Crank Lengths (and Brands)
My wife and I have recently purchased a tandem (2004 Cannondale road) and I am very happy with the purchase with the exception of the cranks. They are Truvative and seem to have a slight excentricity to them that means the front chain can be tight in one position and have a half inch of flex in another position. Reading another thread I began questioning the crank lengths too.
They are currently 175mm captain and 170mm stoker. I have always ridden 175mm although doing some calculations it appears that maybe 180mm might be more appropriate (6'2"/34.5" inseam). My wife is 5'4" and has short legs. (Her mountain bike is a men's Gary Fisher with Genesis geometry that lengthens the top tube as opposed to shortening it as for most women's bikes.) Another problem we have is that as the more experienced biker I prefer a higher cadence than she does. Are there any opinions out there regarding proper crank lengths and if I go longer on mine and shorter on hers might it help aleviate some of our cadence mismatch?
While I'm on the subject, what brands are people impressed with?
I think that's a good idea (longer cranks for a faster spinner paired to shorter cranks for a non-spinner). I posted something before about this, but basically, the further your foot is from the center of the circle, the faster it moves for the same rpm's. I think the difference is around 3% per 5 cm. So if you're using 180's and your stoker is using 165's, then when the tandem is spinning along at 90 rpms, your feet are travelling almost 10% faster than your stoker's around the crank circle. This is why trackies use shorter cranks, because conversely, for the same foot speed, you can turn higher rpms!
It's not uncommon for the sync chain to have some excentricity. Here's a link to Sheldon Brown's site where he explains how to adjust that out. http://sheldonbrown.com/synchain.html
Originally Posted by shider
As far as crank length, I use 172.5 on my single, and the tandem has 175. I can't really tell any difference, but there is such a dramatic difference in the ride and performance of the two that it is probably pretty insignificant anyway. In theory, longer cranks for you would help even out your cadence differences, but you'll still need to compromise to match what she can comfortably spin.
Last edited by twilkins9076; 08-29-06 at 08:00 PM.
With few exceptions, "biopacing" is fairly common on tandem timing / sync chain and chainrings. I'm a bit short on time so rather than elaborting let me suggest you take moment to read through at least the first section on this page at Sheldon Brown's website: http://sheldonbrown.com/synchain.html It explains how you can adjust your chain rings to minimize the biopacing. Reading further will also offer some other interesting tips regarding tandem sync chains.
Originally Posted by shider
There are also some other trivial tips on timing chains at these two previous posts I've made to these forums: Timing chain
A final tip on setting timing chain tension: You'll want to leave enough slack in the chain -- usually an 1" -- before you tighten your eccentric's retention bolts so that you'll still have about 1/2" of slack once everything is snug. Note that, as the eccentric shell (Burley, Co-Motion) or eccentric itself (Trek, Cannondale, Santana) is tightened on a tandem, the BB axle will usually move forward a bit. If you don't factor in this additional slack you may find that it will take several attempts to get your timing chain properly adjusted to have sufficient play to prevent binding.
As for different crank lengths, go with what works best. Some folks are very sensitive to crank length while others (like myself) aren't bothered; I have 165s on my fixed gear bike, 170 on my personal road and offroad bikes, and 175 on our road and off-road tandems.
Yes, you can game the cadence mismatch by outfitting your stoker with 165's but over time you'll eventually learn to compromise on cadence... usually somewhere in the middle and biased towards the captain's preference.
As for "which tandem cranks"? We've been very happy with our daVinci cranks... square taper only (which is OK by me), but very light (about the same as FSA's Carbon Pro tandem cranks), very stiff, and immune to bio-pacing as they use NC-milled 34t timing rings fitted to a square spline on the timing crank arms. Pricing is about the same as the FSA Carbon Pros, i.e., about $550/pr. with your choice of chain rings and bolt circle patterns. Unlike the carbon cranks, you can buff the blemishes out of the polished aluminum daVinci cranks and they are available in lenghts ranging from 160mm through 180mm and the three major bolt patterns: we have 110mm BCD & 130mm BCD spiders on our two road tandems and our off-road tandem has the 94mm BCD spider.
Last edited by TandemGeek; 08-30-06 at 03:34 PM.
Thanks for the replies. I'm still open to getting new cranks but I'll see if they can be adjusted a little better.
Could you run a one or two tooth smaller chainring on the captain crank than on the stoker crank? This would certainly make you spin faster, but then your crankarms' phase would be constantly changing. Anyone try this?
The French company Spécialités TA also has a wide range of cranks for singles and tandems: http://www.specialites-ta.com/index_gb.php
Peter White carries these in the US. I found a pair of 180 mm TA's off of ebay that I run on my tandem. The funny thing is they are for the stoker, so I have a spider on both sides!
The one or two-tooth difference in cross-over chainrings would indeed give you a constant varience of pedal positions. Have not tried it, and is not something I'd like to try.
We pedal 90 degrees OOP.
Longer/shorter crankarms can make a bit of difference; however, tandem riding is more amenable to a compromise cadence. If the stoker ain't happy . . .
Somebody needs to try the different-sized cross-over rings and report back. Can't imagine that not causing problems.