Which Process for a Downhill Mountain Bike Frame?

admin

Alrighty then,Im brand spankin new to this forum Im attending Suny Delhi in NY and majoring in welding technology and also taking metallurgy and blueprinting.Iv already passed the Stick and Oxy and MIG, but yet to touch TIG. I still have much to learn about different metal characteristics and welding processes, but as of right now i am just playing around in a 3D CAD program i have to see what suspension designs and stuff like that, TIG SEEMS to be the craze, but i heard on some sites that oxyacetylene will do the job too? for me oxy is quite easy and fun, but is the tensile strength high enough to pass for a mild steel downhill mountain bike frame? Im also looking at which material to use- if I use one of the allimunum alloy series, I will use most likely MIG it (ugly welds for a mountain bike i know, but for a first handmade frame, why not)Im not planning on officially trying to create a downhill anytime soon at all, as said before i still have lots to learn, but hopefully some smartys on this site can steer me in the right direction with which material, and which welding process!graciasp.s.. suspension design is another issue
Reply:if it were me i would use tig since the weld bead is much tougher than any other process. althought a good quality weld can be achieved with mig the bead is more brittle than a tig. for downhill u do not want your frame cracking thus propelling yr arse at a rather fast pace into the nearest oak!i used to race yeti :-)
Reply:If the oxy acetaline work is done right, it will hold up. Mild steel is very forgiving stuff, and it does not suffer much strength loss from heating, say like heat treated 4130 would. And bead strength, brittleness is not any kind of issue, the welds is the same, if the filler is the same. What can be brittle is the HAZ. And with ox/ acet, you have added gradual heat to all the parts, and on mild, the HAZ is very stable, and not too likely to crack.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:tensile strength isn't the issue (design takes care of that issue)- fatigue isnothing wrong with O/A if the material is steel or a steel alloy. personally i'd braze a steel bicycle frame. whether lugged or fillet brazed you get a very fatigue resistant joint.if you go with aluminium fatigue failure will be a  bigger issue. typical alloys would be 7005 or 6061. most builders have the frame heat-treated after fabrication if using 6061. some would use 7005 naturally aged which is probably a better bet. with no endurance limit, extra care is needed is required to eliminiate any stress risers. TIG is going to be the best option here unless you want to 'hand blend' MIG welds. O/A is going to be a non-starter for Al
Reply:HR is correct about Tig being the best for the job and make sure the filler is at least the strength of the tubing or even stronger.  JohnSMAW,GMAW,FCAW,GTAW,SAW,PAC/PAW/OFCand Shielding Gases.  There all here. :
Reply:gotta disagree about the filler being stronger than the tube. with the joints being the weak spot (regardless of how they're made) the last thing thats needed is a high strength (read hard, brittle) filler. a lower strength filler will be more ductile, the strength is compensated for by the size of the weld.this is the reasoning behind a fillet brazed frame- a largish build up of filler (to provide strength) that also provides a smooth transition into the tubes at the edges of the joint, like a fillet radius- no stress risers
Reply:+1 for brazingAlot of racecar frames are brazed and with brazing you do not change the shape or the workpeice or melt any of the worpeice, like you would with most welding processes.
Reply:I just gotta know---there are downhill bikes and uphill bikes ?
Reply:Yeah, uphill bikes use fourstrokes.  Just use steel for your DH bike, no call for aluminum on a DH racer.I like Rojodiablo's take on the subject.City of L.A. Structural; Manual & Semi-Automatic;"Surely there is a mine for silver, and a place where gold is refined. Iron is taken from the earth, and copper is smelted from ore."Job 28:1,2Lincoln, Miller, Victor & ISV BibleDanny
Reply:I would disagree with TangleDriver, Al. is the best material yet discovered for a DH bike. Steel can work, but I've never seen a steel fram as light as a mid range Al. frame, my Foes Fly is built at 33.2lbs, and there's no way that could be acheived with steel, and still hold up. However if you are going to use steel, I would spend the bucks on CrMoly and go the extra step and get it heat treated after welding. You'll have to run thicker tubes with mild and that will dramaticaly increase the weight, and you really don't want a heavier bike than is absolutly nessicery, I used to ride a 54lb Kona, and for anything except hitting drops it SUCKED, now that I'm riding lighter bikes I can actualy climb on my DH bike and flick it off jumps, instead of just plowing through things.If you do go the steel route, braving or tig is the best way to stick it all together. You could oxy-ace it, but if you're good at oxy, you'll be good at Tig, and the Tig is much easier to work with.
Reply:i actually did alittle bit of tig the other day in class for the first time ever and WOW, soo cool and fun and nice and alot quieter too hah. i didnt use any filler, didnt try fusion (yet)
Reply:well this is my tig stainless, ( no filler unfortunately) for this being one of the first times. Attached Images
Reply:Rojodiablo's answer is right. They used to use A/O to build air plane frames years ago. TIG's faster. MIG would work just fine too.
Reply:I'd go with TIG on steel, for one-off MIG is just too much like hard work if you want decent joints (you can expect to break a lot until you get the bead setup right). Thinwall 4130 is better than mild steel for the bike - it's lighter strength for strength - but more expensive and harder to work. I'm making the transition from brazing to TIG for bikes and while you can oxy weld it, it's very slow and a bit fussy in that if you don't have your insulation set up right sometimes you can't get fusion at all.As far as uphill vs downhill... uphill bikes are the ones you pedal, downhill bikes are motorbikes where the engine is stored in a truck. Very few downhill riders ride uphill on their bikes, and there are no races that I know of that are human powered, it's all about driving up, riding down. Hence the "downhill" part of the name.
Reply:Originally Posted by Joscerb55well this is my tig stainless, ( no filler unfortunately) for this being one of the first times.
Reply:Tig All the way  Mig is not accurate enough , oxy/act is not reliable enough.
Reply:Elmers glue works for me!!!!!! But ya have to blow on it to get it to harden fast!!!!Last edited by Zobman; 04-01-2008 at 05:41 PM.