Roll cage..Cromoly or steel? Options

[brannanjohn]

Which is better?

Is it worth the extra $ to get Cromoly or is the only diff weight savings?

I have also heard that Cromoly requires special preparation such as pre heating etc, that should only be don eby experienced persons.

vs mild steel which is easier to work with and just as strong, but heavier.

Any input?

[GlennCMC70]

mild steel DOM is around $6 per foot. C/M steel is like $18 per foot. C/M is not lighter, just stronger, so you can use a thinner tube. NASA reqiuers .120" wall M/S and .095" C/M steel as an example.
C/M is suposed to be TIG welded, and can be MIG welded, but it takes a very special process of preheating the tube and using heat to bring down the temp after the welding. @ $18 per foot, if you can afford that, you can have it TIG'ed.

it really depends on your goal w/ the car. FYI, the NASA CMC class prohibits the use of C/M for cost reasons.

[brannanjohn]

thanks. I did not realize it was banned from CMC. THis is a weekend play toy and will not be raced in anything bigger than a regional event. I just wanted to ensure safety.

thanks, it look slike mild steel.

[GlennCMC70]

banned in not a good word. its leagal to use for NASA and SCCA. its just not leagal for CMC cause they really try to keep the cost of this class low. C/M tube (i used 80' in my cage) to build a cage will cost you $1500 just for the tube!

[brannanjohn]

The more that I think about, I think I will use moid steel and get the strongest tensil strength. Your absolutely right about price though.

I didnt want to be the only guy running steel cage and have to upgrade later. (more $$$)

thanks

[PeteL]

Although chrome-molybenum alloy steel SAE 4130 can have higher tensile strength...if heat treated properly...than mild steel SAE 1020, it has lower impact strength compared to mild steel. Impact strength, as measured in something like a Charpy Impact test, measures the amount of energy that a material will absorb before fracturing. Simply put, Mild steel will bend in a impact situation where cromoly will break...all else being equal. Considering what you ask of a cage in a crash is to absorb energy while maintaining as much as possible the integrity of the cockpit, the better impact strength of mild steel is an advantage.

Given the difficulty in heat treating a roll cage after welding and lack of stiffness to weight advantage, I have a difficult time seeing why one would choose cromoly over mild steel in this application.

There are certainly places on a race car where Cromoly has advantages over mild steel, suspension arms come to mind, but if I ever put a cage in my car, it will be mild steel.

[brannanjohn]

thanks all. Looks like I am going withthe mild steel cage.