Metals are much easier to recycle than paper products.
To recycle metals you might need to do a bit of pre-processing, grind up everything a bit, and throw it in a crucible to melt it down. 100% recyclable and the same as new when recast.
To recycle paper you have to make sure everything is sorted (no plastic, metal, etc in with your paper) and do a good amount of chemical processing and the end result isn't as easily like "new" paper.
I don't see how this gives cardboard the edge in recycled materials use for a bike.
While metal recycling might be simpler in terms of the process, paper recycling might still be more efficient in terms of energy/volume or money/volume.
Energy used for recycling paper: roughly 1000 kWh/short ton (Wikipedia). Savings of about 60%.
Energy used for recycling aluminum: 700 kWh/short ton. (Various websites.) Savings of about 95%!
I knew aluminum recycling was crazy efficient compared to refining it in the first place, but I'm surprised it's the same or even cheaper per ton as paper.
Sidenote: Recycling aluminum really makes sense when you consider the energy you need to make it in the first place:
"Large, modern cells operate with a specific energy consumption of 5.9 to 6.4 kWh/lb of aluminum. The energy required to produce aluminum from scrap metal is approximately 5% of that required for primary production"
To recycle metals you might need to do a bit of pre-processing, grind up everything a bit, and throw it in a crucible to melt it down. 100% recyclable and the same as new when recast.
To recycle paper you have to make sure everything is sorted (no plastic, metal, etc in with your paper) and do a good amount of chemical processing and the end result isn't as easily like "new" paper.
I don't see how this gives cardboard the edge in recycled materials use for a bike.