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Old 11-16-10, 07:58 PM   #1
jshelly
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Built to fail?

After all these years of dropping my bike off at the LBS I started doing my own work for no reason other then I enjoy it and tonight I noticed what you see in the picture below.

These holes are at the ends of the forks, seat stays, chain stays and even the top tube.

Sure it's a cheap GT Hybrid that's 10 years old but I like it for a beater. I live by the ocean and I'm pretty sure it can't be good to have a bike that's like swiss cheese. You can even see the rust that's developing.

Is there any functional reason for having holes in a frame (on the inside of the tubes mind you)?

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Old 11-16-10, 08:03 PM   #2
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They are there to let gases escape while the end is welded in. Have to have them. bk
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Old 11-16-10, 08:04 PM   #3
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jshelly, I think those holes allow the interior of the tubing to equalize temperature and humidity, helping to prevent condensation inside the tube.

Brad
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Old 11-16-10, 08:04 PM   #4
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Yes there is they are probably there to vent the tubes when brazing.

Thanks for the correction GB.

Last edited by cyclist2000; 11-17-10 at 05:10 PM.
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Old 11-16-10, 08:15 PM   #5
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Brazing.
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Old 11-16-10, 08:16 PM   #6
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Ahhh, learn something new every day.

Now I am afraid she's just gonna fold up on me one day.

Thanks,
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Old 11-16-10, 08:21 PM   #7
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Ahhh, learn something new every day.

Now I am afraid she's just gonna fold up on me one day.

Thanks,
Thats totally normal, I wouldnt worry about it "folding" on you. Gotta have the holes to allow moisture to escape so some rust is normal.
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Old 11-16-10, 08:22 PM   #8
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Tear the bike down and use Wiegle's Frame saver , It sprays inside the tubes , then you roll the parts around to coat the insides

hard to roll a whole bike around enough thats why it has to be apart.. get some in the frame tubes too, thru the BB shell
and more brazing vent holes. like in the seatstays.
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Old 11-16-10, 08:39 PM   #9
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As others have said they're "blowholes".

During brazing the air trapped inside closed tubes expands, and without a vent, would try to escape through the joint. The escaping air can compromise the joint integrity, but maybe more important can be hazardous to the brazer. Some builders rewarm the tubes afterward and do a quick braze plug of the hole, making a weathertight seal.

Spray Frame Saver, Boeshield, or WD-40 into the hole, rotate the fork to spread it then allow the excess to drain. Then dry it in a warm place to dry the air inside then tape the hole closed if you wish.
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Old 11-16-10, 08:42 PM   #10
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Now that I know what they are called a quick google search for "Brazing Vent Holes" and it all makes perfect sense now.

Found at (http://www.southwestframeworks.com/gallery/)






Last edited by jshelly; 11-16-10 at 08:47 PM.
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Old 11-16-10, 08:49 PM   #11
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Now that you know what they are, don't forget to look on the lower inside of your seat stays. Same problem, same solution, so if you plan on doing anything for rustproofing add the seatstays to the list.
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Old 11-16-10, 09:17 PM   #12
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After I braze a frame together and it's cool, I'll plug those holes with epoxy or Bondo before painting. It keeps out moisture and water to prevent future rust (especially for bikes that gets ridden in the rain). Differences in atmospheric pressure places 10x less load on the tubing than the weight of the rider (and that's if you take the frame out into outerspace with a complete vacuum). Cutting apart one of my frames with filled-holes 15-years later shows much, much less rust and corrosion than a frame with the holes left open.

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Old 11-17-10, 01:41 AM   #13
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I had heard to seal them with candle wax dripped in them. I also heard that if you drip wax into the heads of your cleat bolts in your MTB or CX shoes, you won't have to chip all the petrified dirt out when you need new cleats. However, I have never gotten around to actually doing these things.

YMMV
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Old 11-17-10, 09:35 AM   #14
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Quote:
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jshelly, I think those holes allow the interior of the tubing to equalize temperature and humidity, helping to prevent condensation inside the tube.

Brad
They're vents. Without them, the air inside the tubes would expand when heated during the brazing process and push the molten brass out of the joint, preventing a strong joint from forming. After brazing, they allow flushing the flux out of the tubes so it doesn't corrode the tubes from the inside.
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Old 11-17-10, 09:39 AM   #15
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They are also drains for water that gets into tubes, even if you seal the holes condensation will be present, with them open at least it has some where to go.

Last edited by Fizzaly; 11-17-10 at 09:43 AM.
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Old 11-17-10, 11:54 AM   #16
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Spray Frame Saver, Boeshield, or WD-40 into the hole, rotate the fork to spread it then allow the excess to drain. Then dry it in a warm place to dry the air inside then tape the hole closed if you wish.
+1.

I used boiled linseed oil -- poured and sloshed around the entire frame and then dried for 36 hours.
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Old 11-17-10, 04:03 PM   #17
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Yes, vent holes for when the tubes are heated for brazing the DOs on to the stays and fork ends, same at the fork crown and at the rear DOs.
Back in the mid 80's, I also used the holes to fish through my Avocet 20 bike computer wheel sensor wire from the brake caliper area, into the hole on the top of the right fork leg, under the fork crown, down inside the fork leg and out the hole just above the the right front DO, connecting to the magnet sensor mounted to the DO.. No visible ugly wires going down the fork leg for me back then.....but now I just have wireless.....

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Old 11-17-10, 05:15 PM   #18
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Back in the mid 80's, I also used the holes to fish through my Avocet 20 bike computer wheel sensor wire from the brake caliper area, into the hole on the top of the right fork leg, under the fork crown, down inside the fork leg and out the hole just above the the right front DO, connecting to the magnet sensor mounted to the DO.. No visible ugly wires going down the fork leg for me back then.....but now I just have wireless.....

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that must of taken a lot of time and patience.
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Old 11-17-10, 05:31 PM   #19
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If they are just to vent hot air during brazing , you wouldn't need more than a pinhole.
Besides aren't those fork tubes open at the top end?
The stays are open at the BB which is open at the seat tube?

Seems like the tubes already have big holes-BB>Seat tube> fork tubes are open at the crown?

They look like drain holes to me.
Maybe I am missing something?
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Old 11-17-10, 05:43 PM   #20
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If they are just to vent hot air during brazing , you wouldn't need more than a pinhole.
Besides aren't those fork tubes open at the top end?
The stays are open at the BB which is open at the seat tube?

Seems like the tubes already have big holes-BB>Seat tube> fork tubes are open at the crown?

They look like drain holes to me.
Maybe I am missing something?
You probably only need a large pinhole, but drilling tiny holes is more difficult than drilling slightly larger holes, so 1/6" of an inch or so is pretty common. many are punched, which is faster but has a larger minimum hole size.

Most fork blades are closed at both ends since most crowns are closed. Chainstays don't need vents because as you point out they're open to the bottom bracket, but seatstays are closed and need venting. Seat and down tubes are open, and top tubes are usually vented into the seat or head tube and no external vent is needed.

Over 40 years, I've seen them left open, or closed with braze, wax, body filler, or tapered wooden stakes, but there always was a vent at some time during the build process.
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Old 11-17-10, 06:34 PM   #21
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If they are just to vent hot air during brazing , you wouldn't need more than a pinhole.
They're also to allow you to flush the flux out of the tubes after brazing. For that you need a hole on each end. Larger holes mean the tubes drain more quickly, which can be important in a production environment.

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Besides aren't those fork tubes open at the top end?
Some crowns, e.g. Henry James, have vent holes cast into them. With others the crown end is closed and you need to drill a hole. With crowns like Cinelli, several models of which have decorative tangs extending down the blades, we'd drill the upper vent through one of the holes in the tang.


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The stays are open at the BB which is open at the seat tube?
Chainstays are open at the BB end, but still require a distal vent for proper drainage. Seat stays, top tubes, and fork blades are closed at both ends. The top tube is vented into both the head tube and seat tube; the down tube only needs a vent at the head tube end. Seat stays and fork blades need vents at both ends.
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Old 11-17-10, 10:42 PM   #22
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Are we all through venting now?
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Old 11-18-10, 06:59 AM   #23
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*boom-tish*
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Old 11-18-10, 09:00 AM   #24
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Reading this thread got me thinking about the stress concentrations caused by the hole in the bottom of the fork. According to my Design of Machine Components text book you will have about 1.8 times the stress in fork at the points around the hole. Considering that there is usually a Very generous factor of safety built into things that convey humans about, this is not worrisome to me. The corrosion is also a non issue for me as it can easily be controlled via methods explained above. Just fill the hole with wax and don't worry about it.
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Old 11-18-10, 09:35 AM   #25
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Reading this thread got me thinking about the stress concentrations caused by the hole in the bottom of the fork. According to my Design of Machine Components text book you will have about 1.8 times the stress in fork at the points around the hole. Considering that there is usually a Very generous factor of safety built into things that convey humans about, this is not worrisome to me. The corrosion is also a non issue for me as it can easily be controlled via methods explained above. Just fill the hole with wax and don't worry about it.
INteresting thought about stress concentration around the hole. BUt like you said, generous safety factors. I have never ever ever ever seen a failure at a drain hole, and all frames have them.
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