Mini Chopper Frame Material

[WeldingWoman]

Hello! Let me start by saying I am SO thankful I found this forum! I have been searching through all the threads on here for the past few weeks and have come up with some amazing information. There are so many knowledgeable people on here.

So, let me pick some of your brains =)

I am currently enrolled in a metal fabrication class as part of my welding engineering degree. One of our assignments is to design and fabricate a custom welding project. I got the brilliant idea to build a mini chopper.

I have gotten the okay from the school to start my project, so I am now in the market for pipe/tubing to construct my frame. I got a materials list from the local metal supplier, and I am not sure what the best option would be.

My options are: standard black pipe
round welded steel tubing
mechanical steel tubing

What is the difference between pipe and tubing? Which one would be the best choice for a mini chopper? Also, what is a good size?

Thank you in advance! So excited to start building my first mini chopper =)

 

[rocketkart]

i use just standard black pipe for my gokart frames. The purpose with a pipe is the transport of a fluid like water, oil or similar, and the most import property is the capacity or the inside diameter.

 

[mckutzy]

Hello and welcome.

I would say, use/buy the best what you can afford. If you can get a deal on some DOM CroMo tubing, then get it. An good cold finished steel is good.

What is your budget and what other parts do you have for this(Motor, brakes, wheels ect.)

How is your welding skills? Tube/pipe is a bit different than square or flat pieces, so keep that in mind when crafting aswell as the joints.

A mini bike is probably the best as it is not overly complicated(that is if you follow the conventional variants) project and has a simple layout.

 

[Scout]

Tube and pipe are different, and bend differently. Tube is measured by the OD, pipe is measured by the ID. Tube is generally for structural applications, and pipe is for fluids and gasses. I will say that I have seen the occasional jeep roll cage built out of pipe, and one built out of rectangular tube.

Having said all this, I did see a minibike made with a chain frame, he had welded each link together in the shape of his frame. It did seem pretty solid.

 

[Scout]

For a mini chopper I'd use 1" or 1.25" 0.90 wall hrew. I'd think that would be strong enough for a bike frame.

 

[mckutzy]

My ride is a square tube frame(check my sig). For its size it is also pretty light. overall weight is about 100- 120# at most.
I think my rear tire itself weighs more than my bare frame.

 

[machinist@large]

My options are: standard black pipe
round welded steel tubing
mechanical steel tubing

What is the difference between pipe and tubing? Which one would be the best choice for a mini chopper? Also, what is a good size?

Thank you in advance! So excited to start building my first mini chopper =)

OK, quick definitions; some of the others have posted good answers, I'm just trying to tidy things up a bit.

1] Standard black pipe. If you can source good US or Canadian made matl. you can do a lot with it. If it's from China, send it back. The N. American made stuff, the welds are as strong or stronger than the rest of the pipe. I've seen the import crap fail just trying to pressure test an air circuit at less than 75 PSI. Genuine schedule 40 pipe should be good for over 150 PSI using water.

2] Round welded steel tube. This is formed and welded the same way as black pipe, but that is where the similarity ends. As Scout stated, pipe is loosely sized by it's ID, the wall thickness by the schedule rating.

Tubing is rated by it's OD and wall thickness. It's also made from cold rolled or pickled & oiled material instead of mill finish matl. As long as it's sized correctly, it's a very cost effective alternative to our next candidate.

3] DOM (Drawn Over Mandrel) tubing is the strongest on this list by weight, but it's also the most expensive because of the way it's made. This is basically extruded steel. The billet is forced under extremely high pressure thru a combination die/ mandrel that forms the ID and OD at the same time. Think a frosting bag for decorating a cake but using thousands of tons of force.

It also comes in various flavors, as Mckuzty stated. Chrome Moly is stronger than regular steel, but it's more of a hassle to work with. The weight savings on a small project like you're proposing probably isn't big enough to worry about.

Hope this helped. Pat

 

[OzFab]

Tube and pipe are different, and bend differently. Tube is measured by the OD, pipe is measured by the ID. Tube is generally for structural applications, and pipe is for fluids and gasses.

The other significant difference is pipe generally has thicker walls. Yes, you can get thick wall tube but, it's usually close to thin wall pipe...

Welcome to the forum, please enjoy your stay

 

[chancer]

Thanks for the info I was looking to ask these questions when I found this thread. What about the electrical conduit they call "Rigid" would that work for a go kart frame? Of course you would have to grind of the galvanizing before welding but has anyone done this?

 

[mckutzy]

One could use that material(EMT it is called) for a mini bike caliber machine. It only has so much strength as it isnt made to be used in a structural situation. It would fair reasonable well for use in a bicycle type of machine, I even used some scraps I found for an exhaust I made.
In this case if one had EMT or black water pipe Id use pipe.

 

[mckutzy]

3] DOM (Drawn Over Mandrel) tubing is the strongest on this list by weight, but it's also the most expensive because of the way it's made. This is basically extruded steel. The billet is forced under extremely high pressure thru a combination die/ mandrel that forms the ID and OD at the same time. Think a frosting bag for decorating a cake but using thousands of tons of force.

Im sorry I hate to go over ones definitions, just a minor clarification...
What your above description I think you probably meant was seamless extruded tubing.

Drawn-over-mandrel tubing is made from cold-drawn electrical-resistance-welded tube that is drawn through a die and over a mandrel to create such characteristics as dependable weld integrity, dimensional accuracy, and an excellent surface finish.

 

[danssoslow]

EMT is a bit different than Rigid (RMC). Rigid has a much heavier wall, almost like black iron pipe (although its matalurgy may be different). RMC is thick enough to thread like iron pipe.

I can take a piece of 3/4" EMT and bend it over my knee; I couldn't do that with 1/2" RMC.

 

[mckutzy]

Interesting. never knew.

 

[danssoslow]

Yeah, it's kinda weird in that between EMT and rigid, for any nominal size, the outside diameter grows, unlike PVC where different schedules diminish in ID and all schedules fit the same nominal fitings. In fact, if you look at a 3/4" (larger sizes say similar) EMT hand bender, it will also be labled to bend 1/2" RMC or HW for heavy wall.

Now the jump in thickness between them can only be as large as the threads for any given nominal size, a 1/2" lockring will fit any 1/2" connection. I figure this is why I've never seen metal electrical conduit refered to in schedules.

 

[landuse]

I would use 1" square tubing for your frame. 2mm wall thickness. That is what I always use and it is great....and cheap

 

[WeldingWoman]

Thank you for all the great responses!

mckutzy, I am trying to keep the budget down on this as much as possible; I don't want to spend $500 on a school project! But, on the other hand, I want to build something that is dependable and won't having me biting my nails everytime someone takes it for a ride. I would say my welding skills aren't too shabby. I have worked as a welder for a company that produces pressure vessels and heat exchangers for the past five years. I have not bought any of the mechanical parts yet, as I just got the project approved.

Okay...another question! To avoid purchasing a tubing bender (my school doesn't have one so I would have to buy one on my own) I have been thinking about miter cutting and welding the joints that would be typically bent on a frame...has anyone ever done this and, if so, did you have any problems?

Thank you all again for your input! I really appreciate it!

 

[machinist@large]

Im sorry I hate to go over ones definitions, just a minor clarification...
What your above description I think you probably meant was seamless extruded tubing.

Drawn-over-mandrel tubing is made from cold-drawn electrical-resistance-welded tube that is drawn through a die and over a mandrel to create such characteristics as dependable weld integrity, dimensional accuracy, and an excellent surface finish.

Interesting; I'm not saying you're wrong, it's just that all the suppliers that I or any of my employers have ever used have always stated that DOM is seamless. Also, all of the welded round tube I've ever had to use had a weld bead on the inside, the same as welded square and rectangular tube.

What may be clouding the issue is that welded round tube is run thru a final resizing die(s) after the outside weld bead has been milled off (all welded tubing has the outside bead milled flush for both appearance and to make it easier to work with.).

I would use 1" square tubing for your frame. 2mm wall thickness. That is what I always use and it is great....and cheap

Paul, you're probably throwing a little more wall thickness at your work than you actually need for overall strength; on the flip side, you're working with a stick welder, which means that the thicker material makes it easier to weld, and helps guarantee good joints.

Thank you for all the great responses!

mckutzy, I am trying to keep the budget down on this as much as possible; I don't want to spend $500 on a school project! But, on the other hand, I want to build something that is dependable and won't having me biting my nails everytime someone takes it for a ride. I would say my welding skills aren't too shabby. I have worked as a welder for a company that produces pressure vessels and heat exchangers for the past five years. I have not bought any of the mechanical parts yet, as I just got the project approved.

Okay...another question! To avoid purchasing a tubing bender (my school doesn't have one so I would have to buy one on my own) I have been thinking about miter cutting and welding the joints that would be typically bent on a frame...has anyone ever done this and, if so, did you have any problems?

Thank you all again for your input! I really appreciate it!

Cutting and mitering works well; as long as the wall thickness of your material is "in the range" and you add gussets where necessary you shouldn't have any problems.

Now, how to fish mouth/ miter round tubing; I'm going to have to admit defeat here and ask one of the others to link the sticky that's somewhere on the site for the tube mitering calculator, the one where you input the dia. of your tube and the angle, and it hands you a printer ready template that you just print, cut out, wrap around your tube (w/ tape) and grind the profile in with an angle grinder.

The second method I can provide a link for; in it a gentleman from OZ shows a neat down & dirty way to lay out the miter so you can then fish mouth the tube. This is one link I have subscribed to because he is one awesome DYI're. I think you'll like a lot of his postings.

http://youtu.be/iqqLhgVr070

Hope this helps. Pat

 

[chancer]

@ Welding woman
After reading this and shopping around yesterday. I decided to use gas pipe for the frame on my new Kart. This will be my first project too. I found a "pipe" bender at Harbor Freight for $99. Then looked on my local Craigs list and picked up the identical bender for $40 bucks. Tested it out just now and it works great easy smooth 90 deg bends on 3/4 in Gas line. (O.D. 1.05) Cant wait to learn exactly how to measure and bend and keep things plumb, square level and straight. err wait its just a kart right.

 

[chancer]

I am also seeing a lot of posts about trouble "fish mouthing" round pipe. Here is my trick from years ago and it worked great. We used to just take a hole saw and cut the end off the round in a drill press. It has been a while and I am not sure if you would use a 1" hole saw on 1" pipe or if you go bigger or smaller with the hole saw but I do know it works we built a few stock car roll cages for the local short track in the '90s this way. I will try it out tonight and let y'all know.

 

[firemanjim]

Pipe is for mechanical transfer of a product. Tubing is for structural strength. DONT build a frame out of black pipe like from home depot (hardware store). It will BREAK when subjected to severe shock or impact. Tubing is designed for the stresses ..... it wil bend,but stay intact, where the black pipe will break and cause damage,death.....
For a chopper frame, use 1&1/4" x .100-.125" steel tube (welded seam is just fine!) for the backbone. For the down leg parts (engine cradle) , 1"x.09" tube...... make your design. Put it on paper how the bends will be. The radius and everything. Then go to a muffler shop and get them to bend the pieces for ya..... If you can, draw out a full scale print so you can actually check shape of parts while you're at the shop.....