Going to be Unimoged
- Thread starter Fanhunter
- Start date
Deleted Member 183
Well-known member
Spare tire on cab roof.... VERY BAD idea!
MogsAndDogs
Active member
Is it really though? I get the concept but it seems in practice many people do it without issue. Including very knowledgeable outfitters like Atlas4x4 and Unicat.
Deleted Member 183
Well-known member
To each their own!
A MPT 365/80 R20 mounted on beadlock rims weights 191 kg's (420 lbs), then add the mounting structure lets say 46 kg's (100 lbs), then the lifting davit 23 kg's (50 lbs) = 260 kg's ( 570 lbs) minimum.
Add that kind of weight to the top of a structure that is not designed to take anyway near those kind of loads, in addition to being bad for a heavy vehicles center of gravity, and being a plain old pain in the ass to deal with when having to deal with when you have a flat... Oh and lets not forget, almost certain death to the driver and passenger in a roll over accident.
Vehicle "up-fitters" will do just about anything you want if you have the money to pay for it!
MogsAndDogs
Active member
Totally get it. I have no first hand experience or knowledge. Just curious why they would take such a risk. Money is great and all but killing your customers is pretty bad for business and reputation.
Deleted Member 183
Well-known member
If I were to guess... I would say that they feel better mounting the mass to a structure that is rigidly connected to the frame ( like the cab) as opposed to the composite "box's" (habitats) & "floating" sub-frames that they fabricate (aka: cover your back end). I don't know squat about Atlas4x4, but have had several friends over the years with Unicats and for don't believe that any of them had the spares mounted to the top of their cabs.
If you study military medium heavy duty trucks (regardless of country of mfg.), the spare tires are as low to the ground as possible and on some kind of pivoting cradle that can land the tire "tread down" so the one person can roll it into position on the axle hub. Dealing with wheels this size is a TOTAL PITA in even the best situations ( like a level shop floor)!
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MogsAndDogs
Active member
Agreed!
Iain_U1250
Explorer
We have out spare wheel mounted over the cab, but on a completely separate steel frame, as I didn't want to over load the roof, our 365/80R20 on beadlock wheels weight 115kg. The old steel wheel were around 100kg.
The MB Body builder handbook for our model has the maximum roof load of 120kg.
As far as the centre of gravity is concerned, that is quite a complex calculation, the basic Unimog cab chassis has a centre of gravity 370mm - 440mm above the centre of the wheels.
The maximum MB recommend the centre of gravity to be Hmax is different for each truck, depending on the wheel track, for ours it is `1232mm.
Any weight above the basic cab-chassis CG height ( on ours 370mm+503mm) 873mm will raise the CG, anything below that reduced the CG. 870mm is the top of our fuel tanks roughly, and 1200mm is the top of the bumper.
A rough way of calculation the CG is the formula above. The basic cab chassis for us is 4600kg with a CG of 873mm. If all we did was put the spare wheel on the roof and nothing else, then that would raise the CG
H= (4600*873 +125*3000) / 4725 = 929mm - so an increase of 56mm - so not that much in reality. Adding all the other things onto the truck, above the basic CG of 873, in other words, the whole camper box, that does make a difference.
The MB Body builder handbook for our model has the maximum roof load of 120kg.
As far as the centre of gravity is concerned, that is quite a complex calculation, the basic Unimog cab chassis has a centre of gravity 370mm - 440mm above the centre of the wheels.
The maximum MB recommend the centre of gravity to be Hmax is different for each truck, depending on the wheel track, for ours it is `1232mm.
Any weight above the basic cab-chassis CG height ( on ours 370mm+503mm) 873mm will raise the CG, anything below that reduced the CG. 870mm is the top of our fuel tanks roughly, and 1200mm is the top of the bumper.
A rough way of calculation the CG is the formula above. The basic cab chassis for us is 4600kg with a CG of 873mm. If all we did was put the spare wheel on the roof and nothing else, then that would raise the CG
H= (4600*873 +125*3000) / 4725 = 929mm - so an increase of 56mm - so not that much in reality. Adding all the other things onto the truck, above the basic CG of 873, in other words, the whole camper box, that does make a difference.
MogsAndDogs
Active member
Excellent!
Deleted Member 183
Well-known member
That is some excellent information and math Lian! GOOD JOB! You have to love Mercedes for their attention to detail.
Working on American trucks that were intended to only be 2 wheel drive and modified to 4X4 leaves us Gringo's in the dark compared to Unimog "engineering".
Working on American trucks that were intended to only be 2 wheel drive and modified to 4X4 leaves us Gringo's in the dark compared to Unimog "engineering".
charlieaarons
Explorer
The UGN/U500 bodybuilder manual has similar data. Because the cab is carbon fiber, the max cab roof load is 80kg, only 20kg each at 4 specific mounting points.
So Unicat built me an aluminum skeleton of a rack with 2 (Al) Zarges boxes. Where I store my 2 ginormous (1.5” and 2”) kinetic tow ropes, hopefully NEVER needed.
So Unicat built me an aluminum skeleton of a rack with 2 (Al) Zarges boxes. Where I store my 2 ginormous (1.5” and 2”) kinetic tow ropes, hopefully NEVER needed.
Geo.Lander
Well-known member
Depends on the cab. Really doesn't it!