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TT diameter larger than ST diameter

3K views 16 replies 10 participants last post by  Jon Edwards 
#1 ·
Does anyone build frames with a top tube of a larger diameter than the seat tube? If so, how do you detail the tube junction? If not, why not? Is this a bad idea?
 
#2 ·
Hey;

My Recent Fatbike has a 1.25 ST and a 1.375 TT. Mitered and welded them. Slight flattening of the TT sides to flow them into the ST. Never gave it a thought. Imperceptible. The 1.50 DT needed a bit more flattening at the HT, but the manipulation here is equally imperceptible. If you want that big tube stiffness, go for it.
 
#4 ·
I would suggest you make a study of torsional stiffness of tube steel before commenting.

A real world example of why large diameters are used would be to take an 'olde-world' frame that has 25.4mm TT and 28.6mm DT and take it for a ride up hill, and seriously accelerate pulling hard with left hand on the h/bar and max pressure on the right pedal. See if you can't flex the frame. Try a modern 31 x 35mm O/S frame and you will understand why large tubes work. By the way, larger tubed frames can by made lighter than skinnier tubed frames due to the fact that they are thinner walled than the 'olde' style which are thicker to get back torsional strength similar to O/S, assuming you are comparing double butted tubes to double butted tubes.

Eric
 
#5 ·
I suggest you contemplate the meaning of the term "parallel kinematics" in this context. I submit that just by riding a bike it's tricky to separate out the torsional stiffness of the tubes in a structure that has a hinge in the middle. When most riders are cranking as hard as they can, the front wheel moves back and forth. Cycling is full of uniformed opinion, stiffness is overrated.

In any event, we are talking top tubes,not down tubes. The top tube takes very little torsional load.
 
#7 ·
Answer to question:

Yes 90% of the frames I make have ovalized TT/ST junction.

The easiest way to ovalize the end of a tube is to take two boards (I use two 6" x 4" of 3/4" thick MDF) and place them loosely in a large bench vice. Place the tube between the boards and slowly flatten the tube to the desired width. The boards act as a hinge that prevents any unwanted denting of the tube. Make sure you've marked where the butting profile is on the tube first.

*Butted tubing usually has a little bit of bow to it. One has to mark where the highest point is on the tube first, by placing the tube against a flat surface. Then orient the miters of the tube so that the highest point is in the center.
 
#8 ·
Golden Boy

Sorry to take your thread in another direction off topic.

I have used the method that Matty describes and was successful, though I was using .7mm butted end and found the steel to be very springy, that is, it wanted to return to original shape.
Avoid the tendency to over squash as you may then crimp it a bit - longitudinally, which is not desireable.

If you are looking at the purchase of a Tube-Set from Nova, I would suggest purchasing individual tubes instead, as the total price comes out very close. This will let you option your way around the miss-match fit, eg: use Columbus 28.6mm Neuron TT, which will give a strong joint or they sometimes have 29.5mm Genius, no stock at present. The Zona 28.6mm TT in .8/.5/.8 butt would be fine. There is little ride difference feel between 28.6 and 31.7.

Hope this helps you a little.

You doing your bike in 650B again?
I liked your 1st bike, it has excellent proportions.

Eric
 
#9 ·
Thanks, Eric. No apology necessary....you felt compelled to reply to the preceding comment.

I stated in this forum previously that my second frame would be a 29'er, but I think it will end up being another 650b model. Part of me is compelled by the challenge of designing a 29'er frame that I might enjoy, but my gut tells me to stick with the smaller wheel size that I know I will enjoy.

Frame #1 has a 28.6mm TT and 31.8mm down tube. My current design for frame #2 (in SolidWorks, BTW) has a 31.8 TT and 34.9 DT for additional stiffness. But maybe all I need is the larger dia DT and I can leave the TT at 28.6. Then it will blend perfectly with the seat tube. I like the 28.6 ST because I'm building a hardtail and I want the additional compliance, however small, that a 27.2 seatpost will give me. I am not locked in to buying a tubeset. I was already thinking of buying a la carte.

Still enjoying frame #1, but of course there are some things I want to do differently with frame #2:

1. Lengthen the chainstays a bit so I can run a front derailleur. This will also lengthen the wheelbase (all other things being equal) which I think I might like better. A 42" wheelbase can feel kinda twitchy, regardless of the HTA.

2. Stiffen the front end (hence the larger tube dia's) and slacken the HTA from 69 deg to 68 deg.
 
#12 ·
If I may...

I find these pieces interesting. My gut reaction is that it would be best to have complete engagement of the seat post inside the ST for the entire length, but the fact that these are available, and that shims have also been out there for years, suggests that this amount of post-to-tube engagement is acceptable. Yes?

It also surprises me that they are made of stainless. I'm not sure of the reason for that, but again, I must infer that welding SS to CroMo is acceptable. Does it require different rod than S2, or are they only intended for brazing?

Sorry to go off course.....
 
#11 · (Edited)
Quote:I am not locked in to buying a tubeset. I was already thinking of buying a la carte.

Good decision - a la carte. Choices are diverse. I made the 'mistake' on my first 'current generation' frame of getting a tubeset and found myself with the question that you ask.

Tube diameters are increasing all the time it seems, trial and error will get you to the combination you will like most.

You showed in your first frame a degree of daring by going to the front of changing trends by using 650b, and succeeded in my view, if thats worth anything...I don't think it would be a mistake, and because it is new in itself, 650b is a challenge, and you are thinking through the issues which is a good quality.

Keep it up.

Eric
 
#14 ·
Back to the original topic:

If going up 1/8" in tubing diameter, squishing (ovalizing) works pretty well, as described above. You can squish, then miter. Or you can miter, then squish. In the latter case, use the diameter of the larger tube to miter (i.e. 1.25" hole saw for a 1.25" tube), when you squish for the smaller tube, the miter will ovalize as well and come out pretty well.

If going up 1/4" or more (e.g. 35mm TT to a 28.6mm ST), things still work, but the ovalization becomes pretty noticeable. Whether you think this looks cool or goofy is up to you. Other options in this case include piercing the TT with the ST (which I imagine would be very hard to do by hand), or ovalizing the TT horizontally, then pierce. I believe GT used to do the latter in triple triangle frames.



That picture may or may not be a good example.
 
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