Hi! First post on this forum here. The talent is amazing here. I ride a recumbent much like the one pictured here. I am wanting to build one like it for a friend and am going to build out of 4130. My question is on the stays going to the rear wheel. Will it be strong enough if I do a good tight job of fitting them and then fillet braze? It seems like there will be a lot of pressure on the weld because there isn't really much triangulation with the seat stays. Any hints or tips on this?

Assuming good brazing (enough flow to establish an internal fillet, proper sized external fillet) and proper finishing (no undercutting of the tube) it will be a tube selection question. So if the tube choice is up to the task and you do a good job brazing and filing it will be OK. The difference in strength between a weld and a braze depends on how well each is done, not which is done.

But this design relies on the beam strength of the tubing. Adding some triangulation would increase the stiffness and reduce the need for a high beam strength. I have worked on many recumbent with this type of main frame/stays construction and still don't like them from a design standpoint. I understand why this simple design is used compared to a truss one (like a P-38) but I don't like it. Andy.

I would have a really long miter between the main tube and the stays.

The seatstays on a recumbent don't have to be an integral part of the frame. Instead you can bolt them onto the dropouts or tabs on the chainstays if there's enough metal there to provide strength. On many bents, the primary job of the seatstays is to support the seat and adjust seat angle, and they don't contribute much to frame rigidity. BUT, it really depends on your frame design.

I believe the question was regarding the joint between the boom tube and the chain stays. I had the same initial reaction to the question because he mentions the lack of support from the seat stays.

Yes I don't think the seat stays add much strength.

hopefully the seat stays reduce the load on the main tube by transferring it directly to the rear axle