- Our online table of contents is updated at least twice each day. Read all articles published in the last 7 days.
- You can use bmj.com to help you with your continuing medical education. Find out about CME/CPD credits for BMJ articles
- Keep up to date with cardiology: Access the latest cardiovascular medicine resources from across BMJ Group.
- OPEN ACCESS: All research articles are freely available online, with no word limit. Find out more about the BMJ's open access policy. Submit your paper.
- Find out how study types differ in our How to read a paper section.
- Impact factor latest: The BMJ's impact factor is now 17.215. Submit your paper to the world's fourth most cited general medical journal.
Recent rapid responses
Rapid responses are electronic letters to the editor. They enable our users to debate issues raised in articles published on bmj.com. Although a selection of rapid responses will be included as edited readers' letters in the weekly print issue of the BMJ, their first appearance online means that they are published articles. If you need the url (web address) of an individual response, perhaps for citation purposes, simply click on the response headline and copy the url from the browser window.
Displaying 1-10 out of 42 published
2 January 2011
Dr.Groves' elegant study has clearly stimulated a lot of interesting discussion here. Others have already pointed out the limited generalizability of his results. I wish to point out a few common sense (without any level I evidence) with regard to the choice of bicycle for commuting:
1. On snowy and icy days, a full carbon bike with skinny tires just
won't get you to work unless you intend to carry it all the way.
2. A bicycle commuter will never take the elevator! Carrying the extra 4kg weight to climb a number of flights of stairs may sway you to take the carbon bike.
3. The steel frame bike with a set of 28C tires may just be the right bike to ride through the trail instead of the open road saving time and possibly your head.
4. Nearly 2/3 of all commutes are less than 3km. Whatever time gain on the correct choice of bike is negligible when considering other important factors such as theft proofing your bike.
5. Aesthetic is important, even commuting to work. I don't know anyone who would put a luggage rack on a full carbon bike, nor anyone who wears the full spandex outfit while riding an old steel frame bike - that's just not done in any civilized society!
6. If you can only have one bike - get a full carbon! Life isn't about commuting back and forth to work. If you set a higher goal in life, such as to do an Ironman, you'll need the carbon bike!
Competing interests: I use a steel mountain bike only for snowy winters, a Scandium cyclo-cross bike for commuting all other times, and a full carbon bike for triathlon training and racing. Still happily married!
Click to like:
22 December 2010
Considering Parts I & II, what then are the most effective ways for Dr Groves to reduce his commuting times, for the same power output? For constant effort on dry paved roads and without braking, the two main energy sinks are rolling resistance and drag. At lower speeds rolling resistance dominates, while at higher speeds drag does. At an average speed of 15 mph over hill and dale, both are important factors. The following table summarizes the effects of some basic variations in the corresponding parameters. Each row of the table shows the effect of one change, all other parameters remaining at baseline.
Baseline parameter values: bicycle = steel, total rider power output = 130w, rider weight = 76 kg, Crr = 0.007, frontal area = FA = 0.5 m^2, drag coefficient = Cd = 0.9, road grade (average) = -0.13% out, +0.13% back, headwind velocity = 0, distance = 21.75 km each way, temperature =15 deg C, altitude = 400 ft, transmission efficiency = 95%.
Parameter Total Time TimeSaved AvgSpeed
Baseline 1:48:02.4 _________ 15.02
CRP bike 1:47:21.0 0:00:41.4 15.11
Crr=0.006 1:45:52.2 0:02:10.2 15.33
Crr=0.005 1:43:46.2 0:04:16.2 15.64
FA=0.4 1:41:25.8 0:06:36.6 16.00
Cd=0.8 1:44:28.8 0:03:33.6 15.53
Thus the most effective of these changes does not require any purchase but only to ride in a more aerodynamic position. Next would be to consider the rolling resistance tests mentioned above and choose a less hypertrophied tire, if necessary keeping a better lookout for road hazards. These two moves alone could easily allow him to sleep in more than five minutes, and moreover get home another five minutes early for a nap before supper.
For valuable information on many of these topics and many more, including discussions of specific tire models, the reader is advised to examine the well-worn archives and FAQ of the Usenet newsgroup rec.bicycles.tech, such as may be found at <http://draco.nac.uci.edu/rbfaq/FAQ/index.html> or <http://www.sheldonbrown.com/brandt/>.
Competing interests: None declared
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 , 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.
 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.
Competing interests: None declared
22 December 2010
The bicycling community includes many medical doctors, and some have wondered whether their expensive carbon-reinforced plastic (CRP) framed bicycles really give them any speed advantage. In 2010 one of them, Dr Groves, became curious enough to use the methods of his profession to investigate the matter, and we should be grateful to both Dr Groves and the BMJ for the provocative and useful article that resulted. After six months, 56 journeys, and 1500+ miles of riding over hill and dale in English weather, on a 27 mile out and back commute, he found his average speed to be 15.03 mph using his 29.75 lb, GBP50 steel-framed bicycle, and 14.95 mph using his 20.9 lb, GBP1000 CRP-framed bicycle. The average round trip time advantage during the winter was 2 min 35 sec in favour of the steel bicycle, while in the summer it was 1 min 04 sec in favour of the CRP bicycle. The overall average difference was 32 seconds in favour of the steel.
The bicycling community also includes many mathematicians, physicists, and engineers, and most if not all of them have given some thought to the effects of lowered bicycle weight on speed. Perhaps since the dawn of the bicycle age, some of them became curious enough to use the methods of their profession to investigate the matter in more or less detail. In recent decades the state of the art made the endeavour practical enough that a number of them, after a few evenings at home at their desks writing computer programs, made quite thorough solutions available to all via the internet, for arbitrary pairings of bicycling circumstances. A few examples are:
<http://bikecalculator.com/veloMetricNum.html>, <http://www.analyticcycling.com>, <http://www.whitemountainwheels.com/SpeedPower.html>, <http://sportech.online.fr/sptc_idx.php?pge=spen_esy.html>, <http://www.grennan.com/BikePower/> [source code in C]
Using for example the calculating engine at bikecalculator.com with the relevant data contained in Dr Groves' article, and reasonable values for the parameters, I found an overall average round trip time advantage of 41.4 seconds in favour of the CRP bicycle. This is much the same as the 1:04 advantage for the summer months found by experimentation, an agreement that may owe as much to the consistency of Dr Groves' cycling efforts as it does to the heritage bequeathed to us by Galileo, Newton, Euler, and their many successors.
To be sure, this value is an approximation to the actual one for Dr Groves' circumstances, because I didn't bother to use the detailed topography, but only the average slope in each direction; because the other parameter values are also approximations; and because I ignored traffic considerations. Nevertheless, for the given parameter values and unlike for the methods of medical research, it has the benefit of unambiguously demonstrating the effect of weight alone, all other things being equal. Or for that matter the effect of any other parameter alone or in combination, all else being equal.
The differences between our methods can be summarized thus: in science we rely on, and therefore demand, thorough knowledge of the situation; while in medicine the hope is that a randomization procedure will cover up for ignorance.
The latter works passably well for random errors but not for systematic ones. Just because systematic errors (confounding factors) are missed does not mean they are not present. This is why most of epidemiology as it is practised today- perhaps as it ever will be- resembles an elaborate hoax. Dr Groves does far better than most epidemiologists by making a serious effort to account for the confounding factors. I take the opportunity here to call attention to a few more, and at the same time to correct a few errors in his presentation. I continue this in a second part, and close in Part III with a table showing the effects of various ways of exploiting the confounders to lower his commuting times.
1. We are told that in traversing a hill, the extra work done uphill on the heavier bicycle is recovered as extra kinetic energy on the downhill, because energy is conserved. In fact the bicycle is a dissipative system, not a conservative one. Energy is lost due to rolling resistance, drag, and braking; and even to other sinks such as jiggling of the viscoelastic rider and his luggage. All other things being equal, on the uphill a lighter bicycle should essentially always be faster, while on the downhill a heavier bicycle may be either faster or slower, depending on the balance between rolling resistance and drag, as well as such things as the need to brake on sharp curves.
2. Drag, a force, is proportional to the square of the velocity, not the cube. The power lost to drag is proportional to the cube.
3. We are told that the power required to overcome drag is independent of which of the two bicycles is ridden, and that for either one at 15 mph, it is 170 watts. This is incorrect on all counts.
(1) Using the parameter values listed in the article, the power formula 0.5 x 1.2 x 6.7^3 x 1 x 1 gives 180 watts, not 170.
(2) Dr Groves gets this formula from the standard drag-force equation, using a value of 1 for his drag coefficient (Cd), and a value of 1 m^2 for his frontal area (A), their product (CdA) being 1 m^2. He might consider that a frontal area of 1 m^2 would be that of a person of height 2 metres and constant width 0.5 m from top of head to soles of shoes (rather tall and strangely thick), standing fully erect. This seems unreasonable for a thin crouched-over cyclist, and indeed the CdA values typically reported for bicyclists are in the range of under 0.3 m^2 (racing time trialists) to about 0.8 m^2 (fully upright on a city bike; see <http://www.sheldonbrown.com/rinard/aero/formulas.htm>). For my baseline calculations, considering the drop-bars on the two bicycles, I used values of Cd = 0.9, A = 0.5, for a lower CdA of 0.45 (m^2). This is a conservative assumption as reducing drag makes the effect of weight more pronounced. The value of 1 used by Dr Groves was from a citation for a "touring bike"; this may mean one with handlebar bag, panniers, and so on. The bicycles shown in the photograph accompanying the article are better described as traditional audax (the steel) or racing (the CRP).
A robust method for estimating CdA using a power meter is presented in <http://anonymous.coward.free.fr/wattage/cda/indirect-cda.pdf>, while nowadays one can ride with a bicycle computer that calculates it on the fly: <http://www.ibikesports.com/products_iaero.html>.
More on aerodynamics and other matters in the second part.
Competing interests: None declared
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
Competing interests: Triathlete
Click to like:
21 December 2010
My own experience of commuting a shorter distance across Sheffield is entirely compatible with Dr Groves findings, with the optimum journey times being achieved on a mid range road bike; my 3 speed Brompton is hopeless up the hills and the 2.5 inch tyres on my carbon mountain bike have too much rolling resistance.
However, whilst Dr Groves clearly identifies a number of the factors which may effect the length of commute and the amount of effort required to achieve this his conclusion regarding the usefulness or otherwise of a carbon frame is limited to the effect on journey time.
Of equal importance, based on the increased mass of the steel frame together with the likelihood of it being fitted with lower quality components, achieving shorter journey times presumably requires more energy and will therefore deliver greater health benefits.
Perhaps this line of reasoning should be employed when trying to explain to other family members why they really should be perfectly happy with a much less expensive bike than mine?
Competing interests: Currently seeking spousal approval for (another) completely unjustifiable carbon bike
Sheffield Children's Hospital
Click to like:
20 December 2010
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.
Competing interests: Bike commuter for 15 years; Owner of 4 types of commuter bikes, excluding tandem; 1 - 4 hour daily commute; member of a old-timers bike racing club (FOG); back injury attributed to cycling.
FOG (Fast Old Guys)
Click to like:
20 December 2010
So Dr. Groves has figured out that a high-end bike confers no significant performance advantage in a test of low-performance demand. Is anyone surprised?
First of all, what appears "insignificant" on the clock might appear otherwise when analyzed spatially. Groves reports that he rode at an average velocity of 15 mph, or 22 feet per second. If we were to have Dr. Groves race himself during his 27-mile commute on both bikes, record the performances on digital video and superimpose them (as they do in telecasts of the Olympics), we would see the carbon-framed Groves cross the finish line 704 feet ahead of his steel-framed self. In the world of competitive cycling, where the top ten finishers are often grouped within a 3 or 4 meter span, 704 feet would be considered a huge performance advantage.
But this would be better analyzed in a high-performance context. If Dr. Groves absolutely positively had to make it to work in under 1:30, and would be fined by his employer for each second he was over, the advantage of the higher-performance machine would very soon become apparent.
You don't need a Pinarello to pedal to work at a leisurely pace any more than you need a Ferrari to drive to work at city speed limits. But in a situation of high-performance demand, the advantages of high- performance equipment WILL emerge.
Competing interests: None declared
Click to like:
18 December 2010
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.
Competing interests: Only owns one bike (at the moment).
Click to like:
18 December 2010
How I love the 'n of 1' trial where I can be both subject and investigator. Congratulations, Dr Groves, on your motivation to faithfully conduct and write up this study. The rapid responses clearly indicate that similar research questions have been asked and trials performed in the setting of cycle commutes the world over, but the rest of us have lost the race to publication.
I, too, recently purchased a 1000 pound (sterling) carbon frame bicycle on a ride to work scheme after previously commuting on a steel frame bike that is about 4kg heavier. My round trip commute is 24 miles. I find that there's no appreciable difference in the time that it takes me to commute BUT my knees feel less sore whilst I am riding the lighter bike and I've noticed that my quadriceps seem less bulky (observed, not measured) since I began riding it. I found the new bike far less comfortable than the old at first but after my husband, a bike 'anorak', made some adjustments that involved indulging in his favourite hobby - buying bike parts on eBay - the ride became enjoyable again.
Said husband owns a bike that, like the human body, never consists of the same components from one month to the next. He trades bike parts on eBay convinced that finding the perfect mix of parts will lead to the creation of the perfect bike. Husband does not commute on his creation, in fact his bike leaves the house 6 or fewer times a year, most often to be ridden to a local bike shop, but he is very proud of its lightness. James Ward wonders about the effect of this study on marital harmony. I can say that in a study of 1 married couple this wife felt great righteous satisfaction on showing the study to her husband who labours under the (false and untested) belief that the lighter a bike is the better it is. However, as her husband's belief in his rightness was curiously unaffected by the findings of the study, and positions remain entrenched, the overall effect on marital stability was negligible.
Competing interests: owner of several bicycles and frequent cycle commuter; not involved in the selection of this paper for publication
Click to like: