I have often wondered why some regs insist on CDS tubing and non- welded bends for roll cages as I think the spec is over the top.

To my mind a cage built of normal welded tubing would be well up to most requirements - especially as it is normally bolted and/or welded to and already welded structure and most things fail at their weakest point - ie the structure its fixed to or the fixing.

Also all bracing bars are welded to it so there's another weak point.

So bearing in mind that:
1/ In a roll over most loads are pretty transient and not very high so fancy tubing spec is hardly justified.
2/ In a T-bone collision - most probably the worst case scenario for a racing accident the loads are normally straight into bracing bars that are welded onto the main cage and even then loads are limited by the sideways displacement of the vehicle. In this case the fancy bent main bar has little loading.
3/ Welding and fabricating many safety critical items such as suspension brackets, chassis etc are subject to significant cyclical loads that roll cages do not experience is OK

I ask - why the fancy spec for roll cages?

Or is it just to give us and regulators a false sense of security!

In other words - if you are good enough to fabricate a car from scratch and run it without a roll cage why aren't you good enough to fabricate a roll cage from the same materials with the same processes?

just have a look on you-tube at some of the more spectacular catherine wheel like rollovers and I think you'd want the best, the violence of some of these crashes is quite amazing even a surprisingly low speeds.

I'm more impressed at how the cages take what they do bth, was watching some offroad plunges yesterday and 5 gear did a rollover test on a range rover, it took quite a pounding at just 40mph

[Edited on 30/10/08 by Mr Whippy]

I think I'd want a cage which was up to the job.

It amazes me how people - I have to say a lot on here - have rollover hoops which are lower than the tops of their heads

Obviously in the case of a roll you want to be able to be upside down with out your head hitting the ground - this usually means that the bar should be significantly higher than the top of the drivers (or passengers) head.

The cage is there for when everything else goes wrong so it needs to be good.

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Originally posted by mookaloid
I think I'd want a cage which was up to the job.

It amazes me how people - I have to say a lot on here - have rollover hoops which are lower than the tops of their heads

Obviously in the case of a roll you want to be able to be upside down with out your head hitting the ground - this usually means that the bar should be significantly higher than the top of the drivers (or passengers) head.

The cage is there for when everything else goes wrong so it needs to be good.

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even though it may look a little strange
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and it is there to do a job not be a fashion accesory. I could not agree more

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Originally posted by mookaloid
I think I'd want a cage which was up to the job.

It amazes me how people - I have to say a lot on here - have rollover hoops which are lower than the tops of their heads

Obviously in the case of a roll you want to be able to be upside down with out your head hitting the ground - this usually means that the bar should be significantly higher than the top of the drivers (or passengers) head.

The cage is there for when everything else goes wrong so it needs to be good.

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Am I the only one then to be "amused" by the fact that 45mm diameter 3mm wall thickness seamless tube is "essential" for safety, even though it is welded to 25mm 16 SWG ERW tube underneath.

No-one has explained to my satisfaction why you can't make the top out of the same stuff as the bottom.

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Originally posted by smart51
Am I the only one then to be "amused" by the fact that 45mm diameter 3mm wall thickness seamless tube is "essential" for safety, even though it is welded to 25mm 16 SWG ERW tube underneath.

No-one has explained to my satisfaction why you can't make the top out of the same stuff as the bottom.

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Originally posted by Mr Whippy

quote:

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Originally posted by smart51
Am I the only one then to be "amused" by the fact that 45mm diameter 3mm wall thickness seamless tube is "essential" for safety, even though it is welded to 25mm 16 SWG ERW tube underneath.

No-one has explained to my satisfaction why you can't make the top out of the same stuff as the bottom.

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it's simply due to the bar having to span a large distance without much support. Unlike the braced chassis frame. You are meant to weld the rollbar onto large thick plates to spread the load over the chassis anyway.

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lol

i've got a good sized one too. it reaches all the way round infront of my head!

tom

[Edited on 30/10/08 by indykid]

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I am also pleased to say in addition to the people who have too low roll over hoops there are also a good handful who have good sized ones..........ooh err

Cheers

Mark

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So far lots of replies around "wanting the best" but none about by how much it is the best or how justified it is.

For instance I think that in SA many are made of welded bends and tubing (must be because I get blank looks from steel suppliers and recommended cage manufacturers when I ask for CDS tubing ) and they don't fail either in accidents. And our accidents are just as good (bad) as anyone else's

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Originally posted by Ivan
So far lots of replies around "wanting the best" but none about by how much it is the best or how justified it is.

For instance I think that in SA many are made of welded bends and tubing (must be because I get blank looks from steel suppliers and recommended cage manufacturers when I ask for CDS tubing ) and they don't fail either in accidents. And our accidents are just as good (bad) as anyone else's

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This thread seems to be running in parallel with the one posted by maartenromijn in the Chassis category. The respective merits of both one piece and fabricated hoops have been well discussed on his thread but at the end of the day, we are free to make our own choice which is nice in a world where more and more decisions are made for us.

You have no legal requirement (in the UK) to have a rollover bar at all so there is a big choice from nothing to the best which in my view will be the specification laid down by the FIA. In the end as with many things it's probably a question of what you feel comfortable with in terms of cost and specification.

John

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Originally posted by John Bonnett
In the end as with many things it's probably a question of what you feel comfortable with in terms of cost and specification.

John

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As per topic yesterday ( http://www.locostbuilders.co.uk/viewthread.php?tid=99610 ) we concluded that CDS bent roll-bars are stronger than fabricated rollbars. It is best, when you have the choice, to opt for the strongest construction in the interest of your own safety.

I do agree with you that it is hard to tell which strength rollbar would be necessary. I think it greatly depends on your own risk assesment. How is your driving style, in what kind of traffic. Nobody can tell before the accident if the rollbar is strong enough. If you have bad luck, you'll hit a lamp-post as well.

[Edited on 30/10/08 by maartenromijn]

soooo, if you were in a crash, with lots of rolling, landing on the roof, on the sides etc, which of the 2 underneath would you rather have a crash in?



Description

CDS is stonger than ERW. its a fact. yes ERW may well be up to the job, but do you really want to wait til you hit the ground really really hard upside down to find out?
ERW is likely to split, weakening the tube and then on the next bump on the roof, it'll bend, and your head hits the ground.
CDS holds its structure at much higher impact loads.

I'd rather crash in the yellow one. Matter of style....

Your project looks good btw. Have you designed the chassi yourself? Does it have 7-style geometry underneith?

saying that dude...
Rescued attachment truck.JPG

Just as an aside, and I know that this has little to do with the topic - you can have too strong as well, which is exactly why car manufacturers build in deformation into their structures - and no, I'm not saying that roll bars shouldn't be as strong as possible, what I am saying is that if you intend to drive in circumstances that will put your roll cage to the test you better make sure that your other safety equipment is designed around saving you from the type of brain and neck injuries that too rapid a deceleration due to too rigid structures will impart on you. (Remember Senna)

In fact I remember reading somewhere that one of the reasons for the great reductions in F1 driver injuries with the move from steel to fibre structures was the greater controled deformation in an accident that the fibre structures gave. i.e - controlled deformation good - too much rigidity bad.

I think that NASCAR has similar experience.

[Edited on 30/10/08 by Ivan]

When the car's gone past the point of no return and just about to flip onto its roof, you'll be glad you fitted the best! Been there (brand new Mini shell with full Rollcentre cage) and glad I'd spent the months to fit the cage!

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Originally posted by Mr Whippy
saying that dude...

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Originally posted by blakep82

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Originally posted by Mr Whippy
saying that dude...

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yeah, i haven't bought those bar yet, but they will be there!

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Originally posted by Ivan
Just as an aside, and I know that this has little to do with the topic - you can have too strong as well, which is exactly why car manufacturers build in deformation into their structures - and no, I'm not saying that roll bars shouldn't be as strong as possible, what I am saying is that if you intend to drive in circumstances that will put your roll cage to the test you better make sure that your other safety equipment is designed around saving you from the type of brain and neck injuries that too rapid a deceleration due to too rigid structures will impart on you. (Remember Senna)

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what I am saying is that if you intend to drive in circumstances that will put your roll cage to the test you better make sure that your other safety equipment is designed around saving you from the type of brain and neck injuries that too rapid a deceleration due to too rigid structures will impart on you.

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Originally posted by maartenromijn
I'd rather crash in the yellow one. Matter of style....

Your project looks good btw. Have you designed the chassi yourself? Does it have 7-style geometry underneith?

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Sorry, off topic

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nooo, didn't design it myself. its an ex race car. similar suspension etc to a 7 though, but uses a watts linkage at the back.

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i haven't driven it yet, nor have i driven a car with a panhard rod, so haven't had a chance to feel the difference between the two. on paper a watts linkage looks best though, but give there shouldn't be too much suspension travel, i'm not sure it would have made a noticeable difference really

My opinion - The advantage of using the high strength steels is that you can use a thinner tube, therefore lighter.

If you don't mind a thicker heavier tube a less esoteric grade of steel could be used, which is also cheaper.

Note I don't quote actual thicknesses and sizes, that's for each person to decide for himself, as stated above

And that also depends on what you can source and get bent decently. If you have to join, a properly welded joint, WITH A SLEEVE INSIDE, should be as strong as the original material.

Cheers

Fred W B


[Edited on 30/10/08 by Fred W B]

Hi.

The thing is that FIA and in this country MSA governed by the FIA regulations are always being updated. There where new regulations introduced during 2004. And again for the start of 2008. ( not that many manufacturers seem to be tacking any notice )

Having not so long ago had to put a roll cage through the latest testing procedures which now takes into account not just the roll cage but the whole chassis and the distribution of the loads through out the chassis you soon see where and why strict regulations need to be in place.

There is already far too many companies and manufacturers who are selling so called cages / bars that they believe comply with certain regulations but the truth is they do not even comply with the bare basics never mind passing the regulations. Hence there are more and more motor sport competitors being told that the money that they have just spent was wasted as they now have to have a new setup to comply.

As it stands for sports cars not single seaters the larger diameter cages of 1 3/4" are allowed to be used but must follow the drawing in the blue book. Any cages / bars that are of smaller diameter must have been fully tested and certificated by the FIA / MSA if being used in the UK. IF in any doubt seek clarification from the MSA.

The reason there is such regulations is because there are individuals out there who would still rather have a cage / bar made of 1 1/2" dia and 16g to save weight rather than something that will save there life during a big accident. Then there are the cages that turn up where the desighn is sound but assembled with clamps that are of no use or welding that would never hold up.

As has already been said the choice is yours but i would rather enter a situation knowing i had done my best rather than having that nagging doubt that it may not save my life.

Cheers Matt

To answer some of the points in the original post
The logical explaination for requiring seamless tube and contiunous unwelded main hoop would be to minimise the risk of creating additional failure points. The main reason for requiring a more substatial section of material than often used of the chassis is the the roll hoop is far more likely to receive a direct point load a significant distance from a join and most roll structure designs have no/little redundacy in the structure.

For the load cases you describe;
1 - Normal roll over, the loads are generally fairly modest, to compare to production road cars where there is currently very little roll over legislation, the USA (only reg I am aware of) specify a maximum displacement for a load equivalent to 1.5x vehicle mass, which is shortly to be increased to an expected 2.5x vehicle mass, this is far lower than normal motorsport specifications (approx 7.5x vehicle mass from memory), yet most road cars will provide reasonable protection to a straight forward roll over.
The main reason for needing a higher specification is for combined roll and modest impact on the roll structure (eg rolling as a result of being launched to reasonable height), for which the risk of this load case is fairly high in motorsport.

2 - For side impact, this as far as I can tell from the current blue book this is not really covered effectively, with many roll structures built to the regulations often being far from ideal at providing protection My reasoning for this is that the regulations focus is on static strength (with only a vague comment on elongation properties) and many of the available CDS grades obtain the required tensile properties through the cold working which results in a very siginificant reduction ductility (best tubing to use for this type of application is one where the base grade of steel is adequate to achieve the required yield stress, allowing the tube to be used in the normalised condition giving good ductility, but more expensive).
By way of example, for a load applied to the centre of typical side impact bar even when made from larger of the MSA standard spec tubes would only be able to resist a few G acceleration on a typical 7 type car before yielding, once the tube starts to yield providing there is adequated ductility the tube can still help absorb significant energy by acting as a tension member transfering forces into the main structure. For low ductility tubes the failure of the tube will often occur prior to any substantial engery absortption, resulting in very large intrusions into the passenger space.

If designing a roll structure for an application where specific material specifications aren't required there is nothing wrong with using a seamed tube, to use the production car comparision again, in cases where tubes are used in crash relevant parts the decision as to whether the tube is welded or seamless is usually determined by the most effective means of manufacture and the weldability of the steel being used.
The design of the structure and its mounting to the main vehicle structure, are more significant in the determining the effectivness of the structure in protecting the occupants, than the manufacturing process of the tube used.

If however you are building with any intention of racing the car, then you need to use the specified materials / layouts (unless you get your design approved MSA/FIA), but consider that just because it meets the specification does not neccesarily mean it will be fit for purpose. Also not all racing cages are specified to particularly large section or high strength tube, if you are racing in a series where the requirements specified are those for non standard cars then the min is 32mm dia 1.5mm*** wall good quality seamless steel tube effetively braced for and aft of the cockpit

***Changed for 2009 - allowance for different material specs from C(C)39 & 41 removed all cases except single seater and sports racing cars tubing 45x2.5 or 50x2mm min yeild 350MPa.

[Edited on 27/11/08 by Ian-B]

Thanks Ian-B - you answered most of my questions very well and have given me a much better understanding of the issues.

I guess that's why we post here - there's always someone with a much deeper understanding of the subject to answer us.

I'll put it this way:
If you plant your car into a wall head on what part of you are you going to injure? Head, legs?

What about if you get rammed from your side? Arm, ribs, pelvis?

And then if you roll over, head and neck?

Which one is going to kill you most times of ten?