Track Day Roll Cage Material
Discussion
Well I have tried a search to no avail so here goes:
I am thinking of fitting a roll cage to my track day car to add a bit of stiffness and a little additional safety. I am NOT going to race the car or compete in any series, if I decide to go down this route I would purchase a car specifically for it, but want to mount bucket seats and 6 point harnesses properly and increase the stiffness added by my strut braces.
The Custom Cages multipoint cage is over £ and the club kit £ so I was wondering if I could do it cheaper, but still safely, DIY. The good book states that I should use CDS with a minimum of 45mm OD and 2.5mm thickness or a minimum of 50mm OD and 2mm thickness. This is expensive and, since I do not need to pass scrutineering I was wondering if I used a cheaper material but perhaps slightly more of it with additional triangulation and bracing it would work out less expensive. I know that I can get hold of 32mm OD 2mm thickness steel tube relatively easily but would this be to flimsy for a roll cage.
Any comments gratefully received,
Jon
I am thinking of fitting a roll cage to my track day car to add a bit of stiffness and a little additional safety. I am NOT going to race the car or compete in any series, if I decide to go down this route I would purchase a car specifically for it, but want to mount bucket seats and 6 point harnesses properly and increase the stiffness added by my strut braces.
The Custom Cages multipoint cage is over £ and the club kit £ so I was wondering if I could do it cheaper, but still safely, DIY. The good book states that I should use CDS with a minimum of 45mm OD and 2.5mm thickness or a minimum of 50mm OD and 2mm thickness. This is expensive and, since I do not need to pass scrutineering I was wondering if I used a cheaper material but perhaps slightly more of it with additional triangulation and bracing it would work out less expensive. I know that I can get hold of 32mm OD 2mm thickness steel tube relatively easily but would this be to flimsy for a roll cage.
Any comments gratefully received,
Jon
Anyone feel free to prove me wrong...
The rigidity of the roll cage will depend on several factors: your cage design (amount of bracing etc...), the OD of the tube you use and the wall thickness. Your welds are unlikely to have a significant effect unless they have low penetration but we'll ignore that eventuality.
If both wall thickness and OD are multiplied by a factor of "x", the rigidity (torsional and flexural) will increase by x to the power 4, the strength will increase by x cubed and the weight per unit length will increase by x squared. From this, you can see that both wall and OD have a really significant impact on likely cage performance. The FIA recommend 45 x 2.5 or 50 x 2 for the main roll hoop as you said (incidentally, there are some good diagrams on their website: http://argent.fia.com/web/fia-public.nsf/86CFF466E... starting from page 6). Using the proportionality above (check it here: http://desperadocycles.com/The_Lowdown_On_Tubing/C... a 32 dia tube with 2mm wall is only about 1/3 as stiff as a 45 dia 2.5mm wall tube because of the relationship between OD and rigidity.
I came to that because the average ratio between 45 and 32, and 2.5 and 2 is about 0.75 (so x=0.75). Rigidity change = 0.75 x 0.75 x 0.75 x 0.75 = 0.325, hence about 1/3 as stiff.
As you say though, you can design it out to a certain extent (http://www.robinson-race-cars.co.uk/ click on roll cages, then roll cage builder - has some really good pics of increasingly strong roll cage designs) but for the main parts - main roll hoop, front tubes and screen bar, I would push the boat out for something bigger OD. The other sections could probably get away with being smaller OD but it all depends on your design and how well it is fitted IMO.
It would be worth speaking to someone like Andy Robinson Race cars because they do this sort of thing every day and might be able to offer some design advice.
One thing I would ask is if the cheaper material is seamless because I wouldn't personally like to trust a ton odd of car on its roof to the quality of some far eastern tube manufacturer. You might be able to buy some "scrap" from manufacturers that use lots of fairly small tube. If you have your design and can buy specific lengths for specific members, you might be able to get a better deal than having to buy in standard lengths (6 or 8 metres if memory serves).
Keep me posted as to what you do, I am looking into caging my track car (when I get around to buying one) so I would be interested in how it comes out. Good luck Jon!
The rigidity of the roll cage will depend on several factors: your cage design (amount of bracing etc...), the OD of the tube you use and the wall thickness. Your welds are unlikely to have a significant effect unless they have low penetration but we'll ignore that eventuality.
If both wall thickness and OD are multiplied by a factor of "x", the rigidity (torsional and flexural) will increase by x to the power 4, the strength will increase by x cubed and the weight per unit length will increase by x squared. From this, you can see that both wall and OD have a really significant impact on likely cage performance. The FIA recommend 45 x 2.5 or 50 x 2 for the main roll hoop as you said (incidentally, there are some good diagrams on their website: http://argent.fia.com/web/fia-public.nsf/86CFF466E... starting from page 6). Using the proportionality above (check it here: http://desperadocycles.com/The_Lowdown_On_Tubing/C... a 32 dia tube with 2mm wall is only about 1/3 as stiff as a 45 dia 2.5mm wall tube because of the relationship between OD and rigidity.
I came to that because the average ratio between 45 and 32, and 2.5 and 2 is about 0.75 (so x=0.75). Rigidity change = 0.75 x 0.75 x 0.75 x 0.75 = 0.325, hence about 1/3 as stiff.
As you say though, you can design it out to a certain extent (http://www.robinson-race-cars.co.uk/ click on roll cages, then roll cage builder - has some really good pics of increasingly strong roll cage designs) but for the main parts - main roll hoop, front tubes and screen bar, I would push the boat out for something bigger OD. The other sections could probably get away with being smaller OD but it all depends on your design and how well it is fitted IMO.
It would be worth speaking to someone like Andy Robinson Race cars because they do this sort of thing every day and might be able to offer some design advice.
One thing I would ask is if the cheaper material is seamless because I wouldn't personally like to trust a ton odd of car on its roof to the quality of some far eastern tube manufacturer. You might be able to buy some "scrap" from manufacturers that use lots of fairly small tube. If you have your design and can buy specific lengths for specific members, you might be able to get a better deal than having to buy in standard lengths (6 or 8 metres if memory serves).
Keep me posted as to what you do, I am looking into caging my track car (when I get around to buying one) so I would be interested in how it comes out. Good luck Jon!
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