If you’re drilling a seat tube I’d consider:
- the lowest stress / strongest place to drill is the BB shell itself, so if you’re running a configuration where you can route around the bb and spindle (zero contact or you’ll wear the spindle, a centre sleeve to protect is probably a must), do it that way.
- mock up the bend. If it’s in a bad place you won’t be able to undrill.
- don’t drill opposite bottle cage bosses or anything else applying stress to frame.
- centre punch is a great start.
- low speed drill. A pilot hole with a 2mm will be a big help.
- 20-30mm from bottom of tube should be enough to avoid compromising but also still be in the thicker butt rather than the thin central part of the tube. Depending on your frame manufacturer they might actually have the tube spec which would be helpful. Knowing where the butts are is really helpful, I recommend it.
- if it’s steel, use some sugru / kintsuglue / silicone sealant once installed so you can keep water out of the tube. Some nail polish or paint is a good idea.
- if it’s not steel or Ti then it’s probably going to shorten it’s life span drilling it.
- AXS 🫤