GMC Topkick 4x4 Expedition Camper Build

#1
I moved this thread from "Domestic Full Size Vehicles" - maybe more appropriate here...
So to recap:
I retired 3 years ago, and my wife and I got a large 5th wheeler and a 3/4 ton Dodge diesel. We live in SC and have travelled Out West over the last 3 years, including Overland Expo, spending about 6 to 7 weeks each trip in AZ, NM, CO, etc. I was getting tired of towing such a large rig (10,000 lbs) and started getting inspired by some of the rigs we saw at Expo.
In February this year we did a 4 week self drive safari in Botswana and Namibia. A Ford Ranger truck, an Alu-Cab "hard top" rooftop tent, a GPS and the two of us had a fantastic time. A complete opposite to the 5th wheel, and such a memorable trip.
So we decided we needed to downsize. We looked at several different ideas, and as usual got lots of inspiration from Expo. P Finally we setted on a couple of key criteria for us - a bed my wife could walk around to "make", and a separate toilet with a door.
This meant ideas such as Lance slide-in camper wouldn't work. So, doing some layouts, we decided a 14 foot long box would be about right.
We looked at 4WD drive options for Isuzu NPR, but felt the truck too small. We then looked at Isuzu FTR, a 26,000lb truck; but finding appropriate front axles, etc got complicated. We really wanted the FTR as it is a cabover design, so great visibility, and the cab had enough space for my wife to lay down behind the front seats.
We finally settled on a GMC C4500 4x4 Topkick. This is a 17,500 lb GVWR 4WD truck. We found a 2007 single cab diesel with 100,000 miles in FL, so nice and clean.
However, while driving back from FL, I was hit by a "semi" and the truck was totalled.
So bought a 2003 Topkick with 15,000 miles on odometer (?????) but it still had original rear tires, and is exceptionally clean.
Some statistics and our planning:
GVWR 17,500 lb
Chassis / cab weight 5,400 lbs front axle, 2,900 lb rear - at a local scale. (8,300 lbs total)
Payload 9,000 lbs
14 ft standard delivery truck box, 150 lbs/ft = 2,100 lbs
Box contents estimate 2,000 lbs
House batteries - 8 x 6v Trojan golf cart 640 lbs
60 gallons fresh water 500 lbs
Other stuff about 1,200 lbs, - additional diesel, propane, spare wheel, winch bumper, etc
Total "load" about 6,400 lbs
So total truck weight, with tanks full about 14,500 lb estimated.
Trying to keep front axle load at about 6,500, and rear about 8,000 lbs.
Planning on 335/80R20 Michelins or Contis; so with these weights, 33 psi front, 45 psi rear. This is the recommended pressure for highway speeds !!!
It looks like an offset of 5.5 inches on 11" rims should allow me to use same wheels back and front. Two valve holes will be required though.
Planning on 3 point mounting of body using some rubber bushings I found at a company called "Lord". This will require a subframe to be added to the standard box.
So, I'm posting all this here cause I think it's a good idea to "give back" a little to the Forum, as I've got so many good ideas here.
Also I'm very interested in any help I can get along the way.
So I'll be posting photos as I make progress, and reaching out for help, please.
Hope some of you find this interesting
Bob
 
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#4
Front axle twist.jpg
This the front axle out of the wreck. Those are straight edges sitting on the spring perches to show the twist. I'm sitting at East Coast Gear Supply in Raliegh right now waiting for a quote on fixing it.......
 
#5
While all this is going on, I'm planning wheels for the 335/85R20 Michelins. I can only find two choices:
- Hutchinson bead locks at $1400 each
- flat plate centers welded into agricultural 20x11 rims, at about $450 each (Stockton, Rickson, etc)

Welding in a flat, or slightly dished center into the rim, and then planning up to 5,000 lbs on each wheel is a recipe for disaster due to the stresses on the welds. All the OEM dually wheels have a fully dished center 4 or 5 inches deep which is pressed into the rim, and then welded. This is a much stronger and more reliable approach, but is not easy to achieve without some significant engineering and tooling costs.
However, being a retired engineer from Michelin, I have some contacts that can help me.
We are currently in the process of designing and building the appropriate OEM style centers. We're building these centers thicker and stronger than the original OEM wheels, so we're confident in their capacity.
The process will allow us to build centers for FORD F450 / F550 also.
We also have sourced some 11x20 rims, made of the same guage steel as the original 19.5 rims.

So we will have wheels for this truck soon, and will be able to offer wheels for the Fords also. Note these are custom build wheels with whatever offset is required to allow them to be used front and rear, with the same track.
Bob
 
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#7
How are you getting the th XZL's? They are hard to come by new and used. There is Pirelli in the same size brand new for less than 400 a tire, another option.
 
#8
Sounds like you got the rims figured out. I would have recommended military rims with new centers. Part of me would rather go to 22.5 trucker rims and tires like used on Dumptrucks etc. Bit of a road tread to help MPG yet some traction tread for offroad. Just not sure how I could make them match my CTIS (LMTV)
 
#9
Darwin - tires seem to be available on ebay search. I'd probably be fine with Pirelli as well

Coach - the reason for the 33/80R20 is the very low pressure required to carry the load. A regular "semi" 22.5 tire is designed for heavier loads, and are much stiffer sidewalls. A bit of a loss in mpg and noise, but much better ride and off road performance.
 
#10
How are you getting the th XZL's? They are hard to come by new and used. There is Pirelli in the same size brand new for less than 400 a tire, another option.
Also Goodyear in their military tire line, which BTW suggests rims as narrow as 8" for that size. Narrow rims are better as far as bead retention especially if one does not use bead locks.
https://www.goodyear.com/cfmx/web/gov/pdf/military_tire_brochure.pdf
You might check this place out as far as tires and Hutchinson rims:
http://wartimefinds1.homestead.com/Tires-Wheels.html
and another in Holland for 335s:
https://www.vrakking-tires.com/12-20-inch?id_category=12&n=75&p=2
Hutchinsons are worth a lot: lighter, strong, reliable, easy to work on in field, bead lock can be fitted.

Charlie
 
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#11
Charlie have you or do you know anyone that has ordered tires from vrakking and had them shipped to the usa? I have seen the Pirellis on Ebay for 300 plus shipping
 
#12
Charlie,
Thanks for your input. The Hutchinsons at Warfinds are cheap !
Not sure how I would fit them on my axles though. I need a 275 mm (11") pitch circle for the lugs, a 5.5" offset, and also the wheels need to be reversible. My hubs are bigger than the 11.25 center hole, so I couldn't reverse them.
 
#13
Fuel tank brackets.jpg

Took the fuel tank from the wreck - from under the drivers door. Cut the brackets back and raised them, and mounted them just in front of the axle. I now have original 25 gal tank between frame rails, and "auxiliary" 40 gallon tank. I have left the original 25 gal tank as the primary source for supply to the engine, and will use the existing fuel pump in the 40 gal tank to transfer fuel when required. Really very complicated to plumb in both tanks, so this should work just fine.
 
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#14
View attachment 422827

Took the fuel tank from the wreck - from under the drivers door. Cut the brackets back and raised them, and mounted them just in front of the axle. I now have original 25 gal tank between frame rails, and "auxiliary" 40 gallon tank. I have left the original 25 gal tank as the primary source for supply to the engine, and will use the existing fuel pump in the 40 gal tank to transfer fuel when required. Really very complicated to plumb in both tanks, so this should work just fine.
I have a similar set-up with dual tanks. You can always make a provision to just switch the fuel lines if you need to feed from the other tank directly to the engine.
 
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