Jim: You're right, of course. If you look at Berto's numbers, a drop from 5.2 to 5.1mph is a 2% decrease, not 6%. Obviously, other factors are involved in how fast you go, not just the efficiency of the drivetrain.
Applied to my PBP example, going 2% slower would add about 1.7 hours to an 84-hour PBP (again, probably less time would be added when the downhill sections are factored in). --Eric campyonly...@me.com www.campyonly.com www.wheelsnorth.org On Nov 19, 2009, at 7:26 AM, Jim Thill - Hiawatha Cyclery wrote: > Mechanical inefficiency of the drivetrain is not the only thing that > makes a cyclist slower. Let's say, for example, that 90% of the total > inefficiency comes from tire rolling resistance and aerodynamics, and > 10% comes from the mechanical friction and slop in the drivetrain. > Then the total effect of a 6% drop in mechanical efficiency would > translate to something like 10% of a 6% drop in speed (i.e. 0.6%). > Approximately. My numbers are for illustrative purposes only, not > based on actual measurements of inefficiency. > > 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=. > > --Eric campyonly...@me.com www.campyonly.com www.wheelsnorth.org -- 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=.
