I have a Ridley Xtrail that I'd like to start commuting on. There are no rack mounts. How well do the seat post mounted racks work? I have an alloy seat post and weigh about 150 lbs so I'm not too worried about over stressing it. Most days I only bring a set of clothes and maybe lunch so the rack shouldn't need to support more than 5 lbs.
A seat post rack would be ok, it's also the cheapest way to resolve your problem, and as long as your post isn't a lightweight like a Thomson Masterpiece. But there's better options that could be more utility minded (see below). Also not advised to try and p-clamp racks on a bike like that. Doesn't matter how much you weigh, mounting racks on carbon stays that weren't designed to take a cargo load is a big no-no. If you're going to be spending money to haul gear and food might as well take a look at some variation of bikepacking bags, like a large seat pack and a tangle bag under the top tube. Like this: And if you ever get the bug to stay out a night or two on the bike you already have the gear.
*Not mine. Image taken from bikepacking.com simply for an example.
I used a Tubus rear rack with their Quick Release adapters on my old Calfee road bike for commuting using some small panniers. Very sturdy for your purpose. The only pain is if you get a flat and have to remove the rear tire, so you may not want to deal with that. I used tubeless, very flat resistant tires for commuting and rarely had to deal with it. https://ortliebusa.com/product/qr-adapter-set/
Unfortunately this type of rack can swing and is only safe for a fairly light weight. I use a Revelate seat bag for clothes and lunch on a long mile commute and it works incredibly.
![[niner-rlt-9-steel-review-23]](http://www.bikepacking.com/wp-content/uploads/2016/05/niner-rlt-9-steel-review-23.jpg)