Hey Mates - Last time I was on this site was 2008 - Back again and looking to build a "WARRIOR LOW RACER TADPOLE TRIKE" - Can I use alum square tubing instead of steel? - If so what size/thickness? - Thanks for any help Docbob.
Hello there. Welcome back to the pack.
There is a reason why aluminium framed upwrongs use oversized tubing. Rule of tumb, , for the same wall thickness, increase the diameter (or square) with 50%. So a steel 1.5" box section will need a 2" or even a 2" 1/8 box section replacement. Thicker wall won't decrease that a lot, but will make it easier to weld it.
Regular bikes can use aluminium frames because the triangulation is perfect and all the forces run along the tubes. Recumbents use the frame as a beam where the metal is vastly weaker. An aluminium beam will almost certainly fail and fail drastically and without warning. Without substantial redesign I'd not do it. Steel has some give in it which is crucial to a beam.
With the amount of aluminium you need, the weight difference will be small, if there is any.
I have a rear part that is aluminium. But the rest of the bike is steel.
Most weight profit that I hot, was from the steering setup, the wheels and the seat.
Rear triangles are, again, perfectly triangulated so a rear triangle in aluminium can cope with the forces it's subjected to. Those bolted frames look substantially heavier than most steel ones and that's without adding in the weight of the bolts which itself will be no small figure.
This is one of the lightest I've seen and the frame alone is 8kg. They have to use 4 beams to make a box to add enough strength. A Warrior chassis comes in around 6kg depending on length / width etc. Exactly what's defined as chassis can vary from person to person and/or build to build. BB tubes and steerer tubes on a bolt on chassis probably don't count to chassis weight, but they would on a Warrior IMO making the comparison even better for the steel chassis. I'd also expect any bolted aluminium frame to elongate the bolt holes over time.
As it is for weight, than there are other ways to save weight. Use the rear suspension from a mountain bike, use an other seat, change the steering system ad the wheels, use carbon tubes to link the steering.
Then the parts you get. The wheels you choose can make a big difference in weight and roll resistance.
But also with the other parts you can safe weight.