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Cracked My Seat Tube

Old 10-06-19, 12:03 AM
  #1  
patmikel
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Cracked My Seat Tube

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.
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Old 10-06-19, 12:11 AM
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Loose Chain
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I doubt it will not crack again, you should start saving money for a new frame because it will crack again most likely, probably sooner than later. If your seat is raised so high that the seat post remaining in the seat tube is insufficient then your frame is too small or your seat post is not correct to your purpose (as in you need a longer one).

You cannot just lower your saddle, your saddle height is a function of your leg length, you cannot just suddenly grow shorter legs. If the saddle height was set correctly and there is not enough seat post in the frame, you got a problem.
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Old 10-06-19, 06:03 PM
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Originally Posted by Loose Chain View Post
I doubt it will not crack again, you should start saving money for a new frame because it will crack again most likely, probably sooner than later. If your seat is raised so high that the seat post remaining in the seat tube is insufficient then your frame is too small or your seat post is not correct to your purpose (as in you need a longer one).

You cannot just lower your saddle, your saddle height is a function of your leg length, you cannot just suddenly grow shorter legs. If the saddle height was set correctly and there is not enough seat post in the frame, you got a problem.
I think i was riding the seat too high in the first place. It feels way more comfortable at the position it is in now. I was riding the seat really high.
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Old 10-06-19, 06:26 PM
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To get you in the ball park in the future you need to get your cycling inseam, not your pants inseam, and multiply by .883 and set your seat there measured from center of bottom bracket to the top of the saddle measured along the seat tube, or equally good, sit on the bike, maybe secured to a trainer, with your cycling shoes on and pedal backwards, set your seat such that your leg is NEARLY fully extended with your heels on the pedals and there is minimal side to side rock of your hips as you pedal. The later method usually sets the seat a little higher than the Lemond Formula but either should get you to a STARTING point that is very, very close.

Sorry about your bike, live and learn.

Last edited by Loose Chain; 10-06-19 at 06:29 PM.
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Old 10-06-19, 07:44 PM
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Originally Posted by patmikel View Post
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.
Not sure of aluminum, but I can speak on steel.
I snapped an old road bike conversion on the downtube once. I knew a welder, and they offered to weld it back together using an oxy torch. It actually worked, and the thing rode as is for a few weeks. Later, the frame broke again not at the weld, but very close to it (taking some of the weld with it). So I just decided to buy a new one.

Heat affects metal greatly. The atoms re-arrange themselves when heated making the material either stronger or weaker depending on the temperature and how it is cooled (many metals are heat treated).

So I would take that into consideration when getting it welded. If it costs even 1/4 of the price of a new frame I would re-consider. You're also never going to be able to sell it again, or at least not for a decent price.
It could snap, or it could last and that all depends on how it's welded, what it's welded with, and how it cools. You will be changing the material structure of the frame in an area where it was supposed to be consistent. It will never be good as new again.

However, it's not THAT serious, as people braze/weld things onto their frames all of the time.

Keep in mind that my frame snapped on a very critical part of the bike that receives more stress than other areas (downtube).

If you did indeed put the seat too high, if you get it welded and get a super long seatpost, you're probably not going to put a lot of stress on the area that cracked, and that area (someone correct me if i'm wrong doesn't receive as much stress as other parts of the bike if you have a tube shoved in there well. Most of the rest of the bike will be helping with the distribution of the stress.
You're also going to be putting stress mostly not in the area where it cracked since you will be shoving the seat tube lower (but it will still receive stress).
Luckily, this repair doesn't sound like it's going to change the geometry of your frame much which is another consideration.

As always, YMMV, but I would make the price of the weld the deciding factor here. If it's anything even near $100 (which it most likely will be, as most shops have minimums usually set at 1 hr labor minimum), start looking at new frames.

Last edited by BicycleBicycle; 10-06-19 at 07:49 PM.
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Old 10-07-19, 09:57 AM
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If you go the welding route, maybe they can weld another piece of aluminum on the area to help strengthen it, not unlike the pieces that sometimes are welded to the head tube to add strength. Just a thought.
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Old 10-07-19, 10:02 AM
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How much seatpost do you have in the seat tube? Is the minimum insertion mark showing? If you weld the tube and do not have reinforcement welded around it, make sure the bottom of your new post is well below the weld and the heat affected area. (I'd go looking of a really long MTB post. I'd want 2 or more inches of post below the crack.)
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Old 10-07-19, 02:34 PM
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new frame day, the worlds ur oyster.
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Old 10-08-19, 07:08 PM
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I make my living as a welder and I'll tell you now that depending on the aluminum alloy you may or may not need to have it heat treaded after welding to get it anywhere near its original strength. 6061 needs heat treating,it's the cheapest and most common alloy for road/mountain frames. 7xxx may or may not depending on the alloy.

Keep in mind that the heat treating will add $$ to the cost of repair. If your frame is painted then you can say goodbye to that paint job as the heat treatment will destroy the finish.

Edit: the repair could be acceptable if it's a low stress area. Definitely not any joints or a headtube/BB shell.

Welding 6061 without heat treatment will mean that the welds and any area affected from welding heat will be the weakest part of the frame.

6061 joints literally get weaker after welding and need the heat treatment to bring it back up to strength.

The majority of custom aluminum frame builders won't use 6061 because of the extra work involved in building the frame. They use 7xxx because it's pretty much ready to ride out of the jig,with the exceptions of alignment and facing and chasing.

Last edited by BoozyMcliverRot; 10-08-19 at 07:22 PM.
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Old 10-12-19, 09:44 PM
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I would first go get a fit, I just had a Retül fit and the first ride home on my bike with the changes was transformative, I didn't know how bad my position was for me until we tweeked it and got it way better. There were some things I sort of had some inklings about but never pursued them. Then after that I would get a new frame and get something that works well for you.

I wouldn't waste time on a re-welded aluminum frame unless it is something truly special and unique. The cost is not going to be cheap and may not last long. One you get fit and since you have ridden something before you can kind of take a better stab at what you really want. If you can swing it custom steel is nice and certainly great if you don't fit in the geo specs of the stuff out there or have odd requests that frequently aren't found on most fixed gears. If I were going aluminum I would probably go with a Low but I would rather not go aluminum. Steel or Ti is my chosen frame material, however I do like my Langster.
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