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This must be a very basic question, but as it often happens with very basic questions, it is often hard to find the answer by searching.

On a modern bike with metal (welded) frame, is the interior of the top tube (and/or the down tube) accessible through the head tube? I.e does the head tube have holes in it that lead into the other attached tubes of the frame?

From what I could see on the Net, it appears that the head tube is typically a full cylindrical tube, without any holes. Is that so?

Meanwhile, it seems that I have some parts rattling inside the top tube of my bike (Specialized TriCross), which routes the rear brake cable through the top tube. I wonder how they got there, if the interior of the top tube is not accessible (aside from two fairly small cable holes/openings)

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The rattles are often bits of metal from the welding. Even carbon bikes can have debris inside the tubes. I have a $4500 specialized roubaix expert with something in a rear chain stay. Drives me nuts. –  Matt Adams Feb 22 '12 at 19:30
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Well, in my case the rattles appeared after something distinctively snapped inside the tube (or so it seemed) under hard application of the rear brake. Moreover, when I wiggle the brake cable as it enters the top tube, I can hear some part inside rattling against the tube - that's what causing the noise. –  AndreyT Feb 22 '12 at 19:39
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the port for the cable to enter, Is it made of plastic? –  Matt Adams Feb 23 '12 at 3:31
    
In general, there are small holes in the head and bottom bracket tubes to allow airflow through the frame. I wouldn't expect you can get anything large through these holes, but you might, eg, be able to get a small inspection camera through. In your case it sounds like part of the plastic grommet that feeds the brake cable through may have cracked off, or perhaps the cable itself is rattling. –  Daniel R Hicks Nov 3 '13 at 19:21
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4 Answers

From what you described:

  • Rear brake routed through inner top tube;
  • Rattling appeared in the inner tube after hard braking;

I would suspect (95% certainty) that the rattling comes from the very rear brake cabling/housing/routing.

The cables/routings could be removed to check if the rattling persists;

Since the inner tube is not directly visible (and so one cannot know for sure what's going on inside), perhaps contating the manufacturer to ask for advice could be a good idea, too;

Also, a more detailed description of the conditions of rattling (with the bike moving or not, when the bike is leaned, when front/rear wheel is lifted from ground, on certain types of terrain, only while braking, only while NOT braking, etc.) would help to narrow the possibilities.

Hope it helps

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I can't speak for all bicycles, and methods of construction, but I just checked on one of my aluminum framed bikes that I have lying around, and both the head tube and the seat tube had small holes (smaller than the diameter of the top tube, but perhaps 5-10mm in diameter) opening up into the top tube.

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Any welded tubes are going to have holes in their ends in order to let the hot gases escape. So, yes, there is virtually always going to be a hole connecting the interior of the top tube and the head tube (along with every other tube on the bike).

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I have had stuff rattling in my frame as well, I am guessing it was a drip of metal after they welded it. Since it is nearly impossible to access the inside of the tube even with tweezers, the solution that helped me was to use a tube of grease and squirt a little amount into the tube through that small hole, about as mush as you use in toothpaste to brush your teeth. This will eventually catch whatever it was that is rattling around in there. –  BillyNair Jul 18 '12 at 2:02
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Most welded (metal) frames will have vent holes for welding gases and heat from the welding process to escape. That makes it easier to prevent the frame from warping from the heat. That is the 5-10mm millimeter holes described in the answer by @prototoast.

Most carbon frames will have much larger access in to the top tube and down tube, especially recently designed frames.

However, the noise you are describing sounds like it has a different cause, like the cable stop for your brake cable breaking loose inside the frame, or a piece of the cable housing, perhaps.

Have your LBS mechanic check out your frame.

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