About frame stiffness – what is stiff?

During the many discussions with several frame stiffness testing centers and providers my understanding that frame stiffness tests are largely done in a somewhat inexact manner, mostly lacking any statistical validity, was reconfirmed. A quote from one engineering lab in particular sums up the situation nicely:

“And, to be perfectly honest, the bicycle industry has not been particularly demanding or even interested in a higher level of scientific rigour…”

So with that in mind I decided on our future steps which will produce highly credible, repeatable results. For the present however we decided to fall back to the magic of statistics and cohort analysis. We have stiffness data from one of our manufacturing partners and they happen to make several brands of frames that were extensively tested by the Tour Magazin over the years. Helpfully, Tour data shows useful consistency over time. We also know our own frame stiffness data since each of our carbon frames is tested for stiffness as part of our basic QC procedure. Thus with the other brand’s frames serving as a cipher key to correlate Tour test data with our own test data to within 5% accuracy (confidence interval actually) we can now present a comprehensive list of published Tour data for 19 competitors, and for three of our frames: Velocite Magnus, Velocite Geos, and Velocite Selene.

Table 1

The data in the table was sourced from Tour Quarterly online publication, from Tour Magazin print edition for November 2011 and April 2013. Only the data for German market specific online brands was excluded to increase relevance and chart legibility. Some current frame models are missing from the chart, but that is due to lack of Tour test data, not deliberate omission.

STIFFNESS TO WEIGHT RATIO AT THE BOTTOM BRACKET.

Chart 1 shows torsional stiffness as measured at the head tube, ranked from lowest (BMC Racemachine RM01) to highest (Velocite Magnus). This is the traditional way of ranking stiffness when other manufacturers talk about how stiff their frames are. Torsional stiffness at the head tube influences how well the bike will steer, all other things being equal. We can see that the best regarded brands (Specialized, Cannondale, Cervelo) all rank pretty high which is good to see as it shows that there is a degree of competition in the market based on product performance. It also shows that at the high end of the market, riders may get what they are paying for.

Coming to Velocite products, Velocite Magnus exceeds the torsional stiffness of the second stiffest frame by 17% while the Velocite Selene ranks third and Velocite Geos comes second from the end in a field of what are regarded as top end frames.

However as the second chart will show, this is not the entire story. What is not normally emphasised by the bike brands is stiffness at the bottom bracket, the pedaling stiffness. This measure describes how efficient the frame is in moving you forward. In simplified terms, the stiffer the frame is at the bottom bracket, the quicker it responds to your pedaling inputs and the more energy efficient it is. While the frame itself is just about perfectly elastic meaning that when looked at in isolation the frame flexing does not actually lose energy, the energy loss occurs as soon as you place a rider on the bike due to the rider being just about the perfect hysteresis material (damper, energy sink). Thus the stiffer the frame is at the bottom bracket, the quicker it will respond and the more efficient it will be in converting your energy into forward motion instead of wasting it.

COMBINED HEAD TUBE AND BOTTOM BRACKET STIFFNESS.

This second chart (Chart 2) therefore shows a somewhat more complete picture. All the Velocite road frames rank significantly higher than any other high end frame. In fact the Velocite Magnus is cumulatively twice as stiff as the least stiff of the high end frames studied and 51% stiffer than the next stiffest non Velocite frame. This stiffness advantage is not marginal, barely perceptible or within the test protocol margin of error, it is significant and large. You can feel the difference immediately when you ride any of our bikes. What are the drawbacks of this extraordinary torsional stiffness? There are none, except for the higher frame manufacturing costs which we absorb. Despite their stiffness none of our frames are uncomfortable, none experience any rear wheel skipping and none of them are fragile. This exceptional performance is essentially free to the rider.

Why don’t other brands pursue pedalling stiffness? They in fact do and they spend large portions of their technology descriptions, press releases and training materials describing how they focus on gaining the maximum pedalling stiffness. Some even invent new super wide bottom bracket standards in the pursuit of pedalling efficiency. Other brands on the other hand make claims that “beyond certain point, there is no difference” – this is entirely incorrect due to simple physics. Frames operate in a perfectly elastic domain, well below their yield or non-linear behaviour meaning that every rider regardless of their strength is flexing the frame in the same ratio of force vs. deflection or N/mm.

We also did not achieve this performance leadership by making our frames heavy. For instance the Magnus size L frame (57cm) is just 1180g, painted and with all fittings. This compares very favourably with actual (not claimed) weights of the other frames studied.

To account for frame weight, for some time now many brands have been using an abstract measure of “stiffness to weight ratio” (STW) to describe the performance of their frames. The STW ratio is derived by dividing torsional stiffness at the head tube by the frame’s weight in kg. I call this an abstract measure as STW ratio (also known as specific modulus) is only useful to characterise mechanical properties of materials being used. STW ratio is entirely without merit when it is used to imply performance benefits in structures such as frames or wheels. Riders do not experience stiffness to weight ratio, they experience stiffness and weight separately. This is in contrast to the power to weight ratio which is directly related to how far and how fast you will go.

Nevertheless, high STW ratios have become the new marketing battleground with each new exotic and inevitably ultralight frame vying for the STW ratio crown. Thus here is a table listing the stiffness to weight ratios of tested frames, ranked in order of increasing STW ratio as measured at the head tube.

Table 2

Looking at the table above you can see the foundations of the performance claims made by the market leaders. Our first contender, the Velocite Magnus falls to the 5th spot in the STW ratio at the head tube table and the otherwise excellent alloy Selene now owns the last spot in this ranking due to its higher weight.

Let’s then have a look at what happens when the table is sorted according to the STW ratio as measured at the bottom bracket. This STW ranking is occasionally used by some manufacturers to establish dominance over their intended competitor.

Table 3

STRENGTH TO WEIGHT RATIO MEASURED AT BOTTOM BRACKET.

Things look a lot different now when the pedalling efficiency and responsiveness are ranked according to the frame weight. The Velocite Magnus now dominates this ranking convincingly, even against the very expensive, limited run and still very exotic ultra light frames. In fact the Magnus STW ratio is 30% higher than the next non Velocite competitor while the Geos STW ratio is 21% higher. Selene now also returns to the top part of the table with a solid 4th spot.

So what does the high STW ratio tell us when looking at modern bicycle frames? Nothing. Sorry. Please always look for absolute numbers, not a ratio of stiffness per unit weight unless you are choosing which material to use in your next project.

In conclusion, while attempting to maintain some objectivity it is clear that the Velocite Magnus, Velocite Geos and Velocite Selene are very special frames. They are either the stiffest outright or even dominate the synthetic STW ratio measure when it comes to their stiffness at the bottom bracket (pedalling efficiency). They also achieve this performance dominance not through excessive weight, but by focus on real engineering, material choice and advanced manufacturing methods.

To find out more or to buy your own Velocite frame or bike please click below:

Velocite
Velocite Magnus

Velocite Geos
Velocite Selene