To continue the discussion (and the section numbering) from Part I:

(3) Examining the photograph of the two bicycles, it is evident that relative to saddle height, the handlebar height on the CRP bicycle is lower than on the steel bicycle. Dr Groves has kindly confirmed this and by his measurements the tops are roughly 8.5 cm lower (the differing bar shapes reduce the difference at the drops). No doubt this is one of the factors making the CRP bicycle uncomfortable. For this sacrifice, such an adjustment should make the body position more aerodynamic. The wheels on the CRP bicycle are also more aerodynamic, by virtue of having many fewer spokes (they are therefore also much less reliable, and effectively not user-repairable), as well as thinner tires. The steel bicycle has full front and rear mudguards while the CRP has only a vestigial rear one, likely again to the advantage of the CRP bicycle. The shapes, sizes, and configurations of the bicycle frame's tubes, and of its components, also affect the drag force; see <www.sheldonbrown.com/rinard/aero/aerodynamics.htm>.

4. Examining the photograph further, it also appears that the bottom bracket (crankset axle & bearing assembly) of the steel bicycle is lower than that of the CRP bicycle (Dr Groves confirms it is 2 cm lower). As has been known since at least 1939 [1], the geometry of a lower bottom bracket, as of that of a longer wheelbase, smooths the geometry of the rider's travel over a bump, making the ride both more energy efficient and more comfortable. Increased comfort on a ride, in basic position and also in response to bumps, helps the rider to maintain effort over long distances.

[1] Davison AC. Long or short wheelbase? Cycling, May 17 1939: 699- 700. <http://www.classicrendezvous.com/Events/Long-or-short- wheelbase_1939.pdf>

5. Rolling resistance is not the same as friction. A rolling bicycle wheel does have some friction, but precisely because it is rolling, not sliding, this friction is minuscule even for the rear wheel, with effectively the entirety of rolling resistance being due to hysteretic losses in the sidewalls and tread, as the contact-induced deformation makes its way round and round the tire. This means that the rolling resistance is greatly affected by the thickness and viscoelasticity of not just the sidewalls but also the tread, as well as any reinforcing materials.

Dr Groves assumes that tires of the two models used, Schwalbe Marathon 700x32mm on the steel and Schwalbe Marathon Plus 700x25mm on the CRP, have identical rolling resistances and further that their common coefficient of rolling resistance (Crr) is 0.0045, giving a power loss of 26 watts on the steel and 24.8 watts on the CRP. This is incorrect on all counts:

(1) The inflation pressure was not mentioned. This can have a dramatic effect on rolling resistance, although at higher pressures the effect levels off. At the same pressures, all else being equal, wider tires have less rolling resistance but more drag. However, it is not normal to run a 32mm tire at the same pressure as a 25mm tire. For Dr Groves' weight, about 85-90 psi would be a good starting point for a 25, and about 55-60 psi for a 32. See <http://www.precisiontandems.com/photos_files/tirechart.jpg>, <http://www.bikequarterly.com/images/TireDrop.pdf>. As a complication, the actual widths of many tires can be several millimetres more or less than their nominal widths. Even for the same model, some manufacturers use thicker cords or rubber for tires of the wider widths; others may use thinner rubber, while still others keep both the same across a wide range of sizes.

(2) The tires used by Dr Groves are amongst the heaviest, most durable, and most puncture resistant available, of the kind one might consider for cycling in Waziristan or Mogadishu. Most published rolling resistance tests are of much faster tires, and these have shown that even amongst expensive racing and training models of the same sizes and occupying overlapping market niches, Crr values on polished steel drums or plastic rollers- which are much lower, and have much less scatter, than those found for pavement- can range from less than 0.003 to almost 0.007. See: <http://www.rouesartisanales.com/article-1503651.html>, <http://www.terrymorse.com/bike/rolres.html>, <http://biketechreview.com/tires/rolling-resistance/475-roller- data>.

It seems unreasonable to assume that a Schwalbe Marathon Plus, a heavy-duty tire with, according to Schwalbe, 1 cm (!) of rubber between the tube and the road, weighing 590 grammes in 700x25mm, should have a Crr of 0.0045 on English roads, when the Schwalbe Stelvio, a racing tire weighing 223 g in 700x22.5mm, tests out to have a Crr of 0.0059 on a polished steel drum. Further, despite their similar names, the two Marathon models are of radically different internal construction, and also of different sizes, both of which affect the Crr. While the tires on the steel bike are older, and increased wear generally reduces rolling resistance by thinning the tread, Dr Groves has advised that the Marathons had only 400 miles more than the Marathon Pluses, likely not a significant difference.

Dr Groves relied on Schwalbe literature- presumably the chart found at <http://smtp.schwalbetires.com/tech_info/rolling_resistance>- to conclude that both tires have the same Crr, but this is only a coarse classification and can't be relied upon. However, in my calculations, for the sake of comparing the effect of cycle weights alone I also disregarded any differences and likewise kept the two Crr's constant. I did though use a more reasonable baseline value of 0.007- still likely an underestimate, if only given the road conditions. Increased rolling resistance, as with reduced drag, makes the effect of weight more pronounced, but in either case the effect is slight.

This does emphasize another crucial difference between our two methods: with calculation, we can do a sensitivity analysis, such that disregarding the real difference in a confounding factor betters the accuracy and reliability of the fundamental conclusion; while in the experiment, such disregard worsens them.

6. We are told that "lighter rims can confer a significant advantage, but only if there are a significant number of points of speed change on the journey." This would be correct if by "significant advantage" it were meant the tiny number of tenths of a second that could conceivably mean the difference between winning and losing a 40 km criterium race, the type of race with the most and hardest speed changes. In reality even at full throttle the maximum acceleration of a bicycle is so relatively minuscule that there is no difference in effect between weight on the frame or the rims. In fact lighter-rimmed wheels are often slower, because extra weight often goes to a more aerodynamic shape. See <http://biketechreview.com/reviews/wheels/63-wheel-performance>.

In Part III calculations are made to show how the main confounding factors may be exploited to reduce commuting times.

Scoolboy error is actually calling the trial randomised. If you know which bike you are on the placebo and nocebo effects will dominate any actual variation. If you repeat as double blind trial results will still be garbage because speed will be limited by the guide dog

This article was an interesting piece, even if written in a tongue and cheek objective. Yet bike commuting is a serious matter for public health and transportation planning in nearly every city, as BMJ has published on this matter previously. So I'm not surprised if this study will garner many polarized opinions and technical comments. Perhaps that was the real objective?

At any rate, I must agree with many of the commentaters that there are numerous holes in the study:

When riding a new carbon bike, one naturally protects one's interests by riding more cautiously. Carbon fails; steel bends. Hence unless the writer is finacially indiferrent, there's a bias inherent in the type of bike ridden.

Using the median is generally preferrable when speaking about time periods. For instance, in the analysis of waiting lists, medians are reported to remove the skew effect of outliers.

Were "track stands" performed? A full "dab" stopping technique is usually not performed on a high end bikes.

"Non-clips" along with running shoes? Efficiency of high end bikes is greatly increased by pedalling technique. A pounding style, further marred by the hyper-extension of the achilles due to the flexible shoes, is poor form on a high end bike. The full package should, perhaps, be evaluated: tweed- commuting versus racer-commuting.

Tester's physical ability was not standardized. On a track, in controlled conditions, was the tester faster or more effecient on the carbon bike? There is a learning curve associated with high-end, twitchy, bikes. Likely the tester went along this curve as the season progressed, with lower times resulting.

Stopping power was not analyzed, nor the number and type of stops tracked. Commuters must stop instantaneously to avoid death. But the bike setup, braking power, attention, skill and many other factors affect this. Random and regular stops were not included.

Winter? Lycra-clad cyclists have a "wet" bike. And bikes accumulate water in the frame and in the tires, changing the weight and rolling weight. This would create a bias if the same bike is ridden the next day.

Will the tester, or reader, be persuaded to commute again the next day based on the bike chosen? Since the carbon bike requires a more aggressive cycling position, how likely will a L5-S1 bulge or herniation occur over time, thereby curtailing one bike-to-work career?

Speed is an immaterial, if not dangerous, goal during an urban commute. It's not about how fast one gets to and from work; rather, it's about one's physical and psychological state during and after the voyage. A study on the effect of route choices available to commuters according to the vehicle chosen would be more informative. For example, chosing grid- like car routes on a carbon bike versus watershed trails on a mountain bike. Educating the reading public away from car-centric mentality for route choice may lead to more satisfied, and safer, bike commuters.

J.

As both a scientist and a recent convert to the MAMILs I am delighted that the BMJ has finally recognised the importance of the study of bicycles to international health. However, I agree with several other commentators that the article by Dr. Groves is seriously flawed and should not have been published. First reports in a new field tend to be the most cited regardless of subsequent evidence from better-designed studies which contradict them. In this regard, I find it appalling that BMJ has chosen to allow open access to this particular piece of work. Now, rather than being confined to a relatively small number of technical experts with the knowledge, skills and appropriate training to evaluate the weaknesses of the claims being made, the work is readily accessible to a general public seriously lacking these attributes. Unfortunately, among those to whom the author's conclusions are available are spouses, partners and family members of cyclists. The majority of these people are a) greatly influenced by pseudo-scientific claims made by both print and electronic media, and b) extremely reluctant - and in my case, absolutely refuse - to listen to an alternative viewpoint no matter how well-reasoned and scientifically sound. Will the editors accept responsibility for the thousands of us throughout the world who will be told "You don't need another bike, there's an article in BMJ that proves it"? I find it ironic that the author should compare the uptake of new cycling technology to the uptake of new pharmaceuticals when the major error he commits is identical to one so frequently encountered in drug trails: namely the measurement of surrogate outcomes. Frankly, who cares whether you save four minutes riding from Sheffield to Chesterfield? The more important questions are how does the rider feel about the journey on one type of bicycle versus the other; what impact does the bicycle have on the rider's self-esteem and feeling of worth; how are the rider's interactions with colleagues, friends and family impacted by the type of machine he/she rides; etc? Until researchers shift their focus from the purely mechanistic to the psycho-social aspects of cycling many MAMILs will be condemned to riding junk.