In engineering, many optimization algorithms rely on a good initial condition to converge to the optimal solution.
This is a fancy way of saying: engineering is guess-and-check. The better your initial guess, the closer your solution.
Even if you had the most advanced FEA simulation (guess), you would still need to validate it (check) in the real world.
In your case, it’s impossible to know if 31.8 or 28.7mm DT is correct for your first frame. It’s going to be a guess, even for the most experienced framebuilders.
I always err on conservative (stiffer and stronger). There are no real downsides to a bike that is marginally stiffer. However if a bike that is not stiff enough when loaded, you may experience speed wobble or goofy handling.
I think this is a good rule of thumb and I’m in the same camp as you.
Yet I’ll offer a devil’s advocate counter to this aproach…
Going with the stiffer choice, chances are you will never know where the limit of “too flexy” is.
So there’s probably some value in getting experimental and going skinny to learn where that limit may be (for you). Not for the first, second or even third frame though.
@Daniel_Y is right that there aren’t any real downsides to going with a bike that’s marginally stiffer, especially since it sounds like you’re making a touring bike.
But like @JMY I think it’s good to experiment so you actually have personal experience with how a flexy bike behaves. Then all that cool FEA analysis will mean something to you.
@JMY that was exactly my plan, to push it and experiment a bit for this frame, then make adjustment later.
@manzanitacycles I think it will be a light touring at best, I kinda have FOMO on this whole planing/flexing bike thing…And perhaps a bit of mental gymnastics, but I’m justifying this whole flexier approach by me being not the strongest rider and the frame being very small…
(from the Tall Cyclist, which suggest that as a small rider-orange on standard steel i wouldnt have to worry about stiffness too much)
Also, how would one tell where the flex or stiffness is coming from when it comes to Top Tube vs Down Tube please? For fork, I will use the very flexy Kaise Toei which hopefully wont muddy my ability to feel and experiment with TT and DT parameters…
And lastly, what can cause shimmy? I wonder how much of that is wheel and how much of that is differences of TT vs DT flex characteristics. Thanks all!
I’ll give you two more points to consider when selecting the tubes:
The lower the wall thickness, the harder it is to TIG weld. I don’t consider myself to be a very experienced welder, and I wouldn’t attempt welding anything less than 0.8mm I think
Thin butted tubing is a lot more sensitive to dings, especially in the top tube (leaning the bike to a post, accidentally slamming the handlebar into it, generally transporting…) and once a tube has a ding in it, all considerations concerning the strength go out the window since it now has a weak spot.
Considering you are building a pretty small bike, lack of stiffness is not going to be an issue so I wouldn’t even think about any large OD/thin WT tubing.
Now, concerning your question regarding the flex DT vs. TT:
In my opinion, there are two modes of flex which we can actually feel while riding the bike The fore/aft flex under braking and the torsional flex when changing direction quickly or getting out of the saddle in a sprint. This would be equivalent to trying to rotate the headtube around the longitudinal axis while having the BB and ST clamped in.
In the first scenario, it is quite obvious to see that the DT will contribute more to the stiffness because of the angle between the HT and DT vs. the TT/HT being almost 90°. It becomes more obvious if you imagine the joints between the tubes like hinges or balljoints.
The torsional flex is a bit harder to imagine, but again, the different angles do play a crucial role. first, the DT is just longer and (if they were the same dimensions) will deflect more under the same load, and second, the angles of the tubes result in a different relation between shear and bending in the tube. The DT will have more bending stress as a result of that rotation of the HT than the TT, which will have a higher percentage of shear.
This is a bit complicated to get your head around… I will try and make some colorful FEA pictures over the weekend to illustrate what I mean!
I can visualized this, my top tube is 8-9deg but that’s nothing compared to the downtube. Because the bike is short and I want to have a lower BB, DT is very much pointed up: 51deg to the ground. Almost forgot about this but this is why at one point i thought about upping it to 28.6 9-6-9 or just use the seat tube…
I think this is a concern especially for my current design, because the DT is 25% longer than the TT which is a bigger gap than most other designs…
For FEA, may I ask what parameters would be put in? It would be great if you can help punch my numbers in since it’ll be awhile since I can figure out FEA Thanks!
Thanks!