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Welding thinwall cro-mo (structual tip?)

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  • Welding thinwall cro-mo (structual tip?)

    Iv brought up welding this tubing before --- im going to be using my friends tig this winter to build my bike frame, here's what im wondering, has anybody ever tried tigging in structual "gussets" but not like your thinking, im just talking a mild raised straight line that you just hand build with adding from the weld rod, im going to have very good control as my friends got a good machine, so whats stopping from building a little raised area that tapers off, or a bunch of them, you could kinda have a little "spider web" around every joint, just curious as I dont know much about it...

  • #2
    AK...just remember...when you build anything up with weld, it creates tension. The more you build up the more the tension increases. I've used your idea on mild steel but you have to pretty careful when doing stuff like that with moly. Good luck!
    I have tools I don't even know I own...


    • #3
      C.M. properly sized,fitted and welded shouldn't need any reinforcement.


      • #4

        Is this going to be an artistic statement? If I understand the thread correctly, I don't believe the joint is going to be any stronger than without the spider web. The CrMo tube that is tig welded, is the basis for most pro drag racing frames and road racing motorcycle frames, they use gussets where needed, but the other welds are just a simple butt joints. the last road frame I bought that was CrMo and the only gussets were at the swing arm pivot and at the fork headstock.



        • #5
          4130 Chrome moly tubing is designed to be gas welded. It was developed originally for aircraft use and has the maximum carbon content that is considered to be safely weldable via gas techniques. Gas welding normalizes the surrounding metal and produces a more gradual heat affected zone (HAZ) than electric welding methods. This reduces the chance of post weld brittleness and cracking.

          Adding such "risers" with electric welding will only weaken the structure by providing more potential cracking zones.
          Free software for calculating bolt circles and similar: Click Here


          • #6
            Spiderwebbing of joints with weld bead.....

            Several others have already alluded to the reasons why we haven't seen this done yet.
            1) The heat affected zone around a weld bead.
            2) Stress raisers caused by the discontinuity of a weld bead.

            Yes, maybe it would look cool, but I can't really see any purpose in it. Then again I'm not an artist...I couldn't see wasting my time doing it.

            Who knows, you probably would sell hundreds of them. Even cycling is full of people with too much time & money & too little sense. Look at the market for sunglasses.....How many designs are possible ? Convince everyone that your bike frame is a lifestyle statement and get rich....It's not at all about making a well designed, useful product, it's marketing.



            • #7
              Yup. CrMO air hardens. It need to cool slowly if full joint strength is to be obtained. Older aircraft repair texts refer to a "tempering pass" for electric welded CrMo fabrication. An old welder I knew with extensive WW II aviation metalsmith experience told me he favored heating the general area of the weld to a red barely visible in poor light for a full minute then allow to cool slowly in a place free of drafts. I determined this was 1200 degrees. His advise hass been echoed in many texts I've read over the years.

              I ran some tests with TIG weld in CrMo samples for my own information. A strip with a butt weld is likely to brittle fracture when folded back on itself. A strip heathed to 1200 degrees for a minute and allowed to cool in still air was ductile but would still fracture if overbent. A welded strip allowed to cool from red heat in a fluffly theremal fiber blanket (dare I say asbestos? It was that long ago.) was as ductile as an unwelded sample.

              Welding aircraft tubing is well documented. You who desire to fabricate high-strength components having many welded joints should research the available material carefully to avoid the many possible pitfalls. There's no reason in today's glut of technical information for a home worker to wander off into unsafe technique. Adding extra filler metal or extending the bead might serve as an artistic statment but it's not likely to produce a stronger more reliable welded joint.

              Sitting down with a "Today I think I will weld a bike frame. How hard can it be?" atitude is a resipe for joint failure at the worst possible moment. Prepare yourself and develop your skills before you start welding for real.

              What's safe and strong and light all at the same time is never a product of "cool". It's a product of all the traits of nerdiness: study, dedication, hard work, diligence, and especially an other-worldly lack of concern for "cool." What's safe and strong and light is an incremental process where small steps are proven before going on to the next increment. Look at aircraft development in the last century. The pioneers of aviation are survivors of a savagely Darwinian process. Only those who survived those early days went on to make advancements. Many perished in the process.

              CrMo frame construction is but one tachnology that is exhustively presented in welding texts, aircraft maintenence texts of the era, and a large body of welding progedures and engineering papers. This is available to you if you'd look in libraries, Google, weld supply shops, etc. The EAA (Experimenta Aircraft Association) has in its archives a huge connection of aircraft DIY technology. They are well worth an inquiry.

              The procedures governng welded joints are subject to the dryest form of technical writing. Prepare to be bored out of your skull but you have to stay awake. Hidden in a half sentance might lay a jewel beyond price or the means to avoid disaster.

              It's vitally important for a home welder to qualify himself before making high reliability welds. Weld several sample joints, saw them into samples and subject them to hammer and bench vise torture tests layed out in welding texts. The joint fit-up and the first pass are all-important. If the back side of the weld is inaccessible, only good fit-up and first pass procedure will ensure full penetration.

              If you wish to develop alternative joints and welded embellishments, do so on sample pieces and subject them to some form of destructive testing. Only after you can duplicate the strength and ductility of the plain vanilla joints should you attempt these joints on a bike frame.
              Last edited by Forrest Addy; 08-22-2007, 10:43 AM.


              • #8
                Good info guys , thanks for taking the time ----------- I wonder if a fiberglass blanket is good to use to hold heat for normalizing? It maybe has to much thermal mass to begin with and would suck it off the part, esp. thinwall cro-mo, or worst yet melt, Forest, I remember those old asbestos blankets and they were light and fluffy but i have to admit I wouldnt want one laying around the house,

                For the record this wasnt to do with anything esthetic -- I was actually thinking structual and was aware of the fact that it would all have to be re-heated as a unit and then re-cooled but where do you draw the line? wouldnt the entire frame have to go throught this process? otherwise arent you leaving some area's in "limbo" ?

                Evan you have always brought up the gas weld and I believe in what your saying if the material does not go through any further process, from what iv been told though is bike frames go through a process as a unit after the welding takes place, thing is Is iv been around bikes my entire life, Iv seen every material ever used on frames including titainium, kevlar reinforced carbon fiber, ceramic impregnated aluminum alloy and cast magnesium just to name a few,,,
                If there is an advantage to any material or process in the form of strength/weight/durability therein believe me it will be exploited, you can drop over 5 grand on just a frame easy, it is one of the most competitive markets out there and there are some very very high end Cro-mo racing framesets using the best of easton and columbus tubing, these tubes are not only tapered wall thickness and shape (dont asked me how they build them) they have the Meat only where you need it, im talking cro-mo so thin that you can apply pressure with your thumb and cause the reflextion of the light on the show room floor to distort on the painted frame surface, They dont go through these kind of efforts and then just throw in the towel when it comes time to peicing them together,
                And no disrespect to the aviation industry but in many cases these are just over 2 pound items selling for over 3 grand, there is allot of focus, just about everything has been tried, that is why I have to add that if Gas welding was indeed superior I would have seen it bust out big time in the high end cro-mo bike frames, I havent --- Not a one, Not to say they dont exist, its just that the majority are either lugged and brazed with gas or direct tigged, but iv yet to see direct gas connections... or if their doing it their certainly not bragging about it, believe me --- if it was all that and a bag of chips there would be tons of top end companies with pictures of thier machines and in big bold advertizing letters they would be touting "GAS WELDED CRO-MO" Again, not a one...

                Aviation only gets it so far with me --- I think it scares allot of people into thinking "well yeah --- its gotta be good cuz its your ass on the line"
                Try "flying" down a Mt. pass at 70 mph+ on 3/4" wide tires and a bike frame that weighs just over a couple pounds --- believe me -- your ass is on the line also...
                Next time you go and pick up a quart of oil think about something that weighs aprox. the same supporting all your weight for thousands of miles "at speed" and over all types of brutal terrain, hell --- iv pee'd bigger weight than some bike frames, In many ways production Aviation can only dream...


                • #9
                  You won't find gas welding simply because there aren't enough good oxy/fuel welders around. It's out of fashion these days and for good reason. TIG is nearly as good and a lot easier to learn.

                  BTW, the institutionalized paranoia that goes along with aircraft safety isn't so much because the pilots ass is on the line, it's the passengers and/or the people on the ground that are the concern. This isn't usually a concern with bicycles. If your downtube snaps off it isn't going to wipe out a school full of children.
                  Free software for calculating bolt circles and similar: Click Here


                  • #10
                    Originally posted by Evan
                    If your downtube snaps off it isn't going to wipe out a school full of children.

                    Oh you' be surprised!!! (smaller bus and special kids)

                    Honestly, I cannot believe how well they hold up, Iv seen very few real structual failures and tons and tons of repetitive abuse, and they are extremely lightweight... but point taken, but along with it Id add that if there is just a few good oxy/fuel welders and it was the ticket they would imediatly have a well paid job because there is almost no limit to what the performance cycling industry will support in the means of a better end product, the word "production" does not even enter into the equation.


                    • #11
                      not tryin to divert this thread but the thing that really amazes me about a bicycle rolling down a hill at 70mph is the brakes on these things, "what brakes?", what is state of the art in bicycle brakes?
                      "four to tow, two to go"


                      • #12
                        Originally posted by speedsport
                        not tryin to divert this thread but the thing that really amazes me about a bicycle rolling down a hill at 70mph is the brakes on these things, "what brakes?", what is state of the art in bicycle brakes?
                        The nice ones have disk brakes.


                        • #13
                          Disc brakes?

                          Isn't the Bicycle Wheel/Rim a Disc Brake itself? As in the Calipers with Rubber Pads, grab the Rim/Disc.




                          • #14
                            TIG is faster and often "prettier" than oxy/fuel fusion welding. You also have a cleaner bead and area around it after the weld as the shield gas keeps more of the O2 away.

                            TIG is basically the same manual dexterity process as gas welding - heat source in one hand, feed the filler with the other. A person who is good with one technique is likely to be able to pick up the other without much difficulty.

                            If you need to add metal via welding it would make more sense to just add that metal into the tube at the start and increase the diameter. You'd probably find the second moment of area increase from going up to the next size tube is a much more productive use of the metal than trying to add "web" reinforcements of weld bead.

                            1018 is just fine if you will design the frame so it lets the triangulation in the structure give the stiffness instead of hoping some tube isn't going to be bending. 4130 is something I'd rarely think of using on a motorcycle unless it was going on a dirt bike that could use the higher yield point of 4130 to help avoid picking up dents after you drop it in the rocks.



                            • #15
                              25 frames so far...

                              I've built about 25 bicycle frames over the years, all out of various steel alloys offered by tubing suppliers like Reynolds and True Temper. If you're looking to build a nice light stiff frame I'd suggest you look at the various grades of tubes these companies offer. The tubes are butted internally so the ends are thicker and stronger so the HAZ won't be as much of an issue. There is no need to add gussets unless you deviate far from standard construction or want to build a tank of a bike.

                              I've built a 3.0lb 58cm road frame that is very stiff out of True Temper S3 air hardening alloy. Check out their website...


                              Also, Henry James will sell small volumes of True Temper tubing as well as other fittings for bicycle frames.


                              Do you have a jig or a plan for holding all the tubes in place for tacking? How do you plan of cutting the tubing for each joint? I'd suggest a LOT of TIG welding practice joining thin tubes to thicker tubes before you start welding the real thing.

                              Nice tight tube joints is going to make your like a lot easier too...

                              Last edited by glenj; 08-23-2007, 08:30 PM.