40k to 50k used vs NRR at about 15k to 20k used).

I could maybe swap the front axle for the higher rated 2014 NRR front axle? If it was a similar fit...

 

[Gatsma]

You would have to check track widths, spring pad widths, and steering linkage attachments. Might just be a bolt-in, or not. The newer springs would also figure in as they will be rated higher also.
It IS something to think about!