maltess2

Hello, I am deciding between these 2 bikes. In the general forum, a menber explained that in his experience straight frames are more reliable and light than frames made with a more curvy geometry, like the Willy. What do you guys think?

Wilfred Laurier

Put that member on your 'block' list. He doesn't know his arse from a hole in the ground.

Cyclosaurus

Whoever told you that is either stupid or f**king with you.

Wilfred Laurier

Found it:

Maltess2 - please realize that vatdim has no idea what he is talking about. People can have whatever preference they like for whatever reasons they like, but suggesting he can recognize some difference between the bikes based on the curviness of the frame tube junctions is one of the most ignorant and ridiculous things I have ever read on BF.

vatdim

I'm not a pro framebuilder, neither did I say I was. Fact of the matter is, I've seen plenty more curvy frames getting cracked. It's enough reason for me to avoid getting any for the bikes I choose. Make what you want of that.

Blue Belly

It would make sense to me that a tube with a compound curve might be stiffer in a flexy situation. Cracking & breaking would have a lot to do with the construction of that tube & not the shape. Looks are a whole different matter .

Wilfred Laurier

One of your claims was that you read in 'various books' something that agrees with your outlook on frame design. What books were these?

vatdim

I don't remember the exact source where I came across this information laid out with more details (it was about 2-3 years ago). Since I found it made sense in terms of physics, I simply accepted it as a fact at that point. Also, since my own experience and that of people I know of agrees with it, I still feel this way. And I wouldn't recommend something I know I personally wouldn't use, hence my post.

Just something from Sheldon Brown for maltess2 to consider:

And also:

Cyclosaurus

You do realize that the Wilier geometry is still really a diamond frame, right? And that "straight is strong, curved is weak" is such a primitive oversimplification that it is little more than superstition? An arched opening is far stronger than a rectangular one, for instance. You can admit you really don't know what you're talking about or you can dig in and continue to insist on nonsense. Your choice.

Mark Kelly 

I agree with Vatdim. Curvy tubes are the output of designers, not engineers.

As the large companies push the envelope with frame weight, the curvy tubes are disappearing. Why?

Because a straight tube is stiffer and stronger.

It's easy to demonstrate: make a straight tube and a curved tube the same length and with the same layup and compress them axially to fracture. The curved tube will show more strain for a given load (eg it's less stiff) and will collapse at a much lower load. This was predicted by Leonhard Euler almost three centuries ago (Google Euler column buckling).

The same goes with extensional or torsional loads, for different reasons.

An arch is strong when loaded on the arch, which a bicycle tube isn't.

vatdim

Thank you for giving the real scientific explanation to what I'd been trying to get across using my oversimplified terms.
@Cyclosaurus: I already said, I'm not a professional, I'm just a cyclist. I give friendly advice based on what I have read and experienced over time. That's all.

unterhausen

I feel like it's really difficult to come to the conclusion that curved tubes would be less suitable for frame construction. The tubes aren't simply supported, so the stress analysis is not going to be simple. And I'm sure the layup is different for curved tubes. Thus, I have my doubts that there are any significant differences in the ride of a curved tube bike. Having said that, curved tubes are mostly a styling and bull**** marketing exercise

JohnJ80

And that is the essential truth in all of this.

J.

fietsbob

"geometry" in bikes is angles and distances between centerlines ..

When there were rules against Logos on race frames, 1 British company started making Curvy Rear triangle tubes ..

tuz

Curved tubes can be aesthetically pleasing and are sometime functional (chainstays/seatstays for tire clearance, seat tube for short wheelbase, fork blades for some level of suspension or for unicrown designs). But it is true that they are slightly weaker and heavier (longer, and thicker-walled if mandrel-bent) than their straight counterpart. If they are properly designed the resulting bike should be perfectly reliable, and I'm sure the Wilier is.

Canaboo

The shortest distance between two points is a straight line. Put a curved tube in there and it takes more material to bridge the gap. That's weight.

Road Fan

Just speaking as an engineer:

Curved tubes have more length, hence more mass = weight.

Curved tubes are more flexible than an equivalent straight tube of the same material, construction, and dimensions.

If there are high compression loads, the curved tube should buckle (i.e. become destroyed) with less axial force than the equivalent straight tube.

If the curved tube is metal and has been given its curvature by bending, it has already been cold-set one time more than the equivalent straight tube. This means it is closer to its final failure.

The same problems exist with composite tubes, but there is the option to add material or optimize the use of material (asymmetric layup, or control of fiber direction) to manage stress distribution. But this has to add weight and cost compared to a straight tube intended to satisfy the same purpose.

I don't see any benefits to curved bicycle tubes other than aesthetics (the Hetchins) or in some cases providing a degree of bump absorption (curved fork blades).

unterhausen

I am also an engineer, and I say that most of the speculation in this thread supporting the thesis that curved tubes are inferior is without basis. The mental model of tubes as a simply supported beam doesn't fly. I think the long practice of using curved tubes without any problem demonstrates that there are no significant differences between curved and straight tubes in a bicycle structure. Go do a survey of bike racks, and you'll see. Ductile metal tubes are made by repeated cold deformation processes, it doesn't hurt anything. Rolling or bending it any other way doesn't seem to hurt it either, that's not where you see failures

Canaboo

I think people are just pointing out why curved tubes aren't superior which is what people are being led to believe. If something is more complex or requires additional steps to manufacture it has to be better.

Wilfred Laurier

Such a statement I could believe. But Vatdim's exact words were:

and also:

unterhausen

Curved frame sections might well be superior. There aren't too many perfectly straight or symmetric sections in nature. I doubt anyone has really gotten into structural optimization for bike frames. My guess is that if they did, they would find that the sections would not be straight

maltess2

Thanks a lot for all this input. By looking closely at the specs, the Scott has a bit of better components in general except for the wheels, which ara bit better in the Willy. What frame would you think is better constructed and designed? better in terms of quality, both of them are double butler.

Wilfred Laurier

Heh. You gotta be quite wealthy to afford double butlers with your bike frame

In case this was not an auto-correct mistake, the correct descriptor for a tube where the wall thickness is different at the ends than in the middle is 'double butted'

In reality, the quality difference between them is negligible. They are quite likely made in the same contract manufacturing facility in Asia. And if you were concerned about component quality, the wheels are the one component that can possibly make a difference to the ride on a new bike (possibly, but probably still barely or not noticeable). Get whichever one fits better. If they fit the same, get the one you like more for whatever reason (aesthetics, colour, handlebar tape texture, etc). If you don't like one more than the other, buy the one that is sold by the shop you like better.

maltess2

Thanks, and sorry about the misspelling. What about these two frames design wise? is there any of the two more interesting than the other?

Scooper

I'd love to see some finite element analysis images showing stresses in straight vs. curved geo frames.