I posted this thread in the drag racing forum yesterday (https://honda-tech.com/zerothread?id=936287) but I never seem to have any luck with any of my questions there.

I have read that chromoly has a much shorter lifespan than DOM...it fatigues faster and therefore is not a good idea on a car that will see any street time or for a budget racer as it will need replaced more often.

Unfortunately, in all my research about this subject I haven't come up with much info. I have found forums where people will mention that chromoly can turn brittle over time, etc...but what does this mean? What kind of life expectancy is there on a chromoly roll cage?

The bottom line is that with the chromoly cage I will save about 30 lbs (remember this is a drag setup so we get tubing size breaks by using chromoly), but its $400 more. I wouldnt worry if this was a one time cost, but if I am going to have to replace the cage in 5 years since the metal will be too fatigued I don't think the weight savings is worth it. Can anyone help me?

 

You need to get some professional guidance from the people building the cage, I think.

K

 

The cage builder is a great resource but I am trying to get as much info as I can on my own. I am treating this as I do any other part for my car...

I mean, I have talked to the people who are providing me with slicks about what sizes to go with, tube/tubeless, etc. but that still hasnt stopped me from reading everything I can about the subject. Unfortunately I am just having more trouble finding information on this subect that I do with race tires, fuel pumps, etc.

 

It's really not worth the time and money to do a moly cage. It does have a shorter life span, and you will need to have every weld tig welded with moly. To really get the strength out of a moly cage, it should be baked to restructure the molocules in the welds. You really should only think about moly if you are building a drag chassis from scratch! Just go with DOM and be done with it. I believe NHRA allows you to use ERW if not mistaken.

 

The fatigue strength of the Chromoly verses mild steel issue involves more than just the materials involved. When mild steel is welded, the resulting micro-structure in the heat affected zone (HAZ) is usually a combination of large grain Pearlite and Proeutectoid, which are generally relatively ductile. On the other hand, welding say 4130 or other alloy steel, an undesirable and very brittle Martensite micro-structure is formed. To reduce this brittle martinsite to a more ductile coarse pearlite (like in the mild steel case) you would need to anneal the steel. Unfortunately, once the cage is welded into the body structure, annealing is very difficult if not impossible. This is why a DOM cage is practical and desirable in all but a small percentage of the applications.

However, if a Chromoly cage must be built, it is imperative that all joints be gusseted to reduce the bending and other distortional stresses that act within the joints.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Johnny Mac &raquo;</TD></TR><TR><TD CLASS="quote">The fatigue strength of the Chromoly verses mild steel issue involves more than just the materials involved. When mild steel is welded, the resulting micro-structure in the heat affected zone (HAZ) is usually a combination of large grain Pearlite and Proeutectoid, which are generally relatively ductile. On the other hand, welding say 4130 or other alloy steel, an undesirable and very brittle Martensite micro-structure is formed. To reduce this brittle martinsite to a more ductile coarse pearlite (like in the mild steel case) you would need to anneal the steel. Unfortunately, once the cage is welded into the body structure, annealing is very difficult if not impossible. This is why a DOM cage is practical and desirable in all but a small percentage of the applications.

However, if a Chromoly cage must be built, it is imperative that all joints be gusseted to reduce the bending and other distortional stresses that act within the joints.</TD></TR></TABLE>

Hmm good info. It sounds like DOM is the way to go...I will just have to make up the weight penalty somewhere else in the car. 30lbs isnt that much...just turn up the boost right?

Thanks for the help.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bomber &raquo;</TD></TR><TR><TD CLASS="quote">

Hmm good info. It sounds like DOM is the way to go...I will just have to make up the weight penalty somewhere else in the car. 30lbs isnt that much...just turn up the boost right?

Thanks for the help.</TD></TR></TABLE>

that extra 30 lbs is good insurance and just think of how much money you're saving as well by not goin' Chromo.