Curved Top tubes

[Norm]

Greetings, Rob, welcome to Cycle Chat.

I like the look of sloping top tubes, my Secteur rides nicely with one, but then the Tricross and the old steelie ride nicely without.

 

[Smokin Joe]

Thought I'd chip in to this as I have a vested interest. What with me being the editor of Cycling Plus and all. I've just looked back over the last issue and found the offending caption and hands up, I missed it. That said, I'm no engineer so I'm going to consult one to check if this is, indeed, BS. (Yes, it's a bit late now but...)

But a couple of points: Mickle "Cycling Pus is and always has been full of shoot, and don't get me started on that Paul Vincent character." You're perfectly entitled to your opinion about the magazine, and I'm perfectly entitled to disagree but for future accuracy Paul left Cycling Plus some time ago.

And Smokin Joe. Likewise we're all entitled to an opinion but I definitely don't agree with: "As 90% of modern cycling journalists are clueless self-gratification artists who swallow any old shoot fed to them by marketing men they write exactly what the manufacturers intend them to write, ie crap that is designed to make the readers think they must have a different bike for evey day of the week" They're not and they don't.

Rob

We all have our opinions, like you say.

Be good if you stick around and put your side of the story when we criticize C+ (which I buy) and the cycling press in general.

 

[Banjo]

Thought I'd chip in to this as I have a vested interest. What with me being the editor of Cycling Plus and all. I've just looked back over the last issue and found the offending caption and hands up, I missed it. That said, I'm no engineer so I'm going to consult one to check if this is, indeed, BS. (Yes, it's a bit late now but...)

But a couple of points: Mickle "Cycling Pus is and always has been full of shoot, and don't get me started on that Paul Vincent character." You're perfectly entitled to your opinion about the magazine, and I'm perfectly entitled to disagree but for future accuracy Paul left Cycling Plus some time ago.

And Smokin Joe. Likewise we're all entitled to an opinion but I definitely don't agree with: "As 90% of modern cycling journalists are clueless self-gratification artists who swallow any old shoot fed to them by marketing men they write exactly what the manufacturers intend them to write, ie crap that is designed to make the readers think they must have a different bike for evey day of the week" They're not and they don't.

Rob

Welcome to the forums and well done for responding Rob.

In fairness it was only a small caption containing the offending words. When I tried to find it again I struggled to see it and thought for one horrible moment I had seen it in a different bike mag

I would be interested to hear an engineers opinion but do we really need an engineer to tell us if a top tube bends like a leaf spring or not?

 

[Norm]

... but do we really need an engineer to tell us if a top tube bends like a leaf spring or not?

Well, I'm an accountant but, IIRC, all other things being equal and simplifying hugely, a solid bar will bend, a hollow tube will crumple.

This is why things that we want to bend, like springs, are made out of solid bars whereas things that we want to be stiff (like frames) are made out of tubes.

Although anyone who has looked along the wing of an aeroplane in turbulence will know that there is further simplification behind the line that aluminium doesn't flex and it'll break if it does so.

I guess we do need an engineer after all.

 

[Tim Bennet.]

Well here goes!

We all know that a triangle is very rigid in the plane of its component parts. What ever loads are applied to the corners in the direction of that plane are resisted by each of the three constituent parts being either under tension or compression along their axis. The only way a triangle can 'fail' is if one of the sides that is under compression comes out of column (in other words bends or buckles) or a side under tension fails (breaks).

The main frame of a bicycle is admittedly slightly more complicated as it is not a perfect triangle. The top tube is subject to a combination of compression and a bending moment imposed by the leverage imposed by the fork. These forces are resisted by the 'stiffness' of the top tube and, especially with compressive loads, by it remaining as a straight column. Any deformation below its elastic limit will see the tube's 'springiness' return it to its original shape when the load is removed.

Now a 'bent' tube can be equally capable of resisting the bending moments but will be far less capable of resisting the compression loads. As it's already 'out of column' a much smaller compressive load will get it to continue to bend compared to a straight tube. This might be seen as a bad thing, but as long as the 'springiness' of the tube is sufficient to return it to its original shape, then you have a mechanism that can absorb, rather than simply resist the loads.

A common spring of this shape is indeed the leaf spring but the analogy is poor because the way a leaf spring is loaded is rarely by compression and a bending moment imposed along it's length.

The theoretical rationale is ok, whether it works in practice - I have no idea.

 

[Night Train]

I would hazard a guess that it probably works in practice but whether or not it is necessary or effective as a design is another point.

It may add a weight penalty but that depends on how close to design tolerence the simple straight tubes are.

 

[Smokin Joe]

The main frame of a bicycle is admittedly slightly more complicated as it is not a perfect triangle.

It isn't actually a triangle at all, it has four tubes.

 

[oldroadman]

Tubes should be straight, and thin, and light, and made of steel, aluminium, titanium (the best!!) or carbon. There was a silly designwith an unsupported "beam" top tube to give a "softer ride". Which simply flexed the distance betwen saddle and chainset - not the best idea. Some of the bikes pictured would be bought by triathletes or the dafter (more gullible) members of the fraternity. Like a lot of things, it's probably a marketing thing looking for a market, and offers nothing new to what is a virtually perfect design, the basic bike frame.

 

[keefus]

you leave my top tube alone!

 

[slowmotion]

Now a 'bent' tube can be equally capable of resisting the bending moments but will be far less capable of resisting the compression loads. As it's already 'out of column' a much smaller compressive load will get it to continue to bend compared to a straight tube. This might be seen as a bad thing, but as long as the 'springiness' of the tube is sufficient to return it to its original shape, then you have a mechanism that can absorb, rather than simply resist the loads.

Well put. That makes a lot of sense.

 

[sunnyjim]

Well, I'm an accountant but, IIRC, all other things being equal and simplifying hugely, a solid bar will bend, a hollow tube will crumple.

This is why things that we want to bend, like springs, are made out of solid bars whereas things that we want to be stiff (like frames) are made out of tubes.

Although anyone who has looked along the wing of an aeroplane in turbulence will know that there is further simplification behind the line that aluminium doesn't flex and it'll break if it does so.

I guess we do need an engineer after all.

I'm an engineer and IMO bike companies can make more money selling gimmicky shaped frames than boring conventional ones. They've probaby got more accountants than engineers.

 

[BrumJim]

Engineer here:

Concur with the above. Would expect a curved tube to be less stiff, and therefore absorb forces better, but since most loads will be coming up from the road, it is the shape of the front forks and the rear stays that is probably the most important. Would need to fire up an FEA package to look at the real effects, as not too sure what the effects of the various bike loads are on the structural members without spending a bit of timing thinking about it. Can't imagine that the top tube makes much difference, though, to transmitting road loads to the rider. Heard another reason, is that the aesthetics of the top tube are better when bent, as a compact frame looks a bit wrong on a road bike without this.

So, over to the designers on here for comment...

 

[mickle]

Designer here:

http://www.facebook.com/photo.php?pid=418159&l=00520c7a25&id=100000936421951

Sloping top-tubes are nothing new...

 

[dellzeqq]

Well here goes!

We all know that a triangle is very rigid in the plane of its component parts. What ever loads are applied to the corners in the direction of that plane are resisted by each of the three constituent parts being either under tension or compression along their axis. The only way a triangle can 'fail' is if one of the sides that is under compression comes out of column (in other words bends or buckles) or a side under tension fails (breaks).

The main frame of a bicycle is admittedly slightly more complicated as it is not a perfect triangle. The top tube is subject to a combination of compression and a bending moment imposed by the leverage imposed by the fork. These forces are resisted by the 'stiffness' of the top tube and, especially with compressive loads, by it remaining as a straight column. Any deformation below its elastic limit will see the tube's 'springiness' return it to its original shape when the load is removed.

Now a 'bent' tube can be equally capable of resisting the bending moments but will be far less capable of resisting the compression loads. As it's already 'out of column' a much smaller compressive load will get it to continue to bend compared to a straight tube. This might be seen as a bad thing, but as long as the 'springiness' of the tube is sufficient to return it to its original shape, then you have a mechanism that can absorb, rather than simply resist the loads.

A common spring of this shape is indeed the leaf spring but the analogy is poor because the way a leaf spring is loaded is rarely by compression and a bending moment imposed along it's length.

The theoretical rationale is ok, whether it works in practice - I have no idea.

I had always thought that the intention behind bending the top tube was to make the connections at either end of the tube closer to 90 degrees, which would reduce the propensity of the tubes to tear apart at the top when the bike is compressed front to back - the crudest example I can think of would be a rear end impact. The Spesh top tube to head tube connection is broad, and, I would imagine, very stiff and strong. I don't particularly care to think about this, but it might be better able to resist the tension force that gave me a small crack in the lug of my conventionally framed bike.

 

[John the Monkey]

Well, I'm not an engineer, but I much prefer the look of a traditional, horizontal top tube frame. At a pinch, a compact frame would be acceptable. All this sweepy swoopy stuff, no thanks!