AdamR
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| posted on 11/12/07 at 05:37 PM |
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Roll hoops: CDS versus ERW
Could someone who knows please quantify the difference in "strength" between CDS and ERW tube? Let's say 45mm with 3mm wall for the
sake of argument.
Objective replies only please. 
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cossey
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| posted on 11/12/07 at 05:51 PM |
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in reality there isnt much difference as erw very rarely splits along its length and in reality the chassis is likely to fail first. fishers have done
loads of properly braced erw roll bars and i havent seen any fail but then is it worth the risk for a few quid, erw certainly isnt allowed for racing.
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kb58
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| posted on 11/12/07 at 05:54 PM |
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In the U.S., the overseeing racing organization SCCA does not allow EW tubing. If you need to have tubing bent for a cage, may as well have the right
stuff bent up, for peace of mind if nothing else. The local rollcage builder here doesn't even stock EW tubing, it's all seamless/DOM.
Even if you're not going to race, I feel that using the right tubing is a good idea anyway. Besides, the next buyer of your non-raced car may
want to take it to the track. It will be far easier to sell if it has the right tubing.
[Edited on 12/11/07 by kb58]
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blakep82
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| posted on 11/12/07 at 07:00 PM |
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RAC MSA will insist on CDS for a roll bar for racing. there must be a reason why 
I wouldn't use ERW
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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bobster
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| posted on 11/12/07 at 07:59 PM |
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sorry to jump in on your thread. what wall thickness is currently recommended for a roll cage. (38mm cds tube)
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cloudy
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| posted on 11/12/07 at 08:11 PM |
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ERW is fine for use within a spaceframe (compression and tension only), but will fail far earlier than seamless when you add a bending load - so not
used for roll cages
James
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Blakey_boy
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| posted on 11/12/07 at 08:54 PM |
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Bobster 3.2mm
The difference is in case you dont know
ERW is Electric Resistance Welded as others have said can fail quite early and because of the nature of the steel unsuitable for high Impact and
Stress areas.
CDS is Cold Drawn Seamless as it says on the lettering no seam welded or otherwise and is more suitable for load bearing and high stress/loading
areas.
Here endeth the lesson kiddie winkies.
Gene Hunt:- If anything happens to that car, I'm coming round your house and stomping on your toys, Understand?
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Ian-B
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| posted on 11/12/07 at 09:21 PM |
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For info the some of the most commonly available grades of;
ERW - E155+U - typically yield 175MPa 290MPa UTS elongation to failure 15%
CDS - old spec BS6323Pt4 CFS 3BK - new spec - E215+C typically 360MPa yield 430MPa UTS elongation to failure 8%
The common CDS is significantly higher strength, but it is often supplied in the hard condition which is fine for loading below yield, but is not
great for crash structures as it leaves very little scope for further elongation and therefore engery absorption. I would suggest that it is worth
obtaining CDS tubing with good elongation properties, probably the easiest (but not cheapest route) is to buy tube from a roll cage manufacturer.
I would suggest that for the same material grade (of fully weldable low alloy steels), supplied in the normalised condition, there would be very
little difference in performance between good quality welded and seamless tubing.
[Edited on 11/12/07 by Ian-B]
[Edited on 11/12/07 by Ian-B]
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Howlor
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| posted on 12/12/07 at 10:44 AM |
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There soundeth a man that sounds as though he knows a bit about the subject!
Welcome to the forum.
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britishtrident
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| posted on 12/12/07 at 03:25 PM |
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quote:
Originally posted by Ian-B
For info the some of the most commonly available grades of;
ERW - E155+U - typically yield 175MPa 290MPa UTS elongation to failure 15%
CDS - old spec BS6323Pt4 CFS 3BK - new spec - E215+C typically 360MPa yield 430MPa UTS elongation to failure 8%
The common CDS is significantly higher strength, but it is often supplied in the hard condition which is fine for loading below yield, but is not
great for crash structures as it leaves very little scope for further elongation and therefore engery absorption. I would suggest that it is worth
obtaining CDS tubing with good elongation properties, probably the easiest (but not cheapest route) is to buy tube from a roll cage manufacturer.
I would suggest that for the same material grade (of fully weldable low alloy steels), supplied in the normalised condition, there would be very
little difference in performance between good quality welded and seamless tubing.
Good info :-)
Also to add common "as supplied" CDS is difficult to cold bend without first annealing.
[Edited on 12/12/07 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Syd Bridge
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| posted on 12/12/07 at 04:07 PM |
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And annealed cds(or cdw, little difference), .................has the properties of plasticene.
I wouldn't trust my life to it, or yours. After all, what does 'annealed' mean, and do to the metal?
Tell the MSA that you are using 'fully annealed cds', and see what comes back.
As the man said, 'Ask me how I know?'.
Cheers,
Syd.
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907
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| posted on 12/12/07 at 04:44 PM |
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A couple or so questions.
Does CDS gain it's increased tensile strength through being work hardened?
And if so, does welding it anneal it in the HAZ anyway?
Why is there never any mention of seamed pipe that has been welded using a process other than resistance welding?
Just curious.
Paul G
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AdamR
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| posted on 12/12/07 at 04:46 PM |
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Thanks for all the responses. Especially Ian B - that's some great objective information which is just the sort of thing I was after.
Interesting comments regarding the hardness of CDS.
I had two motivations in posting the seemed versus seemless question:
1) I need to have roll hoops made by the end of next week. I have one supplier offering ERW and one offering CFS, the latter being considerably more
expensive. I've ended up paying the extra money but more because I think they'll deliver on time rather than the material.
2) I still find it hard to believe that any *survivable* accident in a Seven could apply enough force to a roll bar to split ERW. I think the mounting
points on the chassis would be deformed out of all recognition before that would cause a problem. It goes without saying that I'm no expert
though!
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Ian-B
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| posted on 12/12/07 at 10:46 PM |
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Regarding the hardness of CDS tubing, all steels will increase hardness through cold working, and therefore the cold drawing process will increase
tensile strength, this also applies for the cold working to form ERW tubing (although to a much lesser extent). To use the example of E215 cold drawn
round tube again;
In the (hard) as drawn conditon E215+C UTS 430MPa elongation 8%
In cold drawn soft condition (heat treated prior to final drawing) E215+LC UTS 380MPa elongation 12%
In the stress relieved condition (heat treated after drawing) E215+SR UTS 380MPa elognation 16%
In the annealed condition E215+A UTS 280MPa elongation 30%
And finally in the normalized condition (heat treated after drawing) E215+N UTS 290-430MPa Yield 215MPa (hence the grade name) Elongation 30%
In all cases only the processing has changed, the material grade is the same. There is a wide range of possible material grades, a some of which would
satisfy the MSA roll cage material requirements in the normalised condition.
As for the effects of welding, on a tube in the hard condition it would be reasonable to expect some annealing effect in the HAZ, but I have no data
to support the extent of the annealing effect, but providing the heat input is not excessive I would guess that the tensile properties would still be
well in excess of the fully annealed condition.
As for ERW being able to resist the required crash loads, I cannot comment specifically as it is far to dependant on the design of the cage and
chassis. It should certainly be possible to design an ERW structure to provide effective protection. But with the readily available material sizes and
grades, it would probably result in additional design work and would be at odds with the recognised (motor sport) standards for roll structures.
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Syd Bridge
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| posted on 13/12/07 at 11:02 AM |
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All of that in layman's terms, says exactly as I put.
Anneal or normalise the tube, and it ends up soft, weak, and stretchy.
The HAZ with welding is always a failure area, and is usually the source of stress cracks which lead to failure. In the HAZ, you have chilling from
the surrounding parent metal, changes in cross sectional area, and a degree of annealing. So, all of these add up to a headache, if you want it to be.
Somethings you have to learn to live with.
Cheers,
Syd.
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Fred W B
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| posted on 13/12/07 at 11:19 AM |
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What would the feeling be in making a roll bar from Schedule 10 one and three quarter inch 304L stainless. (od 42.16 mm, wall 2.77 mm) ?
Cheers
Fred W B
[Edited on 14/12/07 by Fred W B]
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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Volvorsport
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| posted on 13/12/07 at 11:48 AM |
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id say no ! leave that stuff for exhaust manifolds .
www.dbsmotorsport.co.uk
getting dirty under a bus
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Fred W B
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| posted on 13/12/07 at 11:54 AM |
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who makes exhausts with 3 mm wall thicknesses
304L would have a UTS of 585 and an elongation of 40 %
Cheers
Fred W B
[Edited on 13/12/07 by Fred W B]
[Edited on 13/12/07 by Fred W B]
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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907
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| posted on 13/12/07 at 09:13 PM |
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I made mine from Sch10, inch & a half 304.
I still prefer to think of it as a "High level brake light mounting bar", as the thought of rolling over scares me.
Paul G
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JB
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| posted on 15/12/07 at 08:36 PM |
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quote:
Originally posted by kb58
In the U.S., the overseeing racing organization SCCA does not allow EW tubing. If you need to have tubing bent for a cage, may as well have the right
stuff bent up, for peace of mind if nothing else. The local rollcage builder here doesn't even stock EW tubing, it's all seamless/DOM.
DOM is actually seamed tube. It is welded then has a post weld process where it is drawn over a mandrel. It is an improvement on ERW.
I tried to use DOM for my wishbones and i had to squeeze the end flatter to weld on a bearing housing and the tube split along the seam.
DOM is very good for propshafts though as it has a much more concentric bore than CDS.
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