Chassis Flex: Does it matter?

[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)_/¯