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Newsgroups: rec.bicycles.tech Path: nuchat!menudo.uh.edu!swrinde!cs.utexas.edu!math.ohio-state.edu!magnus.acs.ohio-state.edu!csn!rootgroup.com!billp From: billp@rootgroup.com (Bill Pachoud) Subject: Re: crank lengths (contains 'equation') Message-ID: <1993Aug22.070602.22397@rootgroup.com> Sender: news@rootgroup.com (USENET News System) Organization: The Root Group, Inc. X-Newsreader: TIN [version 1.2 PL1] References: <25345s$4m5@morrow.stanford.edu> Distribution: usa Date: Sun, 22 Aug 1993 07:06:02 GMT Lines: 337 Drew W. Saunders (GE.DWS@forsythe.stanford.edu) wrote: : In article <41460@oasys.dt.navy.mil>, : mackinto@oasys.dt.navy.mil (David Mackintosh) writes: : > : > Inseam Crank Arm Length : > (Inches) (Millimeters) : > : > 34,35 170-172.5 : > 36,37 172.5-175 : > 38,39 175-177.5 : > 40,41 177.5-180 : > 42 180 : > : >I guess Big Mig is doing it all wrong, then. His inseam is not likely : >more than 36 inches and he uses 180's. My inseam is 38" and I've got : >175's but he still kicks my ass! Shows what Bicycling knows. : > : >David Mackintosh : Warning: The following may be pure BS, but at least the numbers : work out. : I vaguely remember reading in one of the Bike rags that Lemond had : specified that for a 32" inseam, one should use a 172.5mm crank : length. This is for someone who wants to ride "Tour de France" : style, which the article mentioned would bring about a need for a : longer crank than other riding styles would need. Anyway, if you : change the crank 2.5mm for every inch change in inseam (up to the : longer lengths, where you might want to change 5mm/inch), you : get the following: : (Note that the inseam/crank ratio works out much more closely than : the ratio that another netter posted about the first table that was : re-printed from Bicycling) : Inseam (in) Inseam (mm) Crank Length Inseam/Crank : 29 736.6 165.0 4.5 : 30 762.0 167.5 4.5 : 31 787.4 170.0 4.6 : 32 812.8 172.5 4.7 : 33 838.2 175.0 4.8 : 34 863.6 177.5 4.9 : 35 889.0 180.0 4.9 : 36 914.4 185.0 4.9 : It works for me, with my 30" inseam and 167.5mm cranks, and I think : it works for Big Mig, who has somewhere around 35 or 36" inseam and : uses 180mm cranks. : Take it for what it's worth, but it's at least as scientific as : Bicycling Mag (not that that's saying all too much). : Drew Fred Grosby (fred.grosby@his.com) wrote: : jnewman@shield.dasd.honeywell.com writes: : j> The other day, my husband (6'2'') mentioned changing his 170's to 172.5's : j> on his road bike. After measuring his inseam and looking at the table, we : j> decided not to...perhaps, we should reconsider. : I'm 6'1", have either a 32" or 34" inseam depending on the specific pair of : pants in question, changed from 170mm to 175mm cranks years ago, and have been : quite happy with them. I checked every formula I could find, and the results : averaged between 175 and 177.5mm. 175's you can readily buy, so I got 'em. : And to my recollection, none of those formulas had anything to do with inseam. : : Let's think about this a minute. We've got the thigh, which measures hip to : knee, the lower leg, which measures knee to ball of foot, and we've also got : to take into consideration the fact of being seated, which suggests that a : pelvic bone to knee measurement might be the most appropriate measurement. : Does inseam get at any of this? Not really. Inseam isn't a reliable measure : of leg length, since it doesn't measure either joint to joint or bone to bone. : It also doesn't tell us anything about the relative lengths of the upper and : lower leg. : In helping people fit their bikes to their bodies, I've come to look at thigh : length when trying to figure out crank length. Seems to work pretty well. I am re-reposting a reply I made to this subject a while back that describes my experience with this area, in hopes that it will be of use to the riders of the net. Since that time, I have built a second bike that extends my experience in this area and solves the remaining component questions to my satisfaction. Enjoy, Bill Pachoud Subject: Re: road bikes for tall riders Brian, The best bike that you can buy that will fit your needs (to a degree) that is production or limited production, would be 1. Cannondale made their bikes up to 68cm in a few sizes and up to 66cm (probably closer to your size), in several others. I believe that they are still available by special order, call cannondale. 2. Schwinn Paramount OS ($900 for frame and headset) is made in sizes up to 68cm. A nice bike and has the type of tubing features a big bike needs. 3. Klein Quantum - A nicer version of the c-dale and he used to make them to 68 cm. The cannondale fits your aforementioned budget the best. The other 2 are nicer bikes. If you really want to do it right, you will need a custom bike. The major need you have that is not addressed by the bikes I mentioned is crank length. I spent 2 years and more than $5k solving the problem to my satisfaction. The project involved 4 people, a kinesolgist (cat 2 track sprinter), master mechanic (head mechanic for the Spoke in Boulder, Colo., for 6+ yrs.), and a master framebuilder (John Murphy, Columbine Cycles), and myself. I am 6' 8" and about 236lbs. If you research crank length, and look at the work that has been done, the best available evidence boils down to 2 main points. 1. Pros have empirically determined crank length to be fairly optimal for road riding at about 172.5mm for a 32" inseam (measured by the method in Lemond's book). 2. Crank length for people in the 5-7' range is a linear function for best performance. (Actually, it may be slightly less than linear, but for the range of sizes we are discussing, the difference between the actual function and linear is very insignificant) This means that for me, w/ a 39.25" inseam, I should ride and do ride 215mm cranks. This is a linear scaling of the relationship stated in point #1. It does make a difference, I used to ride 180mm and I notice a difference. I estimate the absolute difference to be small but noticable maybe I get 1.5-2% better performance. As you would expect, I climb and TT better. Sprinting is about the same, better jump, but a bit harder to spin out. By the way, a fallacious argument used against big cranks is that they require much lower cadences. This is false, think about it. If my quadriceps muscle contracts 1", my foot moves much farther than the foot of someone who is average height, because my lower leg is a longer lever, than their's, it sweeps out a bigger circle. So, I can turn larger cranks at the same rpm as smaller riders. The similarity between myself and smaller riders is the speed of muscular contraction, muscle tissue is muscle tissue. My practical experience has confirmed this, I tend to ride at about the same rpm as before. I do ride slightly slower rpms (2-5), because the longer cranks make it easier to turn over a bigger gear for the same speed. This is more efficient for aerobic activity (climbing, TT). My rpms are now about 88-89, where before I used to ride at a little over 90-91 rpms. If you look at the coaching advice, Lemond's book, etc. they would put you on 180mm cranks. The standard is 170-172.5mm for a man of around 5'9", as I stated before. Your inseam is probably in the 36-37" range. The recommendation is that you ride a crank that is 5.8% longer while your inseam is about 17.1% longer than that 32" standard. What is wrong with this picture? The standard wisdom makes sense if looked at from a "what is easily possible" perspective. Most items that are made in sizes to fit a range of people are only made in a range that is one standard deviation from the median. This is because that will cover about 90% of the population and is therefore efficient. (Credit to Jobst Brandt for this observation). That explains why all the large component manufactuers only make cranks up to 180mm w/ the exception of TA and I really don't like the engineering of their crankset, it is too flimsy. You are in luck, see the current issue of Bicycling for an article on aftermarket cranksets. You have 2-3 choices: 1. Bullseye - Excellent engineering, poor execution, the machining quality is very poor. My custom columbine has bullseye cranks, but after the work it took to make them right - Remachine center spindle to make it true, replate (a la crankshaft journal for a car) and machine back down to original diameter. - Custom machine a spider to look good and not flex. - refinish $1500 later I had nice crank. Admittedly, I wanted something that looked very nice like a Campy C-record crank and functioned flawlessly. This crank looks very nice and is very light and stiff. The only remaining problem is the Bullseye stock bearings are really poor, and rather unique, but I may have found a solution to that. 2. Magic Motorcycle Co. These are really nice, and the way to go, about 600. 3. Grafton Speed Sticks, new model. I talked to them about a custom set, and they said they would consider it. very pretty, maybe not quite as trick as the Magic crank. If you are interested let me know, maybe we can work a discount. After you have the cranks, you have to build a frame that will let you use them. There are several factors that you must consider. Getting the cranks 1st will help w/ several dimensions you must fix. 1. The frame will need to be lugless, since you will not be able to get a BB lug that will work with the BB height ( mine is 12.25" for 215mm cranks and I have normal cornering clearance) you need for cranks in the size range we are talking. Same for most all the other lugs w/ a few exceptions. 2. You need to consider several crank clearance issues. - the chainstay width must not be so wide the cranks will hit them at the crank length you choose. This means the Q factor (pedal to pedal width) of your crank will be a bit wider than that of a 170mm crank. This will affect the cornering clearance of your setup, but is not a disadvantage biomechanically as your pelvis is a bit wider than that of a smaller person, assuming you are dimensionally proportionate. - The chainstay length must be long enough that your right heel doesn't strike the rear derailleur. I have short wide feet (size 12) and by actual measurement (pedalling tests), I needed a 17" rear chainstay length. This balanced the bike nicely, it handles well. I like classic 70's racing geometry w/ Lemond/French influences (long [61.5 cm] top tube, relaxed [72.5] seat angle). It is a stage racing bike, predictable, stable handling w /an all day ride. A caveat, I fit the pro flexibility test, I ride relaxed w/ a flat back and am very flexible. If you are someone who rides with a hump in your back, you probably need a very different bike, a la the Bicycle Guide "Roosa Special", or the Clark Kent "Alexi Grewal AX-1). These bikes are built for people that have relatively inflexible pelvic/back junctions and need to relax that angle in their setup. When you see people riding w/ a hump in their back, it indicates they are trying to fit their body onto a geometry that is not right for their physiology. 3. You will find that your gearing will not change much, I ride 52/53-38/39 front and 12-23 or 13-24 rear. I maybe ride one gear larger. Originally I thought I would ride way bigger gears, it didn't turn out that way. I hope this helps you out, I didn't know what I was getting into when I started down this path, it has been interesting. I have learned a lot about a number of areas, as is usually the case when you choose to solve a problem in detail for yourself. Send me email if any of the points are unclear, I will try to respond in a timely manner, but I am very busy these days. Enjoy, Bill Pachoud billp@rootgroup.com ----------------------------------------------------------------- 1993 Notes: I have built a new frameset that builds upon the knowledge I gained from the 1st and come up with a satisfactory solution to the crankset problem. My new frameset is essentially the same as the 1st with a 0.5cm longer top tube and a jump up in stiffness via larger diameter tubes, yet again. The downtube is now 1 3/8", up from 1.25" and the seattube is a new True Temper flared and hardened tube. Fork is Columbus SP /w Henry James tandem crown and chainstays are a tandem gauge. The crankset I am now using is a Magic Motorcycle custom 215mm crank, I can recommend it w/o reservation. It is extremely well made and Alex Pong has really carefully analyzed the problems a crankset presents and engineered a better solution. It is both lighter and stiffer than Dura-Ace or Campy equipment. The net effect of the changes I have made is that my second bike is both lighter (overall) and stiffer than my first bike, which was excellent to start w/. There are now many more trick parts appearing like Ti cogs, etc. to make a reliable 18lbs. stage racer. For me, that would be a 20lbs bike, exceptionally light, given my size and weight. I use a custom Columbine Cr-Mo stell stem and am considering a few new fun things (SRP D-A bolt kit, etc. to drop a lbs or two. That is just for fun, is performance is the issue, training correctly is much more effective! -Bill PPS: Aug. 93. I now have more milage on the 2nd bike I described above. It has continued to function very well. Just for fun I had John Murphy build me a new stem that has some interesting jewelry aspects, it is quite beautiful. It has a columbine flower done in silver and copper w/ a small sapphire. He really is talented if you like pretty things and his bikes are superb! 
