Recumbent - PVC for seat construction?

I'm about to launch construction on my version 2.0 homebuilt, and I'm mulling over various seat designs. My goal is to save weight, and get this beast down to the 30-pound mark. I was just wondering about using PVC tubing for the seat-back. Seems like a suitable construction would be both light and adequately stiff, if properly braced.
Anyone familiar with such construction?

I wouldn't use PVC for any part of a vehicles construction. PVC is rather brittle, even the schedule 80 varieties (which will drive weight up). Shatter a piece of PVC and ask yourself if you want those shards near your vital organs when you have an accident! Ouch.

What kind of design are you going to build? My front wheel drive bike is easily sub-30 lbs, with heavy components.

Hi,

I have not worked much with PVC, but my impression is stay away from it because it's not very strong and this means heavy as well because significantly more material would be required. Of course larger diameters would be stiffer. Also it gets brittle in cold weather.

Depending on the time and effort and money you want to put in, I would recommend molding your own seat (assuming you don't just buy one like in the M5 style and somehow mount it, the first thing I'd try).

Considering how critical the seat is to safety, I think your options are either metal, or fiber reinforced composite /epoxy construction most likely with a foam core. You might be able to get away with fibreglass, but carbon although more expensive will be stiffer and lighter. If you have not done that before, I'd first recommend checking out this company for materials www.fibreglast.com. Also they can give you advice on how to approach the process.

It would be a project, but you could shape some foam core either one part or in separate pieces, and then cover those in laminated layers of carbon/epoxy. Rohacell foam may make a good core, or perhaps honeycomb. My grasshopper seat is constructed this way. There are mounting holes drilled straight through and these are protected by rather large washers to distribute the compressive force of the tightened screw. Its maybe slightly over an inch thick and has virtually no flex.
In some cases it may make sense to build a small metal reinforcement into the seat itself (not sure if thats how the grasshopper seat is constructed);


If the seat can flex there is a chance that the foam core could shear if its not designed properly depending on what it's made out of (shear strength is listed measurement for core materials). A wood core would not do that, but is too heavy. To further guard against shearing, it may make sense to build up the foam core from layers of foam and carbon/epoxy alternating, this would get more epoxy throughout the core. If you are just building the seat back, I imagine this sort of method could work well.

If you want to work with tubes, you could try purchasing or making your own carbon fiber tubes. They sell braided carbon tubes like a 'chinese finger torture' which you can fit over a foam core and layer, then use a solvent to remove the foam later, or just leave it in for strength. Of course these should all be strength tested somehow for safety and mounting points should be reinforced.

These composite things would typically be finished off with a final coat of UV resistant epoxy.

good luck!

It should be easy enough to sculpt the ideal seat shape out of foam, then use it as a form to create a fiberglass or carbon fiber shell.

You can get the foam from a building supply store. Use a grinder or sander to shape the form (either a positive or negative) of the shape you want. When the foam seat is to your liking, you can add layers of fiberglass or carbon fiber to the form. Use the vacuum bagging method to get it thin, strong and uniform.

Sure it'll cost more, but it'll perform a hundred times better, it will look cooler, and it will be lighter. And you'll get to brag that you made your own fiberglass or carbon fiber seat....

Besides that PVC does not like UV exposure over time. Great for light applications (I have a small light mount I made hanging off the bottom bracket) but not for applications that need a lot of strength through low and high temperatures.

What the others have said! By all means use it for the prototype, but don't use it on the bike.

When used for its intended purpose of conveying liquid, PVC tubing is known as "poly-vinyl chloride."
When abused for unintended structural purposes - inducing torque and bending stresses on a material not meant to handle it, PVC tubing is known as "Positively Violent Catastrophe."

Looks like the consensus is a nix for this idea. The design of this beast is essentially one of those "no weld" recumbents, similar to this one (version 1.0)

http://im1.shutterfly.com/procserv/47b5dd07b3127cce908f9df2446e00000016108AcNGzhu3bNn



The seat base will remain the same; what I'll be building new is just the back. I intend to toss all that hefty steel reinforcement and instead run aluminum tubing down to the dropouts. I will also be tossing the "shock absorber" (in reality, a cheapie spring holder) and making the frame rigid.

Unfortunately, I don't weld... I'll be constructing some sort of rectangular frame that I can put a fabric seat-back on.