Roll bar specification

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I,ve recently drawn up some plans for my soapbox and now have the materials on hand to start building! I found the construction tips on steering etc really helpful and I've been keeping an eye on the SCA Soapbox Cartie Technical Specification (2013)during the design process.

What I have realized is, that as I am using BMX forks for my front steering, I'm in a position where I really need a roll bar and harness, to stop me getting caught up in the frontal framework I am proposing to fabricate. I,ve designed the rear end of the cart with a roll bar in mind but no design for such yet. I have questions...

1) The SCA rules say the main roll bar must be diagonally braced both laterally and fore and/or aft. What exactly is meant by this in practice? I have also looked at the FSID rules. It contains a few diagrams for roll bars, but I'm not sure if this is what the SCA has in mind or not. I guess you're looking to put a brace from partway up the roll bar back to the chassis (fore and or aft). But what about laterally? Can it be diagonal from one side of the roll bar to the other (as is seen on some roll bars) or does it have to go back to the chassis from each side of te roll bar? Could one bar pointing backward from the centre of the roll bar to the chassis brace it against movement in all the desired directions? I must really have this point wrapped around my neck!

2) The roll bar may be welded to the chassis, but it must be made without joints. What about the braces? Can they be welded to the roll bar?

3) The SCA roll bar material specifications are tighter than the FSID. Greater wall thickness and must be cold drawn (making it expensive). I understand this is purely for the main roll bar, not the front as well. What do the braces have to be made from? The same tube, or would a bit of box section, for example, suffice?

4) I think I get the line between two roll bars clearing the helmet and hands. The FSID state that this "geometrical line" spans "from the top of the roll bar to the highest point of the front part of the body". As my cart will be a roadster, "the highest point of the front part" will be of substantial strength. So, I won't need a front roll bar will I?


It would be really good if there was a section that gave some practical advice and a few diagrams/photos for roll bar construction on this website, as the principles behind it are no more obvious than such things as Ackerman steering and scrub radius, both of which were entirely new to me. 

I know it seems a lot to ask, but I am confused; evidently! 

  • Posted In
    scottishcarties's picture

    re: Roll bar specification


    I'll try to keep this brief for clarity;

    1) Your understanding of fore and/or aft bracing is correct. Diagonal lateral bracing from one side of the roll bar to the other is acceptable. A single bar "pointing backward from the centre of the roll bar to the chassis" would only satisfy the "fore and/or aft" requirement.

    2) Braces can be welded to the roll bar. 

    3) Bracing bars do not need to be the same material or gauge as the main rool bar, but clearly they must be fit for purpose.

    4) If the chassisat the front is strong enough to act as a roll bar then the forward roll ba is not required.

    I hope that clears things up. Please do ask if there is anything else I can help with.

    scottishcarties | March 20, 2016 - 12:08

    That's brilliant. You're on

    That's brilliant. You're on my wavelength! Thanks.
    A610RACER | March 21, 2016 - 19:57

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