troysmith80

I'm working on a design that is a little different than the norm. Basically a mini-velo but with a really low top tube and seat stays. The problem is that there will be a lot of unsupported seat tube and/or seatpost. How can i make this safe? Compounding that is the fact that the geometry is not nailed down yet, but the ST angle is likely to be in the neighborhood of 58 degrees, which will put more bending stress on the ST and post than would be the case at a more conventional 65-75 degree angle.

I know some of the folding bikes use special, oversized seat tubes, could i use those? This will be my first frame build so I don't have a lot of background knowledge so i'm trying to research as much as possible. Might a regular 30.9 post take the abuse? I'd really like to avoid raising the chainstays or top tube up higher if at all possible.

here is an image from google that is somewhat representative of my problem. I think this is a photoshop mockup and not an actual bike.

Scooper

The lower seatpost on my Brompton M6L folder is 31.8mm O.D. chromoly with a wall thickness of about 2.5mm. It slides into a very beefy 38.5mm O.D. chromoly seat tube that's about 240mm long. The upper seatpost that slides into the lower chromoly seatpost is a standard 26.6mm alloy post. This setup is strong enough to support my 200 pound carcass (now 175 pounds) without much flexing. This is just a datapoint for you. YMMV.

EDIT - The Brompton's STA is a pretty conventional 73.5°.

Here's a photo.

Live Wire 

That sounds like what Shiggy did here: https://forums.mtbr.com/frame-buildin...ke-463275.html
Cool way to get some extra sp extension.
BTW, a bike like the one pictured will ride/pedal like crap with a 58* sta.

Scooper

Yep; that's very similar.

Even with a very beefy steel seatpost (or extended seat tube), though, I'd be very concerned about the bending moment with a 58° STA. At 73.5° the rider weight vector is more vertical.

fietsbob

Bike Friday Manages .. , i have .5M between the base of the folding seat mast,

and the saddle rails..
a thud-buster seat post does absorb what the road bumps that
are often part the bending forces.. 1,25" OD steel, lower half,
then a 27.2 post inside it.

my 'heavy rider option', Bike Friday Pocket Llama frame,
is somewhat like that CAD picture..

i can dismount like a step thru..

MassiveD

What is the wall thickness of the seat boom on the BF, or for that mater the steering extension. I seem to recall it is 58 thou, which is pretty heavy, but I don't remember. I used to have parts sizes for the whole bike, but I lost them during a computer problem, anyone with any bike friday frame info can post it here or PM me, if you would be so kind.

troysmith80

Thanks for the data point scooper.

The reason my STA is so extreme is that i'm thinking of terminating the ST a few inches forward of the bottom bracket, on the DT, instead of at the BB like normal. This can be seen on Workcycles Fr8 and the Quixote/Clever Cycles Xtravois 2.0 among others. So the result is that the "effective" STA will be fairly conventional in terms of where the saddle is in relation to the BB, but the ST and seat post will have to endure a flatter angle.

I might like to use straight gauge 4130 large diameter tube (maybe 1.5"?) for the ST and then braze a sleeve in the top to reduce it for a more normal sized seat post. But then i'm not sure what i use for a binder, and i still feel like i need a stronger post too.

Sourcing the exact same tubes used on scoopers brompton may be a good way to go.

Fietsbob: 1.25 would be a 31.8 post also on the bike friday, so it's very similar to the brompton but with a thinner wall so that it accepts a 27.2 post.

What do these use for binders at the top of the frame and the top of the middle tube?

Scooper

On the Brompton (since it's a folder that needs to be quickly folded to a small package and unfolded) the binders are lever operated quick release cams. I'll take some close-up photos tomorrow.

Scooper

Here are photos of the Brompton seatpost clamps.

This is the clamp on the top of the seat tube that clamps the lower (31.8mm O.D.) seatpost in place.



Here are two photos of the clamp that secures the 26.6mm upper seatpost in the lower seatpost. The first is with the clamp lever closed and the upper seatpost in place; the second is with the clamp lever open and the upper seatpost removed so the wall thickness of the lower seatpost shows.

troysmith80

Scooper, you're a superstar, thanks for the help! Great pictures, that is very helpful.

It looks like there's a plastic bushing in the top of the lower-seatpost, and maybe one in the actual seat tube of the frame as well. Perhaps i should just use off-the-shelf straight gauge tubing with bushings or sleeves in order to make everything fit. I could use a home-made clamp like they've made, or braze on old school seat post binders.

MassiveD

Wow, that Brompton clamp is pretty shameless. Love it.

Ok, be my BFF, and make with the BF info...

troysmith80

I think i'm going to go with a more conventional seat tube angle (abandoning the Adaptive Seat Tube for now), around 71* or so. I will still have a lot of seat post extension though, like around 48cm (19"). I'm thinking i'll use a 34mm seat post like the dahon folders use ( https://www.thorusa.com/accessories/seatpost.htm ).

Does anybody know what size tube i should use for my seat tube to best fit the 34mm seat post? Or, perhaps better, how much difference is ideal between SP OD and ST ID? Should i plan on using a bushing / sleeve, or just choosing the right tube?

EDIT: Looks like it would be more conventional to use a 34.9mm seat post and off-the-shelf 38.1 seat tube, correct?
Edit AGAIN: ...except 38.1 bicycle tube is all goign to be aluminum, isn't it? Straight guage 4130 chromo 1.5" x .065" wall should be pretty much exactly the same though, maybe that's what i'll use.