fooferdoggie 

or bosch powered e tandem right now has 46 or 48t chain rings. But the motor lets the captain from back peddle or the stoker to peddle on her own. this works but it has its issues. anymore it is a issue when my wife peddles faster or harder then I do. if we replace the motor with the newest bosch motor it wont freewheel so now we have a normal tandem setup. so Now I have more of a choice of chain ring sizes. right now we are geared so we can climb steep grades and spin out at around 22mph.

bwebel

Chainring size makes very little difference. I guess you'd probably end up with somewhat different tension on the chain, probably less drag on the bigger rings, but less weight on the smaller. We've used from 34t to 48t and I have never noticed meaningful differences.

Leisesturm

The other poster (and I) assumed standard 'timing chainring' which your thread title asks about and, as they noted, has no effect on what you feel at the pedals, but, as I understand it, 39T to 42T at most, is about right for those, smaller will wear too fast, and bigger is heavier than you need. I am not sure you actually had timing chainrings with the old Bosch drive and I am not sure you will with the new one. Which system is it? To properly size a drive side chainring we would need to know the range of the rear cluster. Also the system probably has a recommended range? If 46 or 48 is the range, those are not timing rings and there isn't tons of difference between them. You'll go a little faster with the 48 and you will rely on the motor more at the climbing end of things. If you really are getting a standard tandem drivetrain with timing rings you will need to keep them properly tensioned. Don't be like me and let it stretch too much and then lose the timing chain on a steep hill.

fooferdoggie 

true its not a true timing chain. the bike has a performance motor with the 2.5 to 1 gear so it has I think a 18t chain ring the back I think is 48 can't remember for sure since it was rebuilt. if we get the new motor it has a standard size chain ring and it wont freewheel so the cranks are locked together like they should be. the bike does have a normal tandem bottom bracket that I had no clue about the first time I had to deal with the timing chain. the bosch cx motor is the cargo/mountain bike motor so it will come with several different sizes of chain ring. we cost when the bike goes faster then 23mph. but we climb a lot of hills. Saturday we did 2500 feet in 15 or so miles with most of it at 10% or more grade. so climbing is the most important we cruise at 18.5 right at the motor cutoff. I think right now its 46 and 11-34

longpete

talking about 11 speed because we ride DI2. 46-34 even onroad with motor not for us. 34-42 on long ofroad climbs with gear foor several days still is too heavy for us.
34-46 was not optimal on 2000 altimeter alpine offroad climbs. This year I' ll see if 51 works with rear derailleur. If not I'll mount a 30 or 32 chainring. Timing chain 42 holds a bit longer than 38 before worn, just like the chain. I was always told a tandem timing chainring should have even numbers 36, 38, 40, 42. Myth or true?

Bezalel

If you use an odd number of teeth without a half link your cranks will be clocked a half tooth apart,

fooferdoggie 

good to know. it will beawhol to change motors the cost is at least 1500.00 the motor is 900 or so. then would need nee crank arms for it and the chainring. but it costs less then a new bike for sure.