I ride a Vision recumbent. It has an aluminum seat frame. Last week 2 of the welds failed and broke. My bike was built in 2001. Vision, before they went out of business, redesigned their bikes and thus the seat frame. I can not find a replacement, but have found a place to repair my current seat frame. You can read about it here; Seat frame broke.
Along with having the aluminum frame repaired I am thinking about having a new one built, using the design of the current frame, angles, same size tubing, etc., out of chromoly tubing. Here are my questions; How do I know what grade or quality of chromoly tubing I need to request that will hold up better then the aluminum? I found chromoly tubing suppliers online that show different grades and specs., but I do not know what would be the best for what I need. I will want to keep it the same tubing size as the current frame, or really close to it. But if I am going to drop anywhere from $0.20 top $0.50 an inch for it I want the best I can buy that will hold up better then the aluminum.
what size do you think you need? I just looked at 1/2" and 1" with .028 wall at McMaster-Carr, and it was under $30 for 6'.
If you have the replacement brazed, flat bar stock is no problem.
I can't really visualize how the seat frame is loaded, hard to answer about strength. The aluminum one broke because of some flaw in the welds possibly combined with a weak design. A steel replacement will certainly be stronger and have fewer problems with fatigue, at the cost of some weight.
I do not know the size. I will measure it and let you know. The aluminum frame is over 7 years old. It failed because of metal fatigue due to my weight, I'm a clydesdale. I knew over time that would take its toll on the aluminum seat frame. In fact when I first got the bike the front boom piece, where the bottom bracket shell is was aluminum. Because of my pedal stroke and style and the aluminum I kept shelling out the bottom bracket shell. My bike shop finally replaced it with a chromoly one and problem solved, haven't had a problem since. Thankfully it was a warranty issue each time it happened, a total of 3 times, with the aluminum boom piece. In fact it was Vision who finally asked how big am I, etc. When the guy at the bike shop told them I was a clyde, they said we'll switch it to a chromoly boom piece. They also adivsed I may have an issue with the seat frame as well.
Does it have to be or should it be chromoly? I found a local supplier who sells high strength tubing. Though it sounds like it may be a bit heavy. Here is a link, page 23; http://www.siouxcityfoundry.com/stoc...book-JAN09.pdf Or is chromoly the prefered material for something like this?
I'd go with one of the big vendors like aircraft spruce or McMaster-Carr. McMaster will probably have tubing on your doorstep the day after you order it. I didn't really understand your PDF. There is a standard for material identification that they may be using, I couldn't tell.
A picture of your bike would help. Aluminum does the big tube boom thing better than Chromo. Chromo does conventional diamond frame well. There are good chromo recumbents, Rans being a good example. With diamond frame either aluminum or steel make good frames, but the recumbent field is a little different where it would be tough to make a boom out of a similar sized piece of chromo simply adjusting the wall.
I assume this seat frame won't work for you since I found it right away, and you could have also:
My Vision is an R40, built in 2001. A few years after and before they went out of business Vision, or rather Advanced Transportation, the company that built Vision's changed the design of the bike, thus changing the design of the seat frame. I can buy a replacement seat frame for the newer Vision style, but not for mine.
I already contacted The Bicycle Man and asked if he had any of the older style Vision seat frames from when he bought some of the Vision product line at an auction because they were going out of business. He does not, only for the new style seat frame.
As you can see I no longer have any faith in the durability of aluminum. I am surprised it lasted this long. I forgot who it was, but someone was qouted as saying "Which would you rather sit your ass on? A metal that has been around for over 2,000 years and is tried, trued and tested or a metal that has been around for only a few hundred years?"
That's pretty similar to the Toxy I used to own, though mine had full suspension on it. I am a Clyde also, but I would imagine the suspension helped spare the frame. No reason why a properly sized aluminum frame wouldn't work for you, though where custom is concerned there seem more folks comfortable working in steel anyway.
I think you could do a seat in the same size of steel, with .035 walls, standard aircraft 4130, and it should be fine. In layman's terms .035 is where steel tubing wall thickness starts to be stout. It isn't the lightest, it isn't heavy, but it is for tough structural uses. Go thicker and it starts to get heavy, and be the sort of part one might machine for a bearing etc... Go thinner and it starts to seem like a fragile wall structure where stiffness is the main design requirement, not strength. One can go to thinner walls relative to larger tubes and get a stiff part, but it needs to be protected from damage, and brutal loads. Also you have brackets that mount against the seat frame wall, another thing that calls for some strength.
So in aircraft terms, your .028 tends to get the jobs like holding the cloth into the airfoil shape; the .035 tends to get jobs like spar or longeron; the heavier tubing tends to be used to take loads off, or for struts, or stuff that has to be fat so it can slip fit to the next size of tube down or up. Obviously wall thickness varies with tubing size to retain the kinds of categories I have outlined above. But these are roughly appropriate rules for bike sized tubes or 1-2 man aircraft tubes.
The tubing in your seat being .875" means it is on the small size, in which case the .035 is proportionately stouter than it would be even in a standard 1" tube or larger. Should be fine, just what you want.
Also the frame parts seem to have a lot of bends. .035 is about the thinnest I would go without worrying about needing a mandrel bender to stop the tube from wrinkling or collapsing.
A new development in the seat frame situation. I am going wiht a replacement aluminum frame after all, along with some pretty major modifications to my bike to accomidate it. It turns out the reason the original frame may have failed is because it was likely not heat treated. The newer ones are. Link; http://johnsbicyclinghobby.blogspot....ame-broke.html
One thing I meant to say was that one probably shouldn't take overweight claims too seriously. Being heavy does not break frames normally. There can be a variety of problems like shimmy that might be traced to weight. A person can break a frame easily enough with dynamic loads and the reason for that is normally obvious. If you have gone through the tubing for a regular tandem the actual changes are not all that great, yet they not only carry extra people, but they support a wider span.