You could use ERW for the wishbones there's nothing stoping you. For the front lower turn the seam round so that it faces the 5mm plate. It only says use CDS as it would seem safer to do so. If you smack the wheel against a curb your more likly to pull the bolt through the suspension bracket, rip the bracket off the 1.6-2mm chassis or bend the wishbone than split the wishbone tubing. As for CDS being stronger it's only stronger in that it's ment to be used for hydrualic lines and hyd. rams for internal pressure as there is no seam to split. There are two main types of CDS in the 19mm - 3/4" and 1" - 25mm sizes hyd and mech. Hyd. grade and is softer the mech. grade ,you could end up with material that could be weaker than ERW. At the end of the day you pays your money you make your choices.
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