frame table and bow in lower rail

[sawdawg]

I was going through the construction pictures of the Lever Family dragster and there was one particular picture that caught my eye. It showed the lower tube rail with an upward bow in it as it was attached to the frame table. When I built my Bonneville lakester, it's like a rear engine dragster but all enclosed, I built it with the lower rail flat and secured to the table. The result was that it has a slight permanent sag in the middle because of all the heat that was applied to the roll cage ( Bonneville cars require more tubes and thicker wall tubes, thus more hear from welding).
On a 200 inch wheel based fed how much bow goes in to it and where is the center of the bow located? Or is no bow needed because less heat is needed to weld the frame together?

[rooman]

It is not the heat from welding as much as the fact that the average front engine dragster does not have a lot of "beam" strength in the frame due to the fact that the frame rails are relatively close together in the vertical plane and are generally made of relatively thin tubing. Just like most aluminum flat bed semi trailers (and some of the better steel ones) the arch is built in so that under load the frame is straight. Although it is not really obvious in the photo the top rail is arched a little as well. "Big Show"  top fuel cars are also that way and most funny cars also have a little arch built in to the front half, especially those with slip jointed uprights.
  The amount of arch varies depending on the frame configuration (number of uprights, diagonals etc), engine location and weight etc.

Roo

[sawdawg]

So are we talking less than an inch and try to have the center of the arch under the motor

[rooman]

Less than an inch is around the mark but again it all depends on the vertical spread of the frame rails and the number of diagonals. The crown of the arch needs to be about a third of the way between the motor and the front end--or at least it does on my cars. More diagonals and/or uprights will make the car stiffer and less arch will be required

Roo