Critique my rack design...homebrew Jeep rack

K

kodiak1232003

Adventurer
ntsqd said:
Above comments made me wonder about this, so I dug up some numbers & ran a few calcs just to see how it worked out.

Density of Plywood tends to be the same as that of the base wood species. In Pine that is 500-650 kg/m^3
(http://www.australply.com.au/ti_char.html)

Density of Steel is 7850 kg/m^3
(http://hypertextbook.com/facts/2004/KarenSutherland.shtml)

Using one of my rack's panels as a sample (53" x 48" x .5"; 1.219m x 1.346m x .0127m) I get a volume of 0.0208m^3
Using the density of pine plywood I get 10.4kg-13.5kg minus any mass added by paint etc.

For the same size panel in steel one would need additional stingers for support since the sheet doesn't have any stiffness or strength in that direction. Ignoring that for the moment, my choice for panel thickness would be either 12 gauge(.1046") or 10 gauge (.1345").
That yields a volume of .00436m (12gga.) or .00561m (10ga.)
Using the density of steel I get 34.2kg or 44kg
Keep in mind that we still need to add structure under the steel.

I don't see steel being anything close to the lightest alternative, even with paint on the plywood.

Density of Aluminum: 2700kg/m^3
http://www.allmeasures.com/Formulae/static/materials/2/density.htm

Running the sheet steel calcs using sheet aluminum:
11.8kg or 15.147kg
This is closer to that of plywood, but we still haven't added the structure needed for it to be stiff enough to walk on. I have walked on the plywood w/o any additional structure.

Food for thot.
Click to expand...


that is very good info.

i'm a carpenter as a second, unpaid, volunteer type position (i build my wife's theatre sets...lol) and i too find that i've had a lot of experience walking on unsupported plywood.

seems to me:
1. its stiff enough to walk on if need be
2. it's cheap enough to replace if damaged
3. it can be sanded, painted and sealed to look just like metal
4. its 1/8 the price of metal
5. drilling into it is very easy for load eyelets...

thanks all so much for the input so far....options are still being entertained, so i'd love to hear other experiences...
 
ntsqd

ntsqd

Heretic Car Camper
FWIW my panels never got the marine paint part. I flat ran out of time b4 going to Baja with the truck and those panels were absolutely going. So it mostly sat here on the coast (within 1/2 mile of the Pacific) for 3 years with only the primer/Penetrol on/in it. There is some mold happening, but the wood has yet to de-laminate or do anything else undesirable.

When/if the rack goes on the '91 Sub I will replace the panels. Not b/c the existing aren't serviceable, but b/c I screwed up in cutting one out and it embarrasses me.

BTW, pic of it here: http://www.expeditionportal.com/forum/showpost.php?p=112568&postcount=4
 
dieselcruiserhead

dieselcruiserhead

16 Years on ExPo. Whoa!!
great comments and advice here. Somewhere is a build-a-custom-roof-rack thread...
http://www.expeditionportal.com/forum/showthread.php?t=5890

Yes the heavy racks look cool but are seldom functional when loaded. The only time I really load mine are when these are very tame and/or highway driving and I need to haul a really big load. Like, moving.. Or Burning Man (sig photo above) when I hauled the gear for 10 people for a week. The vehicle actually hauled all the gear and I have had 500 lbs up there at one point! And it actually handled great, was kind of neat to really get the roof load built up to see how it handled...

My goal now is to build a super light and aero dynamic rack that does not affect mileage and not noticable but still super functional... Probably aluminum and steel combo. Probably rack out of steel, galvanized, and aluminum floor straps. I plan to keep it on the roof full time and build it so it had little or no negative consquences as far as aerodynamics and fuel mileage and weight, but when I need to use it, it is already on, fully functional, and it is easy to use. And can also be super versital, bikes, skis, etc if needed... The key is to keep the floor weight low. Both plywood and steel grate add a lot of weight to custom racks.

With my last completely hand-built rack I had the frame on this one down to about 20-30lbs at most. You can build a very light rack that is very strong with very thin gauge steel using box technques and square or round tubing. I stumbled on this by using ATV shipping racking that the local dealer discards, and it works great. This is how they package and ship 600lb ATVs so it can qork great. You do have to know the limits of the rack. It will never break but can bend if you tweak on it too hard. Buit it can still support 500lbs just fine.

Here is that last all steel rack. It had no real negative attributes other than about 80lbs total and did not have a "finished" look. But I liked it enough that I slept on it and wheeled with it on very hard stuff regularly with no feeling of increased tipping. Hope it helps:
http://www.expeditionportal.com/forum/attachment.php?attachmentid=7148&d=1176849580

Also another good piece of advice IMO is hook points all over. Avoid bars that go across horizonally, they are not necessary as you have to strap everything in place with straps anyway. They look cool but all these do is add wind resistance. Again, with the rack in the link just above I had no loss of power or gas mileage whatsoever... That said the truck cruises through the wind like a massive brick. But my previous full rack did noticably affect power, mileage, and handling and "looked cool" but was a piss poor design, as far as functionality...
 
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K

kodiak1232003

Adventurer
well, i took some advice and reconsidered the thule bar idea

though i had the bars and didn't really have to buy anything, they really didn't go with the theme of my truck:

1.Modifications have to add to the utility or durability of the truck
2. Modifications Can't compromise another part or system.
3. Modifications can't effect my extended warranty. (Real Jeep warranty, not some BS 2nd party one)

the bars didn't go with #2 specifically, so i sprung for a kargomaster congo cage as the base for my rack.

I'm VERY happy with the quality/fit finish, etc.

I'm still working on the sunshade/flooring solution. still might go with a good ole' sheet of plywood.

Brian
 
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