Ok. Here are some pics with my seatpost details. See what you think:


Preliminary bike - wheels not the final ones. Bar ends not the final ones.


Detail of the seatpost (Ritchey Pro alloy 31.6mm x 400mm) with shim. It shows the shim extends well past the joint in the 2 frame parts.


Shim construction: Made from an identical Xootr Swift seat post (taken from wife's Dahon Yeah, she got the Swift post which has a better seat clamp), sawn off at the bottom and the inside widened on a lathe to match the Ritchey seat post. Instead of making a conventional shim which usually have just a slot, this one is like a mini seat tube with short slot and stress relief hole. The location of these match those on the frame. I made a ring for the top to prevent the shim from sliding right into the seat tube.


Here you see the stampings on the original seatpost.


The Ritchey post fits very tightly into the shim - in fact it can't go much deeper, it becomes too tight. So it is a tighter fit than what you would normally find.

With this construction, the bottom QR can be left fastened, and just the top one needs to be loosened to adjust seat height. For folding, undo both QRs to extract post plus shim. Additionally, because this shim has no slot where the bottom QR clamp is located, the shim won't collapse when the QR is fastened.

The only thing I am uncertain about is the shear stress exerted on the shim by the loaded bike. The Ritchey seatpost does not extend past the frame joint so the shim is the only part providing strength here. The shim wall is 1.2mm thick.

Knowing it's not convenient, but wouldn't you rather get an updated frame before you really start test-riding it?

Not sure what you mean about an updated frame...? My frame is brand new.

Considering the previously posted seatpost snappin', I recommend you shoot off an email to Xootr, with these pics, and make sure it's within spec.

By the way, you went with the 3rd party seatpost because...? Or "just because."

P.S. level out ye olde saddle.

I meant if you have this latest frame:

This thread is about making a Swift lighter, so that is why I'm using a different one. The new combination lops off several 100g off the original config. It all adds up. I think I will quite easily come in under 10kg when I'm done.

I have sent a mail to Peter Reich about this, and am making the calclations to work out the shear force and seeing if the alloy is in spec by a good margin. These calcs I will post here too for criticism.

The saddle is indeed looking a bit uppity. Will have to fix that before the next photo session.

Ah. I see. Yeah that's why I asked kb5ql if he could take a measurement of the tubes.

Right, I'm happy that the seatpost shim won't shear off. I calculated it should take a load of about 212kg on the back wheel axle to shear the shim off, assuming perfect "cutting jaws" of the 2 frame tubes butting up against each other. With imperfect "jaws", that force will be even higher. This also assumes there is no give in any other part of the frame, so it is a worst case analysis.

I assumed the seat post is 6061 T6 alloy. Even if it isn't, the shear force should be rougly similar.

I am about 80kg, including all my stuff, so I would have to jump a kerb to reach the 212kg load on the back wheel. I will need to put about at least 4g on the back wheel to reach that value. I guess that won't happen in normal riding, even including the odd jumped kerb or pothole.

Here you go. I just finished putting the bike back together. I got the longest seatpost from Xootr. Weight from Dahon seatpost (~500g) compared to Xootr seatpost (~700g). I gained 200g, but at least the bike won't break in 2...

Original Tubing



New Frame Tubing



Reassembled bike

So they went from approx 3 to 5mm wall thickness on the seat tube.

Did the seat stay and chain stay outside diameters also change?

I'm not sure. They appear (on the outside) to be about the same. I don't have calipers like our friend james_swift to confirm...

I am happy they have increased the wall thickness. It seemed a little thin at the quick-release clamp area.

Which water bottle doohickey is that? I know the similar Profile model, but it mounts on the seatpost, and I don't believe it fits the Swift.

Jack

Tacx Saddle mount. Minoura makes one as well.

There is an available longer stempost if you need it. Yangmusa got it direct from Peter. PM yangmusa for details if interested.

I flipped the stem (7 deg/130mm) and now it is just about perfect.

Thanks for the tip! I ordered one and installed it over the weekend--I like it very much, especially since I can still use my seatbag, which holds tools, tube, etc.

Of course it's made my Swift heavier, since now I'm carrying two cages and twice as much water. But the location is much better than the stem riser setup I had before.

Jack

Can anybody point me to a site where to buy 50g 20"x1.35 tubes?

My Swift is nearing completion, I am still waiting for a few items to come in the mail. Currently it is 9.5kg. I rode it to work today an all I can say is wow! it is fast. I have a 58T ring in the front, it is blazingly obvious I will need the 60T one since I am riding much faster.

Once I have done everything on the plan, it may squeeze in below 9kg.

Photos and full description/specs to follow on completion.

Hmm.

https://www.gaerlan.com/bikeparts/par.../wheelbmx.html

That's where I get my 20 x 1.35 for my Marathon Slicks. Too bad they weigh 110g (3.8 oz). Thanks for bringing that extra weight to my attention Jur...

@Jur - Scroll down - 65g but expensive. Nice people to deal with; I get my Scorchers from them...

https://www.wrhpv.com/small_stuff/tubes.html

Ouch! I'd pay if postage was OK. Alas...

I paid $12.50 for my normal 100g Schwalbe PV tubes just this past week. Bike parts are so horrendously expensive here in Oz.

And just a few days ago I saw on a web page a 53g 20" tube (can't remember price), but damned if I can find it again.

Ha!

https://www.cambriabike.com/shopexd.asp?id=27334

Good price! 20/1.75 though - might be a bit "baggy" in a 1.35?

Jur - where did you get a 58t chain ring? Are you using that ring with the stock cranks?

I bought a bare frame from Peter, so no stock cranks... the chainring came from Loosescrews, they are in fact having a sale on them now, still on, I think. You can get the same ones from Gaerlan.

If the pic is correct, then I am estimating it will be fine, it looks much narrower than 1.75". Maybe that's the maximum what you could use it for, rather than the typical.