Not exactly a Riv-specific topic, but I think some of you folks will find this interesting, and I'd appreciate any insight. Maybe Jan has done a study, or will do one soon.
Over the years, I've used cranks in 165, 170, and 175 mm lengths, and all seem to be ok. I think I prefer the shorter end of the range, 165 and 170 mm cranks (I'm just short of average height at 5'8"), but 175 works well enough. Anyway, it sort of blows my mind that with the wide variance in human sizes, and bikes and parts to address most of that variance, that the total difference from "short" to "long" in the crank world is only 10 mm. So if you're 6'5", you're supposed to ride 175. If you're 5'9" you maybe ride 170. If you're 5'2", obviously, you'd ride 165. My point here is that it's ludicrous that a teeny-tiny 5 mm change in crank length could account for, say, 6-8" in overall body height. It's doubly ludicrous that a person can easily have, for example, 25-50 mm of saddle fore-aft adjustment (with different saddles/seatposts) on any given frame, but only 5-10 mm to be gained or lost switching cranks. It's a tough subject to discuss without anecdotal bias because if you're somewhat close to average, you probably do ok with whatever you're riding. If you're really tall, you could probably be riding 180, 190, or even 200 mm cranks with improved efficiency and ergonomics, but you've probably always been ok on 175. If you're really short, even a "short" 165 may put your knees and hips through the wringer on every pedal stroke. But we've all been forced to choose from these limited offerings, and obviously we're still pedaling, so it must be tolerable. Still, of all the ergonomic considerations on bikes, this is the only part that moves regularly, and repetitive motion injuries are no joke, so it makes sense to get it right, or closer to right. My ladyfriend Abby is pretty short - 5'3" with shortish legs. She has a 46 cm LHT and a 14" Pugsley that fit her pretty well. She had to cut a tour short last summer because of knee pain and swelling after several days on her LHT. The cranks on the LHT at the time were 170 mm Deore with big ring replaced with bashguard, so crank gearing was 24/36/bash. After the trip and the knee issues, I set her up with some 152 mm Sugino XD (same gearing as before) that QBP sells and went as narrow on the BB as possible. So far the shorter, narrower crank seems like a winner, but she hasn't really put it to a serious test yet. My hunch is that better ergonomics equals better speed/efficiency, even if she loses a little theoretical leverage with shorter cranks. Time will tell. But now I'm going to shorten a crank for her Pugsley (hard to find a stock pug-compatible crank under 170 mm). Since I'm not relying on a commercially available crankarm length, I decided to think about it a little more deeply. Tonight I drew a crude picture of some triangles and levers and pivots that represent the leg-to-crank/pedal relationship, and I took "Knee Over Pedal Spindle" (KOPS) as a goal for my analysis. KOPS has some known flaws, but it's close enough for my purposes. Then I started putting all the trigonometry into a spreadsheet, varying everything at different saddle heights and seat tube angles, solving for crank length, and taking the results with a big grain of salt (margin of error). In general, short legs and slack seat tubes mean cranks should be shorter. Longer legs and/or steeper seat tubes mean cranks should be longer. But much more impressive than that observation, is the range of crank lengths this predicts. If you're short, say 70 pbh, and you have a slack seat tube, you will probably do ok on 120 mm cranks. If you're riding comfortably on a 72 cm AHH, 200 mm arms would be more or less optimum if they don't drag on the ground. My computation shows that my own "optimum" is in the mid 160s for the seat tube angles I usually ride, which I know to be true from experience and perception (though not from a controlled scientific experiment). My ladyfriend should be on a 140 mm crank given her PBH and the 73deg ST on her Pugs. I should point out that these specific values are based on numerous wild-ass assumptions, and they should be understood to have a large margin of error. I wouldn't bet my life on any specific result. I would, however, wager that the range of optimal crank lengths is MUCH larger than what is widely available to us. I'd also suggest that tallies and shorties are not being well-served in this department. Here in the Twin Cities we have this guy (friend of mine) who sells shortened cranks: http://www.bikesmithdesign.com/ His page on cranks for children is perhaps inaccurately named, and provides some good general info about why a person would use short cranks. http://www.bikesmithdesign.com/Short_Cranks/cranks4kids.html In particular, for adults, he suggests crankarm length be at most 9.5% of height. For me at 5'8", this linear relationship suggests that I should be on 164 mm or shorter. Abby at 5'3" should be on 152 mm or shorter. Interesting. -- 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?hl=en-US. For more options, visit https://groups.google.com/groups/opt_out.
