Is there any characteristic that differentiates a 2 x 3 full chassis car from a full round tube chassis car when The 2 x 3 is just the main frame rails as far as chassis tuning goes? Will they flex more than a tube chassis main frame rail? Will they handle less power? I know the certification will only allow for 7.50 max rather than 6.00 (told on this forum). I’m just wondering this for personal curiosity as to what future plans for the car will be.

A lot of older racers have told me “back in the day that’s all we had”. I know newer is better but my question is why and how.

Following. Also curious myself. I'd assume it's a subjective opinion.

In my experience, there is no difference in performance, although the 2 x 3 build may be slightly heavier.

Speaking from the experience of a home builder, the 2 x 3 main rails are easier to keep square and aligned as you begin. My setup is home-built tables and jigs; thus the 2 x 3 allows more accurate positioning and holding than round tubes.

You will hear folks say. The round tube has better resale, well, maybe, maybe not. I generally build what I want and like and am comfortable doing.

A .083 wall, mild steel 2 x 3 is completely acceptable and strong. Also, when building a back half, I build the rear rail kick up from .083 2 x 2. I properly braced 2 x 2 rear section will carry you to the winners circle many times.

quote:

Originally posted by Larry Woodfin:
In my experience, there is no difference in performance, although the 2 x 3 build may be slightly heavier.

Speaking from the experience of a home builder, the 2 x 3 main rails are easier to keep square and aligned as you begin. My setup is home-built tables and jigs; thus the 2 x 3 allows more accurate positioning and holding than round tubes.

You will hear folks say. The round tube has better resale, well, maybe, maybe not. I generally build what I want and like and am comfortable doing.

A .083 wall, mild steel 2 x 3 is completely acceptable and strong. Also, when building a back half, I build the rear rail kick up from .083 2 x 2. I properly braced 2 x 2 rear section will carry you to the winners circle many times.

I agree! I have both and my truck is 2x3 and heavier than my car which is round tube but both work great.

So it is seemingly a weight factor maybe high speed crash factor but doesn’t really become a chassis performance factor.

Has anyone spoken to SFI about this and why it can't be used in a 6.0 certification?
It is an expensive upgrade to replace the 2x3 with round tube. I was told somewhere between 8-10K.

quote:

Originally posted by Holytown:
Is there any characteristic that differentiates a 2 x 3 full chassis car from a full round tube chassis car when The 2 x 3 is just the main frame rails as far as chassis tuning goes? Will they flex more than a tube chassis main frame rail? Will they handle less power? I know the certification will only allow for 7.50 max rather than 6.00 (told on this forum). I’m just wondering this for personal curiosity as to what future plans for the car will be.

A lot of older racers have told me “back in the day that’s all we had”. I know newer is better but my question is why and how.

Based on the SFI requirements, you will not get below a 7.50 cert with carbon steel structures, in the sub 7.50 certifications the frame has a bigger role in meeting those specifications.
4130 box tubing is outrageous in price…I don’t know if I have ever even seen 2x3 in CM, but I know 1 x 2 is north of $50 a foot!

From a structural engineering stand point, box tubing of same material is not as strong as round tubing when equally compared (such as 1” x 1” square compared to 1” round).

A 2x3 frame rail is substantially stiffer then a similar design 1 5/8" round tube car. But it is heavier and also won't cert quicker then 7.50 most round tube cars have additional tubing in the center to stiffen them.

Square CrMo tube, under 2” is $50 per ft? WoW

quote:

Originally posted by Jerry Kathe:

quote:

Originally posted by Holytown:
Is there any characteristic that differentiates a 2 x 3 full chassis car from a full round tube chassis car when The 2 x 3 is just the main frame rails as far as chassis tuning goes? Will they flex more than a tube chassis main frame rail? Will they handle less power? I know the certification will only allow for 7.50 max rather than 6.00 (told on this forum). I’m just wondering this for personal curiosity as to what future plans for the car will be.

A lot of older racers have told me “back in the day that’s all we had”. I know newer is better but my question is why and how.

Based on the SFI requirements, you will not get below a 7.50 cert with carbon steel structures, in the sub 7.50 certifications the frame has a bigger role in meeting those specifications.
4130 box tubing is outrageous in price…I don’t know if I have ever even seen 2x3 in CM, but I know 1 x 2 is north of $50 a foot!

From a structural engineering stand point, box tubing of same material is not as strong as round tubing when equally compared (such as 1” x 1” square compared to 1” round).

Is there a difference between stiffness and strength? I would suspect stiffness obviously would mean Flexibility, where as strength would mean how long it can hold its shape under stress?

quote:

Originally posted by Jerry Kathe:


From a structural engineering stand point, box tubing of same material is not as strong as round tubing when equally compared (such as 1” x 1” square compared to 1” round).

You are incorrect.


As for the original question, whether you are using box shapes for the main frame or round shapes, the strength and stiffness comes from the entire chassis geometry and not one part of it!

As for cert's faster that 7.50, I am not 100% sure buy my guess is the failure mode of the connections of round shapes to the box shapes is the issue during a crash. Once any chassis member starts to deform it's structural properties change fast and it's easier for a round tube framed into a 2x3 to deform the 2x3 than when tubes are connected to other tubes. If you want more info look into Punching Shear and you will get a better understanding.

As far as racing performance, they both will work when built and tuned correctly.

quote:

Originally posted by Holytown: Is there a difference between stiffness and strength? I would suspect stiffness obviously would mean Flexibility, where as strength would mean how long it can hold its shape under stress?

kinda, in this case it is because the square will yield at a lower modulus point and fail, the round will do a far better job in retaining its shape memory.

Reason being - Cylindrical shape (round) has the same distribution of load on all sides (360º) as to where square has only four sides for the distribution. The SFI board had it right when they inserted the word "round" in the rollcage/roll bar regulations...no doubt had they not; you would see a lot more "square tube" cages - LOL.

quote:

Originally posted by TOP38:

quote:

Originally posted by Jerry Kathe:


From a structural engineering stand point, box tubing of same material is not as strong as round tubing when equally compared (such as 1” x 1” square compared to 1” round).

You are incorrect.

Actually it wouldn't have been me that was incorrect....its would have been the education that I paid for....maybe you should explain to the SAE board that the physicist who discovered the principles of "moment of inertia" has flawed theory?

From a resale stand point a 4130 car is worth more. But not in all cases is relevant. I work with 4130 almost everyday and it is tough stuff. It drills fine but not like mild steel. Most of what I work with is .058 to .083 as the tubing is going on dragsters. If you are going to build a car for say 9.0 (5.80 1/8) who cares. The tubing will be a little thicker as to cage specs. so a little more weight but all in all a lot of mild steel cars have won a lot of races. I would rather TIG weld even mild steel but a lot of people are more comfortable with a MIG welder. A properly built MIG welded car is absolutely fine for the E.T. stated above. The actual chassis its self is just tubing and a big welding project. All the stuff the chassis holds is the where the big cost is. After the chassis is tin work, mounting the body, all the tabs for attaching what's needed on a race car. The door mounting hinges in the right place for the door to have perfect gaps takes a full day, etc, etc.

/\
All good points.
At the end of the day you will have roughly half the amount of weight in the equivalent chassis design with CM, but at a cost.

We could enter the conversation as to why NASCAR requires mild steel...that could be interesting....lol

quote:

Originally posted by Jerry Kathe:

quote:

Originally posted by TOP38:

quote:

Originally posted by Jerry Kathe:


From a structural engineering stand point, box tubing of same material is not as strong as round tubing when equally compared (such as 1” x 1” square compared to 1” round).

You are incorrect.

Actually it wouldn't have been me that was incorrect....its would have been the education that I paid for....maybe you should explain to the SAE board that the physicist who discovered the principles of "moment of inertia" has flawed theory?

Not to be a wise guy but since you started it,,, you may want to pay more attention in class next time.

I, S and R values are all larger for a 1", .083 wall square tube vs a 1", .083 wall round tube. I'd explain what I, S and R mean, but your education already did that! Oh, I forgot A too. And the SAE board did not discover the principles of I BTW. Ah s h i t, I gave one away....

So to clarify, the square tube will carry more load in both tension, compression and bending and deflect less than the round tube of the same size and wall thickness. However during a crash when stuff takes a hit and parts get crushed, that's a whole different ballgame and gets much more complicated.

Not being or trying to be a wise guy....you said I was incorrect and provided no further comment....try to stay on course here.

You are correct in how you are spinning it, but we we talking failure mode from dynamics not static load.

Actually, I cant even think of anything in the racing game that would even be a static load.....

quote:

Originally posted by Jerry Kathe:
/\
All good points.
At the end of the day you will have roughly half the amount of weight in the equivalent chassis design with CM, but at a cost.

We could enter the conversation as to why NASCAR requires mild steel...that could be interesting....lol

Really?? That amazes me considering NHRA seems to think Mild Steel is inferior. Or is that just due to wall thicknesses and weight issues?

Even though NASCAR's crash every weekend, other than the occasional rollover; a NASCAR crash isn't nearly as violent as a 1/4 mile wreck, even on 150 mph vehicles.

I may not be able to baffle anyone with math equations, but I can tell you I have seen cars with flat tubing punch member tubes right through it in an impact and flex cycles, but have never seen that on an all round tube situation. As I tell my customers, flat tubing is great for street rods but not the best for race cars.