Weird whirr and clunk in tandem drivetrain.
 

Ok this is a problem we have had on out e tandem it has a Bosch motor in front. This lets my wife peddle with me not peddling our it les me back pedal without her peddling.
Other than that the drive train is pretty normal only two chainrings both narrow/wide 10 speed. It has a 11-42 Shimano 10 speed cassette and a DT Swiss 540 Tandem/ ebike rear hub. This noise started sometime this winter. I thought it was my wife over or under peddling because I have been tired and not starting off as hard as I used to. But when I rode the bike by myself it still happened. So you get the wiring noise you hear in the video and sometimes the clunk at the end. It's happens when starting off or peddling really slow or when I let up or let up to shift. I can somewhat eliminate it by starting in a small cog so it takes a lot more force to start. But if we peddle slow enough then it will do it no matter what and the clunk is harder.
The weird thing is the drivetrain is smooth when this happens and you cant feel anything when it makes the wiring sound. But you feel the clunk but I think you feel it in the drivetrain but just in the bike. But it's hard to know for sure. We had it in the shop and the mechanic thought it was the front chainring was worn. It's only 18t because it's 2.5 to one gearing. I have no clue where the sound and the clunk are coming from. There is no slipping like we have with a worn cog or when I used the wrong grease on the drive. The drive feels normal and wit the vide I can see it is acting normal in back. My wife is blind so she could not see if it was working right.
Sorry about the camera angle I shut taped it on the frame.
I replaced the front chainring and the timing chain as it was time the back I just changed a few months ago and the cast only has maybe 200 miles on it.

Before trying to sort this out, I need to understand the configuration.

Typically tandems are linked with a chain loop and must turn together. Then, usually, the rear crank drives the bike.

Your description seems to imply that this is not the case here, and some kind of crank allows one ,(front?) crank to idle while the other drives.

This seems odd because it allows the riders to be out of phase with each other, which can be problematic.

So, please better describe the system, and which crank directly powers the bike.

you have them motor in front. then in back in standard 2 bike two chain ring setup. sorry I meant to post pics of the bike.
https://cimg0.ibsrv.net/gimg/bikefor...d90d9b311.jpeg
https://cimg2.ibsrv.net/gimg/bikefor...f8d120143.jpeg
https://cimg3.ibsrv.net/gimg/bikefor...d25370d5c.jpeg
https://cimg4.ibsrv.net/gimg/bikefor...1f2ad67a8.jpeg

Got it. More questions.

It seems that the rear crank is a classic arrangement driving the rear wheel similar to a solo bike. However is there a freewheel, allowing it to turn without the "motor" chain moving?

Is the motor directly attached to the front drive sprocket, or is there some sort of clutch allowing front pedaling without turning the motor. Likewise how is the front crank linked to the front sprocket? Freewheel ratchet or something else.

Knowing the answers might explain some of the noises, which seem like clutch/freewheel issues.

you got it right. you can pedal backwards in front and only the cranks move. or my wife can peddle while I hold my feet still. other then that everything is normal. At first I thought my wife was peeling faster than me and causing the motor bearing to spin. but thats not happening. peddling is smoother. but when I rode the bike by myself It does the same thing. the bike has 12k miles on ut but the drivetrain was rebuilt after a crash so most of it has about 7000 miles on it. new bottom bracket and new cranks from and back rebuilt pedals and new detailer. I just rebuilt the back and front pedals too but that did not help. I was going to put the wheel the bike came with on and check but its been weeks since we rode because were were sick so I forgot about it.

"Wife and you doesn't help. Let's talk front and rear.

Can the rear coast under front rider or motor power?

Can the front rider coast when the motor is working?

Since the issue seems clutch related, knowing the EXACT clutch arrangement is critical to a meaningful response.

For example, the whirring sounds like a slipping friction clutch or internal belt drive, but wouldn't apply if there aren't clutches or belts.

no if the captain pedals the stoker does too. but the stocker can peddle while the captain does not. The motor has a torquer sensor so it only works if the captain is peddling. when only the stoker is peddling its like a regular bike no assist at all the front chainring freewheels.

no if the captain pedals the stoker does too. but the stocker can peddle while the captain does not. The motor has a torquer sensor so it only works if the captain is peddling. when only the stoker is peddling its like a regular bike no assist at all the front chainring freewheels.

I was recently bequeathed this and when you rotate the crank backwards, the chain ring doesn't rotate.
https://www.giant-bicycles.com/us/explore-eplus-3--2019

Maybe this video explains some about the OP's bike?

Question for the OP.
Does the front ring have any "cadence multiplication" over the captains cadence?
The reason I ask, is my ebike has a 42T chain ring whereas yours looks to be about 1/2 that size.

[QUOTE=Bill Kapaun;23180820]I was recently bequeathed this and when you rotate the crank backwards, the chain ring doesn't rotate.
https://www.giant-bicycles.com/us/explore-eplus-3--2019

yep. Park made their chan cleaning tool to work in the opposite direction for mid drive e bikes. on the tandem I just use the back crank to clean the chain. I think the wirr sound comes from the back but not sure the clunk sure does.

[QUOTE=fooferdoggie;23180827]Too bad that wasn't the question I asked.

I know I won't solve this, but perhaps a useful observation:

The chainring on the front crank is tiny compared to the one the same chain engages on the back crank; That means, for both riders to have the same pedal cadence (not necessarily same phase, just same RPM), the front crank is not connected directly to its chainring, the front crank revs go into the gearbox and some sort of overdrive, to output a much faster cadence/RPM at the front chainring, so that front and back are pedaling at the same cadence.

I asked that in post #9.

Ah, I see that now. I had read it, but it didn't totally click in, a good while later I noticed the chainring difference in the photos.

This is really hard to diagnose without a schematic of the drivetrain.

it has a 2.5 gear so for every crack turn you get 2.5 turns on the chain ring so the 18t chain ring is about 46t. so the system is normal in that way bosch does not use that method anymore but it does not feel any different then a regular chainring. its noise though. I have another bike with that and I cant tell the difference between that and the new regular chainring on my other bosch powered bike.

Wow, 2.5, that's a really big overdrive in one step, at least for a planetary gear, which it's probably not, I can picture in my mind how they would do that. I could see how that would add a lot of whirr noise, and hurt efficiency, I'm sure the driver for the newer simpler system.

no it was pretty efficient. bosch is known for them sot range for the least amount of battery. but that small chainring was noise and wore out faster. Plus the bearing on that side had to be more complex. but thats to the wire sound the tandem as 12,0000 miles on it and the sound is new.

With at least two concentric shafts, both laterally loaded and limited space, yeah I'll bet that the bearings were more complex. Bosch knows their stuff, and it's been my experience over the last 40 years, that Germans are particulary fussy about gear noise, their manual transmissions in particular were less noisy. But other countries have caught up since then.

but they do have a issue with rusting out if you ride in lots of mud. glad they changed over to a normal chainring though. well the sound is coming from the back so I guess I will take the chains off and see if its the bottom bracket. and try our spare wheel.

Notable; SKF bottom bracket cartridges (expensive! 10 year warranty!) offer a stainless steel spindle, makes sense to me, rust would eat up the bearing seals, but they state that it's "more brittle than cromoly", meets city and trekking bikes standards, but not racing bike, so for the hardest hammering riders, they recommended someone else's bottom bracket. A bit surprising to me, as stainless steels have advanced a ton, offering both high strength and hardness, and toughness, but this has mostly been for powder-metallurgy steels, a very expensive process (like an order of magnitude greater cost).

https://www.renehersecycles.com/shop...-square-taper/

two different people told me its either the one way gear on the right side of the motor or I needs new bearings. replacing the one side my mechanic should be able to do it. but I am worried that the other sides bearings need replaced or all of them thats a whole different level and needs a shop that setup for it. but with 14k miles on it I am worried if it did not get a full bearing replacement it would not be a long term repair. I have my spare bosch powered bike that I dont really use and I will see if he can just swap motors. that would save me 900.00 for a new motor.
https://cimg1.ibsrv.net/gimg/bikefor...44ac526e3.jpeg