Hi, a few tubing questions please. I have done some forum search so hopefully these will (edit: NOT) just be another set of repetitive tubing obsessions…
So an Externally butted Seat Tube (EBST) has that bulge at the top, pros is there’s more area to weld on but what about cons, would that thicker 1.2 bulge compensate on compliance for example vs a 0.6 thick top portion?
Would said EBST be good for a downtube as it has a bulkier end to attach to the head tube for example? Or, would it create stress due to that structure? (red arrows on picture)
How much of the bigger butt section (% or mm) is needed or ideal for each tube? I guess it would be mainly TT or DT hear but please feel free to add. Maybe this would depend as each welding method may have a different range for “area affected” as well?
Simiarly, when it comes to the taper/transitional area of butted tube is there a better number (depending on tube length if applicable). Donard mentioned on his website that “The transitions on the 853 tube are longer by 10mm (50mm vs 40mm) Theoretically this means a slightly better distribution of stress along the length of the tube. Practically that’s going to be a very small factor.” , is that agreeable to you?
Thanks!
.6 butt is not thick enough for a seat tube. Those are designed for lugs (which add more thickness). Seat stays cracking the seat tube is the most common framebuidler mode of failure I have seen. I have yet to see a modern, externally butted seat tube crack.
The butt is for strength, not ride quality.
You could, if the lengths happen to work out. The thick section acts like a gusset. Several Taiwanese mountain bike frames are made with a thick 1.4 internal butt at the headtube.
1.6 is overkill though.
“Theoretically” yes. In reality, no difference. Total marketing statement.
Start with fundamentals and design a durable bike with good geometry that fits you perfectly. Then try to tune ride quality.
Tube selection is the final step, and not nearly as important as people think.
Thanks a lot @Daniel_Y , and oops a lot of late night typo…
“will not just be”…
I was hoping that if i use the EBST as a downtube, the extra thickness at the other end would help with braking force (though it might be negligible since it’s a brazed on centerpull rim brake) and front load carriage). So you dont think that there would be potential stress risers where the red arrows are pointed?
You’re indeed right that 0.6 seat tube was for lug. I guess I would need a 1.2 bulge then, esp if I plan to use something like this:
The stress risers occur at the joint because of the geometry and the heat-affected zone. I have never seen a tube buckle at the butt transition under real-world riding.
I’m a big fan of extra material at the TT/SS/ST junction. It’s a highly stressed joint. I added a sleeve to this tube. Also, 635 is a short DT.
One caution - I used this tube for a recent frame and the ID is bigger than I would like for a 27.2mm post. My reamer didn’t remove any material. With what I consider normal tightening, the post slips.
“I was hoping that if i use the EBST as a downtube, the extra thickness at the other end would help with braking force (though it might be negligible since it’s a brazed on centerpull rim brake) and front load carriage).”
While I think a conservative approach when learning is the right mindset you are over thinking it as well. There are down tubes that are 0.6mm wall thickness at the butt and they are used in rim and disc brake road bikes that you can bomb down mountains and cope with the compression forces of a hollow or heavy braking. Steels are generally very strong and quite tough. Obviouslly as you load up the bike with either more weight, slacker head tubes and the rigours of off road you need to make them with a bit more beef in the tubes. Most bikes in that category are well served by a 0.9-1.0 wall DT at the heat tube. My current 170mm enduro bikes uses a 1.0mm wall at the head tube and that is definitely up to the task. Go with a Zona 0.9/0.7/0.9 DT and you’ll be on the right path.
About heat treated tubes (or any super steel for that matter), are there more cons to them than pros? Cons being more brittle and I guess harder to repair (though very unlikely?) for heat treated?
For stronger steel you would use thinner tubings but bigger diameter. Assuming similar stiffness, how would a thinner but bigger tubeset ride vs thicker but smaller tubeset?
You are in luck, I publish a youtube video on Monday about this!
Before you get lost in the details of materials and butting, you need to work backward from what kind of quality and durability you are expecting and then figure out the tubing.
Material selection, heat treatment, butt thickness, are all parameters to tune the ride quality, stiffness, durability, and weight of a frame.
If you’re going full rando style with a handlebar bag and a lowrider rack then the fork is carrying the load. That’s the advantage of that style of bike. You can then make the frame built like a race bike.
Since you’re making a small frame going with something like a 25.4 8/5/8 TT and 28.6 8/5/8 DT would be fine unless you’re a really heavy/strong person who puts out lots of power. You can use 14mm seat stays and whatever chainstays to get the clearances you need.
I’m guessing you’re going with a 1" threaded steerer. In that case you can use a 31.7 HT and braze some reinforcing rings at the HT ends to keep them from ovalizing.
Thank you. Those were the exact tube specs I have in mind, and I was thinking 16 vs 14mm SS (both tapered to 12.5). Not sure if it’d make a big difference in rear load carrying capacity vs compliance…
And, where would the forces that could potentially ovalize the HT come from please?
If you’re also planning to carry a rear load then I’d consider a larger diameter top tube and maybe 16mm stays or else you may get unwanted flex when riding out of the saddle. It’s known as the tail wagging the dog effect. But if you’re going for more of a randonneur style bike, you may want to consider only using front panniers.
My wife is 5’2". I made her a touring bike many years ago. She prefers to use rear panniers. I made the bike with a 25.4 top tube and she complained about the bike flexing on her. When that bike got stolen, I made her replacement with a 28.6 top tube and it solved the excessive flex with a rear load.
Head tubes get ovalized from head-on impacts, like hitting a pothole really hard. They can also ovalize from riding with a loose headset or jumping.
Thanks a lot Nick! Consider how rare i’d have significant rear loads (even then i’d try to keep it at 10lbs max on a saddle bag maybe) I’d stick with the less beefy rear end for now…
Interesting to heard that even such a small bike for your wife (which means stiffer top tube), a 28.6 top tube is still much more preferrable for rear load.
For HT, would going to 1-1.1mm thick 31.7 thick reduce the need for the reinforcing ring or would thickness be unsubstantial in dealing with pothole for example?
I don’t know the answer to that. There’s lots of factors at play: How much you and your baggage weighs, how rough are the roads you ride. IMO it’s better to be safe and add a little extra reinforcement. The weight penalty is negligible.
In the past, a company called Nova Cycles sold a 32.4mm head tube for 1" steerers. That was a good option for fillet brazing or welding without lug reinforcement. Now that Nova is gone and skinny head tubes aren’t common, the parts for making those types of bikes are harder to find. But it’s a pretty easy job to make some rings on a lathe if you have access to one.
Thanks for the video! It was very helpful and answer many of my questions.
So overall, is compliance solely dependant upon deflection? I’m just talking about materials i.e. putting aside the other moving parts like geo…
In that case, I guess to answer the question “how thinner but bigger tubeset ride vs thicker but smaller tubeset” I would need to plug those number into the “area I equation” you put on the video?
Lastly, I guess it would depend on bike (and perhaps where the added loads from baggage are) but for an all road light-gravel bike, would you want the delta deflection of the top tube vs down tube be similar or ideally, should one’s be higher than the other’s?
The equations are not for calculation purposes. They exist to help develop intuition.
For example, beam deflection is L^3 where L is the length of your beam. This means that you bigger frames get a lot more deflection simply because the tubes are longer (and the inverse of that with small frames)
If you really wanted to do analysis, you would set up a FEA simulation in your CAD program. But that won’t tell you how much deflection is what you like. At that point, its easier to build and ride!
People claim “top tube stiffness does XXX, downtube does YYY” but I don’t really believe that. The total stiffness of the frame is a linear combination of the toptube and the downtube stiffnesses. If you need a bit more stiffness, bump up the toptube. That is how it breaks down in pratice.
Specific to your design, you might want to consider the components you build it up with. For example, wide carbon wheels are much stiffer, so you probably want a stiffer frame to match the ride feeling.
The good thing for me is with a very small, rim brake all road frame I wouldnt have to worried about overall stiffness. But, I still want to “optimize” TT vs DT stiffness because most FEA-ish resources suggest that the DT are taking more force? I’m pretty set on a 25.4 8/5/8 TT (45 long) and was set on 28.6 8/5/8 DT (55 long) too until I realized that the delta for the DT in this config would be quite a bit more than the TT, so thinking i might have to go 9/6/9 or even go to 31.8 hmmm
