Custom Bicycle

[rwm]

I am planning a project to build a custom cruiser bicycle to ride to the craft breweries here in the CLT. (OK maybe other places too.) I am just in the planning stages at this point so no pics yet.
I need to decide if I want to start with a donor bike that has a ChromeMoly frame or Aluminum. I can weld either although I am more proficient with steel. My question is bending tube. What kind or aluminum tube is best suited to bending and what do they use on bikes? I saw a frame that was 7075 but I always thought that was low strength and poor form-ability? What say ye?
OK here's a pic:

Robert

 

[richl]

Harbor freight makes a low cost tubing roller that is in the 180.00 area. Get a 20% coupon and it's very affordable. You will be able to roll tubing that looks quite nice. Also take a look at a site atomic zombie for ideas on making fat car tire rims with bicycle spokes. Ratrod bikes is another site to get ideas.
Tubing roller
http://m.harborfreight.com/tubing-roller-99736.html?utm_referrer=direct/not provided

Swag off-road makes a few acc for the roller, dies, hydraulic pump, electric motor.

You can get pretty far without spending on all the equipment.

Hth
Rich

 

[Rustrp]

7075 has tensile strengths in the mild steel range and is one of the strongest as I remember, without pulling the chart.

Most quality tube bending taking place is done with a bender using a mandrel inside that follows the bending process. Without the mandrel you always get mixed results.

 

[mikey]

My question is bending tube. What kind or aluminum tube is best suited to bending and what do they use on bikes? I saw a frame that was 7075 but I always thought that was low strength and poor form-ability? What say ye?

7075 is actually a high-strength alloy and therefore difficult to bend (or weld). I believe frames made from this stuff are actually epoxied into brackets. I don't know much about bike frames but I thought the latest and greatest was titanium.

 

[ddickey]

Carbon is the latest and greatest for many years now. Titanium is probably the ultimate as it never fatigues but is heavier. That's the problem, people want the lightest no matter what. I've seen people pay big bucks to save 50-100 grams. My Ti frame was very nice but my favorite was an aluminum Klein. makes me sad to think it is gone as I sold it years ago.

 

[Rustrp]

7075 is actually a high-strength alloy and therefore difficult to bend (or weld). I believe frames made from this stuff are actually epoxied into brackets. I don't know much about bike frames but I thought the latest and greatest was titanium.

7075 or any of the heat treatable aluminum alloys are tough in their tempered stated but prior to heat treating they are probably one of the most forgivable and easiest to work and weld. All the cutting and fitting processes are easy on tooling and I'm not sure why so many folks think aluminum is difficult to weld. There's not much difference in the melting temps of the different alloys and using the correct filler metal is of most importance along with all the other thing a person should do to produce a good weld.

I'm getting ready to weld up some frames made of 6061 extrusion and most of the welds on the inside I will MIG weld. Apperance isn't critical and it's easier to get the gun nozzle into some of the corners, and a lot faster than me fumbling around inside the frame TIG welding these up. I took about 15 seconds to run the bead and another 2 minutes to load it.

With that said; Finding 7075 in an annealed condition to make one bike frame may be difficult.

 

[mikey]

I have no problem mig welding 6061 but 7075 ... no, not so much. I think I would be reluctant to ride a bike if I wasn't confident in the weld integrity.

 

[Rustrp]

I have no problem mig welding 6061 but 7075 ... no, not so much. I think I would be reluctant to ride a bike if I wasn't confident in the weld integrity.

Weld integrity is always a question when the base metal is hardened/tempered, and there is no way to make sure unless the part undergoes heat treatment after welding. It's always a guess, so 6061 or 7075 may both end up with the mechanical properties of 5052 after welding, or close to it. 7075 has traits and characteristics that require heat treatment after welding and welding should only be done in a 0-temper state. Given that 0-temper isn't going to be available without special order then pre and post heat treatment is required. When life and limb is at stake, don't weld it. If one chooses 6061 based on strength then welds without pre and post heat treatment will result in reduced strength, although 6061 isn't as susceptable to cracking as 7075. Heating a part up and holding it at 400° is difficult to do when most will be attempting to do this with a torch, and there's no accurate way to measure the temp.

To prevent welding on 7075 the industry limits the filler metals available to weld it. I never weld anything where the base metal is in question. It's easier to say no than have calls from attorneys at a later date.

 

[rwm]

I think I hear you saying "Chrome moly!"
Robert

 

[mikey]

Yup, Chrome Moly is easily gas welded; good enough for airframe use, it is probably fine for bikes. Rustrp makes a good point. Welded aluminum will anneal with welding and needs to be heat treated if the hardness is important. On a bike frame, I would think it would be important but I don't have any experience with frames like that so I'm guessing. There is a member who does bike frames but I can't recall his handle.