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I want to route a dynamo light wire internally from the front of the frame to the rear light. The wire enters the frame at the top of the down tube, runs around the bottom bracket, and up the seat tube. Now it just needs a way out of the frame.

Would it be safe to drill a small (say, 3 mm) hole in the seat post just above the clamp? If so, are there any precautions or considerations I should take while doing so? Are there any seat post materials that would be particularly (un)suited to this?

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    I'd wager thick-walled (esp. vintage) steel seat posts would be fine with a 3mm hole, though I'm no material scientist, it might depend on the loads exerted. I wouldn't let a 100 kg man ride downhill on one such bicycle. Question: How do you plan to account for seatpost adjustments?
    – jayded-bee
    Aug 24, 2022 at 7:22
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    I'm 70 kg and the bike in question is a gravel bike. Seatpost adjustments: I'd drill the hole high enough that I have a bit of room for adjustment when necessary, but I'd forfeit the option of pushing it all the way down into the frame. Aug 24, 2022 at 7:32
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    Hm, is routing the cable through the hole at the top of the seatpost not an option? That way you can be sure the post will remain structurally sound, and the adjustment range isn't limited.
    – jayded-bee
    Aug 24, 2022 at 7:38
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    @Michael I'm not sure if it would go down to a 3mm hole but there are hook type deburring tools that will do internal burrs
    – Chris H
    Aug 24, 2022 at 16:00
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    There is at least one seatpost with a hole for a cable.
    – Adam Rice
    Aug 24, 2022 at 17:24

6 Answers 6

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I suggest to avoid drilling holes in the seatpost alltogether. Just find a seatpost that has an opening at the top, there are plenty available usually on older seatposts with two-bolt mounting system. From safety and point of view you want your tail light to be as high as possible, so that drivers are more likely to notice you. Havint the wire go out the top of the seat tube works out in your favor.

I have one in the garage so took a few pictures for you:

Completely disassabled: enter image description here

With the mounting clamp in place. Zip tie added to indicate where the wire would go: enter image description here

With the saddle in place. Note the zip tie is still not jammed: enter image description here

Go to your local bike shop (LBS) and check what they have in stock, I bet you will find a seatpost that works for you.

Things to consider:

  • You will need to secure the cable inside so that it does not rattle while you ride
  • If you frequently adjust the saddle height (two riders one bike), then this setup will become an issue

Additional reading on the subject:

https://singletrackworld.com/forum/topic/drilling-a-hole-in-a-seatpost/

https://electricbikereview.com/forums/threads/bodyfloat-specialized-turbo-x-seat-post-drilling-for-rear-light-wire.4032/

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    "you want your tail light to be as high as possible". No, you want your tail light to be as unobstructed as possible. The OP mentioned a big saddlebag, but I've seen plenty mounted high on a seatpost and completely useless because the rider's coat blocked the light, or a rear rack blocks it from some angles high on a short seatpost
    – Chris H
    Aug 24, 2022 at 20:27
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    You are both right. As high as possible while still being unobstructed as possible.
    – Ted Hohl
    Aug 24, 2022 at 20:57
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    What is local LBS? LandesBauSparkasse? Liverpool Business School? London Borough of Sutton? Lyndon B. Johnson?
    – gerrit
    Aug 25, 2022 at 7:44
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    @gerrit "Local Bike Shop" Aug 25, 2022 at 8:08
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    @TedHohl No. Being unobstructed is a must, height is completely secondary.
    – Nobody
    Aug 25, 2022 at 11:45
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I would consider this unsafe. Even bottle cage screw holes on bike frames are reinforced, even though bike frame is a redundant structure (if one tube fails, the rest will allow safe temporary operation of the frame).

If you want to nevertheless drill that hole and make the hole as safe as possible, I would:

  • Select the heaviest (per unit length -- a short seatpost could be better if it's slightly lighter than a very long but slightly heavier seatpost) seatpost you can find that has walls of non-constant thickness (thick rear and front, thin on the sides)
  • Select a seatpost that's shiny aluminum, not anodized (anodizing decreases material fatigue life)
  • Never drill that hole in front of the seatpost, only on the back or on side, as stresses on the front are tensile and stresses on the back are compressive so front is the least safe location
  • Drill that hole as high as you can, not directly above the seatpost clamp but rather very close to the saddle clamp, as stresses are higher the closer you are to seatpost clamp
  • Investigate the surroundings of the hole often for beginning cracks with a flashlight, let's say every time you refill your tubes with air. If you see any beginning cracks, stop using the seatpost immediately

Considering how difficult it's to do that safely, I would prefer external cable routing. All it takes is few zip ties, and never causes any safety problems.

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    Excellent point about the bottle cage mounts - they are a separate part and work like a hollow rivet. So it should be possible to drill a hole and then put a rivnut through and then run the wire through the rivnut.
    – Criggie
    Aug 24, 2022 at 10:57
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    "[...] bike frame is a redundant structure (if one tube fails, the rest will allow safe temporary operation of the frame)." Is that true? Sounds very unlikely to me. I know I wouldn't trust a frame with a broken tube longer than the time it takes to brake to a standstill. Am I too careful? Aug 24, 2022 at 16:20
  • A frame with a broken tube can be ridden carefully home, but shouldn't be used permanently.
    – juhist
    Aug 24, 2022 at 17:34
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    Bottle mounts are meaty moreso to provide sufficient thread to engage the bottle mount screws, when used in situations like exit points for internal routing, they are barely thicker than the tubing around them. Aug 24, 2022 at 19:50
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    Bottle cage screws can be simply rivnuts, which don't reinforce the frame, just the screw thread. They're not always brazed on
    – Chris H
    Aug 25, 2022 at 7:32
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Safety is a scale with completely unsafe on one end and completely safe on the other end. Just riding a bike is somewhere between completely unsafe and completely safe.

Not drilling a hole is absolutely safer than drilling a hole.

How much less safe is drilling a hole? The only answer that can be given is - it depends.

Here is an incomplete list of variables:

  • Hole size
  • Hole location
  • Seat post material
  • Seat post thickness
  • Riding style

You could provide yourself some hard data to determine if drilling a hole would be in your comfort zone would be to do some carefully measured and logged destructive testing.

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    This is true, but seems to add no insight into the relative danger of the proposed hole. Is that your point, that you have zero idea whether it's very unsafe or a pretty minor thing, without being a materials scientist and knowing how to test material strength, and actually doing it on a copy of that bike or related? Aug 25, 2022 at 14:28
  • @PeterCordes My first point is that "safe" is relative. My second point is that drilling a hole is relatively less safe than not drilling a hole. My third point is that there are a variety of variables that influence how much less safe drilling a hole is - a few of them are even listed. My fourth point is that the only way to quantify how much less safe drilling a hole is would be to test and measure. You don't have to be a materials scientist to drill holes in a seat post and then see how hard it is to break it.
    – David D
    Aug 25, 2022 at 15:45
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If there is room and the seatpost/seat interface would allow it, drilling the hole through the top of the seatpost would avoid all the potential stress riser failure threats of drilling through the shaft of the seatpost.

This approach is not without threat either and may not be achievable based on how the seatpost top/seat rail interface is formed, but I wanted to throw this perspective out there for consideration.

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    Or even better byuing a seatpost with an opening at the top. (Yes they exist). Nothing to drill, no safety to compromise Aug 24, 2022 at 16:52
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    @ArtGertner - I was going to suggest that you post your comment/idea as an answer, but I see you did already. Could be the ideal solution to avoid the issue of weakening the seatpost altogether.
    – Ted Hohl
    Aug 24, 2022 at 17:32
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I'm not sure if this idea fits your aesthetic vision, but personally I'd be tempted to add a hole to the BB shell and have the wire come out there. You could then just route it up the outside of the seat tube. Generally speaking, the BB area is very hefty and an extra hole won't hurt. Evidence: it's seen as safe to add a hole for water drainage purposes.

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    Often there isn't much clearance inside the shell. Aug 24, 2022 at 21:24
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    @DanielRHicks Yeah it would heavily depend on the exact frame + BB configuration used.
    – MaplePanda
    Aug 24, 2022 at 21:42
  • Just to be negative, the goal of the question was to internally route the cable from the front of the bike to the back. Out at the BB and up the seat post seems to serve very little advantage over simple zip ties around the top tube and thus saving the entire argument of drilling the frame.
    – Hursey
    Aug 30, 2022 at 4:22
  • @Hursey I understand your concern and it's 100% valid. I thought I'd leave the value judgement to OP--one could also argue that this entire internal routing scheme is unnecessary.
    – MaplePanda
    Aug 30, 2022 at 5:38
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I remember a seat post test in a magazine where the posts which structurally failed during the test (i.e., broke) all had an embossed logo.

So no, compromising the structure in any way is not safe.

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