CM flexes, watch a top fuel car

I know that, this is from an engineer. and if I were building a car it would be CM. but there is nothing wrong with a MS car.

Not to be a stickler, but when I was in engineering school 20+ years ago it was known as Chrome Moly, which is a steel with significant alloying elements of chromium and molybdenum. One misconception is that a moly car will be stiffer (all other things equal), but 4130 has the same modulus of elasticity (basically spring rate) as 1020, it just has a higher yield and ultimate strength. In other words, if you have two equally sized bars (of 4130 & 1020 steel) and stuck 500 lbs on each, they would deflect the same amount (as long as you are below the yield stress of the 1020). However, there is a load level where the 1020 will be permanently deformed but the 4130 can still spring b

For the chassis guys out there:
I have put myself in a pretty good position. Wife and kiddies all have new or late model cars. Now it's my turn, but I am opting for an upgrade for the 'Cuda. I really want a new tube chassis for her. My questions are:
1) I know C/M is stronger than M/S, but since you are using thinner walls, which is stronger/safer when in use?
2) I know that either way, the final project will be lighter than my backhalved project, but how much weight will I really save using C/M over M/S for a full tube chassis?
3) Rough idea over cost difference?
At this point, I am putting together a high 8 sec project, but you never know what tomorrow brings, so getting it certified to 8.50 or 7.50 is also on the table as far as cost difference. I know that there are a ton of variables, but I want to consider apples to apples. IE; Control arms or struts, ladder bars or four link. I am just considering the chassis with body mounted. Whatever suspension I decide to use, I will install myself, just having all of the mounting tabs welded in place to start with. Thanks for everyone's consideration in advance.
Kenny

and not to mention CM is supposed to be pre heated before tig welding.i don't see any chassis shops doing this.

Just my opinion, but If capable I would go CM, 4-Link & Strut Front End and 7.50 cert .... You will not only end up with a much better handling performing ride.....But down the road if you happen to decide to unload it the return on your investment should be much higher.

Rickster

I agree with everything you said except the thing about the ROI. You make me laugh. My oldest son is just waiting for me to call it quits or pop off. He knows he's next in line for the car. I wouldn't put it past him to wait for me to complete it and then have me committed and claim power of attorney.

IMHO

Slower than 7.50 and not a headsup car where every pound counts, MS all day everyday with ZERO concerns. preferably TIG'd.

Safety, MS no broken tubes or joints like CM in an impact. Seen too many CM cars with cracked and broken tubes without crashing.

Mine is an older Jerry Bickel MS chassis (TIG'd)) with 3000+ runs and is as straight and square as the day it was built, also has never had a cracked or broken joint/tube. It's been 8.0's and should be 7.50's this year and I have zero concerns with it.

What you're seeing re:springback is the proof. The amount of springback is proportional to the strength of the steel if the sections are the same.

Every kind of steel has the same or nearly the same Young's modulus or modulus of elasticity, or spring rate. This data is available.

That's the elastic region. The slope of the stress vs. strain curve is the same up to the yield point, but where the mild steel curve flattens out (yields), the higher strength steel continues upwards and flattens out (yields) at a much higher stress. So you have to put much more stress on the CrMo steel to get it to permanently deform (bend or yield) than you do with the mild steel. When you take off the load, the steel runs backwards down the curve, and because you have to stress CrMo more, it has more distance to return (springback).

The CrMo chassis is NOT stiffer than a mild steel chassis, all things being equal, because of the similarities in spring rate. Also the section of the mild steel tube is much larger so it will be stiffer up to the point of yield.

Strength is very much different. Strength means how far you can bend something or twist, before it permanently deforms. In this the Cr-Mo wins.

Assume a chassis made of the two materials, but equal in diameter and wall thickness.
In a crash, a certain amount of energy is working on the frame trying to deform it. The mild steel will distort and then yield as the yield point is reached. The CrMo will distort and take a lot more energy to get to the point where it finally yields or buckles.

So to offset the lower strength of the mild steel, the sanctioning body mandates larger diameter thicker wall. That should give the same resistance to permanent deformation between the two. In this case, the mild steel will deform less until it fails. The CrMo will deform more before it fails. If the sanctioning body's calculations are correct, they will both permanently deform at the same amount of energy, just behave in a different way as they get to that point.

My thanks to Prof.Ronald Apanian for teaching me how to understand this.

R.

No contradiction at all. This comment indicates the common misconception...perfectly...The steel's spring rate has NOTHING to do with its strength. It's two different things.

That is the hardest concept to "get". They all have the same reaction in the elastic region. They all go up the same path on the Stress vs. Strain graph. It's just that high strength steels keep going up at the same angle, while low strength steels go nearly horizontal way below, depending on their yield strengths.

And THAT is the real reason why there needs to be all kinds of different strength steels. It's how highly it can be stressed before it plastically (permanently) deforms.

Note the graph with curves for different kinds of steels used in industry.

R.

Depending on who you ask, the desired filler for 4130 is ER70S-2, ER80S-D2 or even 312 (stainless). Most seem to lean towards the ER70 because of it's slightly lower strength and weld qualities. It's not quite as finicky as the higher strength material. The ER80 when mixed with the base metal, approaches the hardness of 4130. The ER70 weld doesn't get quite as hard. The result is the weld has a bit of "give" which can help prevent cracking or breaking in an impact situation without sacrificing strength in a properly designed assembly. I use ER70S-2.

TIG welds on properly cleaned, prepped, and welded joints will be pretty. If not, you need to fix what's wrong.