I am totally confused by the blue book as to which tube dimension should be used in a 7 type car.

For example:

section K para 1.4.1

Material Specifications
1.4.1. Specifications of the tubes used:
Minimum Material
Cold Drawn Seamless Unalloyed Carbon Steel,
containing a maximum of 0.3% of carbon.
Note: For an unalloyed carbon steel the maximum
content of additives is 1.7% for manganese and 0.6%
for other elements.
Minimum Yield Strength
350 N/mm2
Minimum Dimensions (Ø in mm)
a) Mandatory tubular members
45 x 2.5 or 50 x 2.0
38 x 2.5 or 40 x 2.0 (For roll cages/bars approved
prior to 1.1.95).
b) Optional tubular members
38 x 2.5 or 40 x 2.0


Section 1.6.2

1.6.2. Sports Racing Cars
The rollbar must conform to drawing K1 with diagonal
brace (drawings K6 and K31). Forward facing stays are
permitted for open Sports Racing Cars.
Minimum height 92cm measured along the line of the
driver’s spine from the bottom of the car seat.
Minimum material
Cold drawn seamless Carbon Steel 350N/mm2
Minimum dimensions
48.3 dia. x 2.6mm.

Which category would a 7 type fit in?

Am I correct in thinking main hoop, diagonal and back-stays must all be the same dimensions.

It looks to me that if I went for 45 x 2.5 and they classed it as in section 16.2 then I would need 48.3 x 2.6. and there is no mention if 50 x 2.0 would cover both?

Do not want to go for heavier material unless I have no alternative.

Thanks

Adrian

[Edited on 3-8-12 by AdrianH]

IIRC 'Sports Racing cars' are things like touring cars and other tin tops.
Use 45mm throughout, and follow the guide drawings fairly closely too.

Hi Loggy, this is where I get even more confused with the Blue book

In section B Nomenclature and Definitions

Sports Racing Cars. Two-seater open or closed
racing car, built for the sole purpose of taking part in
races on closed circuits.

Sports Car. A car having either a non-detachable or a
detachable roof with no more than one door on each
side and a minimum of two seats situated one on each
side of the longitudinal centreline. The two seats must
be crossed by the same transversal plane. Interior
dimensions must be inferior to the minimum
dimensions applicable to Touring Cars.
A minimum of 100 cars must have been constructed
within a 12 consecutive month period.

Hence I believe a 7 type is a sports racing car more say then a sports car, like say an MX5 which is production.

Just thought to myself I am at Halewood car plant on an Autosolo tomorrow so will ask of the scrutineers.

Adrian

[Edited on 3-8-12 by AdrianH]

Hi

A 'Sports Racing Car' is just that a Sports car built purely for racing i.e a Mallock or Radical at one end of the scale or a LeMans prototype at the other.

Our cars are 'sports cars' of 'specialist production' - hence the class structure being :-
"Road going specialist production up to XXXXcc"

So tubes as Loggyboy said.

Though bear in mind if it's compeating as a road going car in any Speed Event then the cages are only 'recommended' anyway so it being exact to the MSA drawings would be a bonus - though a VERY good idea - rather than an essential requirement.

HTH

Jim

Ok good reply, so a minimum of 45 mm and 2.5 mm thick, keep as close as possible to the K1 or K2 if I keep the engine sub 2000cc.

Thanks

Adrian

if it says "minimum is unalloyed steel of a certain amount of carbon" does that mean you can go for T45, which is alloyed? It's 10x as strong as steel.

I know some of my grass-tracker mates are really frustrated because they're restricted to non-T45, and have to use a lot more and a lot heavier steel; for them the regs seem obsessed about wall thickness and not the fundamentals of material performance.

I thought that t45 was great for one hit. so it you plan rolling over and over it quickly loses its advantage as it deformed. can't remember where I heard this but the quote was supposed to have come originally from a bloke who made all sorts of roll bars for a living and had std steel in his race car because of it. love to know he this is cobblers or not.

Hi Mike

I'll call "cobblers" on that. As you move up the pace, T45 is sometimes the only material you're allowed. There are other elements to it as well when specified for roll cages, so it's cold-drawn and seamless, hence often being £50/m plus. I've had a small piece in the vice (3"x1" flat section) and I couldn't bend it with the 2lb lump hammer - I know which I'd trust.

It's also harder to bend because it's so damn strong, but that's it's point. If you don't follow the rules and anchor it well into your car though, it will give at the weakest point and tear out. Strength is strength and as long as you don't deform it past it's maximum point of ductility (if I've got that right) it will survive. Think of Hooke's law with springs - repeatedly stretching a stronger spring to 95% of it's stretch point repeatedly won't do it any more harm than doing the same thing at 50% more often.

One also has to be careful welding roll-cages; the blue book even has a point in that says "if the welding doesn't look neat, odds are high you've got a crap cage". When you look up tig welding the stuff, there is lots of advice on just which filler to use; you need to match the strength of the filler rod with the material you're welding to retain ductility, else you make it brittle at the joint and increase the risk of snapping it. Some people when migging it, may just use mild steel wire and worse, may under-fill it. A joint without filler will never be more than 80% as strong as a filled joint, and the advice I seem to come across is for crucial joints, a little more is better than any risk of undercutting (stress raisers).

so, I'd say if you can have T45, have it. It's lighter and way stronger. It's like the price of a crash-helmet: how much is your head worth to you? Race helmets must be certified to three impacts without deformation and FIA tested. For bike helmets the minimum is (was?) one impact, self certified by the manufacturers. I'm not saying all bike helmets are bad (anything but) but that goes some way to explaining how a cheap bike helmet is £70, and the minimum for a race helmet is getting to £400.