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Aluminum Frames
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Posted by: techman---------------------
I was just looking into issues of aluminum frame design, those being strength/ductility/flex, metal alloy, heat treating and welding - balanced against fatigue life. This prompted me to recollect old internet Q&A's where people said to just "go get your X welded and machined down to Y and you'll be ok" which struck a bad chord with my recent more in depth research (re-tempering small items like frames after welding them seems to be done at least for bicycle frames), and also a biggie Q occurred to me: what about all the modern machines with aluminum frames, does anybody have any clue or insight into how they deal with the issue of finite fatigue life of the metal itself? Or does anybody have any inside scoop on the metallurgies and treatments used in frames and swingarms?
thanks
Posted by: sloppy---------------------
You can pick up a Machine Design Book from your local Engineering College Bookstore that will allow you to calculate the life cycle of any component based upon the type and amount of stress that you put on it. Aluminum has no ability to resist cyclic stresses so it will eventually fail. So your options are; a) you over design the part so it won't fail until you are far past the life of the component or b) you replace parts on a frequent basis so they don't fail during operation.
Automotive and Motorcycle manufacturers use option a).
Airplane manufacturers and Top Fuel Drag Racers use option b). Think about the cyclic loading on an airplane wing next time you go flying. Note: airlines use non-destructive testing (x-ray and sonic) to locate cracks on a regular basis. Top Fuel engines use aluminum con-rods which have to be replaced on a regular basis.
Posted by: cujet---------------------
While it is true that aluminum has a finite fatigue life. For all our purposes it will last forever.
The example of an aircraft wing is a good one. As a very general rule, they are not made "too strong". As a matter of fact, they are expected to flex. They will flex quite a bit (millions of times) over the expected 60,000 hour lifespan.
I am not sure, but I fully expect that motorcycle frames and parts are not stress relieved, heat treated or otherwise treated after welding.
Chris
Posted by: techman---------------------
Thanks for the insight Chris. I was waiting for your opinion. Do you think it might be fair to say, based on wall thicknesses and tubing dimensions of aluminum cradle frames vs steel cradle frames that the aluminum motorcycle frames are simply plenty over-strengthed? (going along with the non-post-treatment hypothesis)
Posted by: MikeS---------------------
Well I am not metal guy, just a good mechanic . From all the reads of frames over the years. Many of the factorys design there frames for a given amount of flex. Reasons being to prevent "premature" cracking and to deal with the short commings of their suspension design or to enhance the handling of the bike.
Now whether they heat treat or stress relieve the frame for proper metalurgy as one would define or not is a big question. Honda for one has the CRF250 R and X models from the naked eye the frames look extremely similar but Honda admits the X frame has more flex as the intention of the bike is for offroad and it will react better to those type coditions where the "R" is more rigid for jumping etc.... One thing I have noticed over the years is that under hard use the frames get weak and spongey on most MX bikes. Some times they crack after the second or 3rd year of continued racing, some sooner. Last year we did such a test. My son had a used 03 KTM250SX, raced by a VETA MXer for a good year, then we trashed it the next year after bit of suspension set up. At the end of last year I has the suspension refreshen for the last 2 events. He liked the bike. I tore it down over the winter and refreshed the motor and swapped the frame to one that had maybe 20 recreational hours on it. Suspension, swing arm, tires even stayed the same. Junior rode it late winter and said if felt tight and he felt the small choppy stuff alot more. A few clicks here and there it was gone. After racing this year so far on the newer frame the bike seemed much better and firmer. It may be our perception ??????
Just some tid bits that really didn't answer much but may open a blind eye or close them ZZZZZ!!!!
Posted by: Ol'89r---------------------
Techman.
From the construction of the modern frames we can assume they are made from a 6000 series aluminum. Since they are mostly a combination of tubing, extrusions and castings. Structural tubing and extrusions are normally manufactured from 6061 and 6063 series aluminum. Casting alloys used are most likely a A357 or similar alloy. All of these alloys are heat treatable and compatible with one another as far as welding goes.
I agree with Chris that the frames are overbuilt for strength. You may remember how everyone complained when the new twinspar frames came out that they were too stiff. Over the years they have added flex to the frames but the physical size of the tubing is the same. Most likely they do this with temper. Since aluminum gets harder with flex it would be better to start with a softer temper rather than a T6 or T65 temper which is harder. By varying the temper you can add more or less flex to the frame.
Minor welding, such as repairing small cracks or replacing mounting tabs will not affect the temper throughout the whole frame, only in the localized area of the weld. Peening the weld can re-harden the temper in the localized area but, if any extensive welding or fabrication is done, imo the whole frame should be re heat-treated. Extensive welding or excessive heat will soften the temper.
A lot depends on what you are welding, where on the frame it is located and what are the strength and flex requirements. We do a considerable amount of aluminum fabrication in my shop and we have always treated frames as though they were a 6000 series with good results. I have tried to find out what alloy the frames are made from. I have asked Honda factory mechanics about this and they could not tell me. Only that they use a 5356 rod as a filler rod. There again, a heat-treatable alloy used with 6000 series alumunum.
Hope this helps.
Just my $ .02
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