I hired a hydraulic pipe bender from Speedy Hire for £26 to enable me to bend the piece of 48mm o/d x 3mm wall black pipe, which I bought from Metal Supermarkets the other week, into a suitable rollbar. I don't intend to attach my seatbelts to the rollbar and I don't intend to race the car formally so CDS (Cold Drawn Seamless) tubing is not absolutely necessary, although still preferable. A rollbar is not required at all for SVA and even for race applications the minimum sizes are either 45mm o/d x 2.5mm wall or 50mm o/d x 2mm wall. My tube has a much thicker wall than is required for a 48mm tube and this should more than make up for the fact that is not seamless tubing.
I decided to weld the bar instead of bolting it on for two reasons:
1. It is much easier than drilling large holes in the suspension turrets and then fitting and welding crush tubes in place to support the bolts. It should also be stronger.
2. SVA regulations have stricter requirements for projections for bolted items than if the item is part of the permanent structure. I can get away with blunted edges for the welded parts of the chassis whereas 2.5mm radii are required for anything which is bolted in place. I would also require bolt covers and scrutiny is likely to be give to the method of attachment and quality of the bolts used.
These points are not particularly onerous but when there is no real disadvantage of welding the rollbar then why bother bolting it? I don't know either...
Sorry, not got any pics of the roll bar fabrication but here is a pic of the double thickness mounting plates where the rollbar is welded to the rest of the chassis.