patmikel

I cracked my seat tube and caught it really early on thankfully. I took it to my local bike shop to get it checked out and they were able to direct me to a local aluminum welder (yay)

The local shop said that it probably cracked from riding my seat too high. I'm hoping that if I lower my seat it won't happen again. I'm curious to hear your thoughts on how aluminum reacts to welds ? Is it going to be weaker or stronger, assuming I have a good welder? This is my first post thanks

headasunder

I'm not a welder but the repaired aluminium seat tubes I have seen have all failed again after a time. If you are comfortable at your current seat height don't lower it just buy a longer post and run it at the same height.

dsbrantjr

In general aluminum needs to be re-heat treated after welding to regain its strength, which was apparently insufficient. I agree with headasunder's suggestion of a longer post. You also do not mention where the tube cracked, which may be relevant. If you have the type with clamping ears and it cracked where the ears were welded on it could indicate you were clamping an undersized post.

mprince

How far down from the top tube junction did it fail? If you had your post near the max insertion mark then it's possible the high force exerted by the post "stub" could stress the top of the seat tube to the point of failure.

A longer post is one solution as more post will be buried inside the frame, but I have doubts the frame repair will last. I'd be shopping for a new frame.

HillRider

If your seat post was so high the leverage damaged the seat tube you probably need a larger frame. A longer post will help but the heat-affected area near the weld is going to be weaker than the original metal.

sweeks

An aluminum repair weld without heat treatment won't *necessarily* fail. I had a crack developing on a Dahon Mu XL folding bike at the junction of the top tube (the only tube) and the head tube. It's had several thousand miles on it since the repair and hasn't failed. Luckily, it's in a very easy-to-observe location. It also probably doesn't get the tensile stress that a seat tube receives (especially on a folder with a loooooong seatpost!).
If the OP's crack was located where the seatpost ends inside the frame, then the advice ^^ for a longer seatpost is likely to prevent a recurrence, or at least delay it with less-favorable mechanical advantage for the "stress-riser".
If the seat tube was cracking around the bottom of a slot where the clamp compresses the tube, then the answer might be simply a larger hole (increased radius of curvature) to serve as a "stress-breaker". I've had this issue on another aluminum folder.
Regardless of the location of the crack and its repair, it's wise to keep an eye on it.

Loose Chain

Well, that sounds like a recipe for disaster.

And I have and do weld aluminum. I have a little TIG set up for home shop use and I work in aviation as a service engineer and welding the "only" tube of the frame to the head tube, whatever, it might hurt a little when it goes boom.

The OP either has a bicycle frame too small for his human frame or he was running a too short seat post.

Sure aluminum frame bicycles are welded but then they are heat treated afterwards to restore the strength to the welded area, my GT 6061 raw aluminum, road performance bike:



Steel bicycles, same thing, some are heat treated post weld to increase the strength, my Pista, chrome steel:



Note the HEAT TREATED decals on both of these welded frames, one aluminum, one steel. That said, I would feel a lot better welding a steel frame for repair purposes, 4130 type steels are weldable and if allowed to cool slowly will be strong. Aluminum is much more dicey. Of course, in aviation, for several reasons, chrome plating is a not smart.

Bicycles and airplanes have a lot in common, both use high strength steels, aluminum and titanium. Both are built to defy gravity and therefore are built very light and (potentially) highly stressed with just enough strength engineered in where needed and very little elsewhere. The Wright Brothers where bicycle shop owners and mechanics among other things and Charles Taylor, their shop mechanic and equally inventive, was the first A&P ratings holder.

sweeks

Maybe so. However, it's my own bike and I feel the risk is low relative to the benefit to me. I watched the crack develop until I was certain it was in fact a crack; it took several thousands of miles to reach a centimeter, which was a very small fraction of the total weld length. Several thousand miles on, I think it was the right thing to do.

All that said, I would not give or sell this bike to anyone because of the high level of monitoring required. Neither would I recommend anyone do the same thing unless they understood the potential complications.

The OP really didn't provide enough information about the location of the crack, other than it was on the seat tube. I agree with others ^^ that if the repair is in the vicinity of a stress-riser, it is likely to fail again and is not a reliable or safe repair.

It's too bad that folding bikes are often made of aluminum to keep the weight down, when the required design features (particularly long seatposts) would be better served with steel or titanium.

WizardOfBoz

I'm assuming that Loose Chain also holds an A&P* rating if he's TIG welding on airframes. I'd tend to give creed to what he says.

The strength of a frame with a repair weld will be less than that of a properly welded and heat-treated frame , unless you heat treat the repair welded frame afterwards. If you do HT the repaired frame you're going to have to repaint.

Lowering the seat of a bike significantly reduces your mechanical and physiological efficiency.

As others have pointed out, a longer seat post reinforces the seat tube to a point, but if your long seat post extends a long way, you still have a larger moment arm. Probably good to think about a larger frame if you can.

*Airframe and Powerplant mechanics rating, needed to work on commercial aircraft.

le mans

Racers tend to go for smaller road bike frames.. extended seat posts for better aerodynamics

Must be flexible

If you're not actually racing and it's a road bike, i agree with the suggestion of getting a bigger frame that fits

Loose Chain

I think you may have sightly misunderstood, yes, I am an A&P, IA, but I have my little TIG unit for home hobby use. I use an oxy-acetylene torch for for welded steel tube (4130) repairs. I can control the heat much better including slowly withdrawing the torch flame. TIG, at least at the level of machine I have I do not have that sort of control.

WizardOfBoz

Loose Chain, I got it. Still, with an A&P rating, you know a little bit about what NOT to do. Like, for example, trying to weld structural aluminum and expecting it to be as strong as the original heat-treated material.

I'm concerned about one aspect of the weld, which is if the repair is a through-weld (which it should be for strength) then there will be weld bead inside the tube and the repair person should ream out the tube to original id to fit the seat post. The alternative would be to weld without full penetration. A partial penetration weld, un-heat-treated, is pretty likely to fail IMHO.

I'd strongly advise the OP to get a longer seatpost if they choose to weld the frame. Dropping your seat and riding in an awkward less efficient position (and too low a seat makes you drastically less efficient) would probably not give satisfaction.

hokiefyd 

I apologize if I missed it in this thread, but @patmikel, have you contacted the manufacturer? Most manufacturers have lifetime warranties on the bike frames. This often applies to the original owner only, but I'd reckon they'd want to know about this condition on your bike, even if they don't give you a free replacement. Maybe they'd give you a credit toward the purchase of a new bike. That may or may not be what you want, but it wouldn't hurt to contact the manufacturer, or have a dealer do it through their channels.

3alarmer 

.
...nothing is forever. This includes bicycles.

davidad

The disadvantage of Aluminum bike frames is that they have a finite life. I would replace it.

Loose Chain

Very few things are infinite. A good aluminum bicycle should last a lifetime. Thing is though, an equally good steel bicycle might just last several lifetimes. "Good" as in quality produced of appropriate alloy and tube size and wall thickness and methods of manufacture that are engineered and controlled. That would leave out most of the cheap Chinese box factory junk. I am quite certain my mid 1990s 6061 heat treated aluminum GT will be going long after I am not. Now, CF, yeah, well, yeah, hmmm, probably a decade or so, maybe.

Wilfred Laurier

A few thoughts:

It's not only the material and construction that determine a bike's life, but the rider and the conditions to which they subject the bike.

There is confusion on this issue, primarily I think due to a misunderstanding of 'fatigue limits' - the amount stress placed on a material that, if not exceeded, will allow the material to last infinitely long. Steel, theoretically, has a fatigue limit, and aluminum does not.
However, just because the material has a fatigue limit doesn't mean that a structure made from the material doesn't experience loads that exceed the fatigue limit. And even though aluminum, doesn't have a fatigue limit, its high strength to weight ratio and robust manufacturing processes generally mean that an aluminum frame could go though billions of stress cycles before failure. Or if the use seen by the frame is higher than expected, it might fail in a surprisingly short time, same as steel. If you convert a lightweight road bike from the 80s for use a gravel bike for a Clydesdale, expect that frame to have a finite life. If you have a robust aluminum frame ridden gently by a 120lb rider, that frame will likely last forever.

FWIW, I also had a mid nineties GT - a Reynolds 520 (steel) tubed Karakoram from ~1997 (IIRC), and the frame broke after two seasons. The aluminum warranty replacement (Aggressor?) didn't fit me well, but I got at least the same usage out of it before I gave up trying to make it comfortable and sold it.

Loose Chain

I never mentioned fatigue limits. I think maybe you are misunderstanding fatigue endurance limits. Yes, both steel and titanium and have an endurance limit or threshold and aluminum (and CF) do not. Steel (frames) not only have a higher fatigue limit than aluminum but also have an endurance limit below which cyclic fatigue stresses, as long as they remain below that threshold, the steel frame will NEVER fail. Aluminum will eventually fail, no matter how small the cyclic stresses are, it WILL fail because aluminum does not have an endurance limit. Which is why it is doubly important that good engineering be applied to aluminum frames and tubing selections, size and diameter and wall thickness, be made that will allow a sufficient fatigue life at the expected stress levels.

I probably will regret bringing CF into the fray, while it does not have a fatigue endurance limit (like aluminum) it does not behave either like an isotropic and ****geneous material. It is more complicated because it is a composite of a usually epoxy binder and a fibrous material, well, anyway, it gets a bit more complicated.

Edit, wow, that is a bit of an overly sensitive word filter that h-o-m-o-geneous is a bad word and needs a PC filter, good grief!

Wilfred Laurier

I understand fatigue limits. What you are leaving out of your explanation is the assumption you are making that, because a material has a fatigue limit, structures built from that material do not see stress above that limit, which is not a good assumption.

Also you are assuming that the eventual failure of the structure made from a material without a fatigue limit will happen in a time frame meaningful to the user. If a 90 lb person is riding an aluminum freeride bike, yes it will 'eventually' break, but 'eventually' could mean a time frame longer than the rider's lifetime. And a lightweight steel bike ridden hard by a bigger or stronger person could be experiencing cyclic stress above the fatigue limit of the material so it would eventually fail.

WizardOfBoz

Kind of like having signs referring to Hong Kong at an NBA game (see below). Say the wrong thing and you are censored. I think sometimes the people (or bots, I suspect) doing the censorship call more attention to the issue by censoring than the actual original words cause. I would think, though, that the bots could be a bit better. Blocking h-o-m-o the slur, but allowing h-o-m-o-geneous. Or h-o-m-o-zygous in a medical forum.

*https://www.washingtonpost.com/sport...-philadelphia/

Loose Chain

I am not making any assumptions, or thinking/saying that bicycles do not see cyclic stresses above the limit. In fact, I am certain they do when people jump curbs, smash into things and especially when the weight limit is exceeded. Bad thing though that the design limit weight is not provided, though I know that most SL/SLX/Prestige and similar tubing steel bicycles were built with a rider weight of somewhere south of 170 pounds in mind. So put a, uh, large 210 pound fellow on one, yeah, it probably does seem some excursion beyond the limit.

WizardOfBoz

You think 210 pounds is large?!

Loose Chain

Yes, I do. And so would most of the manufacturers I bet of bikes with those types of tube sets.

Someday, I would really like to see strain gauges placed on a frame and then put a rider on it and ride it and record the data. I assume this is done at some level on new designs and materials or maybe not. Or put the instrumented bicycle on a machine made to mimic a rider on a bumpy road and the usual curb jumping shenanigans the poor bicycles see routinely not to mention cycle cross, gravel and downhill mtbs. And put a frame in an iron monster that will twist, pull and shake a frame at typical stress levels (determined in the aforementioned tests) until a mean time to failure could be established. Like we do with airplanes and other such machinery.

I bought an old Raleigh step through 3-speed. Began reworking it for my niece as I had done several for myself and wife. Started looking up inside the frame with a borescope (yes, I have one) and was of the mind that I would retire the frame due to rust in the tubes and leaking rust out of the fork weep holes. it looked fine on the outside. Pitting rust on the head tube area and bottom bracket and no telling what in the forks where my scope could not go, no way I would put a child on it.

rickpaulos

In the warrenty fine print, most manufactures define "lifetime" as the useful product time period as they see it, not human lifetimes as they may want you to believe. In reality that works out to just a few years for replacement items. After they run out of old stock, they might choose to issue a credit to buy another of their products. With so many mergers, acquisitions and bankruptcy's, lifetime has a new whole meaning, the life of the company under current ownership.