Chassis Flex: Does it matter?

[manwplan]

Why, no bar that was cut out couldn't be replaced with some new tubing, grinding and welding? How bad do you want to win and how far you willing to go?

No, this car wasn't getting scrapped. It was about 10 races old at the time and after that test I put certain bars back in and that car raced for another 5 years. Won, tons of races.

I will be curious how much multiple cars of the same make and year vary. I have tested tubing crudely for how much it flexes and it varies a ton, especially between brands of tubing. I know 15 years ago I rockwell tested several scraps of tubing (dom and moly, from my drop pile and several friends that build cars too) that where different brands and same brands but years apart on manufacturer dates, needless to say they varied a bunch. Dom was the worst, even the same brand but different lot had a lot of variance. While not very scientific, it did show me some of the problem with a good car and not so good car could be coming from the tube itself. Hell back then you could go to one of the big LM manufacturer place and look at the tubing rack and you would see 5 different brands of tubing. Getting the same brand of tubing every time was and still is sort of impossible unless your ordering 6 months to a year in advance to if you have to wait for a certain brand of tubing it doesn't hold you up. I found one brand of moly that was the most consistent on Rockwell numbers so I built all of the cars out of that, right or wrong but the cars seemed really consistent for the small sample size of cars I built.

You should do some tests on Docol. That is some soft stuff and it is absolutely scary to me that it is the current popular material.

[Lizardracing]

a 100 years ago I got curious about this topic and tried something that Rocketbonehead mentioned.
I chained down 3 corners of the car at the springs pickup points on my dirt modified and hung weight on the unsupported corner and wrote down the flex number after each increment of 50lbs. What I discovered was the first 25 or so percent of the total weight hung, flexed the car the most and after that, each 50lb increment has less and less effect up to where I ran out of suitable weight. It seemed once the chassis was loaded up, it didn't move around as much. I didn't ever go further to perfect the two car garage hilly billy test, I still have the car but I've re-clipped and re-stubbed since then.

[TheJet-09]

It's not my story, and not overly tech related, but a long time hot-shoe from my area went to Rayburn's years ago to meet up with Steve Barnett. Rayburn was putting together a new car for Steve at the time. When the hot-shoe got there, Steve showed him how you could push on the nose some and see the entire roof move back and forth (the car was on jack stands). So the two went to lunch, and when they returned, Rayburn said, "Steve, I fixed the problem with that car you've been showing everyone." They went and looked at it, and all CJ had done was put it down on the ground. When Steve pointed out he hadn't changed anything on the car, CJ replied, "That's right, because you don't race them on jack stands."

More of a Rayburn story than anything, but I couldn't help myself.

[billetbirdcage]

You should do some tests on Docol. That is some soft stuff and it is absolutely scary to me that it is the current popular material.

I've been using Docol for several years now, while I haven't Rockwell tested it, it doesn't seem softer then moly I would say the opposite actually.

The major problem with Docol is RUST, since there is no scale (oxide or decarb layer) it will rust when it sits for a while. The outside is easy enough to clean and prep, but the inside is a pain in the arse. When it's clean and new, it's the cleanest welding stuff I have ever used. Moly has always acted what I call "dirty" when it welds even when sanding inside and outside of the tube.

Even the slightest rust on the inside of the Docol tube will cause problems when welding, you will be welding and its great then you hit a spot that acts like it has paint on it and bubbles and blows holes in it. So do everything in your power to keep the Docol from rusting and don't take a long time to finish a car. Even going back and adding tabs way later you will notice it. I sort of question how long before the cars turn into a pile of rust, lol.

Just for reference on materials:

Material Tensile Yield Elongation Diameter Wall Thickness

Docol 116,000psi 100,000psi 13% +/-.006"+/-.005”

4130 95,000psi 75,000psi 12% +/-.005" +/-10%

1020 Dom 80,000psi 70,000psi 15% +/-.005" +/-10%

If your like me and build a car here and there, the rust issue will be a problem when the tubing has been sitting for a while. Even in an A/C and heated shop, I still get a fair amount of rust on the tubing after a couple months.

Another potential problem is lead time on tubing, since AED is the only place in North America that you can get the tubing from (they bought the rights to north america) if they are out your sort of screwed. Last time I talked to them, they went from ordering to delivery in about 4 to 6 weeks (Docol is made in Sweden) to now 37 to 42 weeks. Also they don't stock every wall thickness in every diameter so some things I still use moly because it's not available here in the US.

[billetbirdcage]

It's not my story, and not overly tech related, but a long time hot-shoe from my area went to Rayburn's years ago to meet up with Steve Barnett. Rayburn was putting together a new car for Steve at the time. When the hot-shoe got there, Steve showed him how you could push on the nose some and see the entire roof move back and forth (the car was on jack stands). So the two went to lunch, and when they returned, Rayburn said, "Steve, I fixed the problem with that car you've been showing everyone." They went and looked at it, and all CJ had done was put it down on the ground. When Steve pointed out he hadn't changed anything on the car, CJ replied, "That's right, because you don't race them on jack stands."

More of a Rayburn story than anything, but I couldn't help myself.

I went to CJ's during the open house during PRI years ago. So there is a car on the jig and you can see it has 3 different types of tubing in it. CJ is talking to 2 guys and I hear him telling them this: See I use DOM here and cheap seemed HREW here (Course he didn't say cheap) and Moly here, because it causes flex here and stiffens here and ETC.

I'm think WHAT? Being the butthole I am, I butt in and ask CJ who's are the 2 frames for that are finished sitting in the paint booth waiting to be painted?

CJ says Don O'neal. I ask why both his cars are completely made out of moly?

That's when the fight broke out! Kidding but he didn't seem to pleased I asked him that.

[billetbirdcage]

a 100 years ago I got curious about this topic and tried something that Rocketbonehead mentioned.
I chained down 3 corners of the car at the springs pickup points on my dirt modified and hung weight on the unsupported corner and wrote down the flex number after each increment of 50lbs. What I discovered was the first 25 or so percent of the total weight hung, flexed the car the most and after that, each 50lb increment has less and less effect up to where I ran out of suitable weight. It seemed once the chassis was loaded up, it didn't move around as much. I didn't ever go further to perfect the two car garage hilly billy test, I still have the car but I've re-clipped and re-stubbed since then.

Pretty much what I see in backyard hillbilly tests I've done too.

Side note: Rocketbonehead is now racing/driving a modified instead of crewing. I talk to him via text about twice a week.

[ZERO25]

I am impressed that Billet cut that whole car up! Must have been one destine for the shelf.

We are setting up for some tests this season. We have measured on-track deflection and are quantifying the stiffness changes for several braces. Next do blind studies with a driver of using those braces in different pre-measured configurations. Data system with load cells on some key locations.

It may not yield any noticeable changes. In that case, additional stiffness is better for repeatability. In the opposite case, complementary suspension changes could be necessary and will need understanding.

Why is this an issue at all? Because some drivers say car to car variation matters. That points to a manufacturing tolerance that needs specification and validation test for each new car. Such that new cars are within a window that is smaller than a driver can discern.

What is a dirt car vehicle stiffness window? Around 2000 lb*ft/deg to 4000 lb*ft/deg. Now you can populate your models or do some napkin calculations. Maybe this can be your teams specification to your chassis builder when you order your next car. "I would like 2500 lb*ft/deg with +/- 100 lb*ft/deg over 40 midwest 1/4mile nights and max 0.125' plastic deformation". Or chassis builder could specify for you "this design is one of our favorite in the 2500 lb*ft/deg variety, with notes of strong restarts and smooth finishes". (corny)

Are your tests for a dirt chassis builder? Just curious if chassis stiffness is "engineered" into their chassis's!

[grt74]

yes, and not flexing is better,

[fastford]

hey billet , what was the actual wall thickness for each of your examples ?

[billetbirdcage]

hey billet , what was the actual wall thickness for each of your examples ?

If you mean on the cutting the bars out, all tubing was .065 moly but the main frame rails so all that was cut was either 1.5 or 1.25 OD tube that was .065 wall.

If you mean something else, I didn't follow

[fastford]

so all three examples on tensil strength were .065 wall ?

[MasterSbilt_Racer]

so all three examples on tensil strength were .065 wall ?

Tensile strength is a material property. Size doesn't really matter except for seam weld where the weld would be a higher percentage of the cross section. Then it's going to depend if a tube was tested or just a section cut from the wall.

All of my materials research and certification experience is from the aerospace industry. I'm not sure how they tensile test structural tubing. The applicable ASTM spec doesn't make allowances for different strength based on diameter. But, the strength ratings are just minimum, where composition of steel and other things have a range to fall inside.

[billetbirdcage]

For reference: HREW (IE muffler tubing)

The seam in the tubing while I'm sure it has an effect, let me say I have never seen a seem split from a wreck. The only time I ever saw a seam split down the length of the tube was on a header so I'm sure heat is what caused that. So why is HREW or CREW so much weaker then say DOM? it's generally not the same material, Dom is 1018 or 1020 and HREW can be several from 1008 to 1010.

Here is a general list for HREW strengths (again it varies due to what material it actually is)

40,000 to 45,000 psi 30,000 yield - 15%

You can see why you don't want to make a racecar out of this stuff unless your using super thick wall stuff and even then it's not worth it.

[fastford]

maybe I should have said structural strength , and that may not be correct because I am no metallurgist , but my understanding is .o65 moly has as much structural strength as .125 mild steel plus memory , which makes for a lighter chassis that will flex and come back , I may be thinking of .095 MS , cant remember what the article I read was

[MasterSbilt_Racer]

maybe I should have said structural strength , and that may not be correct because I am no metallurgist , but my understanding is .o65 moly has as much structural strength as .125 mild steel plus memory , which makes for a lighter chassis that will flex and come back , I may be thinking of .095 MS , cant remember what the article I read was

It can flex and come back. But as I have argued for years, chassis should be designed for a certain rigidity. If that is the case, all steel tubing should be the same diameter unless it is a driver protection part of the car where the extra yield strength is of value.

All steel has roughly the same "spring rate" known as modulus of elasticity. And the stresses should be far enough below the yield point, when you have achieved proper stiffness, that a moly car and DOM car should behave the same when built with same size tubing. If you go thinner tubing with moly, the car can live, but it's now less rigid.