womble

Inspired to ask by this thread, https://bikeforums.net/showthread.php?t=252217, what's reason behind having curvy top tubes on a frame? An obvious example would be on the Nomad.

Does it add signficantly to strength, or is it mainly cosmetic? My assumption is that a straight tube is lighter, but does a curve allow for a lighter tube and an overall weight saving?

the wonginator

it helps for strength, but also greatly improves standover clearance.

Maelstrom 

Its all about the look. It serves no function. What rev says is true, it does do that, but you don't need that obnoxious hump to increase standover and improve strength.

Wheelchairman

I disagree about curved tubes having no function. A great example is the latest Specialised Euduro SL. It has a curve in its down tube to allow the water bottle mount to sit lower and its top tube is designed to get the water bottle out whilst still retaining the low standover height- all this thanx to curved tubes.

Maelstrom 

I suppose. But in my world, I don't consider a water bottle holder functionality.

wheelhot

well for me, the curved shape is too give more standover clearance. But Im not sure if it gives more strength. For carbon bikes, curved shape maybe helps in strength but Im not sure about Aluminium, I though Alu strongest form is cylindrical?

Wheelchairman

THat is true. Circular shapes of any dimension will always be stronger then any other shape. A tube with no bends in it will be stronger. But still, with all this fluidforming tech of late, its possible to regain the strength lost through a bend. I gather you'd simply make the walls in the bended areas a little thicker then the thin wall found elsewhere along the tube.....Or so Ive read in many an Avanti and Giant catalouges

wheelhot

so what is fluidform? I thought its a form of welding XD

Wheelchairman

Im surprised you didnt know this wheelhot, esspecially considering you have a late-model Giant. Did you ever look at catalouges?

wheelhot

well, my dealer doesnt have Giant catalouges, they only have TREK because they are officially a TREK dealer. And I just realize something, on Giant website there are no more info about DiamondMax coating and etc

womble

Thanks guys. It looks like curved frames are cosmetic then... Standover height is lowered on bikes such as the Motolite or Yeti 575 by using straight tubes, and I can't bring myself to believe that improved waterbottle access is really 'functional' either

PiratePete

********** Stronger under tension? shear? bending? less likely to buckle under compression?

The stress state along different parts of the bike frame tubing is very complex and varies depending on what the rider on the bike is doing. Circular tubes are not optimal though. Forces applied to the top tube, for example, are much greater in the vertical direction than in the horizontal so a tube with an oval cross section is more effective.

wheelhot

I wonder how much money Santa Cruz have spent on designing the Nomad curvy top tube.

Wheelchairman

Your nerdy attempt at nitpicking doesn't rate. I said circular shapes. I guess ovals are formed with a series of straight edges then? Because that would mean that ovals don't contain a single, continous circular edge right? right? ......

You're a tool.......and ffs, grow a fricken moustache!

I don't wish to argue about circles and ovals anymore, because that would mean Im arguing about circles and ovals....

PiratePete

I'm not nitpicking, I'm stating that you are simply wrong, circular shapes aren't always stronger. My point was that the shape of something load bearing depends on many things. Look at I-beams.

Wheelchairman

Fine, fine. Acknowledged. But what you are still saying is similar to saying that a Pontiac GTO is a better car then a Holden Monaro CV8.............In the scheme of things WHATS THE DIFFERENCE!?! . An oval is still bascially a circular shape. A GTO is still bascailly a Monaro .


FFS!

Falanx

And curved tubes in the main triangle do not make as strong a frame as straight ones. End of.

[/materials engineer]

Curtis_Elwood

Falanx, while you're here; is a round, or ovalized tube stronger and stiffer than one that's triangulated, such as the shape Marin uses?

Falanx

Right. I want to make sure that the first pont is that we all understand that 'stiffness' is the wrong word to use. Stiffness is a material parameter - Young Modulus in tension, Bulk modulus in shear. Nothing else. Call it rigidity, call it resistance to deflection, whatever you wantwhen you talk about deflection in structures, but it's not stiffness. The same goes for strength, which purely means the resistance to deformation. It sounds like pedantry, but it's done by materials engineers for a reason - to save confusion later on.

Now:

An ovalised section is more rigid than a circular section when loaded in tension parllel to the long sectional axis, and less rigid when vice versa. A circular section however, is more rigid in torsional loading.
Triangles are fun. Equilateral triangles are the most uniformly rigid on triangular sections, but less rigid than a circular section of equal wall thickness and effective diameter in both tension and shear. Isoceles triangles are, like ovalised sections, more rigid than their equilateral counterparts in tensile loading parallel to their long axis and much less so in torsion.

In a classical bicycle frame main traingle, the down tube is meant to be held in tension and the top tube in compression (mostly). Frames are expected to have some flex in torsion, moreso than in tension, giving you some 'whip' to help the frame cope with the impact of obstacles. Most sections are perfectly adequate for this application, within reason and within the bounds of what I've just said...

That clear anythign up?

And: Why does this goddamned interface keep logging me out?

wheelhot

I think you must make it remember your account? (check the rememember me box)

DirtPedalerB

the Marin tubes are teardrop shaped like a rounded triangle with a fat bottom... Marin has hydroformed it's top line bikes this year as well. They do look cool though.

Maelstrom 

There is a natural timelimit for a "session" lasting about 30minutes. If you started the post, surfed on another screen, came back .. etc...it would log you out.

PiratePete

I'm a little confused on your terminology. Don't you mean bending and not tension? "Loaded in tension parallel to the long sectional axis" would be like pulling on the skinny end of the oval.

What do you mean by torsion? Something like a tube twisted around its longitudinal axis at both ends. ie when you press down on the pedal the seat tube would be in bending, deflecting sideways and the down tube in torsion. It seems like the torsion force would be much less than bending forces on the down tube.

Falanx

What I tried to do was limit the discussion to the basic forces involved. Bending is a tensile force (and obviously at the other side of the free surface a compressive one). I simply wrote to demonstrate the difference in behaviour of, initially, an elliptic section and a circular one, and they always come down to straight tension vs. torsion. Yes I meant bending, and I was reducing the forces involved to those in the tensile half of the sample.

Torsion meant rotational tension/compression. Yep, your example is exactly what I meant. Yep, you're right with regards to the degree of one regime or another being exerted on the downtube.

womble

Wow... what have I started?

It's 1am here, my brain is fried and I lack an engineering background. Falanx, you seem to know what you're talking about- could you do a one-sentence answer on whether fancy curves a la Nomad provide any significant structural benefits? Ta.