Frame wall thickness question...

I’m doing my senior project in manufacturing class, a unicycle frame. About how thick do I need my walls to be for 6061 aluminum? Also, what OD would be best? Serious answers only please. This will be my first round crown frame, and my first aluminum frame.

I think it is very subjective to the frame design, but around 0.0625" is a good start. You can reduce wall thickness with increases in diameter, but eventually buckling and dent resistance become serious issues.

Without having at least a sketch design in hand, recommending specs would be misleading.

Giraffe frame? Trials frame? Little kids’ frame?

Trials Frame. Between 1-1.5in. in diameter. Also, where might I find pipe with a 27.2 ID and 31.8mm OD in 6061 Alu.? I know I can build it, I just have to find the right material, the only suitable Alu I have on hand is 2x2 square tubing, not very good unicycle material.

2mm would feel about right to start with. if you want a pipe of such specific dimensions you’re gonig to have to most likely turn it down from solid and rifle drill it.

Contact a metal supply house in or near your area. If you don’t know of any, contact a welding shop in your area and see what they say. As for frame building, you may want to find a bicycle framebuilder that lives close because they may have a set of Chadwick and Trefethen seat tube reamers, but you could always use a ball hone too.

I am personally not a big fan of aluminum for frames, because aluminum suffers from cyclic fatigue, so the question is never IF it will break, it’s WHEN it will break. You see this a lot on bicycles.

Good luck,

Matt.

Contact a metal supply house in or near your area. If you don’t know of any, contact a welding shop in your area and see what they say. As for frame building, you may want to find a bicycle framebuilder that lives close because they may have a set of Chadwick and Trefethen seat tube reamers, but you could always use a ball hone too.

I am personally not a big fan of aluminum for frames, because aluminum suffers from cyclic fatigue, so the question is never IF it will break, it’s WHEN it will break. You see this a lot on bicycles.

And technically, you are looking for tube, not pipe. The highest grade tube you can find, which would be DOM (Drawn Over Mandrel) or extruded. You can always buy a bicycle tubeset that will have the right size tubes, but it may cost more than you’re expecting. Nova cycle supply ( www.novacycles.com ) has a decent selection of 6061, 7005, and Scandium tubes for sale, and you can buy them individually. Columbus, Easton, and Dedacciai all makes aluminum tubes. What you’re looking for is a seat tube, because it will have the proper interior and exterior dimensions for a seatpost. If you do a flat crown instead of a rounded crown design, you can build the whole thing out of straight-guage tubing without having to bend it (which is tricky, even if you do have a hydraulic bender).

Good luck,

Matt.

I also throw my lot in against aluminum. Try steel first, at least.

I’m much more of a machinist than welder, and I am willing to promise you that by far the mot expensive and difficult way of obtaining tubing to your specs is by machining from solid. In the UK they may call deep hole drilling “rifle drilling”, but machinists out here tend to call it “gun drilling”, if you want more information, via google. It requires “strange” drill bits (as compared to regular twist drills), is pretty slow, and generally also needs high-pressure thru-bit coolant. There are special machines made just for the job. Most standard lathes require modification to do good gun drilling. I have made a couple of gun drills (D-bits, they’re sometimes called) for one-off jobs, but they are enough work to be a senior manufacturing project by themselves. And they aren’t cheap bought new.

If you really do get desperate, just buy tube of the proper ID, with walls that are too thick. Then turn to size. If you can’t get that, then tubes of proper OD, then bore out the ID. Warning, a 10" depth, 7/8"ID boring cut is not an easy job, even in aluminum.

Look at aircraft supply places and automotive supply houses. If you do steel, you can get round tube and then ovalize it yourself using a press. That’s how the KH pro was done back in the day, it appears.

You don’t need a bike shop for the reamers. Just weld the seattube to the crown piece, then chuck it up in a lathe before you weld on the forklegs. Indicate it in along its length, then use a boring bar to take a skim cut to size. For my own frames, I’ve always used aluminum that has an undersized ID, then just bored it out to size.

Good luck, but you really need to do more research before you start ordering materials.