We’ll have to learn how to use them…
I can probably draw the basic frame really quickly. If we can run simulations it would allow us to see how much flex when certain pressures are given and where it will flex.
instead of an I-beam, could you braze a 1/4" bar or even a small tube to the outside of your vertical tube? That would increase the dimension of the tube in the direction it flexes, and presumably reduce flexing significantly.
Again, this change would just help patch up the current frame. A better solution would be to use a bit larger tubing in the construction.
Scott
Yeah – Im’ going to add another 1/2" tube parallel to the one going directly to my seat post. It should stiffen it up. It’ll be maybe 1/2" from the original tube. If still isn’t stiff enough, I will connect them at various intervals.
corbin
I am by no means an engineer. 
I should have specified aluminum I-beam, but that’s probably what you were responding to anyhow. I just thought the geometry of an I-beam might resist the side-to-side flex better than tubing, while still being able to be thin enough to avoid a lot of knee strikes. The problem I’m thinking you might have with larger diameter tubing on the forward part of your V is constant knee strikes.
Edit: Maybe try using two different sizes of tubing, with thicker tubing going to the seatpost than to the handlebar.
Nice project, Corbin!
I had significant flex problems with a Nimbus frame too. That had me a bit concerned when I picked up a second hand Hunter frame (w/Schlumpf hub). When I had transitioned from the Nimbus to a KH, the whole feel of the uni was MUCH stiffer- which was (no surprise) really noticeable when climbing steep grades.
When I got the Hunter I initially planned on having flex problems. What really surprised me is that it doesn’t seem to have them. Granted, I’ve got a low gear now, but with the low I’ve taken it up 15%+ grades with a standard Magura HS-33 on it and the frame doesn’t flex enough to cause brake rub. No problems pushing it up too steep of a hill in high gear either. It probably should be noted that the brake mounts on the Hunter are on the back tube and the brake mounts on the Nimbus were on the seat tube stays, but the feel of the frames are entirely different.
I put the calipers on the Hunter last night and found both sets of tubes to be 14.5mm OD. I’m guessing that’s 1/2" tubing + powdercoat. I don’t know what the thickness of the tubing is in comparison to the Nimbus though.
I’m really surprised at how much stiffer the Hunter frame is. I don’t notice much difference between it and my KH. Granted, one is geared and one ungeared, but I’m pretty sensitive to frame flex. There are lots of steep hills around here.
I don’t know if this offers much of a solution to your dilemma. Bigger diameter seat tube tubing should help, but maybe a 3rd loop in the Hunter style may help. I know that the Hunter and Nimbus look similar, but there really seems to be a difference.
yes, gas shrould, less change to base material due to heat!
The Nmbus twin hoop frames also use 1/2 tubing.
I have not noticed any significant difference in frame flex between a single leg KH/Nimbus steel and a twin hoop Nimbus, both in a 26" or a 36". I ride my 36er on real muni and have a very powerful grab bar, and the most flex I see is in the seat and seat post. I don’t get any wheel to frame contact no matter how much I torque the frame, and that’s with a dished wheel.
In a big wheel, wheel flex is so much greater than frame flex, it’s hard to separate the two unless you can go from frame to frame with the same outfitting. Is there any chance that your bearings are flexing due to their being gaps between the bearing-spacer-crank?
Hunter makes a custom frame, so I’m guessing he has access to better quality tubing, likely tubing that is stiffer than a standard chromoly, so specific to bike frames (Presitige, Columbus, Reynolds, etc…). He probably uses TIG, but the weld is not where you’re getting excessive flex or tye weld would break.
Aluminum is stiffer, so if you have access to a TIG welder you could go that direction, though tube bending would be a challenge,but there’s no reason you couldn’t cut the tubing and making some neat looking angles. If you could find a high crown 29" or 32" bike fork, two of those might meet your needs without having to resort to finding tubing and having it bent. I junked my 32" cruiser fork or I’d offer it up.
I just measured my straight blade choromoly forks on an XL Redline Monocog 29" and there was 16.5" from the end of the tubing to the underside of the fork tube. If you cut out the seat tube you could probably get enough clearance to run a 36" wheel. All you need to do is cut out the seat post on one, cut off the drop outs, weld in a cross tube, and weld the hoops to the bearing holders.
Can you buy bike frame specific tubing from Hunter or another builder?
I’m saving my pennies for a TIG welder, just waiting until my wife graduate in May 
The Hunter frames I have seen (and I’ve seen quite a few) are all brazed, not TIG welded. The Nimbus frames are TIG or MIG (probably MIG, based on mass production, and looking at the welds). Note that you can do MIG with alum too…with the proper setup.
I do have a TIG welder (Syncrowave 250), and my plan was to first do a steel frame to prove the concept and then move into aluminum. Aluminum is a little harder to work with, but I think I can make a lighter and stiffer frame with it. I’ll have to machine my own bearing caps, which isn’t a big deal…but adds significant time. I can experiment with steel ones much faster given that I can buy them from UDC.
Yeah, regarding tubing, I’m using aircraft grade Cro moly, which is pretty strong. There is stronger tubing I can get out there, but I don’t think it will make much of a difference.
The older Hunter 36’er frame I have laying around is 5/8" tubing (not 1/2"). But, I know he has changed designs…and smaller frames (muni) tend to be 1/2" (like Tom Holub’s frame). The Hunter frame is definitely stiffer…and heavier, than the Nimbus frame.
I should have more tubing next week and can experiment more with designs.
EDIT: I can swap the wheel in between multiple frames (corbin V-36, and two different KH36 frames. My hunter frame (from Mike) has a different bearing size). The flex is the frame, not the wheel build.
corbin
http://www.unicycle.com/unicycle-hardware/frames-and-frame-parts/frame-parts
Would the Aluminum Nimbus brackets work? There are KH lower bearing caps, but the raw Nimbus look attractive- especially price wise vs. labor of machining your own. The KH would only be the non-threaded caps as well.
Have you considered, instead of building a “V” to make something more like a traditional frame and add a triangle above the wheel? Something more like a “P”?
That should allow your frame to flex a bit without the knee & tire rubbing you’d get from a “V” and still have the added handle strength.
Just thinking out loud…
Well, I’ll be! I didn’t know they had some aluminum ones.
corbin
Yes, I have considered that too…but the P would have issues with leg rubbing based on my riding style.
corbin
I got to working on this some more:
I added a secondary tube going up to the seatpost. That really helped stiffen the whole thing up! It now is awesome. I also brazed on a disc brake mount. I’m going to do at least one “good ride” on it before sanding it down and painting it. I also need to make some custom handlebars.
corbin
make me one!!!
I’d gladly make you one…but it will probably be very expensive.
corbin
I think you should add a bike seat. Otherwise, that is sick!!!
Looking sweet Corbin. When’s the pannier frame coming?
Eventually! I’ll have to use thicker tubing for that concept frame…
going for a test ride right now…
corbin
Nice! Double leg stays, cool connecting hoop, all it needs now is a Flatfish set and some paint
My wife has four months until she graduates and can start working, so by next Summer I’ll be ready to start welding, TIG, SS.
First project: muni bars
Second project: muni bike rack
Third project: 29" guni-muni frame
Fourth project: 36" frame
Thanks for the project updates Corbin!
I’ve been puzzled as to why the Hunter 36 frame resisted twisting (and resulting brake rub) so much better than my Nimbus did. The 2nd tube addition results may offer an explanation. While the tubes of both frames may be 1/2", the hoops on the Hunter are much closer together than on the Nimbus. I realize the Hunter is cro-moly as well, but I suspect the geometry involved is much more important than the difference between the materials.
Pannier frame? Sounds good.
