The power output says 1/8 hp and I assume that is a fixed value, and I discounted aerodynamics and rolling resistance as well for such a small change in speed. Beyond that, isn't it a simple linear relationship between drivetrain efficiency and speed? After re-reading I think there may have been a simple clerical error in the writing, as the numbers do proportion out to 5.1 mph if one uses the 95% (direct drive mode efficiency of the IGH) instead of the 90% number used in the example. That accounts for what I originally thought may have been a math mistake.
--- On Thu, 11/19/09, Angus <angusle...@sbcglobal.net> wrote: From: Angus <angusle...@sbcglobal.net> Subject: [RBW] Re: Internal Hub Efficiency: What the Experts Say To: "RBW Owners Bunch" <rbw-owners-bunch@googlegroups.com> Date: Thursday, November 19, 2009, 9:31 PM Mr. Berto's math is quite close. Power varies with the cube of the speed (speed x speed x speed). 5.2 cubed / 5.1 cubed = 1.060 or about 6% difference in power required. Angus On Nov 19, 8:17 am, Ron Farnsworth <r2far...@yahoo.com> wrote: > Bad math. > If 96% efficient = 5.2 mph, then 90% efficient would be 4.875 mph, not 5.1 > mph. And that's with worst case efficiency numbers for the chain setup and > best case efficiency numbers for the IGH. Based on this, the other > calculations are off too. > Correct me if I'm wrong. > > --- On Thu, 11/19/09, Eric Norris <campyonly...@me.com> wrote: > > From: Eric Norris <campyonly...@me.com> > Subject: [RBW] Internal Hub Efficiency: What the Experts Say > To: rbw-owners-bunch@googlegroups.com > Date: Thursday, November 19, 2009, 12:35 AM > > For those who don't have a copy of "The Dancing Chain" laying around > the house, here's a summary of what Frank Berto said about internally > geared hubs (IGHs), which is itself a summary of extensive bench > testing done in 1998: > > --Standard derailleur systems achieve 98-99 percent efficiency when > the chain is running straight, and 96-97 when in cross-chain mode > (such as small cog/small chainring) > > --Internally geared hubs ranged between 80 and 90 percent efficiency, > achieving higher efficiency in lower gears. One IGH tested in direct- > drive mode was 95 percent efficient. > > Berto compared a typical rider (generating 1/8 horsepower) riding up a > 4 percent grade in a low (31-inch) gear. > > --With a standard derailleur bike operating at 96 percent efficiency, > the typical rider could climb at 5.2 mph at a cadence of 60 rpm > > --With an IGH operating at 90 percent, the same rider would be 6 > percent less efficient, dropping his/her speed to 5.1 mph > > Those numbers don't sound too bad, at least for shorter riders. A 4- > hour ride on a standard geared bike would take about 4 hours and 15 > minutes (or less, since the efficiency of the IGH wouldn't matter at > all on downhills). > > However, applied to an event like PBP, an 84-hour time would become 89 > hours if the bike became 6 percent less efficient (or, assuming that > half of PBP is more or less downhill, 84 hours would become 87 1/2). > > I'll continue to test this in the field and see how my Quickbeam's 8- > speed Sturmey Archer performs. > > P.S. No, the vaunted 14-speed Rohloff hub was not part of the '98 > test. It's hard to see how it could be too much more efficient than > other IGHs, but it could be somewhat closer to a standard setup. > > P.P.S. If you haven't read The Dancing Chain, you really should. It's > a fascinating look back at the early days of cycling, and it proves > once again that everything "new" in bicycling today was in fact > invented 100 years ago. Really. > > --Ericwww.wheelsnorth.orgwww.campyonly.com > > -- > > You received this message because you are subscribed to the Google Groups > "RBW Owners Bunch" group. > To post to this group, send email to rbw-owners-bu...@googlegroups.com. > To unsubscribe from this group, send email to > rbw-owners-bunch+unsubscr...@googlegroups.com. > For more options, visit this group > athttp://groups.google.com/group/rbw-owners-bunch?hl=. -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to rbw-owners-bu...@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=. -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to rbw-owners-bu...@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=.
