I am going to post this interesting but very LONG piece i found a while back , im only posting this for others to read and i am not interesting in getting in a debate nor will i comment on this article . One piece of marketing that is so constantly referred to, that is has become unquestioned - and is again repeated early in this thread - ‘a larger diameter wheel has a larger contact patch'. If 10 bikes with different diameters rims and different widths of tyres all pressured at 30psi, all the same elasticity (ie the same brand /tyre type) were placed with an 80kg rider onboard... Which would have the greatest area of contact patch? The answer is that the area of contact would be the same. Only the shape of that contact area would be different. The reason is 30psi is a measurement of the pressure of the tyre not the volume of air in it. Volume is very different. The higher the volume of the tyre allows a lower pressure to be run due to less chance of rim hits etc. This lower pressure allows a larger area of contact patch to form. More volume also means less sidewall flex (better efficiency) and more capacity for deformation (more comfort is one upside of the latter). There are many positives for large volume tyres. Volume of a tyre is the same calc as a volume of a cylinder: v=r²πh , with r being ½ the tyre width and h being 2πr of the tyre dia taken from the longitudinal axis. It is therefore easy to determine that a smaller dia wheel can have a larger volume of tyre simply by having more width in the tyre. For example. A 29x2.25 Racing Ralph which has a approx dia of 57mm has around 5214cm3 whereas a 26x2.4 Racing Ralph has a approx dia of 62mm has around 5595cm3. The smaller wheel has the larger volume tyre. This would allow the smaller dia wheel to run less air pressure creating more contact patch area. The smaller dia wheel would also have less sidewall flex and less deformation than the larger wheel - not the other way around. A popular race tyre is 26x2.25 Racing Ralph (57mm dia) with 4709 cm3 and in volume terms the corresponding 29 sized tyre would be the 54mm 29x2.1 Ralph with 4669 cm3 . Tubulars are popular in race teams with large dia wheels – a 29x50mm tubular - 4000 cm3 has a little less to a Racing Ralph 26x2.1(54mm) at 4216 cm3. That a large dia wheel is the main determinant for a larger contact patch is incorrect. It is pressure of the tyre that determines these parameters and volume is the large determinant of pressure used and, in this context, the width of the tyre is the main determinant in volume not the dia of the wheel. One downside to a larger volume tyre is the greater mass which leads to the next myth and where this equation is important. The simple formulae for translational energy for a wheel: (1/2)mv². Rotational energy for a wheel: (1/2)Iω² Total energy of a wheel is therefore: 1/2(mv² + Iω² ) For a wheel with all mass at the outer edge: I=mr² For a rolling wheel: ω=v/r Putting it all together you would get: 1/2(mv² + Iω² ) = 1/2((mv² + (mr² )(v/2) ² ) = 1/2(mv² + mv2). The outcome is 2x. This means any extra weight that is on the outside of Wheel A compared to Wheel B is pretty close to double by the time it hits the riders legs to apply power to accelerate it. A real world illustration of this is demonstrated in the power data collected by WC XCO race teams. Early in 2012 I was sent power data sets for different analysis (nothing to do with bikes per se rather robustness of an aspect of the data collected). The sets contained different tyres and setups, some different frame setups, some different wheel dia setups used in short course XC. The majority of data is for tyres - from heavy sidewall, differing pressure, tubs, high pressure 29 skinny to low pressure 26 baggies. The riders were elite girls and guys riding test courses and WC courses in the 2010 and most of 2011 comp. There were nearly a hundred data sets each containing the results of many hours of testing. Every major XCO team on the World Cup circuit would have similar with the three larger corporate teams probably many times more. For example. The stronger sidewall tyres predominantly used and tested in this team weighed an approx 50g heavier than the standard tyres. The additional 100 grams on the edge of the wheels returned between 1.2%ave and 2.9%ave less efficiency ie more power over time, from the different courses of the 18mths of tests. The spread is caused by the varying amount of climbing etc between courses. When the climbs are considered separately the figure got up to a high of 7.5%ave difference. This might be considered a small weight on the outside edge but this amount of weight can be significant on a course with significant elevation, pinches or accelerations. When the rider/bike weight is low, as elite XC racers usually are, it becomes more significant. In all of the data sets any time significant weight was added to a wheel there was an assessable decrease in efficiency of that wheel. So although the laws of physics prevailed, how much loss was dependent on different factors – this data did not show a linear correlation for weight increase. There are many factors that influence power which must be considered in the context of the test circuit. Larger dia wheels are at an inherent weight disadvantage compared to smaller dia wheelsets created by mass difference of the rim and stronger build needed to offset additional flex (about 10% to 14% of the rims and build) and WC teams use different methods to gain parity. For example. Analysis included a 1160g 29 tubular wheelset that when a similar volume tub tyre was added was of comparable weight of the standard team 26 clincher wheelset with a tyre of similar volume. The data returned was within or about the same for both wheelsets. (This team – and most others – judge that due to accuracy tolerances of equipment and other variances that if sets of data return less 1% difference it is too close to call). Another Example. An investigation of a heavier (than the previous 1160g) tubular 29 wheelset with the standard 26 clincher race wheelset with a higher volume tyre to create similar weight. Again the results fell on or under 1%. Also. The larger WC teams use ‘line’ rims and ‘line’ tyres (pulled from manufacturing lines, they are lighter than normal due to manufacturing tolerances) Using these components it is possible with ½ nipples and cleaning up the rims to construct 29 wheels as light or lighter than many 26 wheels used. If that team then used similar volume clincher tyres then the expectation (amongst this team that is without these resources) is the larger dia wheelset can be as power efficient as a team with the smaller dia clincher wheelsets without the disadvantages that tubulars pose. The team determined, as suggested by this data (ie with the short course XC setups, tyres and riders tested), if rotating weight is similar there is no assessable loss of efficiency between dia of wheelsets used. There is some practical restriction in choice of stronger sidewalls/volume etc for larger wheels due to the corresponding increase in weight when it is already compromising to get the larger wheelset down to acceptable weight. Tyres are most easily adjusted to get a wheel to an acceptable weight. For example. When 29 wheels first appeared a 24hr racer (Ed Harris) ran a test in a race – and uploaded it - between the two available wheel dia at the time using powertap hubs. His first impressions were the smoothness and perceived speed of the 29 bike. He discovered after reviewing the power data the smaller dia bike had a 7% power efficiency advantage over the larger wheel dia bike. The large figure had come about because he had used the same width of tyre on both bikes as opposed to the same volume. So as well as the already 10% - 14% weight penalty on the rim/ spokes/nipples of the stock 29 wheels he had added more weight with a larger volume tyre. He may well have had an extra 100 to 200 extra grams on each wheel. Similarly the AMB 26v29 test published in Sept 2011 edition. They used a more accurate crank power meter with more riders and more loops so more data. The article talked a lot about the smoothness of the larger dia wheel through the rock sections and rougher sections of the loops they rode. This may have been taken by the testers to assume that therefore the larger wheelsets were going to return quicker figures over the entirety of the loops. They didn’t seem to take into account that as well as the 10% - 14% weight penalty of the rim/ spokes/nipples of the stock large dia wheels, the testers used the same stock width tyre adding more weight and more volume, to the 29 wheelset which had to be ridden over the entire course not just the rougher/rockier areas. Again there was probably in the vicinity of 100 to 200grams extra on each wheel. The difference was enough to give the small dia wheel a 7.7% advantage over the whole circuit and up to 19% on some climbs with one 1’30” pinch climb giving a 30% advantage to the smaller wheel. These figures are large and were not reproduced in any of the race teams tests for the reason WC XCO racing is not done with stock wheels. However with that much extra weight the numbers would probably be consistent with expectations. Incident angle (the angle the tyre hits an object) and lever point advantage (a longer patch makes contact earlier in front of the axis). These give a larger dia wheel smoothness over rocks or rough ground. (and this would be a factor for more consideration in many cases eg in longer events - this data has context for short course XC only). Tyre manufacturers provide lab analysis from wheels and tyres tested on tyre roller machines to the teams that use their products. There are measurable differences in these lab analyses of lower incident angle and the lever point advantage of a longer contact patch of a larger dia tyre in these studies. However this team’s test results have been unable to clearly demonstrate any incident angle efficiency advantage when those wheels are used in a race setup over the entirety of the test loop. In some courses there was above 1%ave which signify advantage on most others the data fell between + /-1%. This is not unusual and doesn’t mean it is not an important parameter - it is just that any efficiency of the incidence angle, in singularity (ie other parameters are the same on both wheelsets so as to attempt to test a single parameter), was not assessable enough to clearly demonstrate an advantage in this testing within the set boundaries. There are many parts of the bike that, while they have advantage in a section of a course, are difficult to quantify when the results are taken from racing the entire circuit. Setting up the test between similar weight 26 and 29 wheelsets was problematic in itself. The first 29 wheelset was not considered sufficiently robust for racing after a failure of one in testing. This team used 29 tubular wheels to create light wheelsets and while tubulars have some positives they have their own set of drawbacks. This leaves weight in the context of short course XC racing as the overarching consideration for performance in wheels - though tougher sidewalls, tread, high volume, psi are some of the many other tyre parameters to consider when race tactics and courses dictate. This is at odds with the prevailing marketing message of the corporations pushing a wheel size which seems to suggest that larger volume, lower incident angle and lever point but more mass of the larger wheel can somehow defy the laws of physics. And despite the library of power data that the large corporate teams have at their disposal from their race teams, their marketing departments have not been able to find data to back their claims that could be released or leaked, relevant to racing, that is contrary to the data repeated here. At the end of it – in the real world of age group racing that most of us now live in - if someone is putting in double the hours of the next guy, it doesn’t matter about the bike - he should beat him up a hill on a clunker. Equally, if two riders are matched in fitness, and the bikes are of similar weight - then the person who is riding rims/tyres that are 10 to 14 percent heavier is going to have a hard time catching the other guy. Corporations exist to make profit. Profit is made by creating a demand and selling product. It seems hobbies/interests are driven by marketing departments and appear to escape any regulatory scrutiny of their wilder claims. Cycling is no exception.
