Friday, November 7, 2008

Rational Bicycle Frame Design : Giving Consideration To Functionality

Courtesy : AluMatter

A lot of people these days question the age-old triangulated design of the bicycle frame. They think bicycle design is "dead" just because nothing has changed for a close to a century in relation to this structure, the biggest element that stands out when any observer sees a bicycle. And designers around the world are increasingly trying to break free from this constraint and come up with the most bold, radical designs. Some are successful for a while, others are hype that never deliver. Yet, the classic structure has endured through all the competition.

But its nice to ask why, or how this simple design has stuck with us for more than a century. 1887 to 2008 is 121 years!

See, there's a sound engineering principle behind it. Let me explain.

A structural member is weakest when it supports a load which tends to bend it. We all know that. If this member is propped up from below, the load is carried in compression, and much more load can be carried. Still better yet is an arrangement that is strengthened by a cable such that it is in tension when applied with a load. This is the triangulated structure and it is the lightest and strongest structure arranged in such a way that its members are all in tension or compression, but never in bending.

I'm not a historian but I reckon that this brilliant idea might have made its way from age-old bridge and roof design somehow into bicycles. And people soon found out how they could have the lightest structure possible, which is strong at the same time to resist the forces encountered on riding.

Most, if not all, bikes we see these days are derivatives in some fashion or the other of this classic frame structure. The bike may have a few bends and curves here and there, but the fundamental principle still stands. To get a perspective on the road bike business, it is partly around these minimal changes that bicycle companies try to make money off you.

So, if anyone can come up with an alternative frame design that does the job better, without diluting the advantages of the original truss frame, we'll see something new. Until then, keep crying...and pass the hanky. (Note that some folks tried to extrapolate the triangulation to the fork as in the Pederson Bicycle... see below. Yet, interest in these bicycles also diluted with time)

I had to revive a nice snippet of text from David Gordon Wilson's classic "Bicycle Science" (MIT) to share a few other ideas on frame design.

In it, the author talks about the basic premise behind a diamond truss frame that we all commonly ride around, its drawbacks (which we can witness through all the pictures of broken steerer tubes & forks) , and what generally is expected out of a sensible bicycle frame structure. In the following short sections, he quantifies some of the peak forces experienced by the frame at various points, and gives tips on how to set up instrumentation to measure these stresses, both for folks with the equipment to do it and for those who don't have the money.

To put a bottom line to this post. there may be many fancy conceptual ideas floating around on the internet out there that make headlines and often bring much excitement. But if the resulting machine cannot ride properly or snaps under the application of forces, its not only useless but also dangerous for human use. In this regard, it violates the basic canon of engineering ethics which states something like : "Engineers shall hold paramount the safety, health and welfare of the public and shall strive to comply with the principles of sustainable development in the performance of their professional duties."

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Reference Image Of The Pedersen Bike (Visit Pederson Bikes for design features)

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Bluenoser said...

Thanks for that ron. I think the first frame I want to tackle is going to be a cyclocross frame. I wonder what the diference will be between lugged or welded?

Bontrager seems to think lugged can't hold up but Sachs races lugged.

Any thoughts?


Ron said...

By the way, Johaan Musseuw used lugged frames in many of his cyclocross races. He even won Paris Roubaix on them. If lugged is strong enough for him, I think the rest of us flailing amateurs should be pretty well off. :)

The only other thing I can think off is weight of the frame, since cyclocross requires you to pick it up while running across obstacles and climbing up short hills. However, the right technique to picking a bike in cyclocross only involvs a slight twisting of the frame with the pivot point made by your holding arm's elbow and the saddle. I don't think a pound of extra weight is an issue for such a minimal movement.

But if you have the money, get the best equipment for the application.

Phil said...

Ron, also dont forget the rigid rules of the UCI has also been discouraging bicycle makers to make something different with regards to racing bikes. The risk of not being able to produce something that can be pedalled by top competitors in the world, including the loss in business and money through advertisements, must be too great. It is unfortunate that the bicycle is boxed in a corner like this. One one hand, there is an age old design that works but people complain about it, on the other hand...some agencies equivalent to the medieval Catholic Church restrain design and consequent racing on newer designs. Great post!

Anonymous said...

Some tried and tested designs will never leave us. Is the modern passenger airplane looking any drastically different from the vintage ones, although it has gone through several phases? I think while structurally they have not changed, its the materials aspect thats really botching up transportation equipment. Same is true for bicycles, where most of whom I observe are competing on type of material used, to make it stiffer and lighter.

Human_Amplifier said...

Great Post Ron !

Thanks for the refs too .. not seen them before, and I LOVE Ashby tables to graphically compare different properties.

Good point Phil - its a good thing UCI have little control over MTB, folder or HPV development or there would be little new in bike development beyond agonising over a the microscopic difference a the direction of a bit of carbon weave goes in :-)

Finally I guess everyone has seen the "Bicycles & tricycles" by Archibold Sharp, Imperial College, <<1896>> !! ). Which still has loads of relevance today and shows a vast range of alternatives from around the genesis of the diamond frame, but before it had been 'chosen' as THE one. I particularly love the graphic force diagram solutions, which still make quick force analysis a doddle with acruate 2D cad systems.

Now FEA is a a very useful tool to play 'what-ifs' in 3D. But as ever with computors they are just tools, and its creative engineering that the humans bring in.

Anonymous said...

love the hammock seat on the pedersen

Ron said...

Well, for that matter even Merckx based most of his business on high end carbon frames as well, when he never raced on one in his life. Its a good question to ask these guys - why don't you promote the type of frames you guys won all those great races on. :)

Anonymous said...

You cooked this post real good. I hardly thought about these issues before I read your post. Keep it up.

Anonymous said...

Ron - The robots in Asia making the majority of bicycle frames sold throughout the world are called people.
It doesn't matter the material used in the construction, people are doing the work.

A TIG welded steel frame is lighter, as well as cheaper to manufacture then a lugged frame, and it offers the possibility for a greater range in design with regards to geometry as well as tube size & shape.

Any strength that you may gain from a lugged frame can easily be offset in a welded frame using tube size, shape and butting which will create a frame which is still cheaper and lighter to manufacture.

In his day, Merckx was super into anything that would give him a tactical edge on the competition. If carbon fiber frames were made back then, I am sure he would have been one of the first to ride them. In fact Merckx was a driving force in bringing carbon fiber frames to market for this very reason. It might have had something to do with his kid racing and overall support for the teams he sponsored.

Lastly, are you aware of any deign that has been proven to have a superior strength to weight ratio combined with the ability to resist torsional loading while maintaining vertical compliance then the traditional UCI mandated 'double diamond' design of the safety bicycle?
If so, please share this.

- Ryan

Anonymous said...

was just thinking, in designing a bike frame.. wouldn't u have to consider bending moment?