Why Curved Top Tubes?
 

It seems many new road bikes have gently curving top tubes. I'm wondering if there is a functional reason for this, or if it's just cosmetics.

I'm no engineer, but I remember being taught that a round tube was generally the most efficient structure for strength to weight. These were modified by butting because bicycling doesn't stress tubing equally - some areas need more strength than others, so they were given greater wall thickness than areas of the tubes that needed less strength.

To further put strength where it was needed, round tubes were ovalized in different ways in different areas.

How am I doing so far? (Like I say, I'm no engineer - just a curious guy who is trying to figure things out.)

So now see all these bikes with curving top tubes and wonder why?

It's particularly pronounced on mountain bikes where some top tubes resemble the profile of a boomerang or banana. Is that for standover clearance, cosmetics, or something else? It seems as if there would have to be extra material added to match the strength of a straight tube. Strength is necessary, but weight is bad.

Comments?

i would say because it enables bikes to look distinctive.
and hydroforming makes these fancily shaped tubes pretty strong anyhow, and weight can be reduced because of this as well. so with alloys it is probably the case that the 'curved' tubes are not weaker/stronger than straight, butted tubes by a significant margin. perhaps someone with specialist knowledge can comment on this.

i am not sure with mountain bikes hydroforming would be such a consideration, as the bottom end ones which look fancy often weigh about as much as your average suitcase...

The most favorable strength to weight is achieved with the shortest distance, aka a straight line. Routing a structural member via a longer curved route doesn't do any good mechanically. But the most favorable sales to cost ratio isn't always that same straight line.

Some think they look kool.
I guess I'm just old.

From a pure theorical stand point, if the top tube is curve up the the bike would be more prone to flex in the vertical plane. But we all know that preciptable reality does not always follow theory.

It may not make it too flexible, but it will make it heavier.

Curved tubes are only for looks. One would hope that the extra weight isn't very significant.

Marketing.

Part of me says it's marketing with manufacturers asking themselves, "how we can make a bike look different, so people feel they have to upgrade" - like a form of planned obsolecence.. Seems like you can only do so much with a bicycle, yet capitalism is based on buying. Some technologies like disk brakes on a mountain bike fantastic. But going from a 10 spd to 11 spd cassette seems like product differentiation more than performance.

Compliance, shock absorption and comfort might be factors to curving the members, especially on road bikes with no suspension components .Also, perhaps hydroforming metal tubes has become common place and cost to form has really become low cost, making it available to mass production and soon to be found on x-mart bikes.

Good topic, gives us something to think about.

I'm not a mechanical engineer or a metallurgist, but I suspect a curved tube doesn't act like a shock absorber in any substantive way, at least not with the curves we've seen on bikes.

I am a mechanical engineer, and noglider was correct. The slight curves in the top tubes of some new road bikes are all about looks, not about the often claimed "vertical compliance and lateral stiffness" marketing gibberish.

I love it when my intuition is right. ;)

Okay, how about the "hourglass" seat stays on bikes like the Synapse? Do they flex enough to add comfort? I remember a titanium mountain bike that's "suspension" seemed to be frame flex. Anyone remember the one I'm thinking of?

So many bikes these days seem to have curving seat stays. I'm just wondering if they have a function.

Depending on how it's done, curving the seatstays can provide added vertical flex, especially in titanium frames whose chainstays are also more flexible. This is why forks had curved blades for almost a century before it sadly went out of fashion.

But the reality is that in most cases the only function of all these curved tubes is as a sales feature. Like with clothing, many (not all) of the features and special embelishments you see in frames have more to do with fashion trends and marketing than with performance.

Most people who ride titanium mountain bikes, no matter which model or brand, will tell you the frame feels somewhat flexy. I think the builders do this intentionally to improve the ride quality on rough terrain, because ti is not an inherently flexy material.

I ride a steel Mongoose Crossway Hybrid with "Comfort Fit Geometry" and is, in my estimation, real comfort. The frame's top tube and seatstays make a curved arch from head tube to the dropouts, and the bottom tube is curved similarly. Over 5600 miles on the bike, and just recently broke its first spoke, non driveside rear. Presently I'm riding it on dirt (two track) roads in local forests, after changing tires from 32mm to 40mm. This model was discontinued in 2005 (when I bought it for $150).

The 2010 Look 596 frame has a kinked top tube. I've read in a couple of places that Look themselves have admitted that this is purely for aesthetics. What is really ironic is that the catchphrase that they use for this frame is that it has "The aerodynamics of a fighter jet", and yet it would be more aerodynamic if it had a flat top tube. I don't think fighter jets care about aesthetics!

http://www.lookcycle.com/media/catal...f/i/file_7.png

What's funny is that all these manufacturers adopt aesthetics as marketing tool. I thought form follows function. Why do I like a car that looks like a carefully styled airoplane or rocket? Why do I like a black "stealth" like sculptured bike. We are all slaves of the styling maffia it's all about emotion. It will not make you faster, but at least you have to look faster. Oh, I ride a Specialized Roubaix with a curved top tube and I like it ;-)

All true, but sometimes, it comes back around, and the function follows form just when we least expect it.

I always figured the inwardly-curved seatstays were to help avoid heel strike problems on race bikes with short chainstays.

The rebirth of the Hunchback of Notre Dame!

Curved top tubes can have a purpose such as on the Surly Big Dummy. It's top tube is curved in such a way as to lower standover height for a given frame size. It should make getting aboard a loaded bike easier.

I cannot bring myself to like curved top tubes or compact frames. Right or wrong I was brought up to believe that a "boys" bike has a level, straight top tube.

Al

Sure, but that's curved down. Most curved top tubes are curved up. That's not functional, as far as I can tell.

I don't mind straight sloping top tubes. I know the look doesn't please many, because it's a departure from the traditional, but it's OK with me. I don't like all things about the new trends in bikes, but this one is fine with me. My Nashbar generic fixie has this design, and I like it.

I thought they were to increase the tire width while keeping the brake bosses in the correct place.

when I first saw some Specialized with curved top tubes a few years back, I thought they looked like a mixed racing/beach cruiser. Now, I sort of like the look. I guess you can't use full sized frame pumps on those frames.

Bike makers have always done things for purely aesthetic reasons. Think fancy lugs, just for starters.

I can't think of any reason not to build a bike with aesthics and/or unique look in mind.