Another technical question I'm afraid. I've ridden a fair bit and never worried a bit about the small details. As I like the outdoors and exercise I am always able to put up with a bit of discomfort here and there but now that I am on a tandem I find myself worrying about all sorts of things as I want my wife to be as comfortable as possible. Well, here's the question. I noticed that on our tandems the captain's cranks are 170 mm and the stokers are 172.5 mm. This struck me as odd as the stoker is usually the shorter of the two and it is certainly the case in our situation. I am 5' 9" (not so tall) and my stoker is a wopping 4'11 I would say. She's 148 cm in metric. Anyway not so tall. Because of the geometry of tandems is it normal to have longer cranks on the back or do we have them on our bike because that was all that was in the shop and as they were shipping the bike overseas so they didn't worry about it too much? Also, any ideas on what would be a good length for someone of her size. I was thinking about 165mm.
carpediemracing
03-20-07, 09:12 AM
I'm a relative novice on tandems but that's an unusual setup.
It seems like you adjust cranks to help match cadence more - I read somewhere if the captain spins too much then get the captain longer cranks. Perhaps that was the goal with the unusual crank lengths?
As far as length of crank, it depends on the rider. I have relatively short legs - I ride a Small Giant - 44 cm seat tube - and am captaining a M/S Cannondale, the smallest one they make. I use 175's on my road bike with good success - Cat 3, race, and my strength is sprinting (i.e. speed, not climbing or TT'ing). On our tandem 175's for the me would slow down the cadence a bit on the bike overall. Currently it has 170's and I want to spin a bit too much.
cdr
TandemGeek
03-20-07, 11:31 AM
Because of the geometry of tandems is it normal to have longer cranks...
No... it's not normal. Default for past several years has been 175/170 with the occasional 170/170 thrown in. It's hardly ideal but the vast majority of tandem buyers that should probably be on shorter or longer cranks don't know the difference or just accept the marketing spin on why 175/170 is "about right".
Off the top of my head, only Specialites TA, daVinci, (Sugino for a while), Thorn, and more recently FSA have bothered to make tandem crossover cranks in a wider rage of sizes that recognize tandem cyclists come in all shapes and sizes too.
At 5'8" with a 29.5" inseam (30" on a good day), and I finally gave up on 175mm for the road tandems after 10 years this past season and now use the same 170mm as have been fitted to my road bikes for 35 years: it was a welcome change. My fixed gear runs 165mm, but that's a different story. My wife is about 5'2" with a 28" inseam and she's fine with her 170's on the single and tandem. I would note her bike fitter didn't detect any major issues with the 170's and she didn't prefer the 165mm cranks when we experimented with those a while back.
Ultimately, everyone needs to go with what works best. Some folks prefer to push a big gear and long crank, others prefer to spin high revs on smaller cranks, and still others can switch between different lengths without much more than a few minutes of adjustment time. I stuck with the 175's because that's what came on our first tandem and it's taken 10 years of aging and riding for me to realize that 170mm is actually a better fit.
Finally, yes.... you can play crank size to bias pedal cadence but, for the most part, if your stoker really needs shorter cranks because they have really short legs, that should be the biggest concern. Cadence will work itself out through the compromise process... or not.
I was thinking about 165mm.
Sounds about right.... unless she already happens to ride 170mm on a single bike with nary any problems or complaints.
SDS
03-20-07, 03:55 PM
I like 5.29mm of crankarm length per inch of barefoot inseam for a rule. If you find yourself in the middle between sizes, it's usually better to go up to the next size than down.
TandemGeek
03-20-07, 05:43 PM
I like 5.29mm of crankarm length per inch of barefoot inseam for a rule. If you find yourself in the middle between sizes, it's usually better to go up to the next size than down.
Anyone know where I can find a set of 156mm cranks?
Cmwebb
03-21-07, 04:29 AM
Thanks for the advice. My wife is having a blast on the tandem but before we got it the last time she had ridden a bike was when she was in Junior high school and that was on what we would call a womens three speed but is the norm over here even for cool high school boys with the baggy pants and died hair.
When we first started riding the tandem it took me ages to convince her that it was normal to not be able to put your feet on the ground while sitting on the seat. In other words she hasn't ever gotten used to a particular crank length but she has mention that she felt I was spinning too much. I like to spin. I told her that it was because she wasn't used to riding and she'd get used to it. Of course I didn't realize that she was actually using longer cranks than I was despite being about 10 inches shorter. Woops!:)
The next challenge will be of course buying crank arms without having to buy the whole crank set.
ElRey
03-21-07, 07:13 AM
If she's not complaining, leave them alone. I'd go longer on yours first, as this will aid you in putting out low-end torque.
Ben There
03-21-07, 11:34 AM
Anyone know where I can find a set of 156mm cranks?
Yes, Right Here http://bikesmithdesign.com/Short_Cranks/shorten.html
TandemGeek
03-21-07, 12:22 PM
Yes, Right Here http://bikesmithdesign.com/Short_Cranks/shorten.html
Thanks, but it was not a sincere request. I was attempting to point out the weakness of formulas for determining bike fitting dimensions, such as crank length.
The 5.29mm factor may seem to work if your inseam is in the 32" - 34" range. However, once you fall out of that range the linear formula begins to show it's weaknesses. Case in point: I'm 68" tall with a 29.5" inseam in bare feet. Based on the 5.29 formula I should be using a crank arm length of 156mm. Does that sound right about right?
Of course, if I don't like that answer I can use any one of several other factors, formula, or calculators to come up with a different result that might come closer to what I think is probably right and then declare that "tool" to be the most accurate.
While some fitting methods use a single dimension or factor, others require the use of inseam, femur length, lower leg length, and shoe size which account for variations such as Lance Armstrong's freakishly long femur or yet another person's disproportionately large feet. Should shoe size be a factor? Well, if you consider that the length of your foot and where your cleat is mounted helps to determine the radius of you pedal stroke... often times by as much or more than the 2cm of difference that exist when going from 165mm cranks to 175mm cranks ... then yes, it may be a consideration worthy of some attention if your feet are disproportionately small or large.
Just something to consider... never mind personal preference for high vs. low torque feedback or high vs. low cadence.
Cmwebb
03-21-07, 07:09 PM
I just remembered that I forgot to add the most important part of my original query. We're not so worried about speed on the bike when we're out as we just like the exercise and the scenery. I do tend to be a bit of a spinner but the reason for that is I find it is easier on my knees. So...the real reason I was originally asking was I was worried that my wife might hurt her knees if she was using long cranks once we up the milage. Her inseem is 65 cm by the way. If I divide that by 2.5 (I think that's about right) it comes out to 26 inches. Wow! quite tiny. Can long cranks lead to knee pain. If it isn't really related than maybe I won't change them as she hasn't complained and just sits there happily pedaling away.
carpediemracing
03-22-07, 08:34 AM
If you're a spinner and you have the shorter cranks, your wife will have difficulties with your cadence preference if she has both longer cranks and shorter legs. Longer cranks = slower cadence. I've migrated from 167.5's to 175's and my "spin" cadence (where I feel comfy with little resistance on the pedals) went from over 120 to about 95-100 or so. When I first put on the 175's, it dropped to about 80-83.
Knees, from my experience, should be fine. The tendency is to push really big gears at lower rpms when you get longer cranks (this is on a single bike) - no matter what your crank length, that puts a lot of prolonged pressure on the knees which is not good. On a tandem with a spinning counterpart, that probably won't be the case.