Karl and Thomas,
We will be pulling width and rider weight into the equation in part 6 after we talk aerodynamics..which are affected any all of this and more.
However, wider tires will generally shift the breakpoint pressure to the left (toward lower pressure) and also from the yet limited testing we are doing show so far that wider tires have flatter impedance curves on pavement type surfaces..likely because they are better able to bridge the negative spaces within the pavement.
We are continually adding to this set, but of course, each data point on the curve takes about an hour to collect..each curve on the graph is roughly a full day of work for 2 people plus the post analysis and all of that fun stuff! So far we have presented only 25mm data, we have a few runs with 23mm so that gives us some direction here.
We have been using the Chung Method for this and have been using an aero bike and position to try and keep CxA as stable and repeatable as possible..the downside here is that our P5 will not handle anything wider than the 25mm tire on a 16.5c or 17.5c bead width rim so while we plan to test with 28 and 32mm tires, we will have to re-baseline.
Also of interest is that because we are using Tri positioning and a very vertically stiff frame and wheels, some of the pillars of cycling science have asked me to look at this using a more normal frame and fork to see if the particular bike we are using may be exacerbating the suspension losses by transferring even more of the ground inputs to the rider rather than allowing say the flexing of an efficient fork to absorb some of them. The idea being that the steepness of the impedance curve may become flatter with a more flexible bike, and a road position is naturally more flexible than a tri position... this concept currently has me pretty fascinated as it only makes sense that the stiffness of the thing on top of the tire is playing into this..but so far the data here is just too limited. Though I do look at Jan Heine's data using larger tires and classic randoneuring frames and he has found a breakpoint dip in rolling resistance with flatter Crr curves before and after the breakpoint..though he is also doing this with tires on the order of 2x the width as we are studying so again, much more data needs to be collected.
Lastly, Thomas, yes, the pavement situation is partially that it is rough, but also partially we believe that it was soft from being so new and also that it will compact further over time. This is where it becomes really hard to begin defining the surfaces.. what do we call excellent pavement or good? How do we develop a scale that people can use in the real world to make decisions. A year ago I would have said that 4 day old pavement was as good as it gets..now I think that the same pavement a year or so later is much faster if you are riding in the vehicle tracks. However, this clearly has its own limits as eventually that pavement will develop ripples, begin losing some of it's fill, develop cracks..etc and will then be much less good.
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