Chassis tube joints

[jdolecki]

As I build my hypothetical stream liner I wondering which tube joint is better / stronger?
I see lots of race car chassis built with all the tubes intersecting at a common point ( left drawing )
What about the right drawing were the tubes are spaced apart from each other? ( right drawing)

I know some one here has a good answer

[Stainless1]

intersecting together... It's all about load paths... but I'll let one of the structural engineering guys here explain in more detail...
Pork Pie? 

[Interested Observer]

I see lots of race car chassis built with all the tubes intersecting at a common point

Because that is the best way to do it.  Tends to eliminate local bending stresses and subsequent material failures.

[Rex Schimmer]

Your sketch on the left is the proper way to do a tube joint, it provides the highest stiffness and strength. It also can be the most difficult to fabricate which is why you see many chassis with tube joints that look like your sketch on the right. A good example of proper tube joint design and fabrication are the present day off road race trucks, take a look at the effort they take to ensure the proper tube alignment for the fabrication of their chassis. These frames are probably the most sophisticated tube chassis used in racing today and a good example to follow.

If you want to do it right use the left sketch method.

Rex

[manta22]

The non-intersecting joint on the right puts the top tube in bending. You want the tubes to be in either tension or compression.

[salt27]

I've seen chassis built both ways, pros, cons?

  Thanks, Don

[Stainless1]

The bottom line here is you want the the cage and all the parts of the car around you to be as tough, unbreakable and unbendable as possible.  Be sure to read the roll cage sections carefully and follow the pictures as closely as possible.  When in doubt, send your plan to the SCTA chair to approve...
Bonneville cars are stiff chassis... build it so it won't rattle apart on the rough salt we experience a lot.  Don't sweat the weight of all the tubes.... they might save your life.

[Koncretekid]

Here is a simple explanation of how using a truss can tremendously increase the strength of your tube chassis.  I will not do the actual math for you because I am not qualified to design structures (except for my own )



Tom

Upon further calculation, the multiple should be 16x as strong.  It's been a while since I have done these calculations.  The formula for moment of inertia (MOI) of a member is (base x height cubed)/12.  The formula for section modulus (Z) (which is the stiffness factor) is  (MOI)/(h/2).  So multiplying the height by 4 results in a (corrected) MOI of 4x4x4= 64.  But the section modulus (Z) must then be divided by half the height, so the corrected Z factor becomes 64/4 = a factor of 16 times as strong.  I will correct the drawing.

[Koncretekid]

Just wanted to let everyone know I corrected the above text to read that by increasing the height of a simple structure by a multiple of 4 results in an increase of strength of approximately 16 times.  This is not exact because of other factors, but gives you an idea of the reason for doing so.
Tom