What are the differences between a seatpost-mounted and frame-mounted rear rack

Question

I am in the market for a rear rack. I noticed that there are two categories of rear racks:

Seatpost-mounted rear racks, such as this one:

And frame-mounted rear racks, such as this one:

I'm trying to decide between two of these two types of racks. I would like to know what are the pros and cons for each type of rack. I noticed that the frame-mounted rack is advertised to be able to support a heavier load. In that case, why would someone get the seatpost-mounted rack over the frame-mounted rack?

Answer 1

The advantages of seat mounted racks are that they don’t require rear dropout rack lugs; some of the seat mount racks have quick release levers so you can easily take them off (while racing or transporting the bike, etc), and are many times the only option for a rear suspension bike.

The disadvantages of seat mount racks are numerous:

• max luggage weight is limited
• many regular panniers that require a bottom (dropout) hook won’t fit
• some are designed only for proprietary bags and bag mounts
• they are often heavier (thx @michael)
• minimum seat drop becomes limited
• you can’t use many seat droppers and suspension seatposts as there isn’t enough clearance
• seat post damage is possible
• doesn’t fit some oval, rhomboidal, or oversized seat posts
• if you have a round seatpost and even a modest weight on the back and you take a spill or even do a modest spin-turn, it can be enough for the rack to spin the seat post

_{Source: had a Specialized hardrock hardtail back in the day and had a regular rack on it. Switched over to a seat mounted rack at one point as I thought it looked cooler and it does, but quickly realized it was most just looks.}

Answer 2

RoboKaren's answer is great on the pros and cons, so I'll restrict myself to explicitly answering

... why would someone get the seatpost-mounted rack over the frame-mounted rack?

Really, the only reason is that your bike won't support a frame-mounted rack: for example because it has rear suspension. In engineering terms, a frame-mounted rack supports the weight of what you're carrying from below by being attached to the bike somewhere near the rear axle, and the connection near the saddle is mostly to keep the rack in the correct orientation. A seat-post-mounted rack supports the weight entirely by torsional forces through the seat-post, which isn't nearly as sturdy. That means the rack itself is probably heavier and probably can't support as much load. So you'd almost always want a frame-mounted rack if that's compatible with your bike.

Actually, there is one pro of seat-post-mounted racks that RoboKaren doesn't mention. If you're not carrying much weight, a seat-post rack with a small bag on top of it is more aerodynamic than carrying the same load in a pannier on the side of a frame-mount rack. (Actually, GCN's test was with a large saddle bag, rather than the exact situation I describe, but that's basically the same thing.)