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Bicycle Quarterly Rolling Resistance Tests: No Surprises
I just finished going through this past fall's threads on the tire
performance testing reported in Bicycle Quarterly 5(1). I was amazed at the collective amnesia displayed in this newsgroup (rbt). On the correlation between Crrs as measured by drum tests of various surfaces and road tests of various surfaces, see for example: http://www.iea.org/Textbase/work/2005/EnerEffTyre/sandberg.pdf This is a pilot study with no discussion section. For extensive discussion on what the results mean, see this thre ad http://tinyurl.com/2bsqr6 http://tinyurl.com/293kr7 which appeared just a few months before the BQ tire test threads, and which people seem to have somehow forgotten about. In my posts there, I noted that (a) in the historical development of rolli ng resistance tests, which have been conducted by tire manufacturers for a very long time, the drums were originally asphalt or the like (b) there are drums currently available with all sorts of surfaces (c) testers moved more to polished steel drums to r educe the scatter, to make the results more replicable, and (d) now there is a lot of interest in drum testing on rollers with various surfaces, because now there are not just tire manufacturers wanting to reduce the rolling resistance of the tires, but a lso people responsible for the environment who want to reduce the energy losses particular to the road surface. Since these tests are generally for car tires with significant tread patterns, there is also an interaction between tread pattern and road surf ace, so there is also a desire to optimize the pairings. As the pilot study linked above shows, all this can also be measured more cheaply on actual roads using instrumented trailers, BUT... there is an awful lot more scatter with those methods, especiall y, as with the study above, if the trailer is not encased inside a fairing. Thus roller data, with curvature mathematically corrected to a flat surface, is the preferred method, if you can afford the equipment. The amount of scatter observed testing with trailers- an inherently far more controlled system than a bicycle+rider in a rolldown- should be sufficient to make people think twice about the significance of the particular rankings observed in the BQ tests, especially for nearby results. This is even more the case when one considers a statistical point which I bring up further below. In fact, when one observes the scatter obtained even in the results for a single drum surface at two different speeds, one should be even more cautious about the validity of any resulting rankings, especially for nearby results. As is well known, a tight correlation between two methods does not ensure their interchangeability. Again, the statistical problem I allude to above and which I will explain further below makes the situation even more problematic. Nevertheless, although the BQ tests were presented as overturning tests done on polished drums, in fact they are yet another vindication of them. This is best seen by the fact that a transformation based on tire widt h and tread thickness normalized the results: all other things being similar, these are exactly the two factors that conventional rolling resistance theory, as explained in the threads above, would point to. But not all other things were similar, and further below I will single out another factor which was not taken into account in the BQ tests. In short, for the BQ tests, to quote a certain H. Inn, the best surprise was no surprise. Consider the following: -The fastest tire in drum tests was the same as the fastest tire in the BQ tests: no surprise. -Some extremely confusing statements about tire width: ---------------------------------------- #We expected wider tires to roll faster, and the wide tires we tested #rolled slower than the narrow ones. @However, while some are surprised that we found wider tires to roll @faster, that is not inconsistent with previous results. #I like 650B bikes, yet all currently available 650B tires scored average #or worse. %One of our very fast tires, a Mitsubo shi 650 x 37 mm was very wide. --------------------------------------- I take the average of these statements to mean that, all other things being equal, wider tires roll faster. But this has been understood for ages- again, no surprise. -That a 24.5mm tire should be fastest should be no surprise: consider what size Jobst has been riding all these years, and advocating as the best compromise between rolling resistance losses and suspension losses: none other than 25mm. -It was claimed that the pressur e results of the BQ tests, which showed that at higher pressures, not much difference in rolling resistance results from raising the pressure further, were inconsistent with the IRC drum tests. But that is not correct: Señor Fogel already posted the resul ts below, using the rolling resistance figures from those IRC tests: -------------------------------------------- This calculator uses Jobst's rolling resistance data to predict exactly that kind of effect: http://www.analyticcycling.com/ForcesTires_ Page.html [...] The range of difference in a 1-hour 40 km ride at about 25 mph is less than thirteen seconds for inflations from 120 to 100 psi. --------------------------------------------- The BQ tests were done for a 25-30 sec, 184 metre ride. Sc aling the above result down for the distance or time gives a difference of about 0.05 - 0.1 sec, about the same or in fact even less than what was recorded in the BQ tests. Again, no surprises. -The slowest tires in the BQ tests have not been publicly t ested on drums, but were exactly those that would be expected to be slowest, on the basis of conventional rolling resistance theory. The Rivendell tires use the Tornado casing, otherwise abandoned long ago by Panaracer, and for an obvious reason: since th e casing is Kevlar reinforced bead to bead, it is a colossal energy sucker. (It should though be more cut resistant than conventional sidewalls.) Again, no surprises. However, the presence of the Rivendell tires in the statistical analyses therefore does pose a problem. Because of that bead to bead Kevlar reinforcement, they are qualitatively different in construction, and significantly outside the performance range of the other tires. Thus they must function as outliers or leverage points and ought to b e excluded from any regression or correlation analysis. That done, the range of measured variation becomes smaller and the relative size of the noise greater. One then has to start questioning even more the significance of the rankings, especially for nearby results. -The BQ tests showed wearing gloves or not had a noticeable effect. But this was already discussed in http://tinyurl.com/38xthb. In particular, the wind tunnel testing done at MIT shows that gloves or not has about the same aero effect as an aero front wheel or not. Again: no surprises. Now to some apparent anomalies that turn out not to be, and to some erroneous comments: -Some may wonder how it is that the Avocet Duro (700x28) scored worse than the Avocet Cross (700x35). First of all, if it is really a Cross and not a Cross II, then it is really a 700x32 tire, not a 35. The Duro is a true 28. Look at this table: Width Weight (g) Cross Road 28 330 300 (Duro) 32 370 390 (Duro Plus) 35 395 --- In other words, the Duro is a heavy duty tire, the Duro Plus even more so, even more than the Cross. The two series use the same casings and same rubber compound and same construction (save for t he tread thickness, features, or lack thereof), so the weight differences are all due to the tread thickness (tread coverage is also a possibility, but these are similar in style). Thus if the Cross were very worn down, as in fact it was reported to be, t hen we should indeed expect it to be faster than the Duro. Again, no surprise. I note that Jan Heine's remark that: #Just because IRC or Michelin use drum tests doesn't mean it's a good #way to test tires. In fact, both the Michelin Pro2 Race and the Avocet #Duro, which have been optimized on steel drums, scored much less #well in our real-road tests, indicating that the current approach is not #the best. (They both were fine tires, but not as excellent as steel drum #tests would have us believe.) is not apropos: there are no publicly available drum test data for the Duro (or the Duro Plus). The famous graphs are only for the Time Trial (20mm), Criterium (23mm), and Road (25mm) models. These are of a different construction than the Duro, with thi nner rubber and usually, 127tpi casings instead of 66 (old Road were 66). -The Michelin Pro2Race and Continental Ultra Gatorskin in 23mm: the Michelin was reported as better in the Tour tests, but the two scored the same in the BQ tests. However, while the same models (presumably, see further on) were tested, the same individual tires were not. There is variation in production and we don't know how much. For example, while the Tour test quoted had the Crr of the UG at 0.0058 and that of the Pro2Race at .0042, the values given for these tires from some other test that has been oft-quoted in this group (I think done by Le Cycle, on steel rollers as well) are as follows: Tire Prr Crr Speed* Vittoria Open Corsa Evo CX 27.1 W .00334 23.22 Michelin Pro2 Race 29.2 W .00360 23.14 Michelin Carbon 34.7 W .00428 22.94 Panaracer Stradius Pro 35.4 W .00436 22.91 Hutchinson Fusion 39.6 W .00488 22.76 Continenta l Ultra GatorSkin 40.3 W .00497 22.73 Continental Grand Prix 3000 46.6 W .00575 22.50 *185lb rider + bike 250W rider output CdA = .32m^2 (racing crouch, normal road bike) Transmission efficiency = 96% Moreover, the UG was redesigned in 2 006 and it's not clear even if exactly the same model was tested. -The article says that hard rubber, while faster on polished drums, is not faster in the real world. But (at least for smooth tires) nobody has claimed the former: it is universally agreed, and Jobst has said it here many times, that harder, high-carbon rubber is a high loss material, and for better rolling resistance, switch to something else inferior in grip and durability. The question instead is, in the search for a compromise between durability, rolling resistance, and grip, which is better, hard thin high-carbon rubber, or softer thicker rubber? The BQ tests do not and cannot address that issue, but most of us know from experience that for durability and wet grip, you certainly need hard, high-carbon rubber. Therefore, if you want good rolling resistance nevertheless, you have to make it relatively thin. The 1.5mm depth of hard, wide coverage, high-carbon rubber tread on an Avocet Road (25mm, 127tpi, not tested by BQ) seems to be one of the better compromises around. How many miles is a Deda Tre good for? And what's the wet grip? The only circumstance where one might expect hard rubber of a given thickness to give better rolling resistance than soft, is where there is a signi ficant tread pattern, e.g. knobs. The softer rubber will squirm more and so this might overcome its inherent lower rolling resistance. The testers wondered why the Grand Bois tires were only middle of the pack; I don't know their TPI or tread depth, but i f they were e.g. 66tpi or greater and not exceptionally thick, then perhaps the problem is the file pattern tread combined with soft rubber. Get them to make them with a smooth tread and hard carbon rubber, i.e. to replace my Avocet Duro Pluses, and I mig ht even buy some one day... -There is a problem with the conclusion that width and tread depth are correlated with performance, but weight is not, because weight ought to be highly correlated with width and tread depth. This should be explored further. P erhaps because there is a correlation, just not a linear one? -On that note, there are big differences in tire design not just in tread depth and compound, but also in tread coverage. For example, the Avocets have the tread rubber going much higher up th e sidewalls than many other otherwise comparable tires. -The article considered the various widths of the Michelin Pro2Race to form a family identical in all but this one parameter. That does not seem to be correct: if you plot the weights of these tires versus their width- and compare with other tire model series (e.g. Pasela, Continental Ultra 2000, Ultra Gatorskin, Michelin Dynamic, Avocet)- it seems clear that the various widths of the Michelin Pro2Race are NOT all just different widths of the same t ire. Look at this table: Width Weight (g) Pro2Race Avocet 20 210 215 23 220 235 25 265 255 We know the tread depths of these Avocet models are respectively 1mm, 1.25mm, and 1.5m m. Clearly the variation in tread depth between the Pro2Race tires of different widths is even greater. Thus these are NOT the same tire with just the widths differing. -On the difference between rolling resistance and suspension losses, and why one sho uld consider them separately: http://tinyurl.com/2ltu8a (Note Jobst is being sarcastic when he says "I suppose") I add that since jiggling, deforming over obstacles and bouncing, are not rolling, there is no reason why anyone should call the resulting l osses "rolling resistance". They are suspension losses. As to the matter of steel wheels, and why we should not consider it a problem that they are not best on surfaces with texture, while they are on polished drums: steel wheels have no pneumatic suspens ion, and so every minor irregularity exerts what is essentially an unrecoverable braking jolt, or even extra distance to travel, against the system. By contrast pneumatic tires, even at pressures far beyond the ordinary (200psi, as observed by Jobst), are still capable of swallowing normal- sized road irregularities (visibly bulging upon rolling over an obstacle). In an ideal tire, even if the pressure were infinite, a conical object might still deform it to the height of the cone, depending upon the assumptions. That's not the case for an ideally hard metal. One has to be careful about arguing at infinity, it's a strange place. There should be very little discrepancy in suspension losses between different tire models of qualitatively similar design at the same width and the same pressure, since it is the air volume inside them that does the bulk of the bouncing and the shock absorbing. Thus the road surface was smooth enough that suspension losses are doubtful as the explanation for whatever minor disc repancies from the drum tests might have been observed. Perhaps a better one is the following: There was at least one more factor not controlled for in the BQ tests. Changing the diameter of the tire or the pressure changes the ("pneumatic") trail, which, regardless of whether the bicycle had a lot of trail to begin with, changes the handling characteristics (I leave aside any aerodynamic effects, but consider the difference between gloves and no gloves...). This will tend not to worsen the scatter withi n the data for a single tire, but will bias the results between tire models. This may result in systematic errors that further mess up the rankings. Have I covered everything? Well, there is the oddity that tubular tire performance got worse with higher pressure, but clincher tire performance did not. Why? Overall though: no surprises. e |
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