Tandem Timing

[yoothgeye]

I've had a tandem frame for a few years now, every now and then I think about putting it together. I've already got matching bars, grips, and saddles, but what I come back to every now and then is gearing.

My wheelset is from an old tandem, coaster brake, heavy gauge spokes. I have to make a tensioner for the chain in between the 2 cranks, that's no problem.

I have tons of chainwheels to choose from, but this is what I am thinking about. If my front ring and back ring are different tooth counts, will the pedals stay in time (sync) or go out of sync? Once tensioned the chain will always be engaging the same amount of teeth, but if I went "crazy" and ran a really small ring up front with a larger one in back (on the double) would the pedals always turn together?

I'm sure someone knows the answer right off.

 

[jerrykr]

Not knowing the answer, I looked it up. This help?

http://sheldonbrown.com/synchain.html

 

[deorman]

The sprockets on the cranks must be the same.

 

[bikejunky]

You can place a larger chaimring in the middle between the 2 cranksets. It will act as a tensioner, and wont rome, because for every chain pin going back on the bottom, one will be going forward on the top. I have done this on a few builds where I wanted the axle all the way forward in the dropouts to get the proper reveal around the fender.

 

[yoothgeye]

I use a "spooky tooth" tensioner on my tall bike, so I am familiar, but I will be making a standard tandem tensioner since my frame has the tab for it.

 

[The Renaissance Man]

If you want the cranks to turn together, the rings have to be the same size.
You could use a crank with double rings on the middle and use two chains. Then, if the front ring is the same size as the middle ring that it connects to, you can have a larger ring connecting the rear crank to the wheel and they will still rotate at the same rate.

 

[junknutz1975]

2 of those huge exercise bike chainrings would be cool. Bet you could get it up to highway speeds

 

[patman73]

just look at other
tamdems & see what works on stock bike

 

[deorman]

The size of the synch sprockets have no effect on gearing. That is controlled by the drive and hub sprockets.

 

[deorman]

If you want the cranks to turn together, the rings have to be the same size.
You could use a crank with double rings on the middle and use two chains. Then, if the front ring is the same size as the middle ring that it connects to, you can have a larger ring connecting the rear crank to the wheel and they will still rotate at the same rate, but the smaller one will be easier to pedal.

Care to diagram that? Makes no sense to me.

 

[The Renaissance Man]

The size of the synch sprockets have no effect on gearing. That is controlled by the drive and hub sprockets.

You are right! Not sure what I was thinking. I edited that out! The pedals will however turn at the same rate.

 

[yoothgeye]

OK, all of you plus Sheldon Brown have helped me see that my timing chain rings need to be the same tooth count.

I painted my frame/fork with rubberized undercoating and then painted that red oxide primer. I plan to just use generic mountain bike 1 piece cranks (2 American bottom brackets) with double rings, and based on what gearing I want and chain alignment I will decided whether to make the outer or inner rings the timing rings.

 

[The Renaissance Man]

How well did that rubberized undercoating work? I've been planning on using either that or truck bed liner on my latest build.

 

[yoothgeye]

Use bed liner, it was just an experiment and it seems like it comes off pretty easy. I have used bed liner before in vehicles and on other things and it's super tough.

 

[5speed]

2 of those huge exercise bike chainrings would be cool. Bet you could get it up to highway speeds

 

[GuitarlCarl]

Youthgeye, I have two tandems... they both have the type of alignment where the second set of cranks (for the Stoker) has the drive gear on the outside, and there is enough distance between the front crank (for the Captain) and the second, to have a tensioner between them. The two cranks need to be the same to keep the pedals synced, but have no effect on the drive gearing, that is determined by the outside gear on the rear... I have put a exerciser bike gear there it worked well with an old 5 speed rear end set up single speed...

Hey I'm a Youth Director too!

Carl.

 

[GuitarlCarl]

Use bed liner, it was just an experiment and it seems like it comes off pretty easy. I have used bed liner before in vehicles and on other things and it's super tough.

I rolled bed liner on the roof of an old van I had...looked like I had a vinyl top!
I've been wondering about that rubber spray from the commercials with the
screen door in a rowboat... I wonder how evenly I could apply it?

Carl.

 

[rubsterob]

Why do the pedals have to stay synced? Does it throw off the balance of the bike or something? So here is my explanation of how to keep them in sync. Using this picture the small gear (closest to camera) has to be the same size as the front passenger to keep a 1 to 1 ratio and keep the pedals turning at the same speed and the person in front will be using the same cranking force as the person in back no matter the size big or small (this is if the ratio is 1:1). If you don't care if the pedals stay the same speed (never rode a tandem so not positive if it matters) you can go bigger in the front to make the front person work harder but higher top speed. I hope I confused enough people. Not positive if the pedals have to stay insync to keep the balance easy all I know is the chain will not bind if set up like in the picture with 2 separate chains. If you use one continuous chain then the cranks must be the exact same or the cabin will bind.


Sent from my iPhone using Tapatalk

 

[junknutz1975]

Here is a confusing thought........
( think internal combustion engine-wise here now )
what if the front persons pedals are linked up horizontally and the rear persons pedals are set up vertically ?..... Wouldn't that give you the mechanical advantage of it constantly " pushing" ( the combustion cycle ) ?.... Example.... imagine each time 1 pedal is in the UP position ( or top dead center ) ....and you force it down . It would alternate like a 4 cylinder. Yes, it might be difficult to get it started ( because it all lies on 1 persons leg, but it would soon be followed in a 1/4 turn by your passenger..... and so forth...... ( while you are in your mid power band ) yeah, I guess that's what well call it

Make sence?

 

[deorman]

You can do that if the pilot remembers which stoker pedal is up when turning so it doesn't hit the ground. In my limited tandem experience, it's tricky enough to maintain balance and motion with synched pedals. I refer you to the previously linked Sheldon Brown article. http://sheldonbrown.com/synchain.html