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Why canít I put out more power as a stoker?

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Why canít I put out more power as a stoker?

Old 06-03-20, 02:56 AM
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Marilena
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Why canít I put out more power as a stoker?

Hello everyone! My husband and I have been riding our tandem for the last 2 years, primarily as an equalizer, so that we can enjoy riding together. A little bit of information about us. My husband is 6í tall (183 cm), 80 kg and has an FTP of 4.2 w/kg and I am 5í2Ē (157 cm), 54 kg, and I have an FTP of 2.7 w/kg. We both regularly ride our singe bikes and we have Garmin power meter pedals (his are vector 3s and mine are vector 2s) that we use on both the single and tandem bike to track our rides. As I have been getting stronger on the bike I have noticed that my power output is limited by how hard my captain is going. For example, when we are riding up a climb and my husband is averaging 220-230 w, my average power cannot be more than 110-115 w. No matter how hard I push the pedals. I can average around 153 w, only when my husband is riding at his threshold or above. I have no idea why that is happening. I have no problem getting my power as high as I want on my own bike, but there I control what gear and cadence I wanna be in. My husband does not understand it and tells me to just put more force on the pedal. But that simply doesnít do anything. It is very frustrating for me to not be able to go as hard as I want. We are reaching the point where tandem rides are either recovery or extremely easy effort for me, whereas my husband can go as hard as he wants. Given how much lower my FTP is compared to his, I shouldnít be limited to go harder, right? The only hypothesis I have is that the different crank length between the stoker and the captain is setting this power limitation for me. His cranks are 175 mm whereas mine are 150 mm (I am very short, and anything >165mm is giving me really bad pain on my hip flexors). I think that for a given gear and cadence we are locked in my torque is significantly reduced compared to his and that is what is limiting me. Am I crazy to think this is the case? What do you think is the reason? Has anyone else had similar problems? Are other stokers finding it impossible to put as much power out as they want? I would love to be able to get as good of a workout as I would like on the tandem. Right now that is not possible. It only happens if my husband wants to ride really hard.

Any suggestions/interpretations would be very welcome.
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Old 06-03-20, 07:06 AM
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Have you tried riding out of phase to see what happens to your power numbers? My wife and I are 90 deg OOP. (When my right foot is at 12 o'clock, her's is at 3 o'clock). We ride that way so one of us is always coming through the power stroke. I am not sure if it would help, but we have always wondered how in phase vs out of phase impacts power reading for each rider.
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Old 06-03-20, 08:47 AM
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Originally Posted by New2Two View Post
Have you tried riding out of phase to see what happens to your power numbers? My wife and I are 90 deg OOP. (When my right foot is at 12 o'clock, her's is at 3 o'clock). We ride that way so one of us is always coming through the power stroke. I am not sure if it would help, but we have always wondered how in phase vs out of phase impacts power reading for each rider.
Also consider going *slightly* out of phase, perhaps only a chain link or two.

Beloved stoker and I have done so for decades and doing so assists in her being better able to "feel" the pedal stroke and contribute.

Another plus is this minor adjustment will likely not affect the ability of your team to stand on the tandem, which can be an obstacle with 90 degree OOP.
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Old 06-03-20, 10:11 AM
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I think the crank lengths could be a major factor. People always suggest shortening the stoker cranks to make it easier for them.

You could try shortening the front cranks, tall riders can use shorter cranks, in fact a lot of TT riders do to get more aero.

You might also want to diagnose why it is that you can't ride with longer cranks. It could be something you can adapt to with the right training. My partner is the same height and has no issues riding even 175s.
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Old 06-03-20, 11:11 AM
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I have raised questions about the accuracy of power meters in tandem drivetrains in the past with little buy in. From my perspective, I would not expect the stoker power meter(s) to register accurately because they are designed to measure force applied to a single torgue arm driving power through the chain to the rear wheel (the torque arm being the chainring). In a tandem, you have the same torque arm driving the rear wheel, and you also have another torque are "pulling" the stoker cranks forward. Power is (mostly) additive, however, the stresses on the system are not characteristic with the system dynamics the power meters were designed to. My intuition tells me that you as the stoker may be putting out 150W, however, with the captain "pulling" your cranks forward with 250W, you cannot keep the same power applied to your pedals over time since they are constantly being pulled forward. As an analogy, think about pushing a car that is stuck in the snow. While it is stuck, you can feel how hard you are pushing on the trunk while the rear wheels spin. As the car begins to move, you can feel that the force you are applying to the trunk feels less. You are pushing just as hard, but the item you are pushing against is moving away from you.

An experiment to try that would not require you to change the phase of your drivetrain, would be to find an open area with no traffic, maybe a slight incline. Record a couple of short runs with as much power as you two can put out, then swap pedals and positions (you captain, and your captain as stoker) and repeat. I believe you should see your power as higher and your captains as less. If you see the same, results, It could be the same analogy where the captain now stoker is pulling your crank. (If you have a timing chain, see if you can observe the slack in the chain during each run. For the normal configuration, the top should be tight and bottom slack, and visa versa for the swapped configuration).

Last edited by Alcanbrad; 06-03-20 at 11:44 AM.
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Old 06-03-20, 11:14 AM
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Originally Posted by diabloridr View Post
Also consider going *slightly* out of phase, perhaps only a chain link or two.

Beloved stoker and I have done so for decades and doing so assists in her being better able to "feel" the pedal stroke and contribute.

Another plus is this minor adjustment will likely not affect the ability of your team to stand on the tandem, which can be an obstacle with 90 degree OOP.
My wife and I recently did this, with the stoker cranks one link ahead of the captain. She is able to better feel and direct, if she wants, the cadence. I also get some better feedback about when she wants to go harder on the bike. This leads me to shift earlier without a request from her.

We don't have power meters on the tandem, so I can't give you any data on relative effort due to this change. Perhaps if you try a timing change, you could report back and tell us if this actually works. I originally tried the change after some talk from some older and wiser tandem riders in club ride. But that's all it was - talk about feel and perceived effort after a ride.

Last edited by TooMany; 06-03-20 at 11:22 AM.
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Old 06-03-20, 12:19 PM
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At what cadence does your team usually climb? The idea of shorter cranks is that the loss of leverage will be made up for by increased cadence, power being force * speed. Shorter cranks reduce foot speed at the same cadence. Thus a higher cadence is required to generate the same power as longer cranks, using the same pedal force, and your power meter must be programmed for your crank length. Do you also have 150mm cranks on your single? If not, do you reprogram your power meter (like a Garmin) when moving it from bike to bike? Do you ride it at the same cadence as the tandem? The forgoing is one line of inquiry, purely about crank length and cadence.

The second line of inquiry is related to Alcanbrad's. Pedal strain gauges will register your pedal force no matter how hard the captain is pedaling. That said, we each have our pedal strokes programmed into our spinal ganglia. Differences in where the two team members apply power to the pedals could possibly change how hard your programming allows you to push on the pedals at a given point in the pedal stroke. This is what teams refer to as "being synced up." That refers to each team member applying the same proportion of their pedal force as the other team member, throughout the pedal stroke. This is what earlier commenters are attempting to fix by using the sync chain to modify where top dead center (TDC) is between the two cranksets. Many female stokers, I think for reasons of anatomy, have a harder time bringing the pedal over TDC. Having the cranks a bit out of phase is sometimes advantageous in that case.

The first interesting experiment on the flat would be to have your captain unclip, only you pedaling, and him shifting the gears down so that you can move the bike at your preferred cadence, watching your PM. See if you can get the same numbers you see on your single bike. Tell your captain to shift as necessary. That will answer the question as to whether your cranks or the bike's drivetrain have anything to do with it and also what your preferred cadence for max power is on the tandem. This assumes that your PM is programmed for your crank length.

A second experiment would be to move your cranks a link ahead and try to climb, then move a link behind and climb. See if there's any difference.

I've always wished that we had pedal PMs on our tandem, not so much for this reason, but so we would both ramp our power up and down together.
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Old 06-03-20, 12:58 PM
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Originally Posted by Alcanbrad View Post
I have raised questions about the accuracy of power meters in tandem drivetrains in the past with little buy in. From my perspective, I would not expect the stoker power meter(s) to register accurately because they are designed to measure force applied to a single torgue arm driving power through the chain to the rear wheel (the torque arm being the chainring). In a tandem, you have the same torque arm driving the rear wheel, and you also have another torque are "pulling" the stoker cranks forward. Power is (mostly) additive, however, the stresses on the system are not characteristic with the system dynamics the power meters were designed to. My intuition tells me that you as the stoker may be putting out 150W, however, with the captain "pulling" your cranks forward with 250W, you cannot keep the same power applied to your pedals over time since they are constantly being pulled forward. As an analogy, think about pushing a car that is stuck in the snow. While it is stuck, you can feel how hard you are pushing on the trunk while the rear wheels spin. As the car begins to move, you can feel that the force you are applying to the trunk feels less. You are pushing just as hard, but the item you are pushing against is moving away from you.

An experiment to try that would not require you to change the phase of your drivetrain, would be to find an open area with no traffic, maybe a slight incline. Record a couple of short runs with as much power as you two can put out, then swap pedals and positions (you captain, and your captain as stoker) and repeat. I believe you should see your power as higher and your captains as less. If you see the same, results, It could be the same analogy where the captain now stoker is pulling your crank. (If you have a timing chain, see if you can observe the slack in the chain during each run. For the normal configuration, the top should be tight and bottom slack, and visa versa for the swapped configuration).
I think you may be wrong with the car pushing analogy. If you're pushing the back of a car with five pounds of force, it's irrelevant if the car is moving or not, nor how fast. Five pounds of force is five pounds. Where your pushing will be affected is if the speed at which the car is moving away from you is so fast, you cannot apply the five pounds of force. It's like two locomotive engines working together. I always assumed each locomotive had to be PERFECTLY in synch so that each is both putting out the exact same power, and applying the same pulling force. But then I realized, no, it doesn't matter. As long as the two can apply and maintain forward traction force, it doesn't matter if one locomotive is working harder than the other; they are both contributing forward traction force, regardless of the ratio. 50-50 is, I'm sure, the most efficient way, but it doesn't matter mechanically. Additionally, it doesn't matter how fast the front locomotive is moving for the second one to contribute.

So pushing on the pedals is the same. You're either applying a specific force to the pedals, or not. This is not to say cadence isn't a contributory factor. High captain cadence, for example, could indeed prevent a stoker from applying the intended force to the pedals in the first place. But if the same pressure is applied at 80 RPM or 110 RPM, it's the same force.

More than that, I cannot comment since I stopped competitive riding WAY before power meters entered the scene.
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Old 06-03-20, 01:37 PM
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While I agree with Carbonfiberboy and LV2TDM in theory, my contention is that it is different system due to human anatomy. In the case of a locomotive the power curve through the stroke of the engine is same regardless of load (especially with electric motors where the torque cure is linear and constant through the entire 360 degree cycle), however, with us frail humans, the power output is not a constant, there is a bio-mechanical feedback mechanism in place that will automatically adjust your power output when things start to move. I realize I am on somewhat shaky ground here (I am an engineer, but of the electrical variety), but I do know we do not output instant power and hold it constant (as Carbonfiberboy alludes to) through the full turn of the crank. Otherwise, we would be breaking lightswitches and ripping car doors off their hinges as we open them. As a result, I expect the stoker to have a problem maintaining constant peak power through the power stroke due to the crank being accelerated away from them due to the captain. I would think the experiment I proposed of swapping positions and comparing results will either prove my intuition correct or strongly suggest I not quit my day job

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Old 06-03-20, 04:00 PM
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Originally Posted by Alcanbrad View Post
While I agree with Carbonfiberboy and LV2TDM in theory, my contention is that it is different system due to human anatomy. In the case of a locomotive the power curve through the stroke of the engine is same regardless of load (especially with electric motors where the torque cure is linear and constant through the entire 360 degree cycle), however, with us frail humans, the power output is not a constant, there is a bio-mechanical feedback mechanism in place that will automatically adjust your power output when things start to move. I realize I am on somewhat shaky ground here (I am an engineer, but of the electrical variety), but I do know we do not output instant power and hold it constant (as Carbonfiberboy alludes to) through the full turn of the crank. Otherwise, we would be breaking lightswitches and ripping car doors off their hinges as we open them. As a result, I expect the stoker to have a problem maintaining constant peak power through the power stroke due to the crank being accelerated away from them due to the captain. I would think the experiment I proposed of swapping positions and comparing results will either prove my intuition correct or strongly suggest I not quit my day job
I can see where you're coming from. I guess it would take analysis of the captain's pedal stroke to see how much delta v occurs throughout the pedal cycle. Yes, if high enough, I could see him leaving the stoker behind and "dragging" him/her during peak acceleration spots. But I'm pretty darn sure if I were in the stoker position and that happened, I'd be aware of this and would definitely feel the pedals get "light" or even start pulling my foot. Given my abject horror with how, for example, BioPace chainrings felt - and they didn't actually "pull" my feet - I'm pretty darn sure I'd feel it. Actually, after a little thought (duh), when captaining our tandem I get immediate feedback when my stoker starts feeling apprehension on the bike. I feel her let up and it transmits to my feet right away; I definitely feel the change in power output from her end. Sometimes I have to say, "What's up? Were good here; let's keep going." Plus, we communicate regularly through the pedals by letting off, physiologically "telling" the other it's time to either stop pedaling, stand to rest/climb, get the outside pedal down on corners, you name it.

Now this is with clipless pedalsl front and rear, so changes can be felt immediately. Without clipless, it would suck to not have the sensitive feel and have the pedals just leave your feet!

Anyway, I see where you're coming from and it would actually be a great study to conduct: varying power outputs between different tandem teams, given all the possible variables. You could gather a lot of data between different scenarios;
Captain power: X, Stoker power: 0.25X
Captain power: X, Stoker power: 0.5X
Captain power: X, Stoker power: 1.33X
Captain power: X, Stoker power: X
Captain power: X, Stoker power: 1.25X
Captain power: X, Stoker power: 1.5X

Captain cadence: X, Stoker cadence: X
...
Captain weight: X, Stoker weight: X
...
Captain crank length: X, Stoker crank length: X
Etc...

You get the picture. Then imagine all the permutations and results of various combinations. It would be very interesting for a thorough analysis to actually see what's going on and how to remedy issues.
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Old 06-03-20, 05:06 PM
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Originally Posted by LV2TNDM View Post
I can see where you're coming from. I guess it would take analysis of the captain's pedal stroke to see how much delta v occurs throughout the pedal cycle. Yes, if high enough, I could see him leaving the stoker behind and "dragging" him/her during peak acceleration spots. But I'm pretty darn sure if I were in the stoker position and that happened, I'd be aware of this and would definitely feel the pedals get "light" or even start pulling my foot. Given my abject horror with how, for example, BioPace chainrings felt - and they didn't actually "pull" my feet - I'm pretty darn sure I'd feel it. Actually, after a little thought (duh), when captaining our tandem I get immediate feedback when my stoker starts feeling apprehension on the bike. I feel her let up and it transmits to my feet right away; I definitely feel the change in power output from her end. Sometimes I have to say, "What's up? Were good here; let's keep going." Plus, we communicate regularly through the pedals by letting off, physiologically "telling" the other it's time to either stop pedaling, stand to rest/climb, get the outside pedal down on corners, you name it.

Now this is with clipless pedalsl front and rear, so changes can be felt immediately. Without clipless, it would suck to not have the sensitive feel and have the pedals just leave your feet!

Anyway, I see where you're coming from and it would actually be a great study to conduct: varying power outputs between different tandem teams, given all the possible variables. You could gather a lot of data between different scenarios;
Captain power: X, Stoker power: 0.25X
Captain power: X, Stoker power: 0.5X
Captain power: X, Stoker power: 1.33X
Captain power: X, Stoker power: X
Captain power: X, Stoker power: 1.25X
Captain power: X, Stoker power: 1.5X

Captain cadence: X, Stoker cadence: X
...
Captain weight: X, Stoker weight: X
...
Captain crank length: X, Stoker crank length: X
Etc...

You get the picture. Then imagine all the permutations and results of various combinations. It would be very interesting for a thorough analysis to actually see what's going on and how to remedy issues.
The interesting thing about moving from single to tandem is the almost total lack of delta-V in the pedals. On a single, when you push down, the bike "gets out of the way". Doesn't happen on a tandem, where pedal forces are very constant. The technical term is crank inertial load, very high on a tandem. I remember the first time we went out for 20 miles on our tandem and my legs were just destroyed even though my HR never got all that high. I was accustomed to doing 60 mile competitive group rides in hilly terrain, so it wasn't that I was out of shape. My legs just weren't accustomed to that constant loading.

The OP can change her meter display to show instant power instead of 3 second average which most of us use. That'll tell the story, right there.
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Old 06-03-20, 08:35 PM
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Originally Posted by Carbonfiberboy View Post
The interesting thing about moving from single to tandem is the almost total lack of delta-V in the pedals. On a single, when you push down, the bike "gets out of the way". Doesn't happen on a tandem, where pedal forces are very constant. The technical term is crank inertial load, very high on a tandem. I remember the first time we went out for 20 miles on our tandem and my legs were just destroyed even though my HR never got all that high. I was accustomed to doing 60 mile competitive group rides in hilly terrain, so it wasn't that I was out of shape. My legs just weren't accustomed to that constant loading.

The OP can change her meter display to show instant power instead of 3 second average which most of us use. That'll tell the story, right there.
I'm sorry, but if the delta v is either very low or nonexistent, then how will this be an issue? Aren't you effectively rebutting your original point? If your stoker is able to apply force to the pedal and it isn't "getting away" during spikes in velocity, then isn't there no problem? I'm not sure how there's an issue then...

And I've noticed the same as well. Bumping up the power by one rider is indeed very taxing, given the much greater inertia of the additional rider and the bike. And I assume tandem "inertial load" comprises mainly of the additional rider mass of the stoker. That plus the additional weight of the tandem would almost double the mass most captains are used to. Depends though. In my case with a pint-sized stoker, it isn't as much. But I have a VERY heavy tandem, so that makes up for her light weight. Now put me on a carbon tandem and I'd see some nice improvement.
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Old 06-03-20, 09:10 PM
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Originally Posted by LV2TNDM View Post
I'm sorry, but if the delta v is either very low or nonexistent, then how will this be an issue? Aren't you effectively rebutting your original point? If your stoker is able to apply force to the pedal and it isn't "getting away" during spikes in velocity, then isn't there no problem? I'm not sure how there's an issue then...

And I've noticed the same as well. Bumping up the power by one rider is indeed very taxing, given the much greater inertia of the additional rider and the bike. And I assume tandem "inertial load" comprises mainly of the additional rider mass of the stoker. That plus the additional weight of the tandem would almost double the mass most captains are used to. Depends though. In my case with a pint-sized stoker, it isn't as much. But I have a VERY heavy tandem, so that makes up for her light weight. Now put me on a carbon tandem and I'd see some nice improvement.
[chuckles] What do you mean I can't have it both ways? I think the issue is trying to prevent a delta-v, i..e. captain working to drag the stoker's foot over the top without slowing. Not that this is the OP's issue - more my issue. Inside baseball - I always do one-legged pedaling in winter and early spring on my resistance rollers, have for years. 2 minute interval, trade legs, 2 minutes together, the OLP at about 1/2 my FTP, repeat until non-operational legs are achieved. Dear Stoker can't do 1 minute on her trainer, no resistance. As a team, we're about like the OP's, stoker with 1/2 the watts of captain, just not in their league. Stoker loves it though, and that's what counts.

But the odd thing is the low stoker watts, which can't be accounted for by a delta-v. What you save in one spot, you expend in another. Thus two single riders can put out the same watts at the same HR, one hammering the downstroke, the other pedaling circles. They might suck as a tandem team though because the pedals won't respond the way the hammerer is used to - no delta-v. Tandeming is complicated. I ride with a tandem where the stoker gets saddle sores because the captain is a hammerer who rocks his torso over the downstroke, and thus also rocks the bike without realizing it. There's another tandem where it's the stoker who rocks her torso except she's so light that the captain doesn't feel it at all.
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Old 06-03-20, 09:11 PM
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Not a tandem rider but I've known some teams. I read the OP's post and wondered what her preferred cadence is on the single bike and what they ride on the tandem. If the tandem's RPM is higher, yes, I can see that the OP's push into that higher RPM pedal will be a lot less. So before I got far into power meter stuff, I would just look at that one issue. RPM. (Conveniently, that's really easy!)
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Old 06-03-20, 09:32 PM
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Originally Posted by 79pmooney View Post
Not a tandem rider but I've known some teams. I read the OP's post and wondered what her preferred cadence is on the single bike and what they ride on the tandem. If the tandem's RPM is higher, yes, I can see that the OP's push into that higher RPM pedal will be a lot less. So before I got far into power meter stuff, I would just look at that one issue. RPM. (Conveniently, that's really easy!)
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Old 06-03-20, 10:27 PM
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(captain speaking here)

Thanks for all the suggestions and lines of inquiry thus far. Here are some answers to questions I've read through:
  • I know that she prefers a higher cadence than I do on her own bike. I have a 50-34 with 11-34 and tend to spin in the high 70s. She has a 46-30 with 11-34 and 650b wheels and tends to spin around 90 RPMs. Because 1) I'm in control of shifting and 2) I output about twice as much power as her, I think my cadence tends to dominate.
  • As she switches her pedals between the 165 cranks on her own bike and the 150s on our tandem, she always adjusts the crank length, installs the setup angles, and recalibrates (and then after we do our first hard effort after switching pedals, recalibrates again).
  • I think going 90 degrees out of phase would be too much for us, but trying a couple links on either side sounds like a great idea, we can try that when the weather here clears back up and report back.
She originally switched to 150s on the tandem and 155s on her own bike when she kept getting muscle pain with 170s after about two hours of riding. When she upgraded her solo bike, it came with stock 165s. She gave them a try and said that they're totally fine. But she did then try 170s again shortly thereafter and had the same problems with muscle pain. So for her, going back to 170s isn't an option. I'm perfectly happy to try both of us going to 165s though.
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Old 06-04-20, 12:53 AM
  #17  
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Originally Posted by surak View Post
I think the crank lengths could be a major factor. People always suggest shortening the stoker cranks to make it easier for them.

You could try shortening the front cranks, tall riders can use shorter cranks, in fact a lot of TT riders do to get more aero.

You might also want to diagnose why it is that you can't ride with longer cranks. It could be something you can adapt to with the right training. My partner is the same height and has no issues riding even 175s.
Perhaps your stoker has longer legs. I definitely do not have long legs for my height. And I can 100% say that it is not a matter of lack of training. It took me years to figure out how to get a bike with components that fit me properly. I feel the need to give some more detailed explanation because there isn’t enough information out there. And some of you might have partners that are smaller than average like me. Shorter cranks for me means that I don’t have to overextend my knees for every revolution and hit my torso. They also allowed me to get the saddle height much better adjusted. Which also helped a lot with saddle sores (Eventually though I had to change to better fitted seat and now I have zero saddle discomfort). I have 3 different bikes with 3 different crank lengths, 150mm, 155mm and 165mm. The last 2 are on my single bikes and my preferred cadence tends to be 80-90 rpm when climbing. When I had to use some rental bikes with 170s the pain was instantly back, even though I was well trained at that point. Switching back to my own bikes, pain free again. It is a matter of geometry, height and also leg length!
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Old 06-04-20, 01:15 AM
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Than you all very much for the suggestions and comments. I think my husband answered all of your questions. Btw I love how you all assumed that the captain prefers a higher cadence than me! I definitely do not feel like the pedal is being pulled away from me too soon. It is hard to find the right words to best describe the feeling. If anything I feel like I am in too easy of a gear to apply any more force.
  • I know that I can get my power higher on the tandem, ie when we sprint up a small bump, or when my husband is really going hard so there is not physical limitation from the drivetrain. But it can only happen when the captain increases his effort as well. Perhaps that has to do with gear selection or the cadence he dictates. It is hard to say, because a lot of our rides are either ascending or descending. Not much flat ground around here.
  • I would love to be able to switch positions with him, but unfortunately that is not physically possible. First of all I wouldn’t be able to fit on his seat and even if I did I don’t have the upper body strength to captain someone 26 kg (57 lbs) heavier than me.
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Old 06-04-20, 06:30 AM
  #19  
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A little late on the response, but I was in the land of nod while you left coasters were continuing the discussion.

You said:

Originally Posted by LV2TNDM View Post
....

Actually, after a little thought (duh), when captaining our tandem I get immediate feedback when my stoker starts feeling apprehension on the bike. I feel her let up and it transmits to my feet right away; I definitely feel the change in power output from her end. Sometimes I have to say, "What's up? Were good here; let's keep going."
...
I think this proves my point. When the stoker "lets up" you feel it. you didn't let up. F=ma and Power=Fv. If your power did not change, and we can agree the mass of the system did not change, then what you feel in the pedal should not change, you should only feel a change in acceleration and velocity of the mass of the system. When I apply the brakes to slow down, I don't feel it in the pedals, I feel it in the entire system. Given you feel the force of your foot to the pedal, means the power meter strain gauges "feel" it as well and the reading would change. This feeling one gets of the other team member increasing or decreasing power is exactly the effect of one "pulling" or "impeding" the others effort.

This is something that is unlikely to be answered here. The OP said that swapping positions is not possible for them so perhaps another experiment is for them to record an effort where both are pedaling hard and then one of them lets up. I would be surprised if we didn't see the others power reading increase at that instant. The power readings could be easily compared in DC Rainmakers analysis tool.

(Thinking about it, this might make a great doctoral thesis project for an eager grad student.)

Good discussion.
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Old 06-04-20, 08:34 AM
  #20  
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I'm looking forward to the feedback on adjusting the timing chain with Marilena's pedals a couple of links before, and then a couple of links behind cpunerd's. I have my wife's a couple of links ahead so se is feeling the pressure of the power stroke just before me. Happy tandeming!
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Old 06-04-20, 09:12 AM
  #21  
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I really think this is a cadence, crank length issue. In our experience, two sets of Garmin Vector pedals gave us results that were very close to what youíd expect based on data from power meters on our single bikes, and perceived exertion.

If anything the stokers pedals would show a couple of watts of ďghostĒ power when the captain pedaled, and no pressure was put on the stoker pedals. So if anything the stokers power was 2-3 watts high.

It would be interesting to know the stokerís heart rate, and perceived exertion. If those indicate sheís working at threshold, then it would appear it is a measurement problem.

if, however, HR and perceived exertion indicate sheís working well below threshold, then something needs to change with her crank length, cadence or both.
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Old 06-04-20, 10:08 AM
  #22  
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Originally Posted by cpunerd View Post
(captain speaking here)

Thanks for all the suggestions and lines of inquiry thus far. Here are some answers to questions I've read through:
  • I know that she prefers a higher cadence than I do on her own bike. I have a 50-34 with 11-34 and tend to spin in the high 70s. She has a 46-30 with 11-34 and 650b wheels and tends to spin around 90 RPMs. Because 1) I'm in control of shifting and 2) I output about twice as much power as her, I think my cadence tends to dominate.
  • As she switches her pedals between the 165 cranks on her own bike and the 150s on our tandem, she always adjusts the crank length, installs the setup angles, and recalibrates (and then after we do our first hard effort after switching pedals, recalibrates again).
  • I think going 90 degrees out of phase would be too much for us, but trying a couple links on either side sounds like a great idea, we can try that when the weather here clears back up and report back.
She originally switched to 150s on the tandem and 155s on her own bike when she kept getting muscle pain with 170s after about two hours of riding. When she upgraded her solo bike, it came with stock 165s. She gave them a try and said that they're totally fine. But she did then try 170s again shortly thereafter and had the same problems with muscle pain. So for her, going back to 170s isn't an option. I'm perfectly happy to try both of us going to 165s though.
Well, there it is. She needs to spin 90 on those short cranks to develop full power. We went through a sort of similar thing on our tandem. My stoker also spins 90 on her single, as do I, so our cadences on the tandem are great. Our tandem came with 175 captain cranks and 170 stoker cranks. My stoker kept getting leg cramps during long hard rides. I thought about that for a while and finally put on cranks which are the correct length for her legs: 151mm. Her cramps went away. We do fine on the flat, but on steep hills I just can't pedal 90 as my HR skyrockets and my power drops. I climb best at 78 rpm though sometimes we are overgeared on the tandem and it drops to 70, which I can handle. But at anything below about 85, my stoker's power drops off because her torque is limited by her leg strength and the short cranks. Mine would be, too.

After the stoker crank change, our course averages stayed about the same, but our climbing slowed. No more climbing PRs and especially the really steep OOS climbs are more difficult. Looks like we do a little better on the flat and low angle descents where we can both spin it up. She doesn't mind now if we come into a hill at 105 and hold high cadence until we run out of gears. We couldn't do that with her 170 cranks as I'd be pulling her pedals around the circle at anything over 90.

Thus I suggest gearing down on climbs and spinning faster. But . . .hard to say if you come out on it if captain's power drops at higher cadences, even though stoker's power will likely go up. Or, captain could simply train himself to ride higher cadences. That works. Years ago I trained myself up from 85 to 95 on the flat with my single bike over the course of a summer. Research says that the higher the VO2max, the higher the optimal long climb cadence. I've seen the same thing with really good female athletes like your wife.

Funny story: We were out on a hard group ride, trying to keep up with the fast youngers on singles, when we came to a short, somewhat steep pitch. As we started up it, both my legs cramped solid. I hollered, "Pedal!" as she felt the full weight of the bike. She did get us up it! That was with the 170s.
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Old 06-04-20, 10:39 AM
  #23  
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The heart and PE are in line with the power meter readings. I don't think it is a measurement problem merlinextraligh.

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Old 06-04-20, 10:50 AM
  #24  
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Originally Posted by Carbonfiberboy View Post
Well, there it is. She needs to spin 90 on those short cranks to develop full power. We went through a sort of similar thing on our tandem. My stoker also spins 90 on her single, as do I, so our cadences on the tandem are great. Our tandem came with 175 captain cranks and 170 stoker cranks. My stoker kept getting leg cramps during long hard rides. I thought about that for a while and finally put on cranks which are the correct length for her legs: 151mm. Her cramps went away. We do fine on the flat, but on steep hills I just can't pedal 90 as my HR skyrockets and my power drops. I climb best at 78 rpm though sometimes we are overgeared on the tandem and it drops to 70, which I can handle. But at anything below about 85, my stoker's power drops off because her torque is limited by her leg strength and the short cranks. Mine would be, too.

After the stoker crank change, our course averages stayed about the same, but our climbing slowed. No more climbing PRs and especially the really steep OOS climbs are more difficult. Looks like we do a little better on the flat and low angle descents where we can both spin it up. She doesn't mind now if we come into a hill at 105 and hold high cadence until we run out of gears. We couldn't do that with her 170 cranks as I'd be pulling her pedals around the circle at anything over 90.

Thus I suggest gearing down on climbs and spinning faster. But . . .hard to say if you come out on it if captain's power drops at higher cadences, even though stoker's power will likely go up. Or, captain could simply train himself to ride higher cadences. That works. Years ago I trained myself up from 85 to 95 on the flat with my single bike over the course of a summer. Research says that the higher the VO2max, the higher the optimal long climb cadence. I've seen the same thing with really good female athletes like your wife.

Funny story: We were out on a hard group ride, trying to keep up with the fast youngers on singles, when we came to a short, somewhat steep pitch. As we started up it, both my legs cramped solid. I hollered, "Pedal!" as she felt the full weight of the bike. She did get us up it! That was with the 170s.
Thank you for the information, that is very good point you bring up. My captain is concerned about his power dropping by switching to shorter cranks, say 165s or 170s. My power contribution will always be much smaller than his. We do have a question though. You said you switched your wife's to 151s but you never mentioned what crank length you got for yourself to match her cadence. Was it also 151s?

And kudos to your wife!!
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Old 06-04-20, 12:14 PM
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Originally Posted by Marilena View Post
Thank you for the information, that is very good point you bring up. My captain is concerned about his power dropping by switching to shorter cranks, say 165s or 170s. My power contribution will always be much smaller than his. We do have a question though. You said you switched your wife's to 151s but you never mentioned what crank length you got for yourself to match her cadence. Was it also 151s?

And kudos to your wife!!
No, I can pedal fine on the 175s, hitting 115 if I want to. My single has 170s and I don't really notice the difference. It's only like 1/8" per crank, nothing really,. Now 25mm, that's a lot. You raise a good point. If I got shorter cranks, I'd have to spin faster to get the same power and thus my stoker would also develop more power - in an ideal world. The world of our tandem is not ideal however since I don't really want to put on any lower gears than we have on there now. Our cassette is already wide-spaced. So when we slow down on a steep hill, we're still stuck with the same crank RPM but then we'd both have shorter cranks and we'd climb even slower since power = force * speed and force is limited by how hard our muscles can push on the pedals.

In your case, with more power and thus perhaps never having to drop below best performing cadence, shorter cranks for the captain might make sense. So I'd say that if your bike runs out of gears, like ever, captain shouldn't change his cranks. Or you could gear lower, so that didn't happen, and spin faster on his shorter cranks. The crank length formula is 5.5 * crotch to floor in inches = crank length in mm. If your captain's current cranks match that formula or are even close, then he just has to learn to spin faster in a lower gear on those cranks. My ideal crank length by the formula would be 165, but like I say, I don't want to lose power on the steep stuff, so I keep the longer cranks. Your captain is rightly concerned. On my single, I have geared low enough that I don't run out of gears on our usual hills of <10%.

That's a bit incoherent, but hopefully you can make sense of it. Yeah, definitely agree on the kudos. She's a trooper. Does 10-day backpacks with me in the mountains. One can do what one loves to do, that simple. Our team age next week will be 146. I had to double check that on the calculator. Live long and prosper.

Another thought: my wife's trainer has erg mode, meaning power remains fixed even though cadence changes. I have her put it in a gear that gives her sweet spot power and then drop the cadence to 70 and hold that for up to 30 minutes. That's helpful for anyone. I do the same thing with her, but on my resistance rollers using gearing to drop the cadence while holding power. Hurts like the very devil. Some coaches say drop the power further, down to the top of zone 3, and take it down to 50-55 cadence, no upper body movement. I used to do that, haven't in a while.
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