Utility Cycling - building a rack for my bike

i am planning on building a rack for a an extracycle that i am working on building. i have been thinking about making it a 2 part rack. the front part would be bolted or welded to the extracycle and the rear part i would be able to unbolt from the extracycle and bolt onto my mountain bike. i don't think it should be too hard to build the rack or mount it since both bike have a lot of parts that are similar. my main question is how wide should i make the rack or how wide is yours and how well does it work? i know approximately how long i would like to make it

my xtracycle in progress
http://lh3.ggpht.com/_u26Z1RDZ_Po/TIk9zd-oDWI/AAAAAAAAAwo/UuLRUmgXVrU/s800/Picture%20034.jpg
my mountain bike
http://lh5.ggpht.com/_u26Z1RDZ_Po/TIlDx1CwThI/AAAAAAAAAxI/jGfHj4MWAAA/s800/Picture%20006.jpg

Aren't racks usually hard to put on a mountain bike with the suspension like that in the back? Usually you go with a seatpost mounted rack on them.

maybe. i have never really seen any full suspension bikes with a rack. i don't see any reasons why it would not work. there are already a couple of holes for mounting stuff. if i need to fabricate a few bolt on brackets for more mounts i can. the big problem i have is when i make stuff it usually end up too big which is why i'm wondering what would be a good functional width.

i found this on instructables (http://www.instructables.com/id/Bike-rack-for-full-suspension-mountain-bike/)

this is what i'm thinking about doing pretty much (http://www.instructables.com/id/Full-Suspension-Bike-Rack-from-Scrap/)

Axiom makes rear rack for suspension bikes (http://www.axiomgear.com/products/gear/racks/rear-racks/odysee-rear-suspension/). Although, I'd be careful as I'm sure the stays are not meant to carry much weight.

the axion website seems to to be down. i want something that could haul 50-200 pounds which is why i am planning to build my own. i am thinking about making it 8-9inches wide so that i can take my air pig with me if i go for long bike rides out in mono cliffs.

They make these things called "pumps". They're perfectly capable of producing all the air you need on a bicycle. And you can get one that fits in a pocket, if you'd rather not attach it to the bike itself. There's no need to add a rack to bike so you can carry a heavy steal thing so you don't have to work to pump your tires.

the axion website seems to to be down. i want something that could haul 50-200 pounds which is why i am planning to build my own. i am thinking about making it 8-9inches wide so that i can take my air pig with me if i go for long bike rides out in mono cliffs.

You really don't want to put THAT much weight on a rack attached to the rear suspension triangle.

i have never had much luck with air pumps. they always seem to break and i don't really like small bike pumps because they take forever to fill tires. i don't mind carrying a bit extra weight on my bike. it already weighs 45 pounds as is.
i might not put that much weight on it but i would like to be able to if i needed to.

here is a picture of what i'm planing to do for a rack
the red lines are supports and the white lines are where i will put my stuff. i figure if i keep most of the weight to the back it should not effect anything.
http://lh6.ggpht.com/_u26Z1RDZ_Po/TIqy3dvimLI/AAAAAAAAAxc/XccKJhe6cuw/s912/1q.JPG

Well if you do it, try to have it made by an aluminum shop so its light. And I would say 6 inches is the maximum width you want to use.

it is going to be made out of steel. i don't know of any aluminum shops and i don't know any one with a tig welder. i am hoping to get my own tig welder sometime soon but for now i have to work with what i have.