Tandem crank offset front/rear: eccentric bb

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brynpoeth
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Tandem crank offset front/rear: eccentric bb

Postby brynpoeth » 13 Jun 2019, 5:07am

When I see a tandem I check the crank offset
A slight offset up to 30° seems good, spreads the maximum load
Saw a tandem with the cranks offset about 150°, could that be good or bad?
..
Eccentric bottom brackets! Adjusting changes the saddle-pedal distance, should one adjust the saddle too when the bb is adjusted? One reads about small changes in saddle position making a big difference
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roubaixtuesday
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Re: Tandem crank offset front/rear: eccentric bb

Postby roubaixtuesday » 13 Jun 2019, 7:01am

Conventional wisdom is that cranks in line is best, which we've always used on ours.

I've been told that some people run 90 degrees out, which would best spread maximum load I guess.

I think arrangements with "freewheel" style bottom brackets are also possible eg when one rider is disabled and can't necessarily always pedal when the other rider is. That would allow for random offset, presumably.

rjb
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Re: Tandem crank offset front/rear: eccentric bb

Postby rjb » 13 Jun 2019, 7:45am

We have tried offsetting cranks over the many years we have been riding but much prefer to have them in line or as close as possible within the limits of chainring tooth manufacture. It's not always possible to buy matched chainrings with the identical tooth profile matching the bolt holes. If you try offsetting be aware that the stokers crank needs to lag the captain's or you risk grounding the stokers pedal when cornering enthusiastically. Freewheeling arrangements on the rear require a looooong tandem to avoid feet clashing, so will probably require a custom build not an off the shelf model. :wink:
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Brucey
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Re: Tandem crank offset front/rear: eccentric bb

Postby Brucey » 13 Jun 2019, 9:16am

Perhaps I've not been paying proper attention but I've never seen anyone successfully race a tandem with the cranks deliberately out of phase, and I've never felt in the slightest bit tempted to do it either; the slightest mistiming is easily felt and usually disastrous (*). The best setup I've used made a tiny allowance for the bottom run of the timing chain sagging when under normal load, i.e. at rest the stoker's cranks were fractionally advanced, but when going for it, they weren't.

If you are meticulous about setup, then of course you should move your saddle etc whenever the BB is moved.

(*) I think the reason for this is that the machine accelerates through each pedal stroke, and the way this happens is most 'normal' when the cranks are in phase. By 'normal' I suppose I mean most like a solo; it is possible that if you did nothing but tandem riding, you might come to consider something else as 'normal'. But most riders have many tens of thousands of miles of 'solo education' in their legs, such that they have trained themselves to pedal in a particular way, even when half out of their minds with suffering. That education doesn't come cheap, and shouldn't be dismissed without very good reason; any bad habits you might develop through doing things differently won't be easily broken.

cheers
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anniesboy
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Re: Tandem crank offset front/rear: eccentric bb

Postby anniesboy » 13 Jun 2019, 9:35am

I have my tandem set up with me leading by 6 or 7 teeth ,the reason being that my wife not having as much power as me used to complain that she could not push as much as she felt she could.
The comment we get on occasions is for people to ask how we are pedalling at different cadence,clearly we are not.

reohn2
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Re: Tandem crank offset front/rear: eccentric bb

Postby reohn2 » 13 Jun 2019, 2:57pm

We've played with crank phase and settled on Mrs R2 being 2 or 3 teeth behind me,as pedalling is smoother under power.
Alas MRS R2's health is now deteriorating and we haven't ridden the tandem for six months,I'm questioning whether we will again TBH
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NickJP
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Joined: 24 Sep 2018, 7:11pm

Re: Tandem crank offset front/rear: eccentric bb

Postby NickJP » 13 Jun 2019, 10:02pm

We use both systems. On our touring tandem, we have been running the cranks out of phase for over 30 years, with the captain's cranks 90 degrees in advance of the stoker's cranks. That works better for us when climbing while fully loaded. On the racing tandem we have the captain's cranks about two teeth in advance of the stokers cranks, as it's impossible to sprint out of the saddle unless the cranks are pretty much in phase. Getting used to one setup or the other when swapping between them is a matter of a few minutes riding.

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Mike_Ayling
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Location: Melbourne Australia

Re: Tandem crank offset front/rear: eccentric bb

Postby Mike_Ayling » 14 Jun 2019, 12:31am

NickJP wrote:We use both systems. On our touring tandem, we have been running the cranks out of phase for over 30 years, with the captain's cranks 90 degrees in advance of the stoker's cranks. That works better for us when climbing while fully loaded. On the racing tandem we have the captain's cranks about two teeth in advance of the stokers cranks, as it's impossible to sprint out of the saddle unless the cranks are pretty much in phase. Getting used to one setup or the other when swapping between them is a matter of a few minutes riding.

Image

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Is that the Daintree River in FNQ?

Mike

Brucey
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Re: Tandem crank offset front/rear: eccentric bb

Postby Brucey » 14 Jun 2019, 3:13am

If there is a large disparity in rider strengths (or leg lengths...?) I guess it makes some sense to have the stronger rider 'leading' slightly. Mind you, comfort does not necessarily equal speed and/or efficiency. I wonder if anyone has done any tests to establish any measurable benefit ?

R2, sorry to hear about Mrs R2. The happiest miles can linger in the mind, more than the passage of mere time might suggest.

cheers
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NickJP
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Re: Tandem crank offset front/rear: eccentric bb

Postby NickJP » 14 Jun 2019, 3:42am

Mike_Ayling wrote:Is that the Daintree River in FNQ?

Yes, that's the Daintree. We took the ferry across and went up to Cape Tribulation. 1988, so a bit over 30 years ago. Here's another photo from a bit further north:

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