CaptainHaddock

Hey everyone, so as we begin to really dial our tandem-fit in, we're discovering that (well rediscovering really) that 172.5 is too long for comfortable riding for the captain (me). Having poked around on the FSA site, I'm not seeing a size shorter than the aforementioned 172.5 cranks. Does anyone know if FSA makes shorter cranks and I'm not seeing them listed, or am I going to have my cranks manually adjusted? What about the new Shimano cranks?

thanks!

Dean V

What size do you want? 165mm? You could put in some single bike cranks backwards. Then either drill out and heli coil the pedal threads or swap the pedal bodies on their spindles. I have a bike which has the left hand thread on the right side and vice versa. Haven't had any trouble with the pedals working loose yet.

waynesulak

A year or so ago I needed to replace some 2006 170mm FSA captain's cranks and ended up emailing them. They said they did not make them anymore but had a few carbon 170mm in old stock to sell. Guess they think all captain's need cranks at least 172.5mm.

Rather than buy from FSA I went wtih daVinci cranks. Lots of size options. Not cheap but can get down to 160mm for stoker or captain. You can also get 110mm spiders if you like and as light as any tandem cranks other than Lighting Cranks. daVinci has been supporting these cranks for many years. Shimano and FSA sell theirs for a while and abandon sizes or in Shimano's case pull out of tandems all together.

https://www.davincitandems.com/comp.html

CaptainHaddock

So on the topic of captain crank length, how does the combination of shorter captain with longer stoker work? If I'm thinking this through correctly, since they will both have the same size timing ring that at the center point, they rotate at the same speed, but at the crank end / peddle we'll have two differing rotational speeds / cadences. Is this correct? if so, what will be the issues?

thanks!

Dean V

You need to balance the length of crank you like to ride with your cadence relative to the stoker. If, when you are both riding the cranks you like do you tend to have a similar prefered cadence? If for example the captain tends to spin a bit faster than the stoker it may be advantageous for the captain to have longer cranks which will slow down their natural cadence and bring the team more in balance. The same could be achieved by giving the stoker shorter cranks. For me I can't notice any real difference between 170 and 172.5mm cranks. But 175mm definitely drops my cadence and 165mm noticeably raises it.

waynesulak

You are correct that both cranks must turn at the same speed since they are connected by the same size timing rings. The term cadence is generally used to describe the rpm of the timing ring or crank so the cadence must be the same for both the captain and stoker.

Crank length should be determined primarily by the comfort of the rider. Taller riders generally like longer cranks than shorter people but it is a personal decision. Flexibility and coordination can play a role as well as the leg length of the rider.

Small variation can be made to the preferred crank length to help the stoker and captain agree on a cadence. If one rider likes a faster cadence then lengthening their crank will tend to slow them down. Likewise if a rider likes a slow cadence then a shorter crank will tend to help them spin faster.

General information on crank length can be found at:
https://sheldonbrown.com/cranks.html

DubT

The Sheldon Brown article is very good and I think answers about any question one would have about crank lengths.

I find it hard to believe that a 2.5 mm difference can be felt, that is 3/32 of an inch. 1/8 of an inch is 3.175 mm.
5 mm is still only 3/16 of an inch. We run 175s front and rear and are not tall, with no problem.

Wayne

CaptainHaddock

So part of what I've been doing to get my-half of the bike fitted in, is to put the bike on the trainer, adjust the various bits to be close to what I ride on my professionally fitted single and then ride until I find something uncomfortable. What I've discovered is that I "feel" the difference in rotational length between my standard 165mm cranks, to that of the 172.5 cranks. I'm looking to see if I can find a used 170 crank to play with as that's what my wife rides with and to see if I can split the difference.

DubT

I would suggest that you wait until you get it on the road before making any more changes. I have found that there can be a difference between the way you fit a tandem versus the way you fit a single. They are two different animals.

Quit playing and RIDE! LOL

Dean V

Are you quite short that you use 165 cranks? As DubT said I wouldn't bother with 170s, you probably wouldn't notice the difference. Just ride it for a while as it is.

CaptainHaddock

So to that end, we've taken her out on 4 rides now, two of them in XC format, and two of them road. When riding XC, I'm moving about enough that I don't notice. When I'm riding road, I feel like the longer cranks are a bigger reach for my legs.

I am beginning to think that yes, you are right, there are major differences between the two.

CaptainHaddock

I'm 5'4.5", so yeah, short. I and my stoker fit the cdale rt2 M/S fairly well (my stoker who is about 2 - 3" taller). I've been able to fit her into the stoker easier as she fits the standard size models better than I. It's geometry isn't quite everything I'd want, but this is our first tandem and so we're working with prefab sizing over custom fits.

jimmuller 

Check out https://www.bikesmithdesign.com/ if you need shorter arms.

Chris_W

The difference between 172.5 and 170 is actually 5mm because to keep the leg extension at the bottom of the pedal stroke correct, you should place the saddle 2.5mm lower for the longer cranks, then when your knee reaches the top of the pedal stroke it is 5mm closer to your chest than when using the shorter crank. For me, that was a noticeable difference. I spent several rides moving my saddle up and down by a few mm because it felt either too high or too low in different parts of the pedal stroke. On my other bikes, I always used to ride the stock 170s, but I have recently been changing them all to 165 or 167.5 models (the latter being my favorite size, but it's very rare).

I put up with the 172.5mm FSAs for quite a while but eventually bought some NOS Shimano 105-level captain cranks (model 1057 I believe) in 170 length. These are solid arms with a square-taper BB, so I've been wanting to find a lighter option, which hopefully didn't require modifying the crank or pedals. Shimano's new Ultegra-level cranks are offered with 170 mm captain's cranks, and I was seriously considering those. However, I've instead started a major project to retrofit a Rohloff 14-speed internal gear hub to our tandem. This needs only one chainring to drive the rear wheel (although I have also been considering using two rings, with a half-step gearing difference) and so I can now use a single-side drive setup by moving the timing chain/belt to the drive-side, which means that we'll have all of the single-bike cranks available to choose from. I'm currently figuring out the two chainlines and deciding exactly which cranks are going to work best, but we may now finally be able to have lightweight 165mm cranks front and rear, which would be ideal.

DubT

Chris, you are exactly correct about the total travel being 5mm, however that is still only 3/16 of an inch total. I am not convinced that for the majority of us it makes any significant difference, if my numbers are right the 5mm amounts to about a 3 percent difference in total length. Just my opinion, I know some people are more sensitive to change than others, however the human body is very adaptable, Awesome Designer!

Wayne

jimmuller 

I have three rideable solo bikes and one tandem. The tandem and two of the solos have 170mm cranks, the other solo has 165mm. It was not by plan, simply a good option when I built the bike up. I don't think I can feel the difference. If I can it is pretty subtle.

Just personal experience. YMMV.

waynesulak

You may be right for the majority of us however that also implies agreement that there is a minority that it could benefit. Unless cost is prohibitive why not try different length cranks and see what feels best?

jimmuller 

I think one key point here is that if you're going to try shorter cranks, don't mess around with tiny changes. My wife who is 5'2" uses 140mm cranks on the tandem and her solo bike (ridden only on a trainer) and she loves 'em.

DubT

If the implied minority that could benefit from shorter/longer cranks have the resources to experiment then more power to them. How do you really objectively measure the difference? There are so many variables it would almost take a lab environment to determine if there were any true performance advantages.

Wayne

diabloridr

Knee pain is the critical characteristic that matters to my stoker.

waynesulak

I do not expect a dramatic or even a measurable increase in short term power. For some shorter riders shorter cranks allow them to ride longer with less pain in the knees or hips. I feel that at the end of long rides they put out more power without turning circles just a little bigger than what is comfortably within their functional limits.

I use the term functional limits because it is not just height but other muscular and joint issues that can cause problems. It is like raising handlebars with an up turned stem. If it allows a rider to work pain free at threshold longer then it is to me a good thing.

For tandem stokers it allows some short stokers to keep up with the captain's spin so it is just more fun.

zonatandem

Stoker Kay is 4' 10 3/4" 'tall'. Pilot Rudy is 5'7". We both ride 170mm cranks on our tandems.
Have been doing so for the past 37 years.

CaptainHaddock

Which is what I'm thinking I'm going to do, drop down to the 170's in the next few months.