T3.6 Alternative Steel Tubing

[bandi8]

Hi everyone!

I'm writing in order to clarify a doubt I have with this point (T3.6). In the note 3 it says the following:

"Note 3: To maintain the equivalent yield and ultimate tensile strength the same cross-sectional area of
steel as the baseline tubing specified in T3.4.1 MUST be maintained."

Is the cross-sectional area that we have to maintain related to the dimensions of the table in rule T3.4.1 or to the minimum characteristics mentioned in note 4 of T3.4.1?

"Note 4: Baseline steel properties used for calculations to be submitted in an SES may not be lower
than the following:
Bending and buckling strength calculations:
Young’s Modulus (E) = 200 GPa (29,000 ksi)
Yield Strength (Sy) = 305 MPa (44.2 ksi)
Ultimate Strength (Su) = 365 MPa (52.9 ksi)"

I will expose these two cases:

1- If it's related to the table dimensions there is no opportunity to improve the design because although the thickness it's reduced you must increase the O.D. until to achieve the same cross-sectional area, so, no improvement is done.

2- But, if we take into account the rule T3.5, it says that you can play with the buckling modulus = E·I.

e.g. As the Young's modulus is maintained just because it's steel, you can increase the O.D. until you achieve the necessary moment of inertia, being the cross-sectional area smaller than the area in T3.4.1 table dimensions, so, we can reduce the weight.

This is the way I understood the rule T3.6, because unless this point is totally useless.

Thanks beforehand.

Ander.

[MCoach]

Ander,

the point is so that you don't make the entire chassis out of tubing that doesn't meet the base tubes in axial loading, but if you want larger diameter tubes, they can used without a weight penalty as is the case previously for such cases like using 1.25" tubing for the main hoop where the previous limit was 0.083" wall thickness and would make the tube heavier than the standard 0.095" tube. This allows for a lager range of tubing to be used that is equivalent with the minimum.

The new rules do not allow for a weight decrease at all if you've been building to the minimum tube sizes.

If you analyze the Percy and two other templates, the required tube sizes, and structure there is a rough set minimum chassis weight of about 45lbs (20.5kg) if you only build a chassis from the required tubes.