Tandem Cycling - Tamdem Timing Chain Line

In order to correct a problem with the drive side chain line on our tandem (i.e., rear crank to cassette), I swapped out the BB on the rear only. I went from a 122 to a 115. Tandem seems to be working fine in the work stand, but I am wondering if this will create a long term issue with the timing chain and if I should simply swap out the front BB to a 115 also?

Not to hijack the thread, but I have a similar question. We are building a new tandem and the rear BB is 68-118. Is there a problem with the chainline if I were to run a 68-108 for the captain, or should the BB's be the same size front and back.

I always run the same size bottom brackets. This will reduce friction, promote even wear, lessen the chance of throwing a chain and the eccentric stays in the center of the bottom bracket, 7mm is alot.

I always run the same size bottom brackets. This will reduce friction, promote even wear, lessen the chance of throwing a chain and the eccentric stays in the center of the bottom bracket, 7mm is alot.

But it is only 3.5mm on each side. I doubt it will matter.

To correct a Q factor sensitivity problem, I run two different axel lengths on the bb's (5mm) with no detectable loss of performance or wear over the last 4500 miles..my guess would be that at some point, a big enough variance could be a liability but I have no idea what that number might be.

Bill J.

IMHO, it's a non-issue. Check out the chain line on your drive chain and consider how short that chain is compared to your sync chain... which is about twice as long. Just be mindful to keep your sync chain properly tensioned and you should be fine if you stick to chains. The new belt drives are different and less tolerant of an imperfect chain line, hence new off-set adjustable tandem cranks are now available from FSA.

I always run the same size bottom brackets. This will reduce friction, promote even wear, lessen the chance of throwing a chain and the eccentric stays in the center of the bottom bracket, 7mm is alot.


Of course this assumes that the frame is in perfect allignment to start. My bet is that most tandems are off a couple of mm in the timing chain line, and its virutally undetectible, or at least goes undetected.

With a timing belt, the driveline is critical. In installing the belt, I found that the chain line before I installed the belt was off about 3mm. Thus even though I shimmed the stoker's ring out 2 mm for clearence, I had to slide the captain's eccentric about 1 mm right to get the drive line exact. (instead of 2mm left that I would have expected if the bike was perfectly alligned.)

So my point is that 3.5 mm is likely not even noticeable with a chain, and could be compensated at least in part with a slight adjustment of the eccentric.

I had the same issue when I was changing the chainline for the rear. After installing shims to move the BB cup over I did the same for the front. Try using shims for the right side cup for the front [3.5mm] and then it should line up. They were cheap at the LBS. Although it may not be an issue, it would bug me forever [or until I would change it] knowing that it was not aligned.

Can you move the front timing chainring to the inside of the crank?

I did this with my old Schwinn. I have a 115mm BB in the front and a 118.5mm in the rear. By moving the stoker's timing chainring to the inside of the crank arm I was able to achieve a nearly perfect timing chain line. There is less then 1mm difference.

Thanks for the info. After reading replies, and thinking about this more, consider the constantly changing chainline with the rear, it is only aligned (maybe) in one gear per "front" crank position! And as TandemGeek points out, the timing chain is significantly longer than the drive chain.

FWIW, I checked last night and our Calfee and Erickson's timing rings are 5mm and 8mm out of alignment, noting that I have 108mm spindles on the front and 111mm spindles on the rear. I'd assumed they were a bit off and, again, it's just not a big deal given the size of timing rings, length of the chain and the way power is transferred from the captain to stoker's spindle. As noted before, if you mind your timing chain's slack, there shouldn't be any issues so long as the off-set / mis-alignment isn't something you measure in centimeters vs. millimeters.

FWIW, I checked last night and our Calfee and Erickson's timing rings are 5mm and 8mm out of alignment, noting that I have 108mm spindles on the front and 111mm spindles on the rear.

Out of curiosity, how do you check? I assume the bottom brackets are welded to within some tolerance and hence the axles are out of parallel by some (likely small) fraction of a degree. Or maybe it's more. I've no idea what a reasonable manufacturing tolerance is for these things. Point being I wouldn't trust measuring the distance from bottom bracket to chain ring and comparing. Align a laser with one ring and see how far it misses the other by?

Out of curiosity, how do you check?

The same way you measure for drive side chain line, just going the opposite way... you measure from the centerline of the frame to the center of the timing ring at the front and rear bottom bracket shells or using the centerline of the seat tubes.

Also, not all bottom brackets are created equal. Our Phil Wood square taper BB's use a pair of threaded adjusting / mounting rings instead of having a lip on the drive side threaded end of the cartridge with an adjusting ring on the non-drive side. This allows you to off-set your bottom brackets in the bottom bracket / eccentric shell by as much as +/- 5mm left or right of center. You can do this to a certain extent with some other square taper, ISIS and Octalink BBs that don't use with two adjusting / mounting rings and to right of center using spacers on BBs that use the fixed end with the lip. The latter is mostly done to keep chain rings from hitting chain stays or to fine tune drive lines. Up and until recently, I think most of the newer outboard bearing BB designs from FSA, TruVativ, Shimano and Campagnolo did not allow for any off-set. However, as merlin noted, you can always cheat on tandems by offsetting the eccentric a few mm left or right of center. This past year, FSA introduced Mega-Exo cranks that do allow for off-set / chainline adjustment. Not sure about the others.

Again, sync chain drives are pretty crude to begin with and tolerate all kinds of things that a drive chain might not, e.g., excessive wear isn't an issue so long as you keep the worn-out chain on the worn-out timing rings, you have no side loading from derailleurs, you can run just about any width chain that will fit on your timing ring tooth profile and so long as you keep the timing chain relatively tight it won't self-derail at the rear chain ring even when alignment is way off. So, short of perhaps having a little extra chain noise when a set of chain rings are new, I wouldn't be too concerned about a little mis-alignment. After a few hundred miles the chain and chain rings will comform to each other and even that extra noise will go away, again... assuming the off-set between the front & back rings is in the mm range.