when I mechanicked for sidecar race teams, almost all of them were bare metal chassis, this was (mainly and the big point) so that inspection of all areas, for cracks and defects could continually be looked at. It was my idea (when Tony Bateup was building a new chassis for Brian Houghton and the Bevan imp) that a pilot hole was drilled internally at all tube joints, to connect the innerds of them all together, then a 1/4 BSP boss was welded on near the headstock to carry a pressure gauge with another boss and a Schrader valve at the rear, the idea? simply pressurise the whole tube chassis to around 20PSI, and if a crack occurs the pressure drops. This seemed like a good idea but it didn't tell you if a brazed on bracket outside of the tube formations had cracked, or even broken off, as in the case of the rear brake pedal at the IOM in 1975? ! it dropped into the streamlining, fortunately and Brian still won the Southern 100, mainly because he was so far ahead that he was already hard to beat, there was a lot of front wheel lockups though, as the pedal linked all three wheels, and the handlebar lever and calliper was really only for scrutineering, which stipulated separate brake systems.
Then they had riveted/glued aluminium chassis, such as Seymaz, LCR etc and then carbon fibre. sometimes made by Arrows cars.
CLEM