Topic: Aluminum frame.

[hooverhonda]

I am thinking of building a copy of my exsisting cb 750 (k3) frame out if sch 80 6061 aluminum. Other than having the neck milled out if bar stock does anyone have any insights on pitfalls or opinions? I appreciate constructive criticism as well as helpful advice. I am a welder by trade and I specialize in aluminum so I understand the characteristics involved in welding this material. This would be the first time in building a custom frame however. Thanks in advance for your interest.

Sean

[crazypj]

If your going to build a frame build something better than the original.
I would look at perimeter frames, at least you could do top end service without pulling motor plus there is better support for steering head

[bjbuchanan]

Since you have got the skills, materials and place set up a jig so it atleast fits the stock engine to mount in the right places. Then copy some modern frame designs. I'm not sure what look you are going for, or just having fun for the hell of it.

Either way I'm sure this thread catches 754's eye, he is one of the ones to talk to

[hooverhonda]

I thought about building a more modern looking frame but as I'm rebuilding a 73 cb750 cafe style I thought it would be cool to build the frame out if aluminum. Shave some weight. I have already built the seat pan, oil tank and battery box out if aluminum and I'm working on an airbox design. I may even tackle the swing arm if I feel ambitious.

Sean

[iron_worker]

I'm sure you know this already but when you weld aluminum you reduce it's tensile strength to that of the base alloy. You can buy aluminum that is quite strong but as soon as you weld it you've essentially heat treated that joint and it will never have the same strength as the rest of the material again.

If you are aware of this you can design around it and it can be no problem. If your design doesn't take this into consideration then you may have problems ... 

That being said you can probably over build it and still end up with somethign lighter than the stock frame and sufficiently strong.

IW

[hooverhonda]

Very true IW. I intend to gusset in areas the original frame is not. I don't want to ride the bike and crash. I have way too much to live for.

Sean

[70CB750]

Subscribed and waiting for pictures 

BTW do you have pictures of the oil tank?

[hooverhonda]

When I can figure out how to make my pics smaller ill post. They are all on my phone.

Sean

[Duke McDukiedook]

Since this is custom you would want to design this and test it in a finite analysis structural program since the structural characteristics of aluminum are very different from steel or chromoly. It probably wouldn't hurt to be a mechanical or structural engineer either to understand what exactly is going on structurally.

[70CB750]

Finite element method - that's something you don't see here every day. 

 But you are right, Dukie, better to check first than to look for cracks later.

[Swoop]

Sean
If you are interested....I have a Seeley frame that you can come over and take measurements from.
Steve

[754]

I hope you will heat treat it..may want to bore for neck bearings, after heat treat. I would try a nice swingarm before doing a frame.

[hooverhonda]

Where would I find a finite analysis structural program. I don't know any structural engineers. I'm not sure about heat treating the neck.  If whoever doesn't know what they are doing it may become brittle. I'm thinking of getting a brass sleeve made for the alum neck.  Thanks for the advice everyone.

Sean

[iron_worker]

Programs like that cost $1000's (or even 10's of 1000's) and require a lot of knowledge and experience to operate and interpret the results effectively.

IW

[singedebile]

Where would I find a finite analysis structural program. I don't know any structural engineers. I'm not sure about heat treating the neck.  If whoever doesn't know what they are doing it may become brittle. I'm thinking of getting a brass sleeve made for the alum neck.  Thanks for the advice everyone.

Sean

Solidworks is what you want,

[Stilltime]

It would be much more cost effective to have someone that does freelance work with design and analysis help out...  You would have to be able to provide loads and directional stresses however.

You can buy a Solidworks license with analysis and a workstation to run it for around $10k, on top of learning the software and becoming fluent, or find someone to partner with.

It definitely sounds like an awesome project though!  Looking forward to seeing progress and your approach.  The sky is the limit if designing from scratch, make certain aspects serviceable and continue to think how it [the rest of the bike] will all be assembled once complete.

[kghost]

Given that your contemplating 6063.......

I'd think you'd want some sort of heat treatment.

6063-0 has a yield of about 19,000 psi.

6063-T6 about 30,000.

Some sort of heat treatment after welding would be advised. You'll lose about 30% strength. Basically welding returns whatever the alloy is back to the 6063-0 condition.

[hooverhonda]

]i would be using 6061  alum  its a harder alloy  but yes i will definitely consider heat treating.  if they come through here is the bike oil tank and battery box.  the shop it is in is really dirty  i have better pics on a different drive.  ill attach as soon as i have them

[70CB750]

Looking good, thanks for the picture.

Unrelated question: what do you think about welding top steering bracket? I have one broken in the left eye but they all brake pretty much the same.

[phil71]

Welding the top clamp at the break is high up on the list of bad ideas. Find another.

[70CB750]

Of course, but i never asked Al qualified welder.

[phil71]

ironically, I did when I cracked my first K1 top clamp. He simply said "Sure, you CAN, but you shouldn't." That was good enough for me.

[kghost]

I have one that's welded. Been hanging in there the last 10 yrs.....

[Steve_K]

In GP,  when they started to make aluminum frames they copied the steel frames of the time.  The engineers found out that the differences in material made the frame weaker.  If you want to because you can, go for it.

Using the same size tubing will not give the same strength as steel.  Aluminum needs larger diameters and then you can use thinner wall tubing.  I am talking about frame rigidity in a racing sense.   
The aluminum frame would look very cool and one of a kind!
Steve

[Xnavylfr]

I bet EVERY person that has replied to this post have flown in an AIRCRAFT!!!  and NEVER had a thought of how many ALUMINUM parts that made up that aircraft. I know for sure you didn't think of the aircraft parts that had been welded back together.
Definitely HEAT TREAT, but you will have to HT the whole frame at one cook and cool down or you will have worse trouble than to worry about the welds.

 My AIRCRAFT EXPERIENCE started in 1969 so I have a little experience working with aluminum and structural strength.

Xnavylfr(CHUCK)