Round or Square
- Thread starter scuster05
- Start date
General consensus is that round is stronger than square in compression and torsion
pound for pound (or kg for kg). Tension and shear they are about equal. And bending
varies based upon where the force is applied.
Ian
Exactly as Ian says above. To be specific with bending forces, square is stronger when the force is applied in line with an edge (the tube acting basically like a fully boxed I-beam) whereas it's weaker when the force is applie 45 degrees to the edge, in which case the square tube tends to flatten and lose strength accordingly.
you nead to compare the section modulus to get a true indication.
a circular tube of the same thickness and same dimensions I.E. 25 dia as against 25 square
will be stronger in all respects save for bending.
A square tube of same dimension will be thinner to achieve equal mass and may not necessarily be stronger in bending.
For ease of manufacture square tube wins hands down. A combination of square and round can provide the best of both worlds. Square for longitudinals and main crossmembers with round for bracing and struts.
It all depends on what you are trying to achieve.
a circular tube of the same thickness and same dimensions I.E. 25 dia as against 25 square
will be stronger in all respects save for bending.
A square tube of same dimension will be thinner to achieve equal mass and may not necessarily be stronger in bending.
For ease of manufacture square tube wins hands down. A combination of square and round can provide the best of both worlds. Square for longitudinals and main crossmembers with round for bracing and struts.
It all depends on what you are trying to achieve.
Stiffness and strength are being conglomerated here in a big, sloppy mess.
A few generalities:
Alloys used in DOM and CDS round tubing have a higher tensile and compressive strength than commercial square tubing.
The primary advantage of round tubing (besides the slightly higher UTS) is that it is slightly more efficient in resisting buckling per unit weight. (Also contingent on overall design.)
In a space frame design, the layout of the tubing defines the stresses much, much more than whether the tubing is square or round. In addition, the overall design also defines the overall chassis stiffness.
Over a certain strength (necessary to prevent structural failure), overall stiffness is the primary influence to how a car feels and handles.
If you design a chassis to a high-stiffness standard (and use good design practices), it won't make a significant difference in stiffness whether the tubing is square or round. The strength of the round-tube design (given equal weights) will have a higher safety factor though.
A few generalities:
Alloys used in DOM and CDS round tubing have a higher tensile and compressive strength than commercial square tubing.
The primary advantage of round tubing (besides the slightly higher UTS) is that it is slightly more efficient in resisting buckling per unit weight. (Also contingent on overall design.)
In a space frame design, the layout of the tubing defines the stresses much, much more than whether the tubing is square or round. In addition, the overall design also defines the overall chassis stiffness.
Over a certain strength (necessary to prevent structural failure), overall stiffness is the primary influence to how a car feels and handles.
If you design a chassis to a high-stiffness standard (and use good design practices), it won't make a significant difference in stiffness whether the tubing is square or round. The strength of the round-tube design (given equal weights) will have a higher safety factor though.
When I made my engine bay struts for my GTD, the highly qualified engineer worked out what I needed in square tubing as the rest of the GTD chassis is square tubing, but he did say that normally you would expect to use round tubing. Later on the MSA decided they didn't like square tubing anymore for that application.
Then again, on our Lotus 51c (1968 FF) Lotus used square and round tubing for the chassis.
I guess for every section of round tubing there is an equivalent in square tubing but would they weigh the same per equal length? I doubt it.
On a recent FIA roll cage I had commissioned for a car we played around with the different strengths of tubing ie the rules say at least 350 Nm and we used 500 Nm and by playing with the dimensions of the tubes which are not mandatory but optional I have a very strong cage that is 20 KG lighter than one off the shelf bolt in style.
Then again, on our Lotus 51c (1968 FF) Lotus used square and round tubing for the chassis.
I guess for every section of round tubing there is an equivalent in square tubing but would they weigh the same per equal length? I doubt it.
On a recent FIA roll cage I had commissioned for a car we played around with the different strengths of tubing ie the rules say at least 350 Nm and we used 500 Nm and by playing with the dimensions of the tubes which are not mandatory but optional I have a very strong cage that is 20 KG lighter than one off the shelf bolt in style.
Your 51c was more than likely one of many customer cars, and as such ease of construction may have dictated the square tube, particularly as it still probably came in under the minimum weight of 400kg.
Where there is a minimum weight requirement, some will say building as light as possible and then adding ballast to bring it up to the minimum weight can result in a competitive advantage as the ballast can be placed right on the bottom of the chassis to lower the CoG and anywhere along the length of the car to optimise handling etc.
Of course for competitions or classes where there is no weight restriction, lighter is always better, so long as chassis integrity is not compromised.
For a road car, ease of construction and maintenance, and resistance to everyday wear and tear will usually over ride any other considerations.
Where there is a minimum weight requirement, some will say building as light as possible and then adding ballast to bring it up to the minimum weight can result in a competitive advantage as the ballast can be placed right on the bottom of the chassis to lower the CoG and anywhere along the length of the car to optimise handling etc.
Of course for competitions or classes where there is no weight restriction, lighter is always better, so long as chassis integrity is not compromised.
For a road car, ease of construction and maintenance, and resistance to everyday wear and tear will usually over ride any other considerations.
Shaun,
You asked, if useing round tubing what wall thickness should I use...Well there are many variables to consider...but I will just try to give you a general idea...
First , as Russ said you need to decide if you want the ultimate light weight chassis for racing, or a reasonably light chassis for the road, that will last for some time.....
, The quality, and diameter of the tubing you are useing, will dictate the wall thickness, if you use chromolly 1" by 16swg (1.6mm) 4130, then to get the same strenght from 1" cds2...cfs3...dom...etc.....you would have to go up to 14swg (2mm)....(This is just an aproximate comparison) If you were to use 14swg wall, your chassis would be pretty heavy..
The diameter you use will depend on your chassis design. If I was making a space frame from tubing I would use chromolly one inch, and an eighth (28.5mm) x 16swg (1.6mm) for the majority of it, and other diameters where needed
chromolly comes in 4 different conditions
As Rolled
Anealled
Normalised
Hardened and tempered
I would use Normalised. If you Tig weld, or Braze it you wont have to heat treat it afterwards, but if you Mig weld it, you will need to nomalise it again by heating it up, and allowing it to cool at room temp, If you were to pre heat it before Mig welding, then you probably wouldnt need to normalise it after welding. There are a few web sites where I found this information, if your intrested ILL look them up for you and send you their addresses
The best way to help you decide, is to look at other peoples chassis, and ask them what they used. If you look at the forum about Can Ams, and Porches. Just above the paddock forum, you will find some space frame chassis there....... Here are 2 that I just found
David Sturch is building a Chevron B16 Replica
Chris Melia is buiding a Porsche 908/910
I hope this help you
mick
You asked, if useing round tubing what wall thickness should I use...Well there are many variables to consider...but I will just try to give you a general idea...
First , as Russ said you need to decide if you want the ultimate light weight chassis for racing, or a reasonably light chassis for the road, that will last for some time.....
, The quality, and diameter of the tubing you are useing, will dictate the wall thickness, if you use chromolly 1" by 16swg (1.6mm) 4130, then to get the same strenght from 1" cds2...cfs3...dom...etc.....you would have to go up to 14swg (2mm)....(This is just an aproximate comparison) If you were to use 14swg wall, your chassis would be pretty heavy..
The diameter you use will depend on your chassis design. If I was making a space frame from tubing I would use chromolly one inch, and an eighth (28.5mm) x 16swg (1.6mm) for the majority of it, and other diameters where needed
chromolly comes in 4 different conditions
As Rolled
Anealled
Normalised
Hardened and tempered
I would use Normalised. If you Tig weld, or Braze it you wont have to heat treat it afterwards, but if you Mig weld it, you will need to nomalise it again by heating it up, and allowing it to cool at room temp, If you were to pre heat it before Mig welding, then you probably wouldnt need to normalise it after welding. There are a few web sites where I found this information, if your intrested ILL look them up for you and send you their addresses
The best way to help you decide, is to look at other peoples chassis, and ask them what they used. If you look at the forum about Can Ams, and Porches. Just above the paddock forum, you will find some space frame chassis there....... Here are 2 that I just found
David Sturch is building a Chevron B16 Replica
Chris Melia is buiding a Porsche 908/910
I hope this help you
mick
Hi Shawn,
here are some links for you
4130 Steel -- My Observations
Performance Metals Australia- Product Information - 4130 Round Tube
4130 Steel Tubing
Tool Steel Stockholders, Steel Stockists Suppliers Special Steel Stockists
Product Line
http://www.suppliersonline.com/propertypages/4130.asp#Spec
Welding 4130 Steel Tubing - Tig Welding Chromoly Up in Here
mick
here are some links for you
4130 Steel -- My Observations
Performance Metals Australia- Product Information - 4130 Round Tube
4130 Steel Tubing
Tool Steel Stockholders, Steel Stockists Suppliers Special Steel Stockists
Product Line
http://www.suppliersonline.com/propertypages/4130.asp#Spec
Welding 4130 Steel Tubing - Tig Welding Chromoly Up in Here
mick
There's nothing wrong with common old ERW tube. I've built my track car out of that. 1" by 1/16" round and square. See my build thread for further details. http://www.gt40s.com/forum/gt40-build-logs/16138-kiwi-scratchbuilt.html
For a road car I would tend to go with something slightly larger and maybe about 25% thicker.
For a road car I would tend to go with something slightly larger and maybe about 25% thicker.
the "strength" of a column or strut under axial loading is in no way dependant upon the strength of the material in compression, but only on its geometry and the stiffness of the material ( modulus of elasticity)
An individual member made from chromoly is no stiffer in compression than ordinary mild steel.
The structure as a whole may well be a different story, however, that was not the original question
An individual member made from chromoly is no stiffer in compression than ordinary mild steel.
The structure as a whole may well be a different story, however, that was not the original question
I am with you Russ,
ERW has a number of advantages, Easy to fabricate, no special techniques, economical, easy to repair, readily available. Why bother with anything else to make things complicated.
ERW has a number of advantages, Easy to fabricate, no special techniques, economical, easy to repair, readily available. Why bother with anything else to make things complicated.
Hi Shawn,
Yes there are cheaper alternatives. Im dont know what is easly available in USA , but in england we have
CFS3, cold formed seamless,
DOM drawn over mandral
they are very simlar, but made in different ways
ERW electrical resistance welded, The main difference between this and CFS3, DOM, is the ERW tube still has the seam in it, which is cheaper to make , and so is cheaper to buy. The main disadvantage of the seam is when bending with a mandral bender, you cannot get such a high quality bend
If you are looking for box, then the ERW is the only thin wall box readily available
Russ, is obviously a very knowledgable and experianced man, and his chassis is a tested, and proven design, so Russes advice is the best you will get...........study his design, use the same materials, and you will have a very good chassis
Chromolly is not as difficult to work with as some sites suggest. Ive been Mig welding chromolly for 30yrs, I dont pre heat it,I dont normalise it after welding.....I have never had failure with it........
mick
Yes there are cheaper alternatives. Im dont know what is easly available in USA , but in england we have
CFS3, cold formed seamless,
DOM drawn over mandral
they are very simlar, but made in different ways
ERW electrical resistance welded, The main difference between this and CFS3, DOM, is the ERW tube still has the seam in it, which is cheaper to make , and so is cheaper to buy. The main disadvantage of the seam is when bending with a mandral bender, you cannot get such a high quality bend
If you are looking for box, then the ERW is the only thin wall box readily available
Russ, is obviously a very knowledgable and experianced man, and his chassis is a tested, and proven design, so Russes advice is the best you will get...........study his design, use the same materials, and you will have a very good chassis
Chromolly is not as difficult to work with as some sites suggest. Ive been Mig welding chromolly for 30yrs, I dont pre heat it,I dont normalise it after welding.....I have never had failure with it........
mick
Shawn,
Talk to your metal supplier and get the spec's on what they are selling from the manufacturer.
Make sure you have a welder that is up to the task also. I've just upgraded to an older tig machine that will supply 200 Amps at 60% duty cycle. I am adding a water cooler also. Same with the mig also. You may want to run gas to get a nice clean weld. The small Lincoln I had was pushed to the limits.
If you see the drawings of my chassis, it is a combination of round and square. Chrome Molley is nice but it's main advantage is the use of a thinner wall to make parts lighter.
Gas welding is OK too not as hot as tig, R45 rod will do. I would suggest the bare rod with a gas fluxer in the gas line. One of those pistol torches Russ was using on his chassis. I think you have lesser chance of distortion using gas.
Dave
Talk to your metal supplier and get the spec's on what they are selling from the manufacturer.
Make sure you have a welder that is up to the task also. I've just upgraded to an older tig machine that will supply 200 Amps at 60% duty cycle. I am adding a water cooler also. Same with the mig also. You may want to run gas to get a nice clean weld. The small Lincoln I had was pushed to the limits.
If you see the drawings of my chassis, it is a combination of round and square. Chrome Molley is nice but it's main advantage is the use of a thinner wall to make parts lighter.
Gas welding is OK too not as hot as tig, R45 rod will do. I would suggest the bare rod with a gas fluxer in the gas line. One of those pistol torches Russ was using on his chassis. I think you have lesser chance of distortion using gas.
Dave
I'm a round tubing sort.. I always figured if square tubing was better, they'd be making driveshafts out of it. Plus - it's a heck of a lot easier to make bends in round tubing in my bender... Can't seem to bend the square or rectangular stuff without collapsing it...
I've built quite literally dozens of racecars from the ground up. Many more than that of factory tub cars with my cages. Depending on the customer's needs and the rules we were to follow - the chassis / cages were mostly built of DOM or ERW.
I've never yet seen ERW fail when put to the ultimate test.
But yet I prefer DOM Chrome Moly.
The CM is stronger and the wall thickness of the tubing is consistent.
CM also does not seem to work harden quite as quickly as the ERW.
Since I've never known the metallurgical / alloy properties of the ERW purchased, I can't speak to it's purity or suitability over the long haul.
Most of the ERW chassis were destroyed within a couple of seasons of rough racing around the short-tracks.. Those that weren't would start to exhibit cracks at various places in the chassis where they could no longer flex as needed with the multiple impacts.. 4 or 5 seasons would be about the limit for these cars before they needed to be retired and scrapped..
My own belief is that welding CM takes a bit more care in preparation and welding. I like to make sure that the area to be welded is pre-heated to ~100 degrees or more and make sure that there are no fans blowing on/around the work so it can air-cool slowly. A little heat offered up to the weldment via a torch is not a bad idea either.. Treating ERW with the same respect doesn't hurt either but it's rarely done by most shops as most of them use MIG welders which bring much more heat for the thicker material.
I've built quite literally dozens of racecars from the ground up. Many more than that of factory tub cars with my cages. Depending on the customer's needs and the rules we were to follow - the chassis / cages were mostly built of DOM or ERW.
I've never yet seen ERW fail when put to the ultimate test.
But yet I prefer DOM Chrome Moly.
The CM is stronger and the wall thickness of the tubing is consistent.
CM also does not seem to work harden quite as quickly as the ERW.
Since I've never known the metallurgical / alloy properties of the ERW purchased, I can't speak to it's purity or suitability over the long haul.
Most of the ERW chassis were destroyed within a couple of seasons of rough racing around the short-tracks.. Those that weren't would start to exhibit cracks at various places in the chassis where they could no longer flex as needed with the multiple impacts.. 4 or 5 seasons would be about the limit for these cars before they needed to be retired and scrapped..
My own belief is that welding CM takes a bit more care in preparation and welding. I like to make sure that the area to be welded is pre-heated to ~100 degrees or more and make sure that there are no fans blowing on/around the work so it can air-cool slowly. A little heat offered up to the weldment via a torch is not a bad idea either.. Treating ERW with the same respect doesn't hurt either but it's rarely done by most shops as most of them use MIG welders which bring much more heat for the thicker material.
