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Monday
Feb182013

Vent Holes

I am sometimes asked, why are there tiny holes drilled in certain parts of a bicycle frame, like the ones shown on the left?

These are vent holes. During the brazing process the air inside the tube expands as it is heated. The vent hole allows the air to escape as it warms up and also allows for air to enter as it cools.

If the tube is totally enclosed, on cooling the air contracts sucking the molten brass inside the tube leaving a pin hole that is almost impossible to fill.

Worse still pressure can build up in an un-vented tube and hot brass can blow back in your face. Anyone not knowing this will soon learn the importance of vent holes after picking little globules of brass embedded in their face or finds little brass balls hanging like tiny Christmas Tree decorations from eyebrows, mustache or other facial hair

Vent holes are only needed when a tube is closed both ends like the example shown above. The top tube is closed at both ends and is usually vented with holes into the seat tube and head tube. (When the bike is assembled these holes are hidden.)

Seatstays are enclosed with a fork dropout one end and the seatstay cap at the top. The front fork blades are also  enclosed both ends between the fork crown and the fork tip or dropout.

Other tubes like the seat tube, down tube and the chainstays are open inside the bottom bracket shell. These tubes are not totally enclosed so do not need any additional vent holes; neither does the brake bridge because it has a brake bolt hole.

On some of my custom frames you won't see holes in the chainstay bridge like the one in the picture. They are hidden inside the bridge tube by drilling holes sideways through the left and right chainstay tube, before the bridge tube was put into place. Only one hole is needed for venting but often two holes are drilled for better drainage of moisture later.

The vent hole in the seatstays on my frames is on the inside up near the seat lug. You might have to turn the bike upside down to see it.

On my front fork blades I drilled one vent hole in each fork blade near the bottom, but after the fork was fully brazed and had cooled I went back and filled it by brazing a piece of wire in the hole. The heat generated in doing this was so small and the air space inside the fork blade was big enough that it did not cause a problem. This whole process only took a minute to complete.

I did this for two reasons. Front fork blades are highly stressed, so filling the hole eliminated a potential weakness at that point. Also rust needs oxygen, and with the fork blades completely enclosed and airtight, no corrosion inside is possible, even years down the road.

A small and probably unnecessary precaution, but one that took such a small amount of time, I always fugured, why not?

 

                       

Reader Comments (7)

I have had a couple of bikes with vent holes on the inside of the forks and about 100 mm from the yoke, about 4 mm diameter which produce a note like a flute in the wind. It's annoying.

February 18, 2013 | Unregistered Commenterpeter

I now understand the reasons for the vent holes but wonder about the following: I have an older Trek touring bike that sees a lot of wet weather, puddles, small streams... Would it be wise to plug the vent holes in the fork and seat stays to hopefully get a little longer life?

February 18, 2013 | Unregistered CommenterTimJ

Tim J,
My advice would be leave well alone and don’t plug the holes. Very little water will enter these holes and any that does will evaporate through the same holes. If you try to seal the holes moisture might then be trapped inside causing more of a problem.

There was an old trick of shoving a cork in the bottom of a steering tube to stop water spraying up there. This was also a bad idea because moisture couldn’t drain out, and got trapped above the cork.

Unless you live close to the ocean where there is salt in the atmosphere internal rust in steel frames is not a huge problem. If there is salt in the air then external rust creeping under the paint is more a problem than internal.

The steel that frames are made from has a high chrome content so, beyond a little surface rust, will not corrode like old cars made of soft mild steel used to.
Dave

February 18, 2013 | Registered CommenterDave Moulton

Thanks Dave, I'll leave things as they are! We used to plug up the top of open seat posts (on cheaper bikes). Not sure whether that made any difference either.

February 19, 2013 | Unregistered CommenterTimJ

As I previously posted, I appreciate your capturing the process and details of how you built your frames. Now we have another reference to identifying true Dave M frames, vent hole locations! Thanks!

February 19, 2013 | Unregistered CommenterSJX426

I came to ask the same question as TimJ about plugging the holes. Thanks for the article and answering the question.

I'm reminded of the "speed holes" from the Simpsons:

http://www.youtube.com/watch?v=XVV_COOey0E

February 19, 2013 | Unregistered CommenterJeff H

I learnt the importance of vent holes when I built my own frame on one of Dave Yates' frame building courses last year.

February 28, 2013 | Unregistered CommenterYoav

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