Aluminum mountain bike rear suspension TIG weld repair

[jakeru]

Hey guys -

It's about time I posted another project up, so here is one. This is an aluminum rear suspension of a mountain bike I repaired for a customer.

It is tubular aluminum, probably about .065" thick, and it cracked where one of two slender tubes met a large, thicker aluminum piece.

It cracked in the heat affected zone of the original weld. It is likely 6061-T6 (popular heat treated alloy for bicycles... in fact I think it might have even said so on a sticker.)

My thoughts were, I could repair it to its likely original strength, but am probably not going to do any improvement on that, because the heat affected zone will still exist in the tubing right next to the weld. That said, I would do the best I could, prepping it well, ensuring adequate penetration and thickness where welded, and an even (gradual) transition of thickness from one piece to the welded area to the other piece, focusing on leaving no stress risers.

-Before welding-




-Prepped-
I used a flap disc, evenly and with a very light touch so as to not excessively thin the tubing. Note also that I left an area where the pieces would "key" together to ensure proper alignment, although did generally remove the other likely contaminated, cracked faces aside from that. That gave me the best weld quality, without going to a huge unnecessary hassle of aligning the pieces.


-Welding setup used-
Note: 1/16" 5356 filler rod ready to go. Also, sometimes with painted parts, there aren't a lot of places you can ground to, and you do what you can.


-After welding-

[SeanMurphy265]

I ride mountain bikes and if my bike broke like that I would probably replace the part or get a new bike! That's some thins stuff! I new my bike was light, but I would have thought the tubes would have been thicker. Excellent work on your part! I've been wanting to build my own bike frames for a while, but I'm too scared of one breaking under stress. Did you do any pre-heating? I've always been told that when something breaks behind the weld, the weld was too hard. Is that correct? Have you welded on many bikes? When I get comfortable with my tig welding skills I am going to build my own bike.

[67cudafb]

Looks like a good place for a gusset between tubes, not sure there is room with tire though. Must have been tough to weld with tube so thin and high stress area way past my skill good job.

[donor76]

I periodically weld and I strengthen frames on motorcycles my friends which are engaged (Stuntriding), and i would advise to you to strengthen a seam an aluminum plate (better from two parties)
approximately here so---

[Grizzly1944]

Jakeru,

The reason that it broke in the first place is because that the aluminum most likely wasn't heat treated and aged after it was welded at the factory. Whenever you weld a piece of metal it is annealed because it was heated up to the melting point. This weakens it as in this case. The high end bicycle makers will heat treat and age aluminum to bring it back to it's original strength.

I have experience building bike frames and could never figure out why these makers choose aluminum over chrome moly steel. The Aluminum tubes have to be almost twice the thickness of steel to retain the same strength and are heavier than a well built steel frame. Steel tubing is so much stronger that heat treating is not required after welding.

I just wanted to make you guys aware of some of the hazards that can arise when welding aluminum and magnesium.

Best regards!

[SeanMurphy265]

The issue is weight and that is why they chose aluminum. I realize we are not talking about much weight maybe a pound or two. The lighter the bike the less drag it will have.

[Grizzly1944]

Sean,

There is so much b.s. when it comes to selling frames that are made of materials that are lighter than steel. Carbon fiber frames are all the rage now. One guy makes a frame out of bamboo. Aluminum is in fact lighter than steel but twice as much is needed to keep the frame strong enough to handle 3g's of rider weight. The tensile strength of 6061 is 45000 psi before it gets welded together. 4130 chrome moly has a tensile strength of 81200 psi 4130 has about twice the tensile strength of aluminum which means that less material is required to handle 3g's of rider weight. Done properly, a steel frame can be lighter than aluminum, with greater safety in this case.

The bicycle makers are banking on the fact that most riders don't have adequate knowledge about the frames they depend on. They would sell frames made out of denatured beryllium and space aged plastic if they could. Carbon fiber frames make you look cool and fast, but does it make a real difference? If you are Lance Armstrong it would, but with most casual riders, it wouldn't.

Drag is a whole other subject. I understand what you are saying when it comes to weight. Aerodynamic drag is another B.S. selling point when it comes to frames. You can have a frame with aero tubing. It does nothing to reduce aerodynamic drag until about 50mph. The only time the big boys see 50mph is going downhill. 25mph is more like it. The cross section is still the same. The rider is the biggest parasitic drag producer on the whole vehicle.

When you buy a bicycle from the local bike shop it's marked up 500% from when it starts out in the form of raw materials. The shop usually marks them up 100%. I'm not saying that they are ripping people off, it's what the market will bear.

I think that the use of aluminum for a mountain bike is risky carbon or steel would be better. This failure looks to be caused by the lack of torsional rigidity which lead to a fatigue failure. Carbon and steel handle this much better. The rider was most likely off the seat and cranking hard.

I hope this helps!

[Powertig]

Thanks for sharing Jake and I learned a-lot from Donor 76's suggestion. Making things better than new is always the goal of a repair man which frankly is what most of us will be. Materials are too expensive here and labor is too cheap over there to profitably build a great bike here, for all but the elite athlete. Thanks for sharing Grizzly I learned much from your posts and they make sense. How would a Chrome Molly bike stack up cost wise to an Aluminum bike?

[jakeru]

The problem with adding a gusset is that:
1. It may interfere with the tire (this is a big deal for a job I do that someone brings in like this, you have to consider potential interference problems withother parts, etc.), and
2. It does not prevent the reduction of tensile strength in the material that is in the heat affected zone. So at the end of the day, I am not convinced it would add *any* strength. It would make the structure a bit more rigid.

I guess I did add a bit of gusseting however (a very small amount) by building up the weld fillets thicker than they were originally - on both the inside of the "vee" (which I was pretty certain would not interfere with any tire or other component) as well as on the outside.

FWIW, I don't believe bicycles like this are *typically* re-heat treated after being welded. Heat treating after welding I believe, although it is technically feasible and possible for it is be done, happens much less frequently than you'd think on things that are mass produced, to keep costs down, like this part would have been. I totally agree however that it would make it much stronger. It would not surprise me that some bicycles would have it done, but probably only the very most expensive ones. I don't think the heat treating process is very fast, or inexpensive. The proper filler rod would need to be used when welding (for example, there is a specific filler rod to use for welding 6061 aluminum that will be heat treated after welding.)

Personally, I love an aluminum bicycle. Ever since my first ride on a Cannondale. They can be built very rigidly, and very light. It is a great material for many applications needing high strength to weight ratio. Personally, I would love to have a lightweight Lotus elise, with an aluminum-bonded body to drive around. (And FWIW, it's about the lightest currently produced production cars around.)

[sportbike]

The welds would certainly still have low strength if a gusset were used, but the welds would be in an area that had less bending moment, and therefore less stress.

Aluminum weld design can be tricky. Sometimes you will see an aluminum tube that has a short length of telescoping tube just at the end for approximately 1.5 inches or so, and then another weld to join the telescoping part. This is done for the same reason (putting the welds that are at nominal tube diameter in an area away from teh high bending stress).

I have only ever had one welded aluminum component fully solution treated, quenched, and aged after welding. This was a suspension mount for a large unmanned robotic vehicle for which I worked on the chassis design and manufacture.

there is a picture of the vehicle with the suspension mounting bulkhead shown here (vertical item between the middle and rear tires).
http://www.rec.ri.cmu.edu/about/news/11_12_tools.php

[jakeru]

That sounds like some really high-bucks work there, with the DARPA vehicle, John.

I'm curious what equipment or apparatus are used to heat treat aluminum? Is it an atmospheric oven?

[Spike]

I would gusset it with something like 1/8th inch... something more than strong enough, and then anneal the welded area with an oxy torch to remove any 'weld shock' from the base material. If it's a structured part, then it will work harden a bit after you start riding it.
Pre-heating before you weld will help reduce this, also *easing* into the weld, rather than just blasting it with full pedal from the get-go will greatly reduce your heat affected structural problems.

[jakeru]

It seems to me that annealing with an oxy-fuel torch, pre-heating before welding, or welding with lower current (increasing the overall energy inputted into the work), would increase the size of the heat affected zone. (Increasing the portion of the metal that has loses the high strength developed from "T6" heat treatment.)

[CGCINC]

So setting a long upslope on the Tig machine might help reduce the shock? Or does it need to start with lower heat like a torch....