One of the more difficult dimensions to figure out while making cross members and diagonal tubes for your race car is the cope length of the tube you need to build. This is the length the tube needs to be on the deepest part of the cope after it has been cut in your coping machine.
In order to prevent material waste or coping or notching
the tube on the long side of what you think you need, and then coping it 2
or 3 times to get it to fit. I have found a solution that works very well.
A standard tape measure can be difficult to use for this purpose. 
Using a piece of small diameter tubing ( .058 wall chromoly works very well ) insert a length of solid round stock that fits inside the particular size tubing you have selected. The length of tool you need will depend on the length of the diagonal you need to use in your car.
If you start with a 12” length of tubing for your measuring tool and insert a 12” length of stock to slide in it. You will have approx a 12”-24” length checking tool. You may need 2 or 3 different length tools for your project.
In figure 1 you can see I have a length checking tool inserted between the upper and lower rails of a dragster chassis. Since the upper rail runs at about a 5 degree angle a very slight difference in the cope length changes the location front to back where the upright tube will fit. Therefore the cope length is very critical.
I drill a hole and weld a nut to the tubing and use a locking wing bolt to hold the round stock in place once you have your length set. See fig 2.
Keep in mind this is the cope length and you will need to cut your tube longer than the tool by ½” to 1” depending on the diameter of tubing you are fitting.
This tool will save you time and money building you next race car!
Rusty Kramer
KRC