I want to build a simple PVC system to which I can attach a few tail lights. The best place, I think, on my bike to mount it is on the back of my Topeak rack. There is a little plate with two screw holes that will be easy to attach to (see left side of picture below).

My question is, if I drill a hole straight threw a 1" PVC pipe and use that to screw onto my rack, how likely is it that the PVC will fail where the holes are?

can you use a washer to help w/ stress...

True. Although, wouldn't it still make about the same amount of contact with the PVC as the screw would?

I'm an irrigation specialist for a local community college. I think the PVC is very durable stuff if you're talking about the commercial grade versions. Make sure you ask for 200 PSI rated pipe at whatever diameter you select....it's significantly thicker walled (and they do make it in 1"). If they don't have it at the place you want to buy it, then try somewhere else until you find it. You might even have to go to a commercial wholesale irrigation store to get some (most places will let the layperson still purchase).

The only drawback might be the inside diameter....the thicker walls are sacrificed by the ID because the fittings are measured to fit the OD.

One more suggestion....make sure you pre drill to a size not too much less than your screw diameter, but definitely a little smaller so as to make it a snug fit. If your screw is sloppy in the PVC part of your project....it might start working out the hole even larger from constant flexing.

Thanks for the tips! I think some schedule 80 might do? At least for the part where it attaches to the rack.

I also thought maybe I could add a coupling to the mouting points in order to increase the thickness of the pipe.

I'm not too worried about ID... I just need something to immitate seatposts so I can mount my super flashes to them.

There are curved washers to help with the contact area.

And I'd think about putting a spacer /riser inside the tubing so you're not squishing the tubing while tightening the screws

Even cheap, thinwall PVC is plenty strong to run a screw or two through like you're talking about. No need to use the expensive stuff unless you just want to. If you were building an entire rack like that I might use sch40 but for just a talil light mount on an existing rack like that the cheap stuff will be fine...

It's very durable stuff.

Schedule 80 is way overkill....and much, much heavier. You can still use schedule 40, just use the class 200. I have seen the class 160 split easier than the 200 under stress. You might say, "what stress?" Well....who knows. It depends on the kinds of roads/trails you run. Also, sunlight has a long term detrimental effect on PVC....and in that case you might want something thicker too. Why not spend a little more for the initial investment for potential longer term durability? It's not like it's 10 times more expensive. Wait, now that I think about it.....I wonder if they even sell it in 1" in a class 160 anymore? You might not have a choice. We always get class 200 for irrigation purposes regardless. Go put two sticks of them together and you'll see the difference.

You don’t need to drill a hole in the pipe. Just purchase a PVC pipe coupler that is about the size of a seat post, 1/4-20 nut, two 1/4" fender washers, a 1/4-20 bolt long enough to go through the pipe coupler, fender washers, and rack reflector tab.
Assemble the pipe coupler with a fender washer on each end and insert the 14-20 bolt through the washers, coupler, and rack tab. Tighten the 1/4-20 nut so everything is firmly held in place. Install the taillight on the pipe coupler just as you would clamp it on the seat post.

Why not insert a wooden dowel inside the pvc tube to make it crush proof?

Or use a piece of aluminum or steel tube instead of PVC?

I'm actually going to get a small "system" going to support at least two lights. My plan is to have one vertical pipe for each light.

So, that kind of foils the coupler idea. Although, I thought about using some sort of coupling that would normally be used on a metallic electrical junction box... not sure if it would work yet.

The PVC will probably outlast the screws - they'll rust out before the PVC breaks. It's very durable stuff.

Sweet. And I'll be able to keep an eye on the screws because I'll be swapping it back and forth between bikes.

I don't know what kind of rack I have. I bought it from the LBS and they installed it.
Anyway, that little plate broke off and took a blinky with it.

PVC can become brittle in the cold and after UV exposure.
Eric

I think i'm going to use copper instead of PVC. PVC thicker than schedule 40, particularly the fittings, is more trouble to get, and even schedule 80 may not be as reliable in cold weather.

Look at an Ace Hardware for thin walled 1" aluminum tube. They list them on their website. Other hardware stores also carry it (my local one does).

No, it doesn't. It'll be fine. If you're worried about UV, paint it with Krylon spray paint.

Don't use copper - it corrodes when exposed to the elements. PVC doesn't corrode when exposed to the elements (which is why it's used outside!)

Copper is also flexible so it'll bend if you hit it on something. It's also more expensive.

Copper = bad

PVC = good

I would agree about not using the copper. The corroding can be pretty, but the problem you will face is where it touches other metals (the rack, the screws you use, etc). The interface between the two metals will result in a galvanic reaction that will cause "corrosion" at a much higher rate. Thus you need copper or materails that have a sacrificial zinc galvanization to protect them from the reaction.

PVC or that old stand by wood maybe more cost effective and easier. The loads you apply to the pvc are trivial so it may not matter if it gets brittle after a year or so. The cost of replacement is so low even if it does happen to break, replacement is not an issue. But, pvc will break easier on a cold day with an impact and most scheduled pvc are designed to be covered with dirt or are interior to protect them from UV. Maybe the grey conduit (unsure if it is actually pvc) would be a better choice if it is stabilized for UV.

Eric

Honestly, the aluminum pipe sounds like the best idea. It's almost as easy to work with as PVC, looks better and even though I think the PVC will last 10-15 years in this useage, the aluminum will last indefinitely. It;s pretty reasonable too in short pieces.

Thinking about it . Build it out of anything. Use it for awhile. You will decide you want it slightly differently. Build it again. Then you might be happy until just another improvement would make it better. A stick off the tree in your front yard would work well for the first prototypes. Just do not get hung up and not do it over a pvc, copper,or Al decision.
Eric

Yes, the grey conduit is PVC. I've used it on my own tail light and it worked great - and I don't treat it kindly, either.

Personally, I think you should use some carbon fiber tubing. It would provide a stiff but compliant support, and soak up any road buzz. Plus you would save a lot of weight and probably increase your speed by 1 mph. A second choice would be a titanium tube, but it might be difficult to drill holes in. Nix the aluminum -- too much vibration, it dents easily and can corrode if exposed to sweat or salt air near the coast. Steel is out due to rust issues, unless you can find some stainless steel tubing.

Just a thought, but there are tons of old handle bars out there to be had. Go to the LBS and see if they have a bent set of old drop bars lying around you can have. Cut out a straight section from the tops and you are good to go.

Steel would be best of all because it is repairable, man ;-)

Another source of Al tubes is old vacuum cleaners. check your local rubbish tip.