Um yea Jan, hence the portion of the my original text between "resistance" and the period: "... (on smooth surface or neglecting suspension losses), but at very low pressure this change is larger than at higher pressures, and as pressure increases the effect becomes negligable."
That increasing pressure beyond a certain point increases the linear component of drag doesn't mean that reducing pressure below that point doesn't also increase the linear component of drag. This is just the normal nature of a local optimum. Did you measure lower resistance at 30psi than 40psi using a ~23mm tire? I was told decades ago (by somebody I believed) that, even without suspension losses, a tires rolling resistance as a function of inflation pressure will exhibit a minimum, and I haven't forgotten it. On Saturday, January 4, 2014 4:18:40 PM UTC-8, Jan Heine wrote: > > On Saturday, January 4, 2014 12:17:13 PM UTC-8, ted wrote: >> >> 3) For a given tire increasing pressure reduces rolling resistance. >> > > It depends what you call rolling resistance. If you define it as only the > hysteretic losses within the tire, then it's true. However, if you are > looking at the OVERALL resistance of the bike, then increasing your tire > pressure beyond a certain point doesn't gain anything at all! You just > bounce more. So your tire doesn't flex much, but you flex more - the end > result is a draw on very smooth roads, and probably a loss on rougher roads. > > This fact, which is well-documented by now (we ran several tires at > pressures from 30 to 200 psi in 10 psi increments), is the reason why wide > tires can be fast. If high pressures were faster than lower ones, then > you'd have to beef up the casing of wider tires to enable them to run high > pressures, and you'd lose all the suppleness. So you'd have a choice of > either losing speed due to a sturdy casing, or losing speed because you > have to run low pressures. (The load on a wide tire is much greater for the > same pressure than it is on a narrow one.) > > In reality, you can use a supple casing, run your wide tire at relatively > low pressures, and you don't lose anything due to the low pressures, but > gain due to the supple casing. This finding has revolutionized our > understanding of wide tires. No longer is desirable to make wide tires that > can handle 100 psi or more - it's in fact counterproductive, since such a > strong casing cannot be supple. > > Of course, none of this is new, it just had been forgotten for a few > decades. > > Jan Heine > Editor > Bicycle Quarterly > www.bikequarterly.com > > Follow our blog at http://janheine.wordpress.com/ > > > -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To post to this group, send email to rbw-owners-bunch@googlegroups.com. Visit this group at http://groups.google.com/group/rbw-owners-bunch. For more options, visit https://groups.google.com/groups/opt_out.
