You can use the stock 550 shocks with a straight hoop (no upkick required) and a flat bottom tray in a ½ inch hoop without any issues whether welded on the hoop or under the hoop. That is, as long as you are using a stock swing arm.
Stock 550 shocks have 2.2 inches of travel in them. Pull one, take off the spring and measure fully extended and fully compressed. Subtract. See....2.2 inches.
The rear tire, if stock 18” rim, has 3.25” gap from the top of the tire to the bottom of the tray using a 1/2” tube hoop welded to the frame level top edge of hoop to top edge of stamped shock mount and the tray welded to the underside of the hoop. Put the bike up so the rear wheel is off the ground and both shocks installed...ie.: full drop. Lay a flat bar like a ruler into horizontal position on or under the hoop, where your tray will be, and put a square on the tire so it's perpendicular (plumb) and where it intersects the flat bar...that's the max distance the tire can travel before hitting something. This is important, because I won't guarantee that every manufacturer of tires makes their rear tires the exact same diameter, even tho they are supposed to when using the metric or P system. All 130/90/18 rear tires should be within a couple of millimeters of identical circumference....in theory.
The shock isn't mounted 90 degrees perpendicular, it's canted 74 degrees. That means that the 2.2 inches in the shock are reduced to just a hair over 2 inches max wheel travel....so far. Call that vertical measurement side b of a right triangle. We already know the length of the hypotenuse...2.2 inches and two of the angles...90 degrees and 74 degrees.
To find side b, we have to use some trigonometry:
sin(angle)=opposite leg/hypotenuse
sin(β)=b/c
After substituting β=74o and c=2.2 we have: sin(74o)=b/2.2
b=2.2*sin(74o)
b=2.2*0.9613
b=2.1148
The next step for a CB 550, because the rear wheel axle is a nominal 2 inches from the shock connection to the swing arm...is to figure that into the wheel travel. The radian for the arc has been increased. More trigonometry. (many street motorcycles put the shock mount right above the axle so no additional calculations are required at this point. Motocross are a whole other story since the shocks are so far from the axle to increase wheel travel!) I'll jump right to the answer....it's still less than three inches when all said and done.
If you're still apprehensive....form your tray over some 6” schedule 40 pipe and gain almost ¾ inches more gap when you weld it into place. Or you can spend a few hundred bucks on longer shocks.
