I was frustrated by the lack of accurate information on the web (and I include Osprey's own website!) when trying to compare that I hope to provide a simple review and as least to my best to list the spec of the one I bought.
Osprey's own item card list all the weight and volume of all their "packs" but not so on their website where they only specifies the M/L version.
The Synchro 20 S/M is listed as 1lb4oz/0.59kg and 1098cu. in./18L. The weight is pretty spot on on my kitchen scale at 592g and the dimension I measured is 41x26x26cm.
The difference with the M/L is that this one measures 41cm in height vs 46cm and 2L.
Now that's out of the way. How does it perform?
This is a dedicated bike pack with curve back that not only keeps cool but softly molds for excellent support and comfort. Lidlock and built-in raincover and two side mesh pockets are ideal for riding. Three compartment: a small front scratch-free pocket; the middle has mesh for spare tubes and tools plus straps for hand pump; and the main with a divider for hydration pack.
I chose it because it's not 900+g like most others. I have an Osprey Reverb 10 ski pack and I like the build quality and smooth zippers with convenient hoops which is also found here. I chose Synchro because of two other ventilation design: padded mesh shoulder straps and what Osprey calls BioStretch hip belt which is another thin mesh.
I did two bike trips with this bag now totaling 1000km over two weeks. I have not encountered any rain in that time so have not open the rain cover yet! I managed to pack a week's worth of clothing and travel needs without a squeeze during each trip, weighing in at 3.2kg.
Apart from over-stretched side pocket elastics due to one silly action of trying to pack two wine bottles in the side pockets, everything else is still perfect. Shoulder pain isn't an issue because I tried to tighten the waist strap to support the weight there.
Overall a fantastic bag and extremely pleased with it.
