4x4 timbers are usually taken from the center of a tree, and aren't very good for using on a span. A better option would be to double up a couple 2x6's or 2x8's as your spanning lumber, and then try to get some support with 4x4's in the ground underneath. The 4x4's may work, but will probably sag on a 12 foot span with 600+ lbs. It's all in what you want.

 

4x4's are usually used for posts not joists, and a 12' to 15' span is far to long. The strenght of lumber is in its width. So a 4x4 joist is a little bit stronger the 2 2x4 nailed together. I would use 3 6x6 p.t. for the joists ( 3 will transfer the weight better) and then deck over the 3 6x6 with 5/4x6 ( five quarter by six is standard p.t. decking). I have been a custom remodler for 12yrs and I would not drive my little lakota over a bridge made up of 2 4x4 . I would also put roofing tar on the ends of the 6x6 because they will be on the ground and the tar will prevent rot and insect damage. Good luck and if you build it post some pics.

 

I just noticed you said 4 4x4 on each side. My bad. Still I would go with 6x6.

 

Good info, thanks, that's what I needed, somewhat of a professional's opinion! I will go with 6x6s.

 

all you need 2x6x12,6 of them.put 2 side-by-side,1 on each side of your bridge,and 1 in the middle.you can build your deck out of 2x6 as-well.

 

8 4x4s... man, you don't want to end up in the water with a 600 lb atv on top of you.

 

I would also strongly recommend a bridge with a full deck and not ramp style making it stupid people proof. Seeing how it would not be on your land and anyone could use it. When you figure the lengths of you 6x6 make shure they are long enough to span the creek and have pleanty of bearing on solid ground.

 

I know, you never know who will hurt themselves on your bridge. That's the problem though, the banks are not level to build a straight deck bridge. It is gonna have to be an incline going and a decline coming back. Makes it a little sketchier, but this is what I am going to do.

Three 6x6x12's
Dig them all into the bank so it is flush and so that the earth holds the 6x6s.
I will then slam a metal steak (railroad steak) threw each end of the 6x6s, holding them down even more.
Then I will go perpendicular to the 6x6s with 2x6s to make the "deck".
I will place them far enough apart so that the nobbies on the tires may grab if it is ever wet or muddy.
I will make it around 5.5ft wide with 4x4s on the outsides for guides so that you won't fall off track.
There will still be somewhat of an incline once off of the ramp, so I will dig it out to lessen the angle so there are no issues with weaker, 2x4 atvs making it.
I figure it will run me around $120.

I have already preped the area were we will be working by weed wacking all around and down each bank. The banks are cut off and some of the bank actually overhangs the water. I will have to make sure, like suggested, that I have plenty of sturdy land underneath those 6x6x.

 

may i suggest a 15 foot double? aka jump across

 

Already thought of that too, and if I had a sport bike, that would be the solution fo sure. I think my front diff would fall right out of the bottom if I made that launch, hehe.