I am a retail bike shop lifer (actually recently “retired” off the clock after 45 years). I also have been brazing bicycle frames nearly as long. Now with too much time on my hands I return to those dreams one has when the day job gets boring, like make a unicycle.
As I reacquaint myself with current specs and dimensions I am wondering about seat post clamping and it’s twisting when dropped (which will happen a lot initially). Back in the day the 24" one I had (dirt common Asian made chromed with cottered cranks, likely wholesaled for $45 in 1979) suffered from the post twisting off center. No amount of clamping bolt torque stopped this so I did the usual drill a hole and run a bolt through the post and frame.
Now I find much larger in diameter posts and in alloy and will build with this config. But the memory of twisting posts remains.
Is this still a problem with current stuff?
Any suggestions to reduce this, other than a through bolt?
I plan to use my normal two wheel bike fillet brazing with slightly thicker walled tubing (than a bike usually has). There’s a lot more I could talk/ask about but this is enough for now. Andy
Twisting seat is sometimes a problem, but with the two or even three bolt seat clamps it only happens with a really solid thwacking of the seat, usually accompanied by a unicyclist running for their life. On my 20” that I built up for tricks and whatnot I only have a single quick release on it, and sometimes my seat twists, but that thing falls so much I like that it can twist to absorb some of the impact and not transfer it to the frame. Since I added a handlebar to the seat on my 29er it twists a little more easily now, but that’s probably better than sending that force to the junction of tubes at the crown of the frame the way a through bolt would.
The Mad4One three bolt clamp is probably the most secure clamp available. The top screw/clamp is fully on the seatpost, and the bottom two-screws/clamp is on the frame as normal.
These are really very nice, however they are only available for 27.2mm seatposts paired with a 31.8mm frame tube. That might be an issue for a steel frame depending on what tube sizes and wall thicknesses you can get.
For interest, here is also a poll from not long ago on what seatpost clamps people use:
With a good fitting seattube, I haven’t had too many issues, even on a single bolt clamp and 27.2 mm seatposts. When you get unlucky and get a slightly undersized seatpost and oversized seattube, then it can still be a bit more annoying. Since (presumably) you are going to be the one reaming the seattube, I doubt you require anything as drastic as a through bolt or triple clamp.
I’ll be excited to see your work, I would love to build a fillet brazed 29" frame myself, but it’s going to be few years until I have the money to get the tools required.
I have answered my own question about the brazing Vs welding. While I am familiar and have experience with mig and Tig welding I was not aware that fillet brazing it is a preferred method by some frame builders.
Wow! Nice response and no flames. I have attended some email lists and forums with some members who view newbies as fresh bait. Let me go down the replies and give more info and hints to me.
Unigoof- These multi bolt clamps are quite familiar pretty common in the FreeStyle world 9or at least were). While I consider them to be a step up from the cheap tacked on sheet formed bands (that look like a lug) they are not my cup of tea. More further down.
Bug72- If I had a hammer I’d Hammer nails… My hammer is the Ox/Ac torch. Were I to be starting this frame fab stuff again I would most certainly get comfy with a Tig set up. There have been enough studies that show each steel joining method (weld, bronze fillet and Silver) will fail at similar loads but at different locations WRT the joint.
finnspin- Agreed that a good fitting seat tube ID and post OD is right up there. One reason why I like making my own frames is that I am the one controlling every tolerance.
As I mentioned I’m at the very start of the fabbing process. I know my materials, have it all on hand. The components are being gathered, a pause for the holidays. The frame design has moved from mental floss to include pen on paper. I have had a pro level BikeCad program for many years and if it offered a unicycle option I would be doing this on BikeCad. For now I am thinking of an offset seat tube. This will allow an easy to make lower post binder bolt, a second clamp at the seat tubes bottom end. The upper clamp will be the same style, I like to braze on a self made clamp then use a replaceable, my roadie back round… Here’s my Flicker site link. If any of you still look at still photos… Andy
I’m a goldsmith and live and die by the torch. My torch runs 10 hrs a day 6 days a week soldering gold and silver as well welding platinum. I also Tig weld stainless, platinum, gold and silver with an Orion Tig welder. I am experienced In mig welding steel for automotive restoration. I do some brazing on old auto parts, mostly things not under load and those being chrome plated. I have seen Corbin Dunn braze handle bars and wondered if they would be strong enough under load. Based on following Corbin I can see his brazed parts hold up. I was just completely unaware bike frames are brazed instead of welding… live and learn.