Monday, September 27, 2010

Lighter bikes: the basics

It is all right to wish for a bike much lighter than what we have. It is a never ending quest, occasionally verging on the futile, but an entirely rational and justified one: we want to make our daily source of pleasure and transportation as efficient as possible. A light bike is more agile than a heavy one and makes it easier to negotiate the hills—an undeniable fact of life in San Francisco—without heavy breathing. What is wrong with that?

The new Storck Fascenario 0.6 frame is rumored to weigh
less than 600 grams. To save weight, the brakes are 

integrated into the frame. Photo © VeloNews.
But can we go too far in our pursuit? How far is too far? I was pondering these questions this morning, when I was looking at the photos of a sub-6 lbs. complete road bike, shown at Interbike, the North-American trade show that was held in Las Vegas this past week. It was a special build by a renowned Arizona retailer. It is made of carbon fiber with the sole exception of the pedals. Many of its components are one of a kind, custom made for this project by German carbon wizards. The wheels alone are so light and expensive that they will not make into production. Its price tag? Let’s say it competes for attention with the sports cars made by a well known German company. Would I ride this bike or just look at it behind glass?
Professional cyclists would probably scoff at the very idea of making a racing bike lighter than it is, and they would have a point. If they worried about a few miles uphill, when a race usually includes heart-stopping climbs over far more than one hundred miles, they’d probably deduce they should quit their day job. Besides, a race bike should weigh no less than 15 lbs, according to the international race regulations. We common mortals, on the other hand, can use some technological assistance, provided that we don’t weaken the structural integrity of either the bicycle or our wallets in our quest.
If we are considering buying a bike, a light bike, that is, but set a limit to our buying spree, we should think carefully about what our needs are. First, we must consider the bike frame. Most frames, these days, are made of carbon fiber that has been first impregnated with resin and then baked in an oven under vacuum. This combination makes it the lightest material so far available for bikes. It’s a good starting point.
A monocoque frame, such as this BikeNüt Umlaut, is inherently stiffer, lighter,
and less prone to failure than a frame made of several tubes bonded together,
no matter how skillfully.

How does a bike frame work? A bicycle frame consists of a truss, a type of simple but effective structure with which engineers and architects are very familiar. Trusses are made of triangular modules assembled in various configurations. In the case of a bicycle, two modules, or triangles, share a common element, the seat tube. In all trusses the linear elements, the tubes, work either in compression or in tension and exert tremendous strain on the vertices of the triangles, also called nodes. This is the reason why, if the nodes are stiff and the elements don’t move, the rider can communicate all of the leg power to the pedals.
The bottom-bracket area, where several tubes are attached. This node is put to
the test whenever we pedal. It's important that it is stiff, that it doesn't move or twist.
Through this tunnel we insert the axle connecting the two sides of a crankset.
A fast rider will generate a lot of torque in particular on the front and the bottom nodes. Carbon  makes it possible  to strengthen them not by adding material but by enlarging the diameter of the connecting tubes and orientating appropriately the fibers of the carbon layers. Newly designed frames show just how thick these areas can be.
A sealed cartridge component ready to be installed. The cups containing the
roller bearings stick out from the frame, adding to the overall weight.
In the case of the bottom bracket, this trend has given origin to the so-called BB 30, a standard than is taking an increasingly enthusiastic foothold on the industry.  Of course, BB is an acronym for Bottom Bracket, and 30 indicates the size of the axle connecting the two sides of the crankset through the frame—30 mm. in diameter.
These are the few components required for the assembly of a BB 30.
The bearings are contained in the sealed cartridges in the foreground.
They are contained within the frame.

There is a weight advantage associated with a BB 30, a simpler mechanism, consisting of fewer parts than a more conventional bottom bracket. For example SRAM, a manufacturer that offers cranksets in several configurations, indicates that the newer BB 30 Red crankset is 20 percent lighter than their previous models. That’s a savings of 130 grams. We’ll have a more thorough discussion about cranksets in one of the future entries in this on-going series about light bikes.
The length of the connecting tubes is critical in determining the riding quality of a bicycle. It all gets swept under the general category of frame geometry. I have leaned a few rules of thumb about frame geometry: a shorter wheelbase makes a bicycle more responsive, quicker in descents and in cornering, and the feeling of riding one is that, well, it is quick. A longer wheelbase is good for a leisurely ride, comfortable and a bit sluggish, while chatting with friends and enjoying the views.
The fork is a crucial element in the geometry of a bicycle. There are many after-market forks out there that are much lighter than those provided by manufacturers with their bicycles, but we should be cautious in considering an alternative to them.
The Scapula fork, by THM Carbones. Simple and elegant in shape, this is one
of the lightest after-market forks available today. Photo © THM Carbones.

One of the lightest available is made by THM Carbones, the supplier of forks to Marcus Storck, who produces some of the lightest and stiffest frames available to consumers. Their fork is called Scapula—it weighs a mere 235 grams. All of their components take their names from human bones. To adopt a new fork, such as the Scapula, however lighter, than the one that comes with the bike, is to change the geometry and the riding characteristics of the bike. The weight savings would be a little more than 100 grams, but the bike will behave like a strange animal. Is it worth it?
What if you don’t want to buy a new carbon frame and are emotionally attached to your old steel frame? Is it possible to change a few components here and there and make it lighter and even more enjoyable? Of course. I’ll discuss some options in the next installment in this series.

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