Join Date: Jul 2007
Location: Cape Cod
Bikes: Dahon Helios; Jack Taylor tandem; 1970 Raleigh (Carlton) Professional/ Vision R64 Recumbent
Originally Posted by pismocycleguy
Looking at the photos of the Downtube FS's chainline, I see a very long and unsupported drive chain. I have not seen, or had, such a long chainline since I rode a Vision Recumbent. But my recumbent had a chain guide/support half-way between the front and rear sprockets. Dose this long chainline present a problem?
I have for years used a child-back adapted tandem to transport my children, now my grandchildren. When I built this up (with an accessory rear bottom bracket way up on the rear seat tube) I decided to put the drive chain on the front bottom bracket (rather than use a crossover arrangement on the rear bottom bracket, which entails another set of bearings and chainrings. So I have about 20 years of intermittent use on this device (sometimes daily commuting use with a child drop-off at daycare or camp). It has a chainline longer than my Vision recumbent -- 3 entire chains -- and has performed well throughout its lifetime despite that long unsupported chain.
I think the ancient SunTour VGT derailleur on the rear was important to its ability to shift and maintain chain tension over a very wide range of gears (triple front, 5 speed rear). Too bad you can't get this great piece of equipment today!
If you want to, you can have an idler fitted to your bike to support the chain somewhat. This will save wear mainly on the derailleur spring, while putting more bearings into the drive train. Chain wear is not much of an issue with these long runs -- it is inversely proportional to the chain length since it mainly comes from flexing of bushings in the chain. But front shifts can be sluggish thanks to the very small angular displacement by the derailleur (front).