Chassis Flex: Does it matter?

[Ghopper]

Chassis flex: Does it matter?

I am poking the board. Motivated by some brand promotion for simulation software. (if your company is doing simulation of mechanical systems, consider Siemens Simcenter which has many types of solutions under one brand)


https://youtu.be/Ea-MZfhnUmU



Now Billet and Austin can make comments...

[Lizardracing]

I learned in college engineering class, everything is a spring.
If it's consistent, you can work with it, or around it.
If it's not, things get a lot more complicated with never ending variables.

[billetbirdcage]

Chassis flex: Does it matter?



It should but I will pose this question? Say you take a car at a private test session and start cutting bars out of it (completely cut out at each end) and rerun laps after every cut bar.

I know this may be hard to know exactly what bar I'm describing but I'll give it a shot:
1. RF diagonal bar from mid plate upright to main frame rail
2. LF diagonal bar from mid plate upright to main frame rail
3. Diagonal bar from dash bar to RF upper rail
4. Diagonal bar from midplate upright backwards on right side
5. long diagonal bar from right side upright to frame by lower RR 4 link
6. Bar from RF to LF at very front of car in front of radiator (nothing connecting the 2 front upper rails besides removable engine bar to front shock mounts
7. Diagonal bar over drivers head in halo
8. Diagonal bar from dash bar to RF midplate upright


So how much did each bar change lap times? Take a guess?

Some side notes to help:
1. This was in 2010
2. This was a 100% all moly car, all the car was .065 wall tubing besides the main frame rails
3. This was at magnolia and at the time the track/car was in the low 15 second range
4. Only car there so track was not changing and It took me roughly 1 to 2 minutes to remove hood or whatever and cut each bar with a sawsall then back on track for 3 laps.
5. This was at the end of a 7 hour test so track was black and very consistent.

This obviously isn't very scientific as you really have no idea if the car was flexing a ton to start with or if the car was rigid for the time so you can't really say if I made the car 1% more flexible or 50% by cutting those bars out, shrugs. But I can tell you the car didn't like to take a hit and would bend pretty easy with all those bars out of it.

[Ghopper]

6. This car did not have a sway bar.
7. This car had data? or just subjective driver feedback?

I will guess the car got looser overall, but possibly had better corner entry mid way through the cutting session? It could have even got faster when it got looser.

[billetbirdcage]

6. This car did not have a sway bar. No sway bar
7. This car had data? or just subjective driver feedback? No data, just lap times and driver feedback on what it did to car


I did forget to add: SAS spec package at the time, so steel block 358 with 12" spoiler at 2100#. IIRC we where 2150ish before bars cut and spec motors then where in the 660 to 680HP range

[Austin34471]

I will yield my time with the microphone to Billet to hear the results of this test..

[Austin34471]

My guess is that: the driver said it felt a little tighter on entry, less drive, and *sigh* it went the exact same on the stop watch with every bar cut out because dirt late model

[MasterSbilt_Racer]

While we wait for Billet, I know of a car in the mid 2000s that when it stopped winning, it was time to take the roof off, reweld halo cracks, then it would win again. Scary and true.

[fastford]

the theory use to be , in certain conditions , some chassis flex was a good thing , i personally never bought into this and thought of it as a crutch , I think with todays shocks and springs , the less flex the better ....jmo.....

[billetbirdcage]

Now you need to remember this was a while back and 800# zero point shocks and other really stiff stuff was super common at the time, so obviously that should have an effect on the chassis.


1. RF diagonal bar from mid plate upright to main frame rail No change in times - driver could not tell a difference
2. LF diagonal bar from mid plate upright to main frame rail No change in times - driver could not tell a difference
3. Diagonal bar from dash bar to RF upper rail No change in times - driver could not tell a difference
4. Diagonal bar from midplate upright backwards on right side No change in times - driver could not tell a difference
5. long diagonal bar from right side upright to frame by lower RR 4 link No change in times - driver thought he felt a difference
6. Bar from RF to LF at very front of car in front of radiator No change in times - driver could not tell a difference
7. Diagonal bar over drivers head in halo No change in times - driver thought he felt a difference
8. Diagonal bar from dash bar to RF midplate upright No change in times - driver could not tell a difference


Austin cheated as he has heard this story before. It absolutely made no difference in lap times at all, the times the driver thought he felt a difference in those 2 bars he still wasn't 100% sure what it did and was indecisive about it.

[billetbirdcage]

Now, let me go back and say: Do I feel chassis stiffness matters? Yes, I do but I haven't had the equipment like Ghopper to get simulation numbers or any time of real number to see how much change is needed to see a difference and when that difference is noticeable and where.

I actually just revisited this 2 months ago, on a chassis that is much stiffer then the one I did back in 2010. While, I'm gonna keep those results to myself this time it did show on the stop watch with a single bar removal and a real change was felt by driver and we could see it also.

[Ghopper]

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)

[billetbirdcage]

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



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.

[billetbirdcage]

Also how big the gaps and fit of the notches makes a big difference on how much the joint/weld shrinks when it cools and thus pulls on that bar and may preload the chassis one way or the other. So how consistent the builder does things, imo is gonna have an effect. Now is that enough to make a difference, shrugs

Tons of variable here, lol.

Disclaimer, I'm just a dumbass and nothing I say is worth 2 cents

[fastford]

with todays load that is placed on the right front , do you guys think anything could be gained by using the down tubes from the halo to the upper front shock mounts ? the old TNT chassis were this way and worked pretty well in there day , but they stopped doing it after a while , of course then there was not as much dynamic load on the RF like today , just wander if there could be something there that could help with todays set ups , one thing i have noticed is the cars today seem to hop or bounce more through the turns , just thinking undampened flex could cause this ....

[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.

[fastford]

i think rigid in a chassis as for as its normal function is controlled by triangulation more than any thing , now backing one into a wall is a different scenario , like i said , my methods are a bit crude , but I took a piece of .065 moly tubing years ago and clamped it in a pipe vice , hooked a chain hoist to it and pulled it and let off until it didnt go all the way back , then took a piece of dom mild steel and done the same , the dom would not stand half of what the moly would and come back , but that is not what convinced me to use all moly , the weight was , our dom chassis weighed 150 lbs more than its exact counter part did in moly , we were running an all steel ford engine and that weight advantage was wonderful , we could get to min weight and have great percentages , jmo also I am pretty sure the dom was .125 but could have been .095 , not sure now , been a long time ago

[billetbirdcage]

i think rigid in a chassis as for as its normal function is controlled by triangulation more than any thing , now backing one into a wall is a different scenario , like i said , my methods are a bit crude , but I took a piece of .065 moly tubing years ago and clamped it in a pipe vice , hooked a chain hoist to it and pulled it and let off until it didnt go all the way back , then took a piece of dom mild steel and done the same , the dom would not stand half of what the moly would and come back , but that is not what convinced me to use all moly , the weight was , our dom chassis weighed 150 lbs more than its exact counter part did in moly , we were running an all steel ford engine and that weight advantage was wonderful , we could get to min weight and have great percentages , jmo also I am pretty sure the dom was .125 but could have been .095 , not sure now , been a long time ago

A LM stye typical in the 2010 era: (I built these 2 as exact copies of each other to see DOM vs Moly difference on the stop watch vs each other)

Dom car 1.75 main rails .095 - 1.5 was .083 - 1.25 was .065 = no bumpers, powder coat, ect was 324#

Same car but with moly = 1.75 main rails .083 - 1.5 was .065 - 1.25 was .065 = 274#

[billetbirdcage]

Sorry, I forgot those 2 where actually 2.0 inch round frame rails not 1.75 but wall was still what I listed

[MasterSbilt_Racer]

i think rigid in a chassis as for as its normal function is controlled by triangulation more than any thing , now backing one into a wall is a different scenario , like i said , my methods are a bit crude , but I took a piece of .065 moly tubing years ago and clamped it in a pipe vice , hooked a chain hoist to it and pulled it and let off until it didnt go all the way back , then took a piece of dom mild steel and done the same , the dom would not stand half of what the moly would and come back , but that is not what convinced me to use all moly , the weight was , our dom chassis weighed 150 lbs more than its exact counter part did in moly , we were running an all steel ford engine and that weight advantage was wonderful , we could get to min weight and have great percentages , jmo also I am pretty sure the dom was .125 but could have been .095 , not sure now , been a long time ago

You have to have triangulation. But you always have unsupported spans. And no matter how much triangulation you have, the triangles with the larger cross section are stiffer, assuming we have materials with basically the same modulus of elasticity.

[fastford]

A LM stye typical in the 2010 era: (I built these 2 as exact copies of each other to see DOM vs Moly difference on the stop watch vs each other)

Dom car 1.75 main rails .095 - 1.5 was .083 - 1.25 was .065 = no bumpers, powder coat, ect was 324#

Same car but with moly = 1.75 main rails .083 - 1.5 was .065 - 1.25 was .065 = 274#

dont know about your chassis , but in 2006 we had a thoroughbred dom LM chassis and switched to a moly TNT that had basically the same amount of bars and weight was just shy of 150 lbs less , I still have my notes where we weighed the two and looked again tonight to be sure , but either way , I will never use any form of mild steel again , even on our modified ...

[MasterSbilt_Racer]

I think Ghopper is the only other guy here that says you design for stiffness. If you do, weight will be the same for both cars, aside from making the cage of the stronger material. But even there, if you downsize tube wall, torsional stiffness will be impacted.

You build a DOM car of typical tubing and the moly of the typical thinner tubing and compare the torsional stiffness, they will be different cars. The moly car will be less rigid. At that point, they are 2 different designs and one is less optimized. You can't even compare them. The moly car is lighter because it is a lighter design, not an equal design.
If you use all moly for better crash worthiness, that's fine. It will likely have better fatigue life as well, provided it was welded correctly. But, there is nothing to support going lighter and having the same car. It's a misunderstanding of the end goal.

[fastford]

I respect both you guys opinion's , but I know what my scale says , I know of no new LM chassis that is not built out of moly or docol , either way , I appreciate both of your contributions and have learned a lot from you both , one more thing , both my chassis were scaled rolling , but all components were swapped from one to the other , so IDK .....

[MasterSbilt_Racer]

I respect both you guys opinion's , but I know what my scale says , I know of no new LM chassis that is not built out of moly or docol , either way , I appreciate both of your contributions and have learned a lot from you both , one more thing , both my chassis were scaled rolling , but all components were swapped from one to the other , so IDK .....

I'm not arguing it's lighter. Of course it is. But it was just randomly lightened with the thought the material was stronger. You could use thinner DOM too, and may not see any difference until it got hit. It's yet another thing that hasn't really ever been approached correctly in our sport and smart people are changing that.

[fastford]

agreed ..........

[billetbirdcage]

at 150# lighter, that DOM frame had to be made with some really thick wall stuff. Which is why I listed the wall thickness on mine which is more typical of what most guys use on Dom vs moly. It's usually same or one wall thinner size on thickness for the moly.

Right or wrong, that is sort of the industry standard as I have seen it.

[Lizardracing]

Based on internet findings on a metal sales website

Chrome-Moly Tensile Stength = 90,000 psi
Diameter (in) Thickness (in) ID (in) Cross Sect. Area (in^2) Tensile Strength (lbs) Weight (lb/ft)
1.75 0.120 1.51 0.61 55,303 2.089 lb
1.75 0.095 1.56 0.49 44,453 1.679 lb
1.75 0.083 1.58 0.43 39,119 1.478 lb
1.50 0.120 1.26 0.52 46,821 1.769 lb
1.50 0.095 1.31 0.42 37,738 1.426 lb
1.50 0.083 1.33 0.37 33,253 1.256 lb
1.25 0.120 1.01 0.43 38,339 1.448 lb
1.25 0.095 1.06 0.34 31,023 1.172 lb
1.25 0.083 1.08 0.30 27,386 1.034 lb


DOM Tensile Stength = 70,000 psi
Diameter (in) Thickness (in) ID (in) Cross Sect. Area (in^2) Tensile Strength (lbs) Weight (lb/ft)
1.75 0.125 1.50 0.64 44,668 2.169 lb
1.75 0.095 1.56 0.49 34,575 1.596 lb
1.75 0.083 1.58 0.43 30,426 1.478 lb
1.50 0.120 1.26 0.52 36,416 1.769 lb
1.50 0.095 1.31 0.42 29,352 1.426 lb
1.50 0.083 1.33 0.37 25,863 1.256 lb
1.25 0.120 1.01 0.43 29,819 1.448 lb
1.25 0.095 1.06 0.34 24,129 1.172 lb
1.25 0.083 1.08 0.30 21,300 1.034 lb

HREW Tensile Stength = 40,000 psi
Diameter (in) Thickness (in) ID (in) Cross Sect. Area (in^2) Tensile Strength (lbs) Weight (lb/ft)
1.75 0.120 1.51 0.61 24,579 2.089 lb
1.75 0.095 1.56 0.49 19,757 1.596 lb
1.75 0.083 1.58 0.43 17,386 1.478 lb
1.50 0.120 1.26 0.52 20,809 1.769 lb
1.50 0.095 1.31 0.42 16,772 1.426 lb
1.50 0.083 1.33 0.37 14,779 1.256 lb
1.25 0.120 1.01 0.43 17,039 1.448 lb
1.25 0.095 1.06 0.34 13,788 1.172 lb
1.25 0.083 1.08 0.30 12,172 1.034 lb


If 2 chassis were identical, the Moly car would weigh less and be stronger over all but flex about the same.
If you go to mix matching tube sizes and types, there's a lot more math involved.

[MasterSbilt_Racer]

Based on internet findings on a metal sales website

Chrome-Moly Tensile Stength = 90,000 psi
Diameter (in) Thickness (in) ID (in) Cross Sect. Area (in^2) Tensile Strength (lbs) Weight (lb/ft)
1.75 0.120 1.51 0.61 55,303 2.089 lb
1.75 0.095 1.56 0.49 44,453 1.679 lb
1.75 0.083 1.58 0.43 39,119 1.478 lb
1.50 0.120 1.26 0.52 46,821 1.769 lb
1.50 0.095 1.31 0.42 37,738 1.426 lb
1.50 0.083 1.33 0.37 33,253 1.256 lb
1.25 0.120 1.01 0.43 38,339 1.448 lb
1.25 0.095 1.06 0.34 31,023 1.172 lb
1.25 0.083 1.08 0.30 27,386 1.034 lb


DOM Tensile Stength = 70,000 psi
Diameter (in) Thickness (in) ID (in) Cross Sect. Area (in^2) Tensile Strength (lbs) Weight (lb/ft)
1.75 0.125 1.50 0.64 44,668 2.169 lb
1.75 0.095 1.56 0.49 34,575 1.596 lb
1.75 0.083 1.58 0.43 30,426 1.478 lb
1.50 0.120 1.26 0.52 36,416 1.769 lb
1.50 0.095 1.31 0.42 29,352 1.426 lb
1.50 0.083 1.33 0.37 25,863 1.256 lb
1.25 0.120 1.01 0.43 29,819 1.448 lb
1.25 0.095 1.06 0.34 24,129 1.172 lb
1.25 0.083 1.08 0.30 21,300 1.034 lb

HREW Tensile Stength = 40,000 psi
Diameter (in) Thickness (in) ID (in) Cross Sect. Area (in^2) Tensile Strength (lbs) Weight (lb/ft)
1.75 0.120 1.51 0.61 24,579 2.089 lb
1.75 0.095 1.56 0.49 19,757 1.596 lb
1.75 0.083 1.58 0.43 17,386 1.478 lb
1.50 0.120 1.26 0.52 20,809 1.769 lb
1.50 0.095 1.31 0.42 16,772 1.426 lb
1.50 0.083 1.33 0.37 14,779 1.256 lb
1.25 0.120 1.01 0.43 17,039 1.448 lb
1.25 0.095 1.06 0.34 13,788 1.172 lb
1.25 0.083 1.08 0.30 12,172 1.034 lb


If 2 chassis were identical, the Moly car would weigh less and be stronger over all but flex about the same.
If you go to mix matching tube sizes and types, there's a lot more math involved.

If you look at 1.50 x .120, there is no weight difference. Which, really is more reflective of dimensions being the same. The other ones have weight difference because cross section is different due to tolerances allowed for the tubing type.

There is no value here to determine flex. That is determined by modulus of elasticity when you are below yield. You better be about 45% of yield at most, on any chassis members, with maximum loading during a lap, or that thing will be junk very quickly.

[MasterSbilt_Racer]

Tensile stress = load to this value, tube is in 2 pieces

Yield stress = load to this value, tube is one piece, but permanently deformed

Modulus of elasticity = slope of the stress/strain curve until you load to the yield point. Think of this as the spring rate. All materials deflect with load. If you don't reach yield, the dimensions remain unchanged when you unload it. Operations below yield mean you use this to calculate deflection, just like a spring. For example, 300# on a 100#/in spring compresses it 3 inches.

I'm taking a lot of liberty for simplicity as axial and bending loads are handled differently.

[Lizardracing]

I think I wrote that wrong. I was trying to agree that according to those numbers, if they are legit, flex is in the mechanical design structure of the chassis and not because of the tube being DOM or Moly. Assuming identical chassis, identical tubing sizes used.

To put this another way, 2 10’ sticks, hanging off the edge of a table 5’ with 200 pounds hanging from each will bend downward the same amount and spring back when the weight is removed. The Moly tube, with more elasticity, will be able to hold more weight before it bends permanently.

Back to the flex and handling effects.
Billet says in his tests, cutting out bars and all that, didn’t appreciably effect the handling of the car or the driver feel. Can we infer that flex changes the sensitivity of the chassis adjustments? IE, a flexable car needs bigger changes to effect the car handling while a stiff car might be so sensitive the car is tough to keep on point?
What I mean by that is dirt tracks that change a lot all night, a stiff car might be good for a lap or two but begin falling off at a faster rate while a more flexable car will be good over a broader range of track conditions. If that is indeed true, one might see why an asphalt car or a drag car can be stiffer, while a dirt track car appreciates some flex.

I can tell ya on my modified, made with 1.5 and 1.25 .083 wall DOM tube, is very insensitive to changes. I know this, and just make bigger swings as the night goes on. It also doesn’t sit flat on four jack stands so maybe I’m ignoring something I shouldn’t be! Hahaha!

[talclipse]

Glad to hear bonehead is doing well Billet. Sure miss his input around here.

I think cars today are more consistent than they ever have been. Most chassis builders have went to some form of CNC tube notching and bending so their fitment is much better than they were 10 years ago. (I can remember picking up a modified chassis and it having 3/8" weave welds where their fitment had gotten sloppy.)

With that said I will die on the hill that cars built from 4130 need some form of stress relief/ normalization. I know rocket uses a mechanical stress relief device that mounts under the jig and uses ultrasonics to vibrate the chassis at high frequency to achieve this. I've also been told it was down at one point and they built a few cars without it and there was no difference.

I know a local "CHASSIS BRAND by DRIVER" built a fixture where they would secure the chassis, then use a jack with a load cell to raise the RF corner of the car a given measurement. They had found that their stuff worked better in a specific range. They went as far as to having different numbers for SLM, crate, steel block, etc.. Its probably worth noting that at one point they were replacing a chassis after only a handful of races. ¯\_(ツ)_/¯

[billetbirdcage]

talclipse: With that said I will die on the hill that cars built from 4130 need some form of stress relief/ normalization

They are when they get powder-coated, least I've had people tell me that. Look up normalization and see if you think 400 degrees an oven might get is gonna get the job done. . .shrugs/smiles

[Lizardracing]

No.
Annealing temp are around 1500' for several hours and a control slow temp decrease and normalization/stress is around 1200' for 24 hours.
450' for 30min ain't doing nothing......Measurable anyway.
Maybe part of the process is a 12-1500' to bake off impurities first, the coated, then baked at 400 for 30minutes and you have an argument.

[Austin34471]

"We here at * ______ Race cars* perform a special stress relief and annealing process in an large oven that is big enough for an entire chassis while using a special coating to prevent oxidation and further material degradation over time."
Oh so you powdercoated it. Nice.

[Ghopper]

It is hard to "like" a comment here. The forum needs to adapt to 2022 and mimic some disruptive social media trends.

You guys are really getting into the weeds of component details. Tubing thickness and gusset designs are all over the map in this market. Now everyone is guessing on residual stresses like they are talking about dark matter (https://en.wikipedia.org/wiki/Dark_matter#:~:text=Dark%20matter%20is%20a%20hypot hetical,the%20matter%20in%20the%20universe.&text=Because%20no%20one%20has%20directly,and%20rad iation%20except%20through%20gravity.) Don't most people vibrate at time of weld?

The answer is to measure the current spring rate of the car, and tune this to be in a window that is determined from objective/subjective feedback of multiple track test sessions. Billet was already doing some of this.

No dirt chassis manufacturer has contacted me about measuring their vehicle stiffness. I will just end up knowing it for their cars. Cup teams have chassis-only twist rigs to check variability in cars. I was lucky to have a friend that got this rig off FB Marketplace to make it easier find stiffness of the whole car assembly.

[grt74]

alot of over thinking going on here, just saying, its more important to just and some bars in a particular area,

[ZERO25]

So the real question is.....can it be measured accurately and predictably to enable us to adjust our wheel loads effectively?


Side note.......Hoffman says he buys 15 chassis at a time. They come in pieces, in a box, pre-bent from a cnc company. His results speak for themselves!

[MasterSbilt_Racer]

So the real question is.....can it be measured accurately and predictably to enable us to adjust our wheel loads effectively?


Side note.......Hoffman says he buys 15 chassis at a time. They come in pieces, in a box, pre-bent from a cnc company. His results speak for themselves!

You can easily build a test setup. It's not about adjustment. The car is either in a range you know is good or you don't use it. That's what one should do with the data.

We live in a world though where the chassis is blamed when a guy has something else screwed up and thinks the chassis is wore out. It's expensive, but allows egos to remain intact.

[grt74]

You can easily build a test setup. It's not about adjustment. The car is either in a range you know is good or you don't use it. That's what one should do with the data.

We live in a world though where the chassis is blamed when a guy has something else screwed up and thinks the chassis is wore out. It's expensive, but allows egos to remain intact.

completely agree with this one !!!! ill say this and leave it alone, all chassis take a set per say, thats why you can build 2 cars the same but one is going to run better, put that one on a pull down ,get the numbers the same and there its is (its amazing that we are looking at 1/16th of an inch and less these days) ,now you have the same car (notes are the big key here), of coarse one will always feel a tick better than the other, just the way it is, now the issue with tubing isnt racing the car that is the issue, its when you stuff her in the wall at over 150mph, at that point its becomes a saftey issue, thats why you get mandates from nhra,cup, and others in the racing world, we are getting closer and closer that point, over 150 is a factor and every 25mph after is, thats why youll always see cup under 200 for the most part
just my 2 cents
consistency is going to be the biggest factor, on all parts, tubing,welding, testing, tweaking, then great notes,
hoffman is winning because of his complete package and consistency, not the chassis by itself

[Ghopper]

You can easily build a test setup. It's not about adjustment. The car is either in a range you know is good or you don't use it.

I reject this. Change the stiffness matrix of the existing car (which is an assembly of many parts). There are many parts on a racecar that changed after they leave the factory; Tires, shocks, motors are matched to the changing needs of the consumer.

How: Maybe you are changing motor mounts, number of bolts in components, adding a brace, cutting a brace.

[MasterSbilt_Racer]

I reject this. Change the stiffness matrix of the existing car (which is an assembly of many parts). There are many parts on a racecar that changed after they leave the factory; Tires, shocks, motors are matched to the changing needs of the consumer.

How: Maybe you are changing motor mounts, number of bolts in components, adding a brace, cutting a brace.

I didn't say you couldn't bring it in.

[ZERO25]

I was hinting at making it easily adjustable........at the track if need be! Theres plenty of super stiff bump springs that could be hidden in the right places.

[TheJet-09]

Somewhat off topic, but I was curious...how many man hours do you think go into building a chassis (on average) from the first piece of tubing on the jig to ready for powder coat? And having asked that, where do you think the largest percentage of cost in building that new chassis comes from...material (but I doubt that)? The design itself? Labor to build it?

And I don't ask because I think I have an answer, I'm truly just curious.

[billetbirdcage]

Somewhat off topic, but I was curious...how many man hours do you think go into building a chassis (on average) from the first piece of tubing on the jig to ready for powder coat? And having asked that, where do you think the largest percentage of cost in building that new chassis comes from...material (but I doubt that)? The design itself? Labor to build it?

And I don't ask because I think I have an answer, I'm truly just curious.

Designing car, making jigs, and labor to do that stuff is a lot to recover so that cost has to be spread out into the number of cars your gonna build. The number of hours you have in just drawing tabs, brackets and etc is overwhelming. I know on one of my cars I have over 90 different tabs/brackets and lasered parts just for that one style of car.

Material (before stuff got crazy) cost just for the main frame (no bumpers and etc) is gonna run close to 2K if you buy enough to do multiple cars (say 5) and then you have lazered parts, brackets, tabs, and etc (that can easily run 700.00 per car).

Time to build a car from raw tubing to finish ready to powder coat: That's gonna very a ton, things like how often you do it and your equipment change that a lot. For example, I've had a guy weld for me before and he's really good and fast as hell. A car all tacked together on jig took him 6.5 hours and then 1.5 hours out of jig to weld underside that you can't get to on jig. I did the same car and didn't keep real good time but I was around 14 hours just in jig part. Course he put his helmet down and didn't lift it until he was done.

2 guys working can usually do one in around 40 to 50 hours, if you done a couple before. Again that's no bumpers, fuel cell cages, etc just a bare frame.

Honestly frames should cost way more then they do, If a guy was honest.

#Note: this is bending the tubing and notching it, nothing CNC'ed prebent and notched

[ZERO25]

Do you think buying cars in a box, CNC'ed, will lead to more consistency in rigidity?

[fastford]

do they have CNC,s that can weld an entire LM chassis ?

[ZERO25]

do they have CNC,s that can weld an entire LM chassis ?

Dunnu, but there was a go kart mfrer that used robots ,a few years ago, to weld their frames. They won a ton of races!

[powerslide]

Dunnu, but there was a go kart mfrer that used robots ,a few years ago, to weld their frames. They won a ton of races!

That would be phantom. They are the largest manu of karts. They recently posted about the process only taking one man from start to finish to take the tubing from one machine to the others. They have fallen off winning big races in recent years. They somewhat remind me of GRT, always trying to sell a trick of the week. Or in this case new frame design of the year

[Punisher88]

That would be phantom. They are the largest manu of karts. They recently posted about the process only taking one man from start to finish to take the tubing from one machine to the others. They have fallen off winning big races in recent years. They somewhat remind me of GRT, always trying to sell a trick of the week. Or in this case new frame design of the yearI had one of their nemesis carts. Got it new in 03 I believe. Hated that thing. Won a pile of races but every week you were moving washers on the front getting all the percentages back right on it. The cross numbers would change dramatically after every race. Went back to running a tempest by millennium. Winning just as many races but with way less work.

[fastford]

That would be phantom. They are the largest manu of karts. They recently posted about the process only taking one man from start to finish to take the tubing from one machine to the others. They have fallen off winning big races in recent years. They somewhat remind me of GRT, always trying to sell a trick of the week. Or in this case new frame design of the year

your exactly right , I was at a sentman trade show at Memphis back in the early 2000,s and GRT had a new improved frt lower control arm , I ask Joe about it and he said you had to have one , I studied this thing for a while , pulled out my tape measure and did a little basic measuring and could not see a difference geometry wise , so I asked joe to explain how it improved performance , he looked at me with that ole smurk he had and said , " there really isn't , but we have to sell parts " now I did not think a whole lot of Joe , but he was honest , every time I see a new improved gadget , I think of ole JOE ......