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My wife and I have recently purchased a vintage tandem, and are willing to switch to Shimano SPD pedals (M520).

The problem is that there are 3 "chainring" sides on a tandem, 2 of which are located on the left side of the bike. So basically I don't see any way to fit the pedals in the correct position, except for the stoker right side, because the threads will be the wrong way round.

How do I fit normal pedals onto tandem cranks?

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    Your question is unclear. The left pedals go on the left side and the right pedals go on the right. – Daniel R Hicks May 16 at 23:30
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    On a regular bike, there is a dedicated left pedal and a dedicated right pedal. They have opposite threading. The crankshaft arm (on the right) can only be mounted with the right pedal. Problem is that on the tandem, there are 2 crankshafts located on the left side. If I mount my "right" pedal on the left side, the front and rear of the pedal will be reversed. Note that I am talking of SPD which have a front and a rear, and cannot be used if reversed – magicloic May 16 at 23:32
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    I will note that, if the bike has 3-piece cranks, there's a vague possibility that, in it's prior life, the original crank arms failed and the person making repairs reached into their spare parts pile and substituted "normal" crank arms, putting right arms on the left side, etc. Then they would have installed pedals on the "wrong" sides to match. If this is the case your best option is to replace the crank arms with "proper" ones, though finding the correct arms could be difficult. – Daniel R Hicks May 17 at 13:17
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    @DanielRHicks ...and the easy way to check would be to pay careful attention to whether the current pedals remove in the expected way (be that clockwise or anticlockwise) – Swifty May 17 at 15:22
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    If by chance or rather poor repair one left arm has been replaced by a right arm it is best in the interest of safety to put a proper left crank-arn back instead as the pedal may come off in a worst case scenario. – Carel May 17 at 18:19
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Tandem cranks are special. The pedal thread directions on each arm are oriented correctly to prevent loosening from precession, as on any other bike. You just need two sets of normal pedals.

If somehow non-tandem cranks have been installed such that either of the left cranks are right-threaded or the right captain crank is left-threaded, that situation is not acceptable and threadlockers will not make it so given the danger level of the crash that could result. Either get tandem cranks or get the offending crank(s) helicoiled to have proper thread orientation to resist loosening.

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    Tandem cranks are also fun for making left-side drive bikes. – whatsisname May 17 at 2:41
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    It will be easy to check whether the pedals are correctly threaded by taking them off. The right one turns anti-clockwise while the left one turns clockwise. – Carel May 17 at 18:25
  • Are tandem front-cranks and track bike left-side cranks essentially the same ? – Criggie May 17 at 22:03
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    @Criggie: Regarding threading, yes, probably the way you are thinking. There are a handful of drivechain configurations for tandems, so there isn't a single crank style necessarily, but I imagine you are thinking of a left-side spider and right side "bare" crank arm. That's what I was referring to in my comment. – whatsisname May 18 at 4:25
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    @Criggie : Left-side driven track bikes are purpose-built and use cranks that have been specially made with correctly threaded pedal threading, lefty-lefty, righty-righty. Anything else is certainly illegal by UCI-rules out of safety concerns. – Carel May 19 at 19:23

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