tomtomtom123

Can a 16" wheel clear a Shimano GS rear derailleur, and not have the cage touch the ground? Perhaps extending only to the edge of the rim and not past the tire?

Iride01 

Which flavor DR and model? My 105 5800 GS when in the 11 cog is more or less 8 inches from the axle.

Andrew R Stewart 

Back in the 1990s when Recumbent Cyclist News was running this thread was a frequent topic. IIRC the long cage, MtB, ders got pretty close to the ground with a 16" tire. The trike guys had it easier with no leaning in righthand turns Can you mount and measure? Then there's the question as to what's acceptable ground clearance... (the obvious answer is "the amount that defies the laws of physics") Andy

tomtomtom123

Hm... I looked at my 4700 GS while on a 34T cog, with the cage extended so that it's radially in line with the hub axle which should be the furthest it would ever extend. The tip of the cage was almost to the edge of my 406 rim, so if I assume it's somewhere around the bead seat diameter, then the cage fully extended at 34T cog is around 203mm from the axle. A 16" 305mm rim with 47mm tire would be around 200mm radius, so there is a chance that the GS cage would touch the ground if using a 34T cog. With a 349mm rim, there would be maybe 20mm of ground clearance. But a flat tire might mean the cage hitting the ground.

Well, maybe I'll think about an SS short cage and exceed the chain wrap capacity.

Iride01 

On a regular bike in the large rear cogs, the DR will be pulled up and away from the ground because more chain is wrapped around the gears. So in that case who cares if it extends to it's fullest potential as long as it doesn't interfere with anything else.

It's the small cogs that the DR will be closest to the ground because unused chain has to have it's slack taken up.

How does all this relate if you are talking about a recumbent.... I have no idea.... what do other recumbent riders have and do?

alcjphil

With a 16" rear wheel, why would a 34 tooth large cog ever be necessary?

tomtomtom123

Yes, if you have a single chainring, the cage will be pointing forwards when on the largest cog so the longest extension of the cage will still be above the ground.

However, if you have a double crank, depending on the jump between the two chainrings, when on the largest cog and the small inner chainring, the cage might be pointing straight down towards the ground.

Also, on my 20" wheel, the innermost cog actually sits within the profile of the 2.15" tire, so on a 16" wheel with the same tire width, a long cage might actually rub on the tire.

Iride01 

Perhaps if you don't have too much difference in chain wheel size, you can take more links out of the chain to give more clearance. A 3x front my give you more bang for whatever you are trying to accomplish.