Landracing Forum
Tech Information => Technical Discussion => Topic started by: ronnieroadster on November 02, 2011, 06:09:44 PM
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Is it legal to use 4130 chrome moly for a lakester roll cage? My build requires 1 5/8 diameter tubing if moly is legal I need to know what the tubing thickness should be.
Thanks in advance for any guidance.
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Hi,
I can not answer your question directly, but let me be the first to ask you if you've acquired a rule Book yet? I don't recall what numbers it give for CM wall thickness, but those that I've talked to about my planned project have recommended not using Chrome-moly tubing, something about it becoming brittle, and snapping in a crash versus folding up, helping to absorb an impact.
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Is it legal to use 4130 chrome moly for a lakester roll cage? My build requires 1 5/8 diameter tubing if moly is legal I need to know what the tubing thickness should be.
Thanks in advance for any guidance.
1 5/8 x .095 4130 was legal this year.
You should be highly familiar with the welding characteristics of 4130 before
using it. It is excellent stuff done properly.
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As Carroll Smith put it some decades ago (paraphrased, memory isn't what it was):
"The advantage of moly vs. mild steel creates a narrow safety envelope in which, to have value, the anticipated crash must be serious enough to overwhelm the lesser metal, but not sufficient to overcome the greater. It's pretty small."
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There have been quite a few discussions about proper construction techniques here on this site.
The other qiute from Carrol Smith (on 4130) is this "if you don't heat treat you end up with an expensive part with the same strenth as 1020- and brittle weld areas".
John
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Thanks for everyones impute. Yes the material cost is more the Tig welding will be done by a certified welder but considering the weight advantage I feel its worth it.
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Less weight in an LSR vehicle, isn't always a good thing.
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Thanks for everyones impute. Yes the material cost is more the Tig welding will be done by a certified welder but considering the weight advantage I feel its worth it.
Unless you're looking at the outer limits..... like a liner, the rewards are debatable. It's stronger for the same weight , or lighter for the same strength but time and time again we see the same advice, use mild steel tubing.
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Mild steel is so much more practical and readily accessable, and facilitates quicker repairs and remedies, if necessary.
Ask yourself this question - "If I needed to fix or add a weld to my chassis on the salt, would I have the capability to do it?" You said that the chassis builder would be a certified welder - is he coming with you if something breaks?
Yeah, there are hundreds of folks at Speedweek who can weld anything from tungsten to wicker, and a number of them would be happy to step up and lend you a hand, but you kind of need to be as self sufficient as you can.
Keep it simple, keep it practical, keep it fixable.
I'd spend the extra money on dyno time.
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As Mr. Midget says, think of it like you are going to Mars and need to fix it to get back home. Murphy's Law has a front row seat on the salt. :-o Tony
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If you're planning on running on the salt weight often becomes your friend. This is like no other form of racing. If you check the build threads you'll find that more often than not weight must be added to increase the speed. Air becomes tough stuff to push.
If I were building a cage for the salt I'd invest in a load of DOM tubing.
Pete
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Some of you guys think totally different than I do and that is NOT meant as bad.
I have a VERY strong aversion to checking out before my time.
If I decide to go LSR racing, I would want to be in a car that was built WAY over
enginered to be as safe as possible. That would include the strongest alloy steels
and engineering practices available. If I did not have the money to have the work
done by qualified people, I would go to school and learn how to do it myself, which I did.
Previous crashes at the salt can be analyzed and studied but you can never foresee the future.
Previous crashes at Indy, formula 1, World of Outlaws NASCAR and others can be studied
but eventually there will be one that is different. The same applies to Bonneville.
It boils down to each crash is different. You can't build a car too strong.
The rule book is a guide. It doesn't hurt a thing to go WAY past that...
If you can't afford it, either wait till you can or gamble with an early racing carreer at that
GOLDEN SALT FLATS in the sky.
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Some of you guys think totally different than I do and that is NOT meant as bad.
I have a VERY strong aversion to checking out before my time.
If I decide to go LSR racing, I would want to be in a car that was built WAY over
enginered to be as safe as possible. That would include the strongest alloy steels
and engineering practices available. If I did not have the money to have the work
done by qualified people, I would go to school and learn how to do it myself, which I did.
Previous crashes at the salt can be analyzed and studied but you can never foresee the future.
Previous crashes at Indy, formula 1, World of Outlaws NASCAR and others can be studied
but eventually there will be one that is different. The same applies to Bonneville.
It boils down to each crash is different. You can't build a car too strong.
The rule book is a guide. It doesn't hurt a thing to go WAY past that...
If you can't afford it, either wait till you can or gamble with an early racing carreer at that
GOLDEN SALT FLATS in the sky.
I think you'd be surprised at the even spread of opinions.
I don't think there would be many experienced racers who would disagree with the intent of your post, but I think you might be surprised at the reaction to regarding all who come here as an homologous group.
Your contention that people regarding mild steel as acceptable as being tantamount to "gambling" makes me think you may have not considered the nature of injuries sustained in an accident and the possible and practical ways to avert those injuries.
As Carroll Smith put it some decades ago (paraphrased, memory isn't what it was):
"The advantage of moly vs. mild steel creates a narrow safety envelope in which, to have value, the anticipated crash must be serious enough to overwhelm the lesser metal, but not sufficient to overcome the greater. It's pretty small."
If you give me an explanation in your own words of the meaning of the above statement I will put more weight in your opinion.........
So , what you contend is, at the very least, we should use CrMo of the minimum dimensions , or bigger or we are foolish?
Roll cage material is but a small part of the safety package....fire-suits, extinguishers, proper design of the cab and intrusion protection...the HANS, the helmet...the chassis..........
Mostly we can't defend our brains against sudden deceleration.
The car I drive is so over engineered in the drivers area it is ridiculous. The chance of me being crushed or mortally injured by a physical object in an upset is , I hope, small. If I somehow get it into the air at 200mph and then stop abruptly It's not going to be the cage material that determines my outcome.
I can choose between super-strength and protection or ultra-lightweight in an effort to minimise inertia............. it's not a simple argument.....
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Is this poster R Sangiovani? Or someone using the same nick?
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My advocacy of mild steel is purely practical. You need to build your car out of the legal material you feel most comfortable racing with.
Of the few accidents that I'm aware of on the salt where a driver was injured, I can't think of one where the difference between mild steel and chrome moly would have made a difference in the outcome.
While Pete1 is correct when he says no two accidents are the same, the nature of the crashes that tend to occur in NASCAR, Outlaws, Formula 1 and Indy are those of the collision variety, where a car hits another car, a wall, or some sort of obstruction. Rollovers and getting airborn are, unfortunately, the common crash scenarios in LSR. I would recommend building accordingly.
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The rules require a heavier wall thickness for mild steel tubing which will likely more than make up for the difference in material strength. DOM tubing is very high quality tubing, much tougher to bend than the equivalent ERW. It is probably much less likely to fracture beside the weld than the 4130N given the difference in wall thicknesses and the effect of the heat affected zone in 4130. The N is very important if you are going to use chromemoly tubing.
Pete
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I would want to be in a car that was built WAY over
enginered to be as safe as possible.
Easiest, cheapest, most foolproof method of making the cage stronger:
add 1/2" to the OD.
Comparing moly to mild, perhaps a 30% advantage in strength (viz., before separation) with the same wall thickness. If wall is thinner, strength is a match, but small disadvantage in stiffness.
Comparing minimum OD to +1/2" OD, using 1-5/8" as a base, same wall thickness to remove a variable:
1-5/8" at .083" = .360 "stiffness units" (dimensionless)
2-1/8" at .083" = .834 "stiffness units"
The 2-1/8" is about 32% stronger, and more than twice as stiff.
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Yes Tman this is Ron San Giovanni.
Peter Jack I agree when running on the salt weight is your Friend we learned that when helping the team get the D/BSTR record a few years back I was the one who made the final call to add weight depending on the salts condition prior to each run so this area of LSR racing is not new to me.
But excess weight can also be your enemy during an accident there is always going to be a fine line about such a decision. My interest in weight reduction if I decide to use the chrome moly gives me the opportunity to place the weight where I feel its needed.
Ronnie Roadster
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Yes Tman this is Ron San Giovanni.
Peter Jack I agree when running on the salt weight is your Friend we learned that when helping the team get the D/BSTR record a few years back I was the one who made the final call to add weight depending on the salts condition prior to each run so this area of LSR racing is not new to me.
But excess weight can also be your enemy during an accident there is always going to be a fine line about such a decision. My interest in weight reduction if I decide to use the chrome moly gives me the opportunity to place the weight where I feel its needed.
Ronnie Roadster
Thought it might be you from the HAMB, have admired your cars for years.
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Last I spoke with Steve Davies (chief inspector) they strongly prefer mild steel to moly though latter still legal. Regardless of diameter and "stiffness" as Panic mentions however, minimum wall thickness with mild steel is nominal .125 (.120). Though have seen no suggestion, I would not be surprised to sometime see a switch to outlaw moly. As others have said, too much may be just enough cage strength wise ( or not enough depending on point of view :-o). And I agree about the weight comments, I just don't like it in the back of the car.
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Last I spoke with Steve Davies (chief inspector) they strongly prefer mild steel to moly though latter still legal. Regardless of diameter and "stiffness" as Panic mentions however, minimum wall thickness with mild steel is nominal .125 (.120). Though have seen no suggestion, I would not be surprised to sometime see a switch to outlaw moly. As others have said, too much may be just enough cage strength wise ( or not enough depending on point of view :-o). And I agree about the weight comments, I just don't like it in the back of the car.
I surely hope they don't outlaw it, just because an inspector prefers one over another. :mrgreen:
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My experience is with chrome-moly motorcycle frames. They are not uncommon and they work satisfactorily, for the most part. They are lighter. Occasionally they would crack and I would weld them up. My welds looked good and they worked OK about half of the time. The welds that went bad did not break. The metal near the weld cracked. The cracks did not happen just after the welding. They occurred later when the frame was stressed. This is a big problem. A person does not know they have a bad joint until it is stressed and it cracks.
Ask your welder if he knows how his work has held up during hard use after he welded it. This is the critical thing.
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Some comments on the above discussion:
“Certified weldor” -- Certified by whom, to do what, with what materials, with what technique, under what conditions? Unless the answers to these questions fit your application, you’re whistling in the wind.
4130 -- As has been stated, 4130 offers some, but marginal advantage in strength over mild steel unless it has been properly frabricated and heat treated. It also offers a proclivity for brittle fracture. It is most useful for piece parts that can be machined from heat treated material, or roughed out, heat treated, and then finish machined. For welding, what qualified weld procedure would you use? What pre-heat? What qualified filler metal? What welding technique and joint design? What post-weld stress-relief for all the welds? For space frame construction, do you know somebody with a pretty large heat-treat facilities? Can you live with the resulting deformation? Not to mention non-destructive testing of the joints for cracks, even assuming all the previous steps were properly taken. Then, how do you repair the cracks without invoking the whole process again? Welding alloy steels is not a trivial undertaking.
Carroll Smith is correct and some of the headaches outlined above are why the conventional wisdom, also stated in previous posts, is that mild steel is hard to beat in this application. Using 4130 borders on foolishness.
Peter -- Mild DOM tubing has some strength advantage due to the cold-work involved during its forming, and can have a nice finish for the same reason, but can lose that effect in the heat affected zone of a weld. It can also have more wall thickness variation than ERW tubing.
Panic’s “stiffness units” -- What Panic is referring to is the bending moment of inertia of the tube cross-section. (Would be nice to use more descriptive, or even correct, terminology). This doesn’t apply to axial stiffness, and may or may not be an indication of the actual bending capacity of the as-fabricated tubing frame elements. The joint design would have to be adequate to support the capacity of the tubing, and there is a real possibility of local bucking of the tube wall before the cross-sectional capacity is reached.
Wobbly -- The cracks were probably there, it just took a while to become easily noticeable. That’s why alloy welds need to be stress relieved and NDE’d.
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"I would not be surprised to sometime see a switch to outlaw moly."
I don't foresee that happening in the near future -certainty not in 2012.
DW
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"I would not be surprised to sometime see a switch to outlaw moly."
I don't foresee that happening in the near future -certainty not in 2012.
DW
I realise that this is the information age, but what the hell is "outlaw moly"??
I do know that there are more/divesified alloys available in yuur-up. I dealt with some in karting years ago, all seemed to respond well to being treated like 4130 (post annealing and pre heating).
John
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I realise that this is the information age, but what the hell is "outlaw moly"??
outlaw as prohibit
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I realise that this is the information age, but what the hell is "outlaw moly"??
outlaw as prohibit
Yeah. He's using it as a verb, not a noun. :wink: :cheers: Wayno
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Duh..........
here in the state of Confusion (California), English is a second language.
The Brits have been using a form of alloy that is not 4130, supposedly made for race car chassis (easy to weld/repair etc.)
J
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"I would not be surprised to sometime see a switch to outlaw moly."
I don't foresee that happening in the near future -certainty not in 2012.
DW
May have stirred up something here :-(. I would emphasize my use of "sometime". In 2010 with a new car we were really given a hard time in inspection about the moly cage though were eventually passed on it. It did meet rulebook specs and then some. Anyway, I was just speculating here based on these discussions and not really meaning to be predictive. Certainly if such a change were made I would expect a lot of grandfathering and warning about it.
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The top fuel and funny cars are Chrome moly and they front and back half them 2 or 3 times a year due to fatigue and bending.
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The top fuel and funny cars are Chrome moly and they front and back half them 2 or 3 times a year due to fatigue and bending.
This is a great example of apples and oranges, the stresses are completely different. I'm sure they would grandfather in established vehicles. If not, there would be a lot if protesting going on. 8-)
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I'm sure they would grandfather in established vehicles.
Kind of like what happened to the four wheel drive roadsters. :evil: Tony
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The top fuel and funny cars are Chrome moly and they front and back half them 2 or 3 times a year due to fatigue and bending.
I believe chrome moly is required in top fuel but not sure.
Wish I could put more stress on our chassis [like hook up like a top fueler] :-P
JL222
ps ..working on it :-D
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is using mild steel erw and 4130 moly on same chassis legal ?,(example would be mild steel frame rails and 4130 roll cage)
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Hotschue (Udo Horn) in the Bonneville Bugeye (see the build diaries) did that. Inspector gave us hard time about it but passed it. Hard time based on my prior comments- preference for mild steel rather than moly in general as has been tossed about here at some length, and tube diameters also commented on and the fact that the "cage" extends to the feet as well as around the body. Door car cages are different from special construction by nature so depends on what you are building and the mix and match pieces.
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I don't want to rattle anyone's cages here, but it might be a cause for concern. Following is a photo of two pieces of my frame tubing, CREW 1" x .060" wall, TIG welded together (by me under the watchful eye of my frame welder, who is a certified airframe welder.) After I welded these two pieces together, he put it in a vise and pounded it with a 2 pound hammer until he bent it. He said to me "if that had been chrome moly tubing, which I use exclusively for J3 Piper frames, it probably would have broken." Hopefully, no one will be testing this theory. They use Chrome-moly for airframes, race bikes, and dragsters, because weight is a concern. Whichever you use, make sure it is properly welded and annealed if necessary.
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what kinda car would work better in lsr on the salt ? a nhra pro stock or a nascar assuming they both had same motor and wheels and tires
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Neither would be legal for anything but "time only" so sorta a moot question. :-(
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I now have my popcorn and a comfortable seat, carry on.
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:-D Since you asked- don't see Pro Stockers crash often, but the Nascar guys seem to do it for fun and games regularly. Had my druthers in a crash on the salt, think I'd pick the Nascar one as they seem to walk away mostly intact from some pretty horrific stuff.
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The strength (safety) of a race chassis does not come from the choice of construction materials, it comes from a proper design appropriate to the material chosen.
You can build a chassis out of wood that is stronger than chrome Moly if you design it right. Pound for pound wood is stronger than steel if stressed properly (in compression), it is also stiffer and better at dampening vibrations. That does not necessarily make it the ideal construction material (although the guys that rode British Mosquito fighter planes and American PT boats into battle might have a strong case to defend the strength of properly designed wood construction).
Point being chrome moly is not some magic material that instantly gives extra safety. It has certain physical characteristics that allow you to build a very strong (very expensive) light weight chassis, but you can build a chassis just as strong out of mild steel tubing for lower cost, and it will fail more gracefully in a catastrophic accident consuming energy bending stiff ductile tubing where the same accident would likely break chrome moly tubing chassis of the same strength when it was over loaded.
I will take an over engineered mild steel tube chassis every time over chrome moly.
The only practical advantage of chrome moly is that it allows you to build a stronger chassis for the weight, but you give up money, stiffness ease of repair in the field and graceful ductile failure in the bargain.
Larry
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A note regarding TIG welding aircraft structure 4130 material. For years the only accepted method was gas welding, which, if done correctly, anneals the weld area and greatly reduces joint failure in the heat effected area just outside of the weld fillet. As I have posted a number of time on this site I built and modified a number of off road motorcycles back in the 70s with 4130 tubing that I gas welded. I raced them all for a number of years, both motocross and District 37 desert racing and never had a frame failure. I generally avoid 4130 today as I do TIG weld it and have had failures related to fracturing in the heat affected area.
I do agree with hotrod regarding being able to build very satisfactory structures using mild steel tubing of a heavier wall than 4130 and being probably a better option, however the property of a mild steel structure that hotrod seems to like," graceful ductile failure" I don't think I could support if the "graceful ductile failure" happened to be the roll cage folding in over my head! I think that we have all probably seen Top Fuel dragsters crashing at some pretty high speeds and their driver survival cages certainly appear to hold up well to such crashes. So again it is the design and fabrication techniques used to be appropriate wit the material that is important.
Rex
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as a hypothetical , is it considered stronger if the roll bars of the cage are welded to the bottom frame rails and 2nd rail into that or weld on top of the shoulder hoop. also if the roll bars have to be 1 continuous piece
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however the property of a mild steel structure that hotrod seems to like," graceful ductile failure" I don't think I could support if the "graceful ductile failure" happened to be the roll cage folding in over my head!
You are misinterpreting my comment. The idea of graceful failure is well understood in engineering design.
You can never design a structure or component so it is absolutely guaranteed to never exceed its design limits and fail.There will always be that 1:1000 or 1:1000000 event where all your design planning assumptions turn out to be too conservative or the loads are way beyond anything you planned for.
Example suppose you design the roll cage over your head to survive a 20 G impact after the car flies at 200+ mph and lands on its top. Ungraceful failure would be for the car when experiencing a 21 G impact for the roll cage structure to break apart and the car parts and driver to fly in loose formation until they all come to rest.
Graceful failure of the cage would be for when subjected to a 21 G or 30 G impact for the cage to remain intact but deform significantly, perhaps putting a scuff mark on the drivers helmet where the top of the cage comes down 2 inches but still manages to protect his head.
Alive and injured is always better than almost alive. Mild steel structures when loaded beyond their ultiment strength tend to bend and stretch to release some of the over load without breaking.This in safety cage structures can be the difference between a deformed but still useful safety bubble inside the cage and a cage loop that buckles and breaks at the welds coming off the frame leaving the driver with ZERO protection.
Personally I believe rule making bodies should include energy dissipation structures into their safety cage designs. When they were designing safety casks for radioactive materials they tried several different designs to come up with a structure that would survive really extrodinary extreme impacts. Things like being hit by a high speed freight train at a RR crossing or having a truck loaded with a D-9 bull dozer cross the center line and side swipe the safety cask with the bull dozer blade at highway speeds (140 mph closing speed), the final designs have fins on the cask (they look sort of like an air cooled engine cylinder). Those fins are designed to take such an extraordinary impact and deform using up energy so that the cask structure itself although mangled still survives and maintains it integrity.
Rule makers are finally starting to consider such energy absorption structures. Champ cars now require an energy absorber on the back of the transaxle unit so if the car backs into a concrete wall at 200+ mph there is some give and dissipation of energy in the structure before an immovable force (concrete wall) meets an extremely rigid metal structure (transaxle/engine assembly) and gets pushed through the drivers seat back.
NASCAR after the Earnhart accident also included energy absorbing foam structures in the doors and of course went to the energy absorbing barriers.
Roll cages should have an energy absorbing structure on the top/outside of the primary cage tubes to absorb catastrophic impacts and use up energy before the cage structure has to deal with the impact.This way the impact of something like an engine block striking a main roll bar tube, would instead of denting or cutting the outer wall of the tube, would mash the external crush tube, blunting the impact on the structural tube, and protecting it from that point impact.
The ideal low cost crush structure could be something as simple as a second external tube of lighter wall thickness, designed to crush on hard impact and protect the structural tube it is attached to.
Larry
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Nice discussion Larry.
Reminds me a bit of a conversation we had with a guy we were buying a Valkyrie kit from. (Valkyrie was sort of GT40 lookalike kit car, corvair running gear, Small block chevy mid engine. 40 years ago we built and went autocrossing, never licensed). Seller, after a question about the apparent light structure of the frame, said "there are all sorts of collapsable members up front to absorb an impact"..Wag in the family commented "yeah, like our legs and feet". I nearly had occasion to find that out. Old Marlboro sports car track in southern Maryland. We were autocrossing, running backwards of usual and at the beginning, now end, of the long back straight is a very tight hairpin turn, bordered by a solid wall. The valkyrie was light with a fairly healthy 327 in it and I was wailing down that straight. Inadequate brakes!!. OOPS. Just made it around that corner but scared the bejesus out of me. And roll cage? whats that? barely had seat belts. :cheers:
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Larry, You make a really interesting point about secondary tubing. I am actually building this into my 26-7 T Coupe bodied Comp Coupe. With the roof being flat, I am building a complete steel framework for it out of 1'' square, .083 tubing. I am going to attach it to the cage with some short pieces, and make slip joints with Grade 5 bolts, so I can remove these bolts and then lift the body off the frame for major change/maintenance. What do you think?
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Sounds like the idea has possibilities. Basically you want an energy absorber to undergo as much destructive crushing as possible to use up energy.
In some racing classes they specify a block of high density foam in the drivers side door to help provide crush in case of a drivers side T-bone. There are several materials that can be used for crush elements. High density poly urethane type foams, and aluminum honey comb both use up a tremendous amount of energy as they are crushed.
When I was a teenager my Mom was trying to understand how the then "new fangled" padded dashes prevented injury because to normal finger pressure they felt pretty darn hard. I had a wild hair and ran into the bed room and grabbed a block of 1.5 inch thick styrofoam I had been using for model making and dashed back into the kitchen placed my hand on the floor and set the styrofoam on top and told my Dad to stomp on it. He immediately figured out what I was up to and promptly stomped on the block of foam as my Mom let out a yelp of protest. I showed her the block which had a clear imprint of the back of my hand on one side and the heel of my Dad's shoe on the other. Then the light dawned for her that under an accident the forces were so high that that "hard dash" was much softer than the typical pressed steel dash boards of the 1950's.
Same goes for a crush zone on a body or above a roll cage. You would want to create a crush element that used up the maximum amount of work possible as it was being crushed between the roof skin and the roll bar tubes.
It could be a thin wall square tube like you are talking about filled with high density foam, or a thin wall tube like a 1.5 inch exhaust tubing tacked on the top of the roll cage tube. The impact would have to crush that thin wall tube flat before the main roll cage tube really had to do any work. If you wanted more crush resistance you could nest several smaller tubes inside a large diameter tube.
For example a 2 inch exhaust pipe tube with several pieces of 1/2 inch electrical conduit inside, tacked together then filled with high density foam. You would want it attached strongly enough that it would not "squirt" out of the way on impact but stay in place to be crushed.
Square tubing filled with balsa wood or any other crush-able material could be devised depending on what you had on hand.
It might be interesting to have a local engineering school graduate student figure out how much energy it would take to crush flat several different simple designs.
As the saying goes, it is the sudden stop that hurts you. If you can just double the distance it takes to stop from 1-2 inches to 3-4 inches of crush, when the car impacts a very hard surface, you have made a huge reduction in peak G loads. That difference just might be the difference between breaking major structural welds and serious injury to the driver.
Larry
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is using mild steel erw and 4130 moly on same chassis legal ?,(example would be mild steel frame rails and 4130 roll cage)
I would not think that it would be a good idea to mix the two types of steel in the same structure. The mild steel will need to yield in a crash situation, i.e. bend, stretch, compress, or do whatever the situation requires. The higher strength steel will not yield (bend, stretch, compress) as much before it reaches it's breaking strength. I think the result will be that all the load will come onto the stronger members before the mild steel has had a chance to it's job, which is absorbing energy. The welds would probably fail, and the entire structure would no longer act as a unit.
A similar example of this is the use of too much reinforcing steel in a concrete beam. A beam with the proper amount of reinforcing steel, will actually bend, stretching the steel (which becomes stronger as it stretches), and hence the failure mode will be slower and noticeable. A beam that has too much steel, will literally explode, as the concrete exceeds it's compressive strength, and fails catastrophically before the steel has reached it's yield strength. Think about that when you're driving beneath all the overpasses on the way to the races!
Tom, B.S.M.E.
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Hmmm......Looks like I need to find an Engineering Student capable of being `Salt Trained`...... :-P
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There should be no troubles beyond those already expressed for 4130 if it is welded to a mild steel main frame rail. As a matter of fact 4130 is generally welded with a mild steel rod, 70S2. This is a double deoxidized rod with no moly. The welding engineers I've talked to and the articles I've read all seem to feel that enough molybdenum is picked up from the base metal that the weld is more than satisfactory. A weld between 4130 and mild steel should gain the desired characteristics between the two alloys and would likely be a little more malleable than a weld between two pieces of 4130 and thus be a little more forgiving. Always remember that the most important parts are the design of the joint and the weld and the skill of the operator applying the weld.
Pete
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A friends single seat off road race buggy was built out of mild steel with chrome moly members welded on it to hang the front suspension from.
It took a lot of abuse and he broke a lot of parts but I don't remember the mild steel to chrome moly welds ever being an issue.
Other than that I know nothing. :-D
Don