Just curious, how does having a piggy back on an air shock affect the riding characteristics?
I know they help keep it cooler, so prevent fade on long descents, but for non Alp downhill how would, say, a monarch plus feel better than a monarch?
Cheers

You are really talking about 3(and a half) major factors here:

1) Oil temperature: By allowing the damping fluid to be pumped external to the main damper piston you can reject more heat, this helps the shock fluid stay cooler and maintain a more constant viscosity and hence damping co-efficient. Having more oil volume helps in this respect too

2)Generally, the larger volume of the system also allows the designer to better compensate for changes in system volume through the travel range. Because the piston rods are single sided (generally) as the shock compresses, this rod “takes up” volume in the shock. This means you need somewhere to allow for that expansion. Piggy back canisters allow you to have a nice large nitrogen chamber behind a diaphram or second piston, that can absorb that volume change without an excessive pressure change (PV = nRT), again maintaining consistence of the damping co-efficient throughout the whole shock length

3) A remote reservoir opens up possibilities for “twin tube” design, where the fluid flow is in one direction only, and as such allows better use of the control orifices and crucially, tends to automatically bleed any cavitated fluid out of the primary damping stacks. Again, we are talking about consitence of the damping co-efficient at the full range of damper rod velocities

3.5) (The “Half point) Generally, a larger physical volume shock allows the designer more freedom to orient, mount and design the oil flow paths through all the damping elements. This results in less cavitation, less fluid power losses (effectively sticktion) and wider, yet more linear control authority (from the various damping orifice control knobs)

Simples!

(the downsides are that cost, weight and size all increase compared to a more conventional damper…..)

Okay thanks!
so the main riding benefits would be : (?)
Consistency on long runs
more tuneable
plusher
less harsh bottom out

Does that sound about right?

And they look cool which is worth another couple of points!

A remote reservoir opens up possibilities for “twin tube” design, where the fluid flow is in one direction only

Romic twin tube shocks were pigyback-less.

The advantage of a twin tube over a de-carbon damper is the volumetric flow of the oil, you can use almost all the oil for damping which means lower pressure drops in the orifices/shims and less chance of cavitation. Its nothing to do with the pigyback.

A pigy back really just provides volume for the oil displaced by the shaft, smaller shocks just have the ‘pigy back’ inline with the damper inside the shock. It also makes it easier to fit more adjustments onto the shock as the pathways transporting the oil from the shock to the pigyback are a convenient place to put them. Think about the old fox van-r vs the old van-rc or dhx, the r didnt have an external resevoir but would have had one inside.