Two rod straight joints

[raptor93]

Hi everyone,

We are a new FSAE team. We are running short of chassis rods and ordering new rods is not possible.
Is it allowed to join two rods end to end in a straight line? and is that a violation of the triangulation rule?

[Claude Rouelle]

A. I don't understand your question. Maybe others will but I don't. Can you elaborate? Offer a sketch?
B. Why is ordering new rods not possible?

[Big Bird]

I think the original poster is asking if he can butt-weld chassis tubes. My responses:
- irrespective of whether it is legal or not, it is a really bad idea. I sincerely doubt you will match the original tube for strength or straightness
- why can't you order more material? If budget is that tight that you cannot afford some steel tube, then you are in for a rough time when this project starts throwing some real expenses at you

Kind regards,

Geoff

[raptor93]

Dear Big Bird,
yes, we were talking of butt welding. This kind of joint is done only for one straight pipe of chromoly 4130 which is less than half a metre after two straight pipes are butt welded. ordering such a small pipe is not possible because of no access to such pipes in our region. That was unfortunately the last pipe to be welded in the chassis with no leftovers.

Kindly tell if it will be considered as a bent tube requiring any support according to the rule T3.5.5

Thanxxxx in advance.

[Big Bird]

Hi Raptor,

Firstly, I am not someone who can give you official judgement on whether this arrangement is legal or not. I repeat, it is not a good idea though.

A deeper issue is that of your use of 4130. Please excuse me for shouting, but:
YOU DO NOT NEED TO MAKE YOUR CHASSIS OUT OF CHROMOLY

Just in case you missed the point:
YOU DO NOT NEED TO MAKE YOUR CHASSIS OUT OF CHROMOLY

Chromoly, 4130, whatever you want to call it, is a WASTE OF MONEY. It is no stiffer than mild steel, and the extra yield strength is wasted as a chassis should be nowhere near yield anyway.

This point has been made many many times on this forum. Make your chassis out of mild steel - cheaper, and doesn't need heat treatment after welding.

Kind regards,

[Luniz]

Plus: A PIPE is a thing for liquid or gaseous substances to go through, hence it is referenced by its INSIDE Diameter. A TUBE however is a thing made to build structures from.

[Tim.Wright]

Hi Raptor,

Firstly, I am not someone who can give you official judgement on whether this arrangement is legal or not. I repeat, it is not a good idea though.

A deeper issue is that of your use of 4130. Please excuse me for shouting, but:
YOU DO NOT NEED TO MAKE YOUR CHASSIS OUT OF CHROMOLY

Just in case you missed the point:
YOU DO NOT NEED TO MAKE YOUR CHASSIS OUT OF CHROMOLY

Chromoly, 4130, whatever you want to call it, is a WASTE OF MONEY. It is no stiffer than mild steel, and the extra yield strength is wasted as a chassis should be nowhere near yield anyway.

This point has been made many many times on this forum. Make your chassis out of mild steel - cheaper, and doesn't need heat treatment after welding.

Kind regards,

At the risk of opening this can of worms again... wouldn't you accept 4340 as a chassis material for reasons of crash/damage resistance? With the same modulus but higher yield and UTS I'd assume there would be more strain energy to failure for a 4340 structure compared to a 1020 structure?

Perhaps its overkill for FSAE... But if I was designing a steel spaceframe for hillclimbs I'd put a decent amount of thought into what happenes when you slide sideways into a tree.

Tim

[Big Bird]

Hi Tim,
Yes, I certainly would accept chromoly in a form of the sport where it might be likely that you could impact a tree, or another car, or a rhinoceros, or a Bridgeport mill. But in our sport we are exceedingly unlikely to encounter most or all of those things. And when you have situations like this, where a team is compromising their budget and their structural integrity, for the sake of persisting with a material spec suitable for an impact that isn't going to happen, or for a material spec suitable for repair and re-use when effectively the car will only be used once anyway, then we are spending money on a spec not suitable for us.

[Drew Price]

Raptor,

Ask the rules committee directly.

The answer may actually depend on which tube in your chassis you intend to make from more than one piece of material.

If it's the front or main roll hoops for example, the rules specifically say those parts of the structure must be made from one, continuous, un-cut length of material.

No one here can give you an official answer.

And you can weld plain carbon steel tubes to 4130 if that makes it easier for you. Use the same welding rod (or wire) you used for the rest of the chassis.

[Claude Rouelle]

Raptor93,

I was confused by the word rod, rods end etc... You would have used tube (never pipes!) I probably would have got it quicker.

That being said
a. Without any spare tube if you have a crash or a chassis failure and you can't order more tube, what is your plan B?
b. I agree with Geoff that 4130 is a unnecessary expense for a FSAE. What made you decide to got for 4130?
c. If you have headache once or twice a year what do you do? Take an aspirin right? That is what I would do. You will work on the illness ignoring its cause(s). But if you have a headache every day you will probably want to consult a doctor and work on the causes: food allergies, breathing polluted air, too much stress, maybe time to change girlfriend...? I have not seen your chassis drawings but I have seen seen so many first year chassis with so many conceptual design mistakes (even if the chassis passes technical inspection) such as lack of triangulation, suspension pick up points not at a chassis node etc that I fear that you won't be the exception and therefore that would make the choice of 4130 even more a engineering mistake and waste of money. Work on the causes of chassis deflection or break before you work on its consequences.

[Z]

Plus: A PIPE is a thing for liquid or gaseous substances to go through, hence it is referenced by its INSIDE Diameter. A TUBE however is a thing made to build structures from.

And a ROD is a long cylindrical piece of metal with a diameter somewhere between a WIRE (smaller) and a BAR (larger).

A ROD made out of wood is often called a DOWEL.

A BAR with a hole running through the middle of it is called a HOLLOW BAR. Since it is referenced by both its inside and outside diameters, it might be called a PUBE!!!???

Off-road racecar frames are often built out of PIPES. These frames have to carry large loads, and can potentially have very large impacts. So making them out of relatively thick wall, high quality, piping, such as is used in the Petro-Chemical industry, makes a lot of sense. Consider that such PIPES often carry highly toxic or explosive chemicals at extremely high pressures, yet still have to be reliably welded together at their many hundreds of butt joints. (Edit: the internal pressures can exceed the yield stress of mild-steel! Racecar frames are girly-boy stuff... :))

As for the OPs original problem. I suggest you go to your local cafeteria and, ahem, "beg, borrow, or buy" one of their chairs or tables. Should be enough tubing there to finish your frame.

Z

[Charles Kaneb]

What load is this tube carrying? Is it a regulated tube?

Can you switch to a sleeve over the junction?

Can you upsize the tube to use a mild steel one? If you have zero spare 4130, how will you confirm that you're using the right filler wire for the mild-to-4130 joints and practice those joints?

If it's only one tube you need, can you get it from a different supplier at a higher cost?

As-welded 4130 is about 15% stronger (60* vs 50 ksi yield) than 1018. Additional strength is possible in 4130 with heat treatment, but there is an advantage as-welded. You should be below the endurance limit in normal service and below the yield limit in your anticipated overload conditions, but they've got a saying about actual vs anticipated loads: "That's Racin'!" You'd think we'd test in wide-open lots with well-defined courses away from obstacles, but I managed to drive our 2011 car straight-on into a curb at around 40 MPH two days before Texas Autocross Weekend a few years back. Somehow, it handled better afterwards - with new lower control arms.

The advantage of 4130 over 10xx for American teams has little to do with material properties and everything to do with Aircraft Spruce stocking 100 sizes of 4130 tubing to ship out on UPS. If you specify that you want manufacturing test papers for one tube in your order you might get certification papers for all of them.

*on spec sheet - YMMV.

[Rex]

"That's racin" indeed. I follow the mild steel argument but I struggle to agree with it. I have personally seen a fair number of these cars crashed over the years. Side impact with a pole, front impacts with buildings or curbs, rollovers, etc. 5+ examples I can think of right offhand that resulted in real damage. Bad decisions in course layout? Sure. But who hasn't made a bad decision or two. If 4130 has an advantage in crash worthiness I would argue it at least warrants considering. Spending money on safety in racing is usually the best money you can spend. Would we suggest buying the cheapest helmet and say that buying anything else is a waste of money? This is a huge change from my position 10 years ago when I was building cars (when I thought these cars would never hit anything), but when you've seen some scary stuff your opinion tends to change.

[raptor93]

All that I can say is that these yellings and sarcastic comments were indeed a good lesson learnt. We do consider the arguments above as correct and makes us consider these flaws in our project. Due to lack of experience we committed this mistake and didn't think so critically as we lack time and everything seems new and complicated. We aim to successfully represent first and learn for the coming years.

Thanxx :D

[raptor93]

Rex that's a heads up for us even though we agree 4130 is a bit unnecessary. Being a first year team lacking experience in building and driving we are definitely worried about impacts and failures. Hence i agree a little extra expenditure would save the money spent to repair the broken bones :P

[Big Bird]

Fair call Raptor, my sincere apologies for yelling. As a long term contributor to these boards I find myself often repeating the same points. It is more the fault of the difficulty in finding info / random scattering of information across these boards - and my lack of patience - than your inexperience. Lesson learnt.

[raptor93]

Big bird we take yellings as a matter of concern. Shall keep this in mind for the next time. But just a worry will this have a negative impact on the design judging wherein the only reason for the use i can give is the safety.

[Claude Rouelle]

I cannot speak for Geoff "Big Bird", (although I bet he will think as I do) but as a design judge in many FSAE / FS competitions I can assure you that any question simple or sophisticated, educated or lees educated, where ever it comes from will never ever create any "pre-judging" in my mind or any other judges/ mind. At the contrary the daring of asking questions (whether the answers in this forum are smooth or brutal) shows the interest of the student and his desire to learn. It will be pleasure to meet you one day at one the competitions and put the name and the face together. At the design event it is your car and your knowledge that will be evaluated, not your forum participation.

[raptor93]

Hi all,
Our Main Roll hoop is of mild steel. And the side impact structures are of chromoly 4130 and We used ER70S-2 as filler material for the dissimilar metals. Is it fine to do so?? Or does it fringe any specific rule??

[Drew Price]

No Raptor, that's fine. ER70-S2 is the correct filler for mild-steel to mild-steel, milt to 4130, or 4130 to 4130 (unless you are going to heat treat it - then you have some rreading to do.)

[raptor93]

According to rule T3.5.5 "The support tube must have the same diameter and thickness as the bent tube." Will a tube of same diameter but larger thickness will also be acceptable as the support tube?

[Claude Rouelle]

Why would you want to do that?

[Charles Kaneb]

If you're going to weld in a mild steel tube to replace the 4130 one you don't have, you'll need to upsize it either in diameter or wall thickness. Changing wall thickness needs less redesign work, changing diameter will be lighter.

[Markus]

Raptor: I think you will have no problems in using an "oversized" tube in such a case.

Claude: Given the subject of the thread, I would presume they want to use a different tube size because they're lacking tubes of the same size.

Charles: That is in the case you design for strength. If you design for stiffness the difference between mild steel and 4130 is neglible thus there shouldn't be a need for redesign.

[raptor93]

Thank you for the responses.

Markus you interpreted our problem just right.

Thank you for helping.