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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 09-18-09, 10:00 AM   #26
Alex & Deya
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Pedal based power meter...pics from Interbike courtesy of Google wattage group.

Some photos from the pre-Interbike open house:

Information sheet, page 1:
http://www.flickr.com/photos/djconnel/3930884632/

Information sheet, page 2:
http://www.flickr.com/photos/djconnel/3931039857/

Blow-up from wall poster:
http://www.flickr.com/photos/djconnel/3930768660/

Souvenir water bottles :
http://www.flickr.com/photos/djconnel/3930768710/
Wow that looks very good! that looks like a great option for tandem. Any idea of price?
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Old 09-18-09, 10:09 AM   #27
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I hope it works well.

I may end up selling a few pairs of Keos

Quote:
Originally Posted by Hermes View Post
Pedal based power meter...pics from Interbike courtesy of Google wattage group.

Some photos from the pre-Interbike open house:

Information sheet, page 1:
http://www.flickr.com/photos/djconnel/3930884632/

Information sheet, page 2:
http://www.flickr.com/photos/djconnel/3931039857/

Blow-up from wall poster:
http://www.flickr.com/photos/djconnel/3930768660/

Souvenir water bottles :
http://www.flickr.com/photos/djconnel/3930768710/
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Old 09-18-09, 10:42 AM   #28
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Wow that looks very good! that looks like a great option for tandem. Any idea of price?
$800 including pedals is what I remember.
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Old 09-18-09, 10:55 AM   #29
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Why not the Polar chain-pickup system? That's what I'm thinking about. One pickup for the timing chain to get captain's power, another for the chainstay to get combined power off the stoker's HRM. Subtract for stoker power. The Polar recording HRMs move from bike to bike, so all you need is multiple pickups. Riders I know using the Polar power setup say they work +- about 5% compared with SRM.
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Old 09-18-09, 04:12 PM   #30
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Why not the Polar chain-pickup system? That's what I'm thinking about. One pickup for the timing chain to get captain's power, another for the chainstay.
Because it won't work with a belt
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Old 09-26-09, 02:27 PM   #31
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I agree: Vector appears the most promising solution, especially if they release a Frog version (Speedplay road pedals being more oriented towards competition than rest stops). Frogs share the same spindles as X-series and Zeros, but it's a matter of Metrigear devoting the resources to calibrating frogs. In due time , I am sure.

The negative torque discussion is quite interesting. I'd never considered that before: only the stoker drive-side spider never sees negative torque. Vector will have no issues handling negative torque. And with the Quarq Qollector, you should be able to collect captain & stoker synchronized data, alleviating the challenge of aligning independently recorded ride files on two separate head units.

BTW you no longer need to rely on my flickr shots: it's now all on the Metrigear web site.
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Old 09-26-09, 03:47 PM   #32
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I sent an email to Quarq, and asked if they were aware of any tandem compatible cranks that would work with a Quarq. I haven't gotten a response yet.

I'm betting you could use any of the Quarq cranks now available turned backwards for the captain's crank, and etiher get the pedal threads redone, or use locktite.

Stoker's crank might be more problematic.
Well, Quarq is coming out with a CinQo for the Lightning...

Interbike 2009 - New Quarq Power Meter Crankset Options


Quote:
INTERBIKE 2009 - Quarq has been busy putting together powermeter rings for lots of crankset brands, with models for the following manufacturers coming out within the next two months:

* Rotor Agilis 3G
* Rotor 3D
* SRAM S975
* Cannondale Hollowgram SI / SL
* Lightning Carbon
* TA Specialties Carmina

All of these are pictured above or after the break, along with a prototype Collector data retrieval “black box” for cyclists that don’t want or use a big cycling computer on their bike (think track cyclists) but still want to collect power data.



...and Lightning is coming out with a tandem crankset.



Last edited by Ritterview; 09-26-09 at 04:29 PM.
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Old 09-28-09, 04:47 AM   #33
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Okay..... let me make sure I understand the problem with each position:
  • Captain crankset: supports captain power only; supports negative torque, so must be a track version of a Quarq or SRM to operate correctly. Negative torque may occur when captain applies negative torque but the stoker applies a positive torque of greater magnitude such that the net drivetrain experiences positive torque and therefore the freehub does not disengage.
  • Stoker crankset, timing chain: same as captain crankset, minus drivetrain losses in timing drivetrain.
  • Stoker crankset, drive side: supports captain + stoker power - timing chain drivetrain losses, so can be used for total power. However, a triple crankset seems to be the preferred solution here, and I don't believe there's any sets which work with triple cranksets.
  • Rear hub (PowerTap): reports total power - total drivetrain losses

The negative torque issue is a big one. For example, the Polar estimates power by estimating chain tension from the primary mode of chain vibrations. But this doesn't work if the monitored segment of chain (the top part) is slack. It also hasn't been calibrated to belt drives.

SRM makes a track version ($$$$$) and Quarq has one in development. So then perhaps you have a PowerTap for total power (- drivetrain losses), SRM track for captain power. But to display the difference would require a very clever head unit, and none are available yet (Qranium might do it; I'm not sure). Post-processing Qollector data would work, but that's not visible on the bike.

Metrigear Vector is the most promising solution, but may end up delayed from its '00Q1 release projection, especially if mountain bike pedals are to be used (Speedplay Frogs are not part of the Q1 projection). Brim Brothers (cleat system) is another possibility, as is the pedal system revealed at Eurobike. But I don't know if it works with mountain bike shoes, or for which pedal systems it is designed. There's another system which is designed to measure crank arm bending, and that would also be similar, but I really really doubt it's available for tandems.
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Old 09-28-09, 07:19 AM   #34
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Very interesting discussion. I don't have a negative torque issue on the Davinci of course. Our stoker crankset is to say the least a bit unusual for crankset solutions might be out. A pedal-based solution could be very interesting if SPDs were supported. iBike might be entertaining, but in the absence of wind I could estimate power output myself and I'm very suspicious of the ability of a $700 device to accurately measure relative wind direction and velocity and calculate its effect. In any event wouldn't take a tandem into account (or does it). A pure headwind isn't too bad, but 30 degrees one way or the other and we feel like two 1/2 bikes.
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Old 09-28-09, 08:20 AM   #35
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You could run a full right-side-drive system with the existing technology, I think.

Or put your Cinqo/SRM on the left side of the stoker crank - no reverse torque. But the crank would be turning backwards, so I don't think a stock Cinqo would work. I don't know if the SRM knows which direction it is turning.
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Old 09-28-09, 09:19 AM   #36
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Quote:
Originally Posted by djconnel View Post
Okay..... let me make sure I understand the problem with each position:
  • Captain crankset: supports captain power only; supports negative torque, so must be a track version of a Quarq or SRM to operate correctly. Negative torque may occur when captain applies negative torque but the stoker applies a positive torque of greater magnitude such that the net drivetrain experiences positive torque and therefore the freehub does not disengage.
  • Stoker crankset, timing chain: same as captain crankset, minus drivetrain losses in timing drivetrain.
  • Stoker crankset, drive side: supports captain + stoker power - timing chain drivetrain losses, so can be used for total power. However, a triple crankset seems to be the preferred solution here, and I don't believe there's any sets which work with triple cranksets.
  • Rear hub (PowerTap): reports total power - total drivetrain losses

The negative torque issue is a big one. For example, the Polar estimates power by estimating chain tension from the primary mode of chain vibrations. But this doesn't work if the monitored segment of chain (the top part) is slack. It also hasn't been calibrated to belt drives.

SRM makes a track version ($$$$$) and Quarq has one in development. So then perhaps you have a PowerTap for total power (- drivetrain losses), SRM track for captain power. But to display the difference would require a very clever head unit, and none are available yet (Qranium might do it; I'm not sure). Post-processing Qollector data would work, but that's not visible on the bike.

Metrigear Vector is the most promising solution, but may end up delayed from its '00Q1 release projection, especially if mountain bike pedals are to be used (Speedplay Frogs are not part of the Q1 projection). Brim Brothers (cleat system) is another possibility, as is the pedal system revealed at Eurobike. But I don't know if it works with mountain bike shoes, or for which pedal systems it is designed. There's another system which is designed to measure crank arm bending, and that would also be similar, but I really really doubt it's available for tandems.
THe Metrigear solution is ANT+ technology so a Garmin 705 will work. The captain and the stoker each have a 705 that is coded to each set of pedals so that each on measures individually. IMO, experienced riders will be able to use that data effectively. Garmin would have to make a firmware solution that would allow totalized power for the the captain and stoker which is possible if they elected to do it. I also think it may be possible to feed information from one Garmin to another but I have not done it.

If I were Metrigear, I would focus on Shimano and Look pedals next. I am not sure why they picked Speedplay first other than Speedplay probably made an investment in the company. I am interested in a Metrigear solution for Shimano so that I can put power on my track bike without spending 3700 Euros.
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Old 09-28-09, 10:26 AM   #37
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Metrigear Vector is the most promising solution, but may end up delayed from its '00Q1 release projection, especially if mountain bike pedals are to be used (Speedplay Frogs are not part of the Q1 projection). But I don't know if it works with mountain bike shoes, or for which pedal systems it is designed.
Speedplay has displayed a new mountain pedal protoype, the SYZR. So if Metrigear was inclined to work on a Speedplay mountain pedal, it would more likely be on the upcoming SYZR rather than the Frog.



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Old 09-28-09, 10:33 AM   #38
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If I were Metrigear, I would focus on Shimano and Look pedals next. I am not sure why they picked Speedplay first other than Speedplay probably made an investment in the company. I am interested in a Metrigear solution for Shimano so that I can put power on my track bike without spending 3700 Euros.
I think they picked Speedplay because it was easier to work with their spindles (axles).

As far as the Vector system on a tandem, I think you could get away with one set, and two headunits (or the cool Qollector for just post ride analysis). Left pedal for stoker, and right pedal for captain. Of course if you have some significant difference in power between L&R then the reading based on one side will not be as accurate.
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Old 09-30-09, 02:28 PM   #39
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we have characterized the power at each position during stationary tandem cycling and we have some data on the open road as well.

briefly, we used an srm at each postion and ran a right side only setup with the inner ring serving as the timing chain. we also had a powertap on the rear wheel.

check out some video at: www.pedalingforparkinsons.org

we have did a fair amount of testing to ensure accurate and consistent readings at both positions. at some point i may write it up and submit to a journal..though not too sure if there would be interest.

jay
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Old 10-01-09, 06:55 AM   #40
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I think they picked Speedplay because it was easier to work with their spindles (axles).
Speedplay seems to have a whole lot longer axles than Look, Shimano...
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Old 10-16-09, 03:24 PM   #41
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Nothing new, just an interview with a Metrigear rep at Interbike.

MetriGear Interview

MTBR Interbike Interviews:
MTBR Virtual Booths

RoadBikeReview at Interbike:
RoadBikeReview Youtube Channel

Last edited by masiman; 10-16-09 at 03:35 PM.
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Old 10-19-09, 06:13 AM   #42
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Clark Foy of Metrigear has been testing the Vector at the Low-Key Hillclimbs. He posted some of his data on their blog. As you can see, it's still prototype at this stage: some algorithm tuning is still occurring.

Algorithms are non-trivial. The naive approach would be to assume power = F · v. But if I push forward on both pedals and pull back on the handlebars while the bike is coasting, F · v > 0 while the bike is not being propelled, and muscles are doing no work. It's all Newton's Third Law. So the pedal needs to isolate the rotational velocity. This is just my wild speculation, but it seems to me from the data since they showed force data and not velocity data, the velocity data is the one for which algorithms are being tuned.

The data are very cool, though.

He was back at the next week's Low-Key Hillclimb, as well, this time with two pedals working together: Clark's on the outside in this photo going into the steep section.
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Old 11-15-09, 06:01 PM   #43
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It looks like the Metrigear will also record cadence as well according to the article.
http://www.bikeradar.com/news/articl...etrigear-23513
I wonder how this will fit with the cadence/speed sensor that already comes with the Garmin 705. You will have two pickups for cadence displayed on the 705 unit, unless you use just the Metrigear cadence and the GarminGPS for speed
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Old 11-16-09, 01:00 AM   #44
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Has anyone run power measurement of both cranks and also the rear wheel to see how it all adds up. I am just wondering what the inefficiencies of a tandem are and what may be done to reduce them.
I noticed it when I did a race with a stoker of similar ability to myself. On solo bikes we can hill climb at very similar speeds to each other. When we raced on the tandem we were noticably slower climbing than when solo even though the power to weight ratio is the same. So I can only assume that power is being lost somewhere.
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Old 11-16-09, 08:21 AM   #45
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Dean,
I assume you and your stoker have similar climbing cadences. Did you experiment with OOP? Are your respective HR in the same zone were you to be climbing solo? It seems possible that either differences in cadence or in where in the pedal stroke power is being applied could cause some inefficiency. How big a difference in climbing speed did you experience?
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Old 11-16-09, 01:06 PM   #46
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It looks like the Metrigear will also record cadence as well according to the article.
http://www.bikeradar.com/news/articl...etrigear-23513
I wonder how this will fit with the cadence/speed sensor that already comes with the Garmin 705. You will have two pickups for cadence displayed on the 705 unit, unless you use just the Metrigear cadence and the GarminGPS for speed
I have the Quarq and Power Tap wireless ANT+ with the Garmin 705. I have the Garmin 705 speed/cadence sensor on the bike as well. The reason for the sensor is that I had it prior to getting power. Both the Quarq and Powertap provide cadence info. The firmware in the Garmin prioritizes Quarq and PT cadence over cadence from the Garmin sensor.

Last edited by Hermes; 11-16-09 at 01:58 PM.
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Old 11-16-09, 01:45 PM   #47
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Dean,
I assume you and your stoker have similar climbing cadences. Did you experiment with OOP? Are your respective HR in the same zone were you to be climbing solo? It seems possible that either differences in cadence or in where in the pedal stroke power is being applied could cause some inefficiency. How big a difference in climbing speed did you experience?
We do have similar climbing cadence, but in saying that I do tend to spin faster climbing on the tandem than solo. Haven't tryed OOP and weren't using HR moniters, all I know is that we were being dropped on the hills by guys we could keep up with normally. I have to say that I used to use HR and power meters years ago when I was racing more competitively. But now that I am riding and racing more for fitness and enjoyment I do not bother with it anymore. I like riding more without these things telling me what to do, and reminding me of how slow I am compared to 20yrs ago.
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Old 11-16-09, 02:40 PM   #48
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But now that I am riding and racing more for fitness and enjoyment I do not bother with it anymore. I like riding more without these things telling me what to do, and reminding me of how slow I am compared to 20yrs ago.
+100

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Old 11-23-09, 09:14 AM   #49
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Some climbing data....
  • Brian and Janet on Mt Diablo in the Low-Key Hillclimbs: 110.3% of the male single median speed.
  • Janet on Alba in the Low-Key Hillclimbs: 81.3% of male single median speed.
  • Brian on Old La Honda at Low-Key Hillclimbs: 131.8% of median single male speed.
Okay: I'll assume the ratio of masses of Brian to Janet is 4:3. Then I should weight their speeds (roughly proportional to power:mass) proportionately. Doing this, using their individual speeds normalized to the median male single speeds, I get 110.2% for the predicted tandem speed. As a tandem they were 110.3%.

Not bad, all things considered.

I was interested to see how out of phase their cranks were: 90°:


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Old 09-28-11, 07:43 PM   #50
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For anyone still tracking this thread, we've successfully implemented two independent power meters on our daVinci tandem. The DV drivetrain "made it easy" - no math needed, just some metalwork on behalf of our dealer to mate the Quarq units to the DV cranks. No need to flip the pedals (we ride Look, so it'd be messy to reverse them), and Quarq was able to program them to read power in the opposite direction. Unfortunately it's not a solution for standard tandem drivetrains.
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