tre5
+1y
My view on this is: If you're asking, you shouldn't be doing it. But, in order to not sound like an asshole, LOL...
Basically you can go at it two ways. First is to design it the way you want it to look and build the rest around that and second would be to design the cantilever around what you have. The reasoning behind that is... it doesn't really matter how big you go with the arm. It could be 4 feet long or 12" long. The distances that matter are the lengths between the center pivots and outer pivots. You would want to start at a 1:1 ratio and go up from there. As you go up in ratio the length from the center to the bag becomes smaller in relation to the length from the center to the axle (or arm or whatever you are pushing on to lift the vehicle). So let's say the distance from the bag to the center is 12" and the distance from the axle to the center is 12"... you would have a 1:1 ratio. Doing it like this you will get the exact amount of lift the bag will provide. So a bag that gets 7" of lift will lift the same amount, 7". Now, if you go to something like a 2:1 ratio the distance from the center to the bag (for example) would stay at 12" and the distance from the center to the axle would increase to 24". This will make you bag that gets 7" of lift obtain 14" of lift when utilizing the cantilever. You can setup your distances however you like. Hell, you could put the bag on the front bumper and pivot the cantilever on top of the cab and then have an arm pushing down of the axle. As long as the ratio between the distances stays in check. I would probably try to stay within the 2:1 range, but going a tad over isn't going to kill it.
One thing to remember when doing a cantilever is considering the direction of travel of all your pivots. Obviously things are going to rotate. So remember this when setting up your bag, as well as the link from the axle to the pivot.
The last thing I will add is... doing a cantilever using a bag on one side and a coilover on the other side makes absolutely no sense to me. There are now 2 spring rates and the spring on the coilover becomes basically useless unless you find a really soft spring rate that works well with the spring rate of the bag. And the spring on the coilover being useless basically means it's not moving which also means the shock part of it isn't moving either.
Good luck!