Derailleur gear Thorn Adventure and Discovery - through drive

[MartynA]

Anyone here got experience of the same side sync chain Thorn tandems 2000-2005 vintage?

We had poor results with the original steel 27t sync rings on out year 2000 Adventure but the 30t Al alloy Thorn/Talon rings on our 2001 S & S coupled bike have been fine. I bought the last of the alloy rings from SJS in 2019 and have also now got some 30t TA Chinook rings but these also appear to be getting very scarce.

[The French Tandem]

What kind of problems did you have with the 27t steel rings?

We have no specific experience with Thorn tandems, but we use a homemade right hand side sync chain on our tandem, with a 1x10 final transmission, and are very satisfied with it. The only issue we had was the rear chain falling off quite often. Problem was solved by fitting back a dummy front derailleur as a chain guide.

A

[LittleWheelsandBig]

Tiny synchronising chainrings kill the tandem’s rear BB in crossover cranks and the front BB in same-side cranks. Bigger rings reduce chain tension and chainring wear and are simply better all round.

[Brucey]

+1 on that.    Using 46T chainrings instead of 30T ones on one tandem meant that the timing chain stayed on instead of falling off on a regular basis, (even though it wasn't slack when checked).   I'd rather eat worms than use tiny timing rings again.

cheers

[MartynA]

What kind of problems did you have with the 27t steel rings?

We have no specific experience with Thorn tandems, but we use a homemade right hand side sync chain on our tandem, with a 1x10 final transmission, and are very satisfied with it. The only issue we had was the rear chain falling off quite often. Problem was solved by fitting back a dummy front derailleur as a chain guide.

A

The gap between the 27t steel ring and the granny ring was just wide enough for the drive chain to get into and stick also the thin steel rings wear fast and when the sync chain got even a little slack it tended to drop onto the granny ring. Quite fun if you don't realise it has happened as drive continues, provided you don't try to use the granny ring, but the crank sychronisation walks.

The Thorn/Talon 7000 series Al alloy rings overcome these problems.

I'm not convinced that we are likely to damage bottom bracket bearings due to excessive loading, more due to water and muck ingress but certainly tooth and chain wear is accelerated by using small rings. I've got hold of a reasonable stock of TA Chinook 30t 64mm BCD rings over the last few days, to save the Thorn/Talons for the back. I had the last one 30t TA from SJS and Chain Reaction/Wiggle plus a couple from Rose. It might be possible to run a 30t 104mm broad narrow BCD at the front with a bit of adaptation.

Thanks for the comments,
Martyn