unterhausen

Yes, Milan Sanremo has me thinking about droppers. I don't think that's too much of a spoiler, go watch the GCN highlights if you don't know what I'm talking about.

Some time ago I rode a 7 mile downhill on my gravel bike, and really wished I had a dropper. It's not a steep downhill, but it's bumpy. I like that route, but the downhill has me avoiding it.

I'm really, really soon now (can I trademark this?) going to build a new gravel bike frame with lugs. And the only seatpost that works takes a 30.6 post, which is inconvenient even with a solid seat post. So I had the bright idea of modifying the lugs. I went and looked at seat tubes again, thinking about turning down the external butting so it's close to the lugs ID. But even the ones made for 30.9 posts look like they wouldn't work. It's only .3mm on the id. But the OD's I'm finding are a millimeter bigger.

I'm just typing this out so I can do some measurements later.

guy153

Since you have a lathe could maybe make a custom dropper. Just thinking outside of the box here.

unterhausen

I definitely have thought about turning down a dropper. Maybe the next time the bike shop throws one out, I'll ask them to save it for me so I can look and see if there is anything I can do to the post. Dropper post companies have failed to make good posts, so I'm not sure I want to mess with that rabbit hole.

I think I could modify the seat lug without issue, but I'm worried about the bb shell.

dsaul

You can get 27.2 droppers and use a shim.

unterhausen

True, but I can't get used to the idea

himespau 

Can you permanently affix the shim into the tube so you can think of it not as a shim but "extra butting"?

unterhausen

You will occasionally see evidence that builders do that. Usually in the form of a complaint though.

79pmooney

The builder of my bikes thinks nothing of shimming his ti bikes down to 27.2. I never think about it.

obrentharris 

Didn't Cinelli shim down their seat tubes to 26.2mm? At least you have time-honored tradition on your side!
Brent

unterhausen

I imagine the top of your ti bikes seat tubes are flat. I'm building a lugged bike, so it's anything but flat. If I was building a bike without lugs, I would put a dropper specific seat tube on it and be done without any thought.

degull

how about creating your own bilaminate lugs?

Darth Lefty 

Clearly the problem is the lugs, not the dropper. “I want to put lugs on my dropper frame, what lugs can I use?”

PVD is now building his “all road” bikes for 210 droppers so he can corner like motoGP

unterhausen

The seat lug is no problem modifying or making my own, but I would like to use the bb shell.

scubaman

Wow, that seems really scary - at least if any of the lower part of a dropper post were to extend above the seat clamp and thus be directly weight-bearing. But I'm not an engineer.

unterhausen

It is probably a bad idea. But that's why I only said I was thinking about it and also I wanted to get a dead one from the bike shop to see how much wall thickness there is.

79pmooney

Suppose you ran the seatube to just short of the lowest point of the lug top. Make a shim with the upper portion the lug ID and simply run it past the high point of the lug then have an attractive machined radius or taper to the seatpost. This shim could be used with any type of lug or lug shaping and styling in the future so you could make a few while you have the lathe set up and easily offer dropper post bikes with that seat tube diameter in the future.

duanedr 

Are you using one of the 33mm externally butted ST? Making the seat cluster lug is the easy part. I like a sleeve there anyway and have found that HT material from the top of the tapered HT's works for the sleeve on the 33mm section if you lightly bore it on a lathe (increase ID by .002-.004" - YMMV). Then I would cut the ST socket from BB and do a sleeve treatment to match the DT socket (bi-lam style). It's going to be a tricky miter/fit up (it would be for me!). If the BB shell is one of the double oversized, you may not have much room on the shelf of the shell for the ST to sit against once you remove the socket. What size are you using for the DT?

unterhausen

As you say, the bottom bracket is the real problem. I don't have a seat tube that will fit a dropper yet, I'm trying to figure out how feasible this is.
I was looking at the 31.1 ID Zona tube https://framebuildersupply.com/colle...5-9-length-550
I assume that fits a 30.9 dropper?

What I currently have is a non-dropper zona tube https://framebuildersupply.com/colle...8-6-length-635


I had a vision in a dream last night that sectioning the seat lug/bb shell and welding them back together is the best way to do this.