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curving seat tubes

2066 Views 11 Replies 9 Participants Last post by  natzoo, 
B
Hi all

I flipped back a few pages in the forum as I haven't been tracking this forum much.

I built a few frames back in the around 2000 time frame, picked up a HJ access 65 fixture and then let it sit/loaned it out and have never used it. In the time since I bought it, Boost became a thing, so my friend misplacing the rear axle part of it may not come back to irritate me quite as much.

Anyway-

I bought some basic 29er tube set from Nova w/ the curved seatstays and chainstays thinking I'd make some kind of Chameleonesque geometry bike as a fabrication project.

I was thinking, boy, it sure would be fancy to get a curved seat tube as well to get more tire clearance back there. I don't really have any options for rolling the one that came w/ the Nova set do I? I think it's a .6mm wall after the .9mm butted section at top.

My other issue is that since I've last done this, I've gone well past the 40 year old mark and I apparently have to take my glasses off to see TIG welding now. Yay.

Is segmenting the seat tube at the bottom a horrible idea? I segmented and butt welded chainstays on a trials mountain bike years ago and somehow they're no worse for the wear, although I suspect the wall thickness on the stays was substantially more than the seat tube tubing will be.
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pvd
You can segment the seat tube. Just don't be silly and ensure that it's well done.

Ensure that you understand what you are doing with the tube and why; Professional Seat Tube Design | Peter Verdone Designs

Also, nobody knows what "Chameleonesque geometry" means.
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pvd said:
You can segment the seat tube. Just don't be silly and ensure that it's well done.

Ensure that you understand what you are doing with the tube and why; Professional Seat Tube Design | Peter Verdone Designs

Also, nobody knows what "Chameleonesque geometry" means.
Click to expand...
Thanks for the links.

Sorry, I wasn't entirely clear- it was intended to suggest I am not pretending to have any insight about how or why I'm picking geometry. I have a bike I like riding, I'm going to fab a smaller size version for my girlfriend. :)
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S
You can buy bent seat tubes from Bike Fab Supply.

www.bikefabsupply.com

BENT SEAT TUBES — BICYCLE FABRICATION SUPPLY

BICYCLE FABRICATION SUPPLY OFFERING OF 35MM OD BENT SEAT TUBES FOR BICYCLE FRAMES. USA MADE 4130 .035” WALL 1.375”. TUBING BENDS ARE IN 6.75” AND 9.00” RADIUS WITH 10-30 DEGREES OF BEND ANGLE. OPTION FOR CUSTOM BEND LOCATION AND DEGREE IS AVAILABLE.
www.bikefabsupply.com www.bikefabsupply.com
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S
On the seat tube, Ben Land created more clearance on his recent build without bending by joining it to the down tube, ahead of the BB: http://instagr.am/p/CRjXChoF3N7/ . He also has a video on Youtube with more details about that chainstay yoke.
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4
Speaking of chainstay yokes...
Bicycle Bicycle tire Bicycle frame Bicycle fork Plant

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2003 era cousins of the plate gusset - these were intended to be used with rectangular stays for reasons I can't even remember. The only set that got on a bike were ones I sent out for a friend to incorporate into his design trials frame he had a pro builder fab up.

Wood Table Hardwood Flooring Wood stain


This might be generally more interesting to see. Really positive location for mitering the BB interface. And easy to make.
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J
I did the offset thing on a recent build. It is a kid's bike though, so strength is pretty much a non-issue. I'm not sure if it would flex too much or be too weak on a different design. Anything works if you overbuild it enough.
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AdamR83
6
Hello to a fellow trials rider ;)

I am currently building a frame which gave rise to similar ponderings. In the end I made a 'prop' to lean the seat tube against (a stick that bolts between the dropouts and is ~10mm longer than the radius of a wheel / tyre, to make sure I had clearance) and offset the ST at the BB shell. Wasn't as extreme as the solutions above - 17mm offset in my case - so the ST still has some weld contact with the BB shell.






One of my other options was the cut n shut seat tube, as you mentioned. I would have had no reservations doing this in 0.9mm wall (my ST is 0.8mm). With a perfect fit-up (top tip: clamp a belt sander in a vice and run the tube ends against that) you can do a perfectly strong join without filler. If you have a pulse setting on your welder, or a very steady hand, it will be barely detectable after paint.

I built a few headers (manifolds) like this in the past, then later learned that this is how they do it in F1. A back purge is really useful in this instance.

Automotive exhaust Rim Automotive exterior Bumper Bicycle part


Water Liquid Fluid Amber Natural material


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J
Nice, Caterham?
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Ben.land101
stuartm2 said:
On the seat tube, Ben Land created more clearance on his recent build without bending by joining it to the down tube, ahead of the BB: http://instagr.am/p/CRjXChoF3N7/ . He also has a video on Youtube with more details about that chainstay yoke.
Click to expand...
I should note, PVD actually gave me that idea for the seat tube a while back. Gotta give credit where it's due.
One of the biggest unknowns is how the bottom bracket will react to heavy pedaling forces.
It's not been a problem whatsoever on a "enduro" bike. We just pedal to the top at a reasonable pace without killing ourselves.
Although, I'm going to do the same thing on my next bike, which will be a singlespeed. I'm gonna test it out myself and see how it reacts when you put some real horsepower behind the pedals.

Also worth noting, I make sure that the seat tube is placed well within the long-butted section of the downtube. Maybe not %100 necessary, but my arm-chair engineering analysis told me it was a good idea.

But here's another idea I was playing with a while back. Some people call them "pie cuts". Also very common in motorsports, etc.
If you haven't seen it before, all you need to do is cut a bunch of angled pieces from a straight tube, then weld them back together to create the bend. You can get some pretty crazy CLR's and radii when you start playing around with the numbers.

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Ben.land101
bentmettle said:
My other issue is that since I've last done this, I've gone well past the 40 year old mark and I apparently have to take my glasses off to see TIG welding now. Yay.
Click to expand...
Also, get yourself some of these if you haven't already.
Cheater Lens - Amazon Link
Basically reading glasses for your welding helmet. My vision is still "perfect", but I still use them. Makes welding thin-wall tubing so much easier when you can see it properly.
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N
3
This is the method most builders have converged on for mountain bike seat tubes. I found this is the best compromise due to awkward seat post diameters and availability of tubing:
  1. 1.375"x.035thk straight gauge (34.9x.9thk) chromoly
  2. bend seat tube 12-20deg for tire clearance
  3. weld on a seatpost topper:

Comments:
  • The reasons for the topper:
    • It is thick (1.7thk), making it much stronger
    • the thickness reduces distortion from welding
  • Adjust your bend, offset, and actual STA to:
    • get the tire clearance you need
    • get the effective STA you want


Example:
This is an example of the bends to create a chamelonish seat tube angle.

Bend: 15 deg
offset: 28mm
Actual STA: 72.5deg
Bend centerline radius: 230mm
clearance: 29x2.6 @ 420mm chainstay
Effective STA: 75deg



Bicycle Bicycle frame Bicycle wheel Bicycle tire Wheel

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