Why do derailleurs hang downwards?

[snailracer]

How about if the derailleur was also 'flipped' through a 180 degrees?

You'd then need to pedal backwards to go forwards, cos the back wheel would effectively also be flipped.

 

[deptfordmarmoset]

That test would be thwarted by the freewheel.

the backpedalling bit would create enough nasty graunching to demonstrate the effect though, wouldn't it>

 

[snailracer]

the backpedalling bit would create enough nasty graunching to demonstrate the effect though, wouldn't it>

Err no you'd just spin the freewheel, right?

 

[deptfordmarmoset]

Err no you'd just spin the freewheel, right?

I just got onto the turbo and tried. It sounded nasty enough for me to stop doing it sharpish

 

[Linford]

The meshing needs to be done before the chain goes around the cog, not after. All ofthe force goes across the top of the chainset.

 

[snailracer]

I just got onto the turbo and tried. It sounded nasty enough for me to stop doing it sharpish

Yes it will sound nasty, but you are only subjecting the derailleur to gear-shifting force of a few kilos, at most, with your fingers.
If the derailleur was above the sprocket, it would be subjected to pedal forces, which are tens of kilos, with your legs.

 

[Cubist]

whatever happened to those cages the early MTBs all had?

The margin by which stuff hits rocks and ruts is pretty small. For example, I get more pedal strikes with thicker pedals, let alone longer cranks. The positioning of the hanging cage in your photo means it will never be out of the way of rocks, whereas if it isn't there rock strikes are far far less common. The derailleur sits well within the perimeter of that cage, and sothe cage ads excess weight, and makes rock strikes far more likely. What's more, my shadow design (low profile) derailleurs show scuff marks, obviously from rock strikes, but are very very rarely hit hard enough to damage them or affect their working.

 

[Ex EnergyMan]

Gravity !