Anyone use a chrome-moly rollbar?

[jlandreth]

I'm hoping to save some weight. I helped Harv install his Autopower bar and that sucker is heavy!

[jlandreth]

I'm hoping to save some weight. I helped Harv install his Autopower bar and that sucker is heavy!

[jerry]

generally, chrome-moly refers to minute amounts of chromium and molybdenum added to steel to increase hardenability, especially in thicker sectioned parts. you will not gain any weight savings with these alloys over plain carbon steel. you will generally gain higher strength and toughness when heat treated properly.

i doubt that any manufacturer of roll bars is using anything but these cr-mo type steel tubes such as 4140 or with Ni addition 4340.

[jerry]

generally, chrome-moly refers to minute amounts of chromium and molybdenum added to steel to increase hardenability, especially in thicker sectioned parts. you will not gain any weight savings with these alloys over plain carbon steel. you will generally gain higher strength and toughness when heat treated properly.

i doubt that any manufacturer of roll bars is using anything but these cr-mo type steel tubes such as 4140 or with Ni addition 4340.

[MrNvgtr]

I thought they weren't allowed in most sanctioned track events. Just something to remember even if you don't plan on tracking the car any time soon. If you sell it, the next owner might have those plans.

[MrNvgtr]

I thought they weren't allowed in most sanctioned track events. Just something to remember even if you don't plan on tracking the car any time soon. If you sell it, the next owner might have those plans.

[jlandreth]

I guess I should have done my homework before posting the question. I just hate to put that much weight, that high in the car. On the other hand,I attended a D.E. event a few months ago and saw an E-36 M-3 smash the b-pillar on the driver side, to the top of the driver seat .He was lucky to escape w/o injury

[jlandreth]

I guess I should have done my homework before posting the question. I just hate to put that much weight, that high in the car. On the other hand,I attended a D.E. event a few months ago and saw an E-36 M-3 smash the b-pillar on the driver side, to the top of the driver seat .He was lucky to escape w/o injury

[TobyB]

Yeah,

some sanctioning bodies allowed it,

but most now do not, because it's a devil

to weld and then heat- treat back to appropriate strength...

I strongly suggest, if you're having something built,

to have it done at a race shop to SCCA specs.

It'll be easier to sell later...

t

[TobyB]

Yeah,

some sanctioning bodies allowed it,

but most now do not, because it's a devil

to weld and then heat- treat back to appropriate strength...

I strongly suggest, if you're having something built,

to have it done at a race shop to SCCA specs.

It'll be easier to sell later...

t

[bmw1602.com]

it's lighter because you can use a smaller diameter and have the same strength. Also, since it smaller diameter you have more headroom (in a cage).

It's not allowed because you need to heat treat any cro-mo part that you weld.

In any case don't take my word for it. Here the master's words explaining it accurately.......

From "Engineer to Win" by Carroll Smith

" Welding 4130

There are a lot of myths surrounding the techniques of welding 4130 steel. Most of them are just that - myths. To be sure, care must be exercised. We have a narrow choice of suitable filler rods. Pre- and post-heat are important and become critical in thicker sections and all welds should be stress relieved after welding. But there are no great secrets involved.

Most racing welders do not believe that it is necessary to stress relieve welds made in 4130, especially TIG welds. They are wrong. Maybe explaining why will make some converts to the cause of righteousness.

Contrary to popular opinion, the necessity for stress relieving 4130 weldments has little if anything to do with carbon content or weld contamination, and the superior shielding afforded by TIG welding does not help at all. In fact, we shall see, the need for stress relief is GREATER with TIF-welded 4130 than it is with oxyacetylene welds.

4130 is an "air hardening" steel. During welding, by whatever process, both edges of the joint are heated to the molten condition and then allowed to cool in air. This results in the creation of a series of "layers" in the heat-effected zone adjacent to the weld itself. In a short distance measured at right angles to the line of the weld we will find metal that has been heated considerably above the critical zone, just above it, not quite up to it, nowhere near it, and only slightly heated. As the weld puddle progresses, the heated metal behind it will be cooled both by exposure to the air, and by the quenching affect of the adjacent cooler metal. The heat=effected zone will thus cool more rapidly than if the whole part had been evenly heated and cooled as a unit. The weld is, in effect, quenched and is therefore considerably more brittle than the surrounding material. It doesn;t take a cold liquid to achieve a quench - all it takes is a source of RELATIVE cool. The weld is liable to be largely martensitic in structure and to fail under impact. In addition, the grain structure will vary throuought the entire heat-effected zone, and with larger grain size being found in the portion that has been hottest.

With TIG welding, the heat effected zone is geographically narrower than it is with torch welding. Far from being an advantage, this produces a steeper temperature gradient in the heat effected zone and, with air-hardening steels, a more brittle weld than a good torch weld - regardless of how skilled or careful the operator is about slow withdrawl and post purging.

The solution is simple. At the completion of welding, the entire area surrounding the welds should be evenly heated to about 1200*F (somewhere between dark cherry and medium cherry) and allowed either to cool in STILL AIR or covered in LIME or DRY SAND. Still air is best arranged by placing shields around large parts (like tube frames) or by placing a sheet metal box over the part until it has cooled. This, as you will have deduced by now, is also the reason that 4130 should always be pre-heated (gently warmed to 300-400* F) BEFORE starting a weld. It is a case of keeping the temperature gradient in the heat effected zone as gradual as practical. With higher-alloy steels, such as 4340, pre-heat and post-heat are an absolute necessity. Strictly speaking, it probably isn;t NECESSARY to either pre-heat or to normalize welds in 4130 if the sections are 0.065" or less in thickness, but it is damned good insurance. "

Most cro-moly road bicycle frames, off-road A-arms, plane airframes structures are ALL sent to a heat treating facility with large specialized ovens.

1020 DOM has a tensile strength of about 80,000 psi, and 4130N is about 90,000 psi. Once heat treated it goes up to to about 130,000 psi !!!

Since it's kinda hard to find a heat treatment facility that will heat treat a chassis with a cage in it. You end up with brittle welds and weaker than DOM metallurgy.

Which is exactly why cro-moly is no loger allowed in SCCA, many clubs and I belive the FIA as well.

Cages will tend to fail few millimeter away from the beads on impact. Exactly where the heat affected zone ends (HAZ). It all comes down to knowing and having the experience to be able to weld it correctly. I'm not going to say 4130 is difficult to weld but you do need to control your heat input a little better when compared to mild steel. Mild steel is very forgiving, 4130 is a little more picky with the haz.

In the end YMMV, so do what you want. I'd stick with DOM. It's usually cheaper anyway.

[bmw1602.com]

it's lighter because you can use a smaller diameter and have the same strength. Also, since it smaller diameter you have more headroom (in a cage).

It's not allowed because you need to heat treat any cro-mo part that you weld.

In any case don't take my word for it. Here the master's words explaining it accurately.......

From "Engineer to Win" by Carroll Smith

" Welding 4130

There are a lot of myths surrounding the techniques of welding 4130 steel. Most of them are just that - myths. To be sure, care must be exercised. We have a narrow choice of suitable filler rods. Pre- and post-heat are important and become critical in thicker sections and all welds should be stress relieved after welding. But there are no great secrets involved.

Most racing welders do not believe that it is necessary to stress relieve welds made in 4130, especially TIG welds. They are wrong. Maybe explaining why will make some converts to the cause of righteousness.

Contrary to popular opinion, the necessity for stress relieving 4130 weldments has little if anything to do with carbon content or weld contamination, and the superior shielding afforded by TIG welding does not help at all. In fact, we shall see, the need for stress relief is GREATER with TIF-welded 4130 than it is with oxyacetylene welds.

4130 is an "air hardening" steel. During welding, by whatever process, both edges of the joint are heated to the molten condition and then allowed to cool in air. This results in the creation of a series of "layers" in the heat-effected zone adjacent to the weld itself. In a short distance measured at right angles to the line of the weld we will find metal that has been heated considerably above the critical zone, just above it, not quite up to it, nowhere near it, and only slightly heated. As the weld puddle progresses, the heated metal behind it will be cooled both by exposure to the air, and by the quenching affect of the adjacent cooler metal. The heat=effected zone will thus cool more rapidly than if the whole part had been evenly heated and cooled as a unit. The weld is, in effect, quenched and is therefore considerably more brittle than the surrounding material. It doesn;t take a cold liquid to achieve a quench - all it takes is a source of RELATIVE cool. The weld is liable to be largely martensitic in structure and to fail under impact. In addition, the grain structure will vary throuought the entire heat-effected zone, and with larger grain size being found in the portion that has been hottest.

With TIG welding, the heat effected zone is geographically narrower than it is with torch welding. Far from being an advantage, this produces a steeper temperature gradient in the heat effected zone and, with air-hardening steels, a more brittle weld than a good torch weld - regardless of how skilled or careful the operator is about slow withdrawl and post purging.

The solution is simple. At the completion of welding, the entire area surrounding the welds should be evenly heated to about 1200*F (somewhere between dark cherry and medium cherry) and allowed either to cool in STILL AIR or covered in LIME or DRY SAND. Still air is best arranged by placing shields around large parts (like tube frames) or by placing a sheet metal box over the part until it has cooled. This, as you will have deduced by now, is also the reason that 4130 should always be pre-heated (gently warmed to 300-400* F) BEFORE starting a weld. It is a case of keeping the temperature gradient in the heat effected zone as gradual as practical. With higher-alloy steels, such as 4340, pre-heat and post-heat are an absolute necessity. Strictly speaking, it probably isn;t NECESSARY to either pre-heat or to normalize welds in 4130 if the sections are 0.065" or less in thickness, but it is damned good insurance. "

Most cro-moly road bicycle frames, off-road A-arms, plane airframes structures are ALL sent to a heat treating facility with large specialized ovens.

1020 DOM has a tensile strength of about 80,000 psi, and 4130N is about 90,000 psi. Once heat treated it goes up to to about 130,000 psi !!!

Since it's kinda hard to find a heat treatment facility that will heat treat a chassis with a cage in it. You end up with brittle welds and weaker than DOM metallurgy.

Which is exactly why cro-moly is no loger allowed in SCCA, many clubs and I belive the FIA as well.

Cages will tend to fail few millimeter away from the beads on impact. Exactly where the heat affected zone ends (HAZ). It all comes down to knowing and having the experience to be able to weld it correctly. I'm not going to say 4130 is difficult to weld but you do need to control your heat input a little better when compared to mild steel. Mild steel is very forgiving, 4130 is a little more picky with the haz.

In the end YMMV, so do what you want. I'd stick with DOM. It's usually cheaper anyway.

[Guest Anonymous]

Which is exactly why cro-moly is no loger allowed in SCCA, many clubs and I belive the FIA as well.

SCCA, NASA, PCA, & BMW Club Racing, and Grand Am have released their 2008 rules, and all of them still allow 4130 chro-mo as roll cage material. Which club's rules were you thinking about? I do know places that don't allow it, but for the type of racing many of us are likely to experience ourselves here in the US, 4130 is a legal cage material.

In most cases, I think mild steel is better for production-based racecars, and I usually try to talk folks out of using 4130, esp. when there's little hope the welds gets stress relieved. Mild steel absorbs energy better which is a good thing for a safety structure.

There are some pro cages in 4130 that get done correctly at great expense, but I think many more are not stress relieved & are subject to the ill effects you cited from Carroll Smith's book.

[Guest Anonymous]

Which is exactly why cro-moly is no loger allowed in SCCA, many clubs and I belive the FIA as well.

SCCA, NASA, PCA, & BMW Club Racing, and Grand Am have released their 2008 rules, and all of them still allow 4130 chro-mo as roll cage material. Which club's rules were you thinking about? I do know places that don't allow it, but for the type of racing many of us are likely to experience ourselves here in the US, 4130 is a legal cage material.

In most cases, I think mild steel is better for production-based racecars, and I usually try to talk folks out of using 4130, esp. when there's little hope the welds gets stress relieved. Mild steel absorbs energy better which is a good thing for a safety structure.

There are some pro cages in 4130 that get done correctly at great expense, but I think many more are not stress relieved & are subject to the ill effects you cited from Carroll Smith's book.

[Guest Anonymous]

I believe they still allow it in the regs but not at a smaller diameter to save weight except in pre 1994 cars that already have current log books.

http://sports.racer.net/docs/rules/2006/GCR2006.pdf

GCR in 2006

section 18.6.C

C. Minimum tubing sizes for (all Showroom Stock, Touring

and Improved Touring Category auto-mobiles registered

after June 1, 1994) for all required roll cage elements (All

dimensions in inches):

Up to 1500 lbs. 1.375 x .095 DOM / Seamless / Alloy

1501-2200 lbs. 1.500 x .095 DOM / Seamless / Alloy

2201-3000 lbs. 1.500 x .120 DOM / Seamless / Alloy

1.625 x .120 DOM / Seamless / Alloy

1.750 x .095 DOM / Seamless / Alloy

(American Sedans may construct to these specs regardless of weight.)

3001-4000 lbs. 1.750 x .120 DOM / Seamless / Alloy

Over 4000 lbs. 2.000 x .120 DOM / Seamless / Alloy

Note: ERW tubing is not permitted in any car registered with

SCCA after of 01/01/2003.

So yes you can run it. No it won't save you weight. Yes it will be stronger if welds are done properly. Yes it will cost atleast 2x as much. No it won't make a difference in winning. The reason people run mild steel is tech inspectors have a trepidation against 4130 and know how to check DOM with ease. That and it's arguably as strong in a crash because it deforms and not shatters and anyone can make it safely with a mig gun instead of a tig artist.