Vents and Drain Holes

Vents and Drain Holes

There seems to be some disagreement as to when and where it is necessary to drill holes in a bicycle frame, so I decided I should at least be consistent in my own practice. My thoughts on the subject crystallised when I offered to service a ten year old steel framed (Reynolds 725) trekking bike from a well known manufacturer and discovered what can happen.

This particular frame had been tig welded. It has two, what I presume are vents, at the top and bottom of the under surface of the down tube. There were also small vents drilled on the inside surface of the lower fork blades chain stays and seat stays.

IMG_1419 IMG_1422 IMG_1424

I tend to assume that it was normal practice in the factory not to close these vents. There was no drain hole in the bottom bracket shell, the most dependant part of the frame, so any water entering the vents was destined to collect inside the bottom bracket. After drilling out one side then finally removing the bottom bracket itself a large amount of rust was evident inside the frame, the seat post also being seized.

IMG_1414

 

Recently when I came to fit a Campagnolo external bearing bottom bracket to my newly built steel frame the instructions were clear: ” Make sure that there is a water draining hole on the bottom of the bottom bracket shell. If there is no such hole, do not simply drill one. You must contact the manufacturer for further information and clarification in this regard.” As I was the frame manufacturer I got out my drill! I am now clear in my own mind. Drill vents but seal up the exposed ones before painting, usually with brass or silver solder. Drill a drain hole in the bottom bracket shell if there is not one pre drilled.

I have to say my 40 year old lugged Bob Jackson frame showed no significant internal rust when it went for a respray, and not a sign of a vent hole or drain hole anywhere, all the tubes were completely sealed. I suspect vents were drilled during brazing (I think they have always used silver solder) and than filled afterwards. It has been said that the commonest source of water ingress into a frame is at the seat post binder bolt, and that this can be alleviated by proper greasing and sealing, making a drain hole unnecessary. some bicycle manufacturers suggest you remove the seat post and invert the bike to drain out any water if you ride in very wet conditions, not something I imagine most cyclists would get around to or even contemplate. I do not believe you can rely on everyone looking after their bikes diligently, so a drain hole would be good insurance and I cannot foresee any disadvantages other than cosmetic.

Possible source of water ingress

Possible source of water ingress

 

 

 

 

 

 

 

 

 

 

 


VENT HOLES
The purpose of vent holes is of course to vent expanding air as the tubes and lugs heat up during brazing. The theory is if there is no escape for the hot air the tube may be pressurised inhibiting the flow of brass or solder. Of course the air has to go somewhere so it will alternatively escape from the joint, possibly forcing the brass or silver back at you. There may be an additional anxiety that moisture may be trapped in the tube after brazing and encourage rust later. It may be possible to work out how big a vent hole is required in different situations using the physical laws available but I do not know if anyone has done this, nor do I know if any experiments have been carried out on heated tubes to try and directly determine  ideal hole sizes. My guess is that the air could potentially expand up to 4 times it’s volume and that the hole sizes in widespread use are probably adequate for all eventualities.

The Paterek Manual give sizes of vent holes for all the usual situations and recommends the same for both lugged and fillet brazed frames. I have decided to use roughly the same sizes though converted into metric as Paterek still sticks with imperial sizes. He also suggests every tube to be brazed has a venting of some sort at both ends of each tube, which should ensure efficient exhausting of gases.

The sizes I use are:-

HEAD TUBE, to down and top tubes, 10mm. Head tubes that are purchased pre drilled may have vent holes of  up to 19mm.
FORK CROWN,  to upper fork blades, 6mm. There are many different designs of fork crowns, many of which permit venting through but will likely need the steerer to be drilled for venting to take place. If the top of the fork blade would seal into the crown socket without venting through the crown I would probably drill a smaller 3mm hole through  the upper inner fork crown tang and blade and ensure there was a vent in the lower fork blade.
LOWER FORK BLADES, 2.5mm on inside about 25mm up from the end when using slotted or plug dropouts. Socket dropouts usually come with pre drilled hole.
NON-DRILLED BOTTOM BRACKET, to seat tube and down tube, 10mm. to chain stays,  6mm.
UPPER and LOWER SEAT STAY and  LOWER CHAIN STAY, 2.5mm, as for fork blades about 25mm from lower end, though again socket dropouts usually have a vent hole pre-drilled.
If I am brazing a “fast back” seat stay into the back of the seat tube I tend to drill the vent hole direct into the seat tube rather than the upper end of the seat stay.
BRAKE BRIDGES and CHAIN STAY BRIDGES also need venting. Again I tend to do this by drilling a
2 – 2.5mm hole into the seat stays and chain stays directly under the ends of the bridges so they are hidden.
TOP TUBE, to seat tube, 2.5mm into seat tube hidden by top tube. If you have a decent sized hole in the head tube end many would not bother with a vent at the seat tube end of the top tube. I have also heard of a single small hole only being drilled in the centre of the underside of the top tube to enable the top tube to be left sealed, this hole being filled later.
If you drill your bottle boss holes before brazing (recommended), these would also act as vents and may influence sizing of holes elsewhere.

DRAIN HOLES

In order to try and understand the size of the bottom bracket drain hole required I drilled increasing size holes in the bottom of a sealed bottom bracket half full of water. I was surprised to find that at rest the negative pressure generated in a sealed chamber which is trying to drain, plus the surface tension of the water prevented free drainage until the hole size reached 12mm when free drainage occurred. This I suppose simulates trying to drain a completely sealed frame.

Surface tension holds water at 10mm

Surface tension holds water at 10mm

Breakpoint at 12mm

Breakpoint at 12mm

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I repeated the experiment with the bottom bracket ventilated at the top with a 2.5mm hole and was equally surprised that free drainage occurs through a drain hole of only 2mm. And this, not unsurprisingly, was maintained with larger holes.

Free drainage at 2mm

Free drainage at 2mm

 

  My conclusion from this is to make sure there is some, albeit tiny, ventilation from some part of the frame connected to the bottom bracket (this could even be a small hole in the top of the seat tube), and drill a small drain hole into the under surface of the bottom bracket. I’ll probably stick to 5 or 6mm, small holes could get blocked. The pre drilled drain slots in many bottom brackets seem more than adequate.

 

CORROSION INHIBITORS

It is also worth noting there are some products which can be sprayed into steel frames after painting to inhibit corrosion.

JR WEIGLE’S FRAMESAVER is probably the cheapest and available from Ceeway and others.
DINITROL FRAMESAVER is available through Bob Jackson Cycles.
I have also used CORROSION X, which is a very good penetrating oil so beware of leaving to much of it on screw threads as it encourages loosening.

Dinitrol_500 Frames3new-corrosionx

3 Comments

General Strike

about 4 years ago

Bob Jackson always made a big thing out of not needing to drill vent holes in stays because they were silver brazing at a much lower temperature. I don't know if that approach was extended to the rest of the frame.

Reply

Stephen Hilton

about 4 years ago

I can believe you may be right with that assertion. I have got away with brazing dropouts etc without vents, often when I forgot to drill them. I think it is easier with silver because of the lower temperature (and probably superior capillary flow) as it is also less troublesome fillet brazing without vents than lug brazing, but in my experience things go more smoothly with vents. Especially with narrow tubes where I have experienced huge suction effects which can be alarming watching large amounts of expensive silver disappear. For the novice I feel sure venting the tubes properly is the best plan but I think an experienced framebuilder can use their skill to mostly get around it.

Reply

Richard Wilkin

about 2 years ago

I'm researching the minimum size for a drain hole that drains freely (i.e. no capillary bridging due to surface tension) for a treehouse. Thanks for sharing the findings from your scenario.

Reply

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