Chassis Tubing (Really Need Help)

[Erick Scarpone]

Hi I'm working on the selection of the tubing material for our next car... So basicaly I'm looking for less weight, good mechanical properties, and low cost. And that we can do the Safety Structure Equivalency Form if we get to use a diferent W.T. or O.D than the ones listed in rule 3.3.3.1.

So i made an Excel file with all the different W.T and O.D of 4130 in chassis shop, I was hoping to find out that I could use less O.D with the minimum W.T required by the rule 3.3.3.2.2 but to my surprise I just got that using O.D 1+1/8'' and W.T. 0.049 I comply with the minimum required buckling (E*I) and is actualy 0.086lb/ft less than using O.D. 1'' & W.T. 0.065 (Actually for the Side Impact proteccion it says that a minimun W.T of 0.049 but it does not comply with the E*I, at O.D of 1'')

For Bracing, harness Attachment Etc. I got to the conclusion that using the same O.D. 1+1/8 with W.T 0.049 wpold be the way to go less weight and still using 4130.

For the Front & Main Hoop I got that the way to go would be using O.D 1+1/8 and W.T 0.065 insted of using O.D 1'' and W.T of 0.095 couse I be saving 0.1823Lb/ft.

So my big question is WHAT IM I DOING WRONG??? I'm asuming that something is wrong because i did not saw cars with tubing of big Outside Diameters at the competition. Then I Started thinking that maybe people would use minor O.D and really big W.T but this configuration does not save weight, and doing some resarch I read about that in the second moment of inertia if the mass is way more far of the deforming axis it is better for torsional stiffnes (For Example is better for Torsional Stiffnes a round hollow tube than a round solid bar).

Anyway I'm watching that using chromoly and SAE1010 the add up the same weight, and SAE 1010 is cheaper. So what are the real advantages of using chromoly?

Im using the area inertia ecuation of (Weakest axis)Ix=pi/64*(O.D^4-I.D^4) and E is very similar for SAE 1010 and Chromoly?
Help please Help!!!
Sorry for the lenght of the post but I do need help and whant people to understand my situation thanks!!!

[Erick Scarpone]

Hi I'm working on the selection of the tubing material for our next car... So basicaly I'm looking for less weight, good mechanical properties, and low cost. And that we can do the Safety Structure Equivalency Form if we get to use a diferent W.T. or O.D than the ones listed in rule 3.3.3.1.

So i made an Excel file with all the different W.T and O.D of 4130 in chassis shop, I was hoping to find out that I could use less O.D with the minimum W.T required by the rule 3.3.3.2.2 but to my surprise I just got that using O.D 1+1/8'' and W.T. 0.049 I comply with the minimum required buckling (E*I) and is actualy 0.086lb/ft less than using O.D. 1'' & W.T. 0.065 (Actually for the Side Impact proteccion it says that a minimun W.T of 0.049 but it does not comply with the E*I, at O.D of 1'')

For Bracing, harness Attachment Etc. I got to the conclusion that using the same O.D. 1+1/8 with W.T 0.049 wpold be the way to go less weight and still using 4130.

For the Front & Main Hoop I got that the way to go would be using O.D 1+1/8 and W.T 0.065 insted of using O.D 1'' and W.T of 0.095 couse I be saving 0.1823Lb/ft.

So my big question is WHAT IM I DOING WRONG??? I'm asuming that something is wrong because i did not saw cars with tubing of big Outside Diameters at the competition. Then I Started thinking that maybe people would use minor O.D and really big W.T but this configuration does not save weight, and doing some resarch I read about that in the second moment of inertia if the mass is way more far of the deforming axis it is better for torsional stiffnes (For Example is better for Torsional Stiffnes a round hollow tube than a round solid bar).

Anyway I'm watching that using chromoly and SAE1010 the add up the same weight, and SAE 1010 is cheaper. So what are the real advantages of using chromoly?

Im using the area inertia ecuation of (Weakest axis)Ix=pi/64*(O.D^4-I.D^4) and E is very similar for SAE 1010 and Chromoly?
Help please Help!!!
Sorry for the lenght of the post but I do need help and whant people to understand my situation thanks!!!

[Chris Clarke]

Generally you want to increase the dia and reduce the wall thickness to increase EI and reduce weight.

We have used larger dia roll hoops and side impact tubes in the past and been approved for SSEF.

If you look in rule 3.3.3.2.2 it states the minimum wall thicknesses. These are:

Roll hoops - 0.083"
Bracing, front bulkhead, harness attachments - 0.065"
Side Impact - 0.049"

So you cant use O.D. 1+1/8 with W.T 0.049 for the bracing/harness, and you also cant use O.D 1+1/8 and W.T 0.065 for the roll hoops.

For the roll hoops we used 1.25" Dia x 0.083", this ends up being heavier than the minimum, but is much stiffer. Our last chassis was overkill, so I think I will be going with 1" dia for the next roll hoops.

We used 1.25" Dia x 0.049" for the lower side impact tube, which did end up being lighter (barely).

When using tubing with two different dia, you also have to watch the mitres, as you can't really mitre a 1.25" dia tube to fit a 1" dia tube. Also, if you want to reduce the 1.25" dia back to 1" somewhere along its length, you will have to machine a reducer. This all adds up.

I am curious to know what square tubing people have used for side impact and had approved for SSEF. Especially seeing as we will have to prove equivelancy for the front bulkhead bracing for 2006.

For the mild vs chromoly, there is tons of stuff on that arguement here and on the internet/library.

[Matt Gignac]

For the side impact, we used 1" square, .049 wall for the bottom side impact bar. Technically, a .035 wall would have given sufficient EI, but there's a minimum of .049 wall.

I think it ended up having an EI about 25% more than the minimum from the rules, and a little bit heavier.

And maybe my calculations were off, but 1 1/8 OD could be used for the main hoops with .083 wall, and should be stiffer for pretty much the same weight. And 1 1/8 is probably harder to find.



Matt Gignac
McGill Racing Team

[Erick Scarpone]

The rules say that if we use a thinner W.T. then we must use a bigger O.D. so for the main hoop wouldn't be ok to use an O.D. of 1+1/8 and 0.065 W.T??? If we can't use this type of configuration what would be the point of the SSEF??? There would not be a way to reduce weight and still get a minimum E*I.
Cool way of testing our imagination...don't you think?