Drilling a hole in a seatpost

Converting my cross/gravel/gnarmac/whatevs bike to dyno lighting. Normally I use mudguard-mounted lights but I’m not going to do that in this case, I’ll be using a seatpost mounted rear light.

I can route the wire internally down to the BB and then I’ve got the choice of either taking it up the outside of the seat tube or making a modification to a seatpost to bring the cable out near the light, which would mean the only exposed wiring would be on the fork leg. I quite fancy a go at the latter, and I have a choice of seatposts I could commit to this (BBB Skyscraper, Raceface XY, Salsa Shaft, possibly others, all 27.2).

Before I end up regretting it, anyone done this?

Not massively sure whether drilling the head or the shaft would be wiser (obviously depends on which post I use). I guess it’s going to need a 3-4mm hole. I have a fair amount of exposed post to play with when deciding where to put it (see below) and I’m guessing that once you’re a couple of inches away from the head or the frame the stresses won’t be problematic… hopefully 🙂

The other option is a hole in the frame at the back of the seat tube, but that seems a bit too bold.

(And yes, I know that at some point in the future I’ll remove my seatpost, snap the wire, and end up swearing like a docker for half an hour. So be it.)

I’d drill the head. There’s a lot more material there and it won;t be flexing and using the hole as a point to create stress risers like the tube of the post would.

(I am not an engineer)

How about running a plastic shim to insulate the seatpost from the frame, then just connecting the wire to the seatpost so that the seatpost is part of the circuit? Of course, you will need an earth wire from the light back to the frame, but that could just go down to the seatclamp.

Or just using a shim and running the wire through the gap in the shim?

In the very top would be the best bet as mentioned above. No way would I be putting a hole in the shaft part unless you want your post to end up in your bum!

You will want to seal it with some silicone though as water splashing up to the bottom your eat will go down and increase the amount getting into your BB area.

why do you need wires? What is wrong with a light with batteries built in?

Given that most two bolt seat posts have the clamp section pressed onto the tube, is there a post that you could simply thread the cable out of the top of around the clamp with no drilling?

why do you need wires? What is wrong with a light with batteries built in?

Why not drive a car? They have headlights built in and you don’t have to get cold or do any pedalling

In through the top of the head. Lots of posts already either have a hole there or a thin section. Anything on the shaft will eventually cause an issue. A well finished, deburred hole in the head will make no difference.<span style=”font-size: 12.8px;”> </span><span style=”font-size: 12.8px;”>(i am an engineer)</span><span style=”font-size: 12.8px;”> </span>

Just have to make sure whatever adjustment system you have doesn’t clash with the cable.

Also could have a look at an in line connector to release if you do something daft…….

Probably worth having a peer up inside each post to see what the head looks like from the inside.

How about  just connecting the wire to the seatpost so that the seatpost is part of the circuit?

Because I don’t trust electricity and, at speed, dynohubs put out lots of volts which I don’t want injected into my thighs 🙂

Or just using a shim and running the wire through the gap in the shim?

Yeah, nice idea, would work great, but it’s a 27.2 frame so that would probably mean using an old 25.0 USE post. (Which I happen to have, but a. it’s silver, b. it has no layback, and c. 14 stone vs USE seatpost has not historically been an entirely happy affair.)

why do you need wires? What is wrong with a light with batteries built in?

Thanks for that 🙂

In through the top of the head. Lots of posts already either have a hole there or a thin section.

Mm, unfortunately none of the posts I have do 🙂 I think the XY’s head will be impractical to drill. The Skyscraper might be viable, I’ll need to dismantle it to see, but it means losing layback, which isn’t ideal. I think I looked at the Shaft and decided it would be trick to drill, but I didn’t fully dismantle it to check.

Hey, how bad can it be? 🙂

My ringpiece clenched in horror at those pictures.

I’d take the view that a mountain bike post, at your body weight, will be fine with a hole drilled in it.   In the unlikely event it did fail it’s unlikely to kill you.

I would have thought it was a small risk especially as some posts are thicker at the front and back. The traditional holes for lightness approach does not always end well. I would put a grommet or a blob of silicon sealer in to avoid chaffage.

Yeah, same here Simon. Clearly it depends on the profile of the shaft, where in the shaft you put the hole, what diameter the hole is, and how well you finish it, but I’m inclined to think that in most cases it’s possible to do it thoughtfully without realistically compromising the longevity of the post. (And in any case even an aluminium post won’t fail catastrophically without a visible and/or audible crack propagating for a little while first.)

in the spirit of not not answering your question…. is there an internal port on the chainstay for rear mech cabling? (I think I can see a blue port cover…). then run a light on the rear seat stay.

A friend put a small nick in his seatpost to mark the correct position. 6 months latter it snapped white he was out riding

is there an internal port on the chainstay for rear mech cabling? (I think I can see a blue port cover…). then run a light on the rear seat stay.

No, sorry, it’s just some blue graphics 🙂 Seatstay mounted lights suck anyway: they end up obscured by the wheel from lots of directions.

A friend put a small nick in his seatpost to mark the correct position. 6 months latter it snapped white he was out riding

Yeah, obviously. That’s because he put a massive stress riser right at the point of greatest stress. (The smaller the radius of a fracture, the greater the stress riser, and a small nick creates an extremely small radius at the bottom of it.)

Drilling a smoothly finished, sufficiently large hole away from highly stressed areas is a totally different proposition.

Some Taiwanese post manufacturers have posts with holes in the head for e-bike light wiring so there will be some ready-made options available before long.

Yeah, seen those. Hoping to avoid a new post when I’ve already got a good five or six to choose from, though 🙂

I’ve got two Skyscrapers kicking about so, provided they don’t put my seat too far forward, I think I’ll check the wall thickness of those (and whether the head looks like an option) and might have a go at perforating one of them.

Hole right at the top of the post. Leverage is the lowest there.  Post actually needs far less material there but they make them as straight tubes so they’re the same all along.  Front or back of the post, depends on your appetite for a wet BB vs an extra bit of wire.  Not that I think water in the seat tube is an issue, if you grease your bb on insertion, the plastic tube thingy on a BB30 keeps water out.

Add a connector and enough slack to expose it when you remove the post.  That way when you whip the post out you’ll just pop the connector and be able to pop it back on.

I’m not too concerned about water ingress: I’ve got mudguards and between the saddle bag and one or two lights it’ll be well out of the firing line.

Internal connector is a good call, though, mainly because it’ll make threading the cable trivial 🙂

(It’s not BB30, though. Good old threaded FTW.)

Just seen this useful video about how to get a cable thru a frame

How To: Thread Internal Cables Without the Headache

Hole right at the top of the post. Leverage is the lowest there.

I was thinking along those lines – good point.

A friend put a small nick in his seatpost to mark the correct position. 6 months latter it snapped white he was out

I’ve done this on my brompton but went for a wider gently filed patch.  Not sure if that’s a better idea or not.

Yes, it is. Plus a Brompton post is steel rather than aluminium so will fare better, all else being equal. (Anodized aluminium is especially bad, because near the surface the material is more brittle.)

To be extra safe, have a go with the finest wet-and-dry you can, and then finish off with some Brasso.

I knocked out the boss for the water bottle mount on mine, which is just the perfect sized hole ie you won’t die.

Plus a Brompton post is steel rather than aluminium

or Titanium 😉

Touché 🙂

I’d try and do something like this if you can…

Guessing that a di2 cable in the pic and seems quite a common way to fit a junction box under the saddle.

That would be my choice of hole

Guessing that a di2 cable in the pic

Nope. It’s a light cable.

Is there a stress-relieving hole at the top of the seat tube, below the seat post clamp?

I’d drill through the seat post there where it’s supported and bring the cable through the frame. Leave some slack cable and the seat post can be removed using the slot in the frame to allow the cable to be freed

A bit of slack in the cable would allow you to remove and remake the terminals at the light if necessary for fettling, consider an inline connector inside, or a short length of cable to bridge the seat post wall with an inline connector inside and outside

IANAEngineer 🙂

Is there a stress-relieving hole at the top of the seat tube, below the seat post clamp?

That’s pretty much the point of highest stress on the seatpost, although it is in compression, not tension. Also, it would be a pain in the arse to fit the seatpost and cable if you drill there and the wire is liable to get damaged if you are careless.

Right then. Decided I didn’t want to drill the XY that’s currently fitted, so offered up the Slyscraper. Not enough layback.

The Shaft on the MTB does have enough, though, so I took the saddle off and cleared away the pile of crud on the head. Found a rubber bung covering… a hole!

So the Shaft goes on the cross bike for easy dyno routing, the MTB gets a Skyscraper because it doesn’t need layback, and I can put my black XY on the tourer which currently has an incongruous-looking silver one.

I’m winning! For once it’s all going my way! 😀

And, for the record, the Skyscraper also has a handy orifice which will take a cable.

Shermer75 – it was a valid question. I was curious as to why an all in one rear light was not acceptable as a rechargeable one lasts ages. Front light I can understand as they generally use more power. No need to be a bellend.

AndyL – battery lights are cheap, bright, and whether last a decent amount of time but still end up failing at an inconvenient moment.  Once you’ve run decent dyne lighting and no longer have to think about it hard to imagine going back.

OK. So, with dynamo lighting:

– I never need to charge anything, or use single-use cells

– I never get caught out by forgetting to charge something

– The bike is always ready to go whatever the light/weather

– I never have to make the decision of “does the light/weather/etc now warrant switching lights on”?

– I never have to wonder whether a battery will last for the whole of a big ride, either when planning one or having battery anxiety during one

It basically means that lights are something that I can completely forget about. I like forgetting about things while I’m on a bike.

I do normally end up having a battery powered second light, either because I’m using a Fly 6 or because I just want the redundancy, but again having endless dyno power means I never have to think about anything.