TechCorner by joe timney
Box vs. Round Tubing
There is generally no advantage to a box frame car. Most people think
that this design is stronger, when in fact the strength of any racecar
comes mainly from the roll cage being properly tied-into the chassis. Th
secrets are the triangular tubing layout, and properly installed
gussets. A standard door car cage with engine bay bars and x bars from
the top of the cage back to the upper rear shock area will more than
double the chassis stiffness. I must admit that an inexperienced first
time builder may be uncomfortable with a round tube project but with all
of the tube notching equipment available, it is not the problem it once
was. Another advantage to a round tube car is that it sits lower to the
ground, due to the fact that the frame isnt as tall. One more plus for
a round tube car is the $130 you will save not buying the box tubing.
Mild Steel vs. 4130 Material
There is a myth that a chassis constructed of 4130-chrome molly is
stronger than mild steel one. While 4130 is a stronger material, the
rules allow us to use a thinner wall thickness (.083 & .065) Therefore,
the 4130 structure will be 25 to 30% lighter than mild steel. This may
or may not be a consideration in your planning. Other factors to
consider are the cost (about $900 more for 4130) and ease of
construction. A mild steel chassis can be MIG welded (Metal Inert Gas)
while the 4130 tubing requires the TIG method (Tungsten Inert Gas) and
must be done by a professional.
Mild Steel Wall Thickness
Although the rulebook only requires .120 wall tubing thickness in a
mild steel application, it would be wise to use .134 (11ga.). You must
be aware that .120 wall tubing can vary by production runs and can be
within acceptable industry mill spec standards yet being under the
minimum .120 nominal wall thickness! Since NHRA has instituted the .118
minimum, I have had to repair many new cars as thin as .113!
When ordering tubing from a steel supplier, specify ASTM A513 Type 5,
DOM (Drawn-Over-Mandrel). This material is a cold drawn electric
resistance welded tube with all flash removed. Each tube is tested for
soundness of weld. It is made from 1020 steel in walls up to 10 gauge
and 1026 steel in walls heavier than 10 gauge. Normalizing and cold
drawing over a mandrel makes DOM a uniform and precision Product. It is
preferred over seamless tubing for its excellent O.D. & I.D.
concentricity.
Cold Drawn Seamless Tubing (ASTM A519) is another good choice but
availability in .134 wall thickness is limited. It is made from 1026
steel, produced to OD & ID dimensions; and it is furnished in As
Drawn condition. Cold Drawn Seamless offers good surface quality and
increased mechanical properties over hot finished seamless tubing.
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