As for seatpost rack stability i ve found it depends on the attachment.
Seatpost bike rack reviews.
Blackburn sp 1 seat post rack bike rack user reviews.
The spx 1 fit just fine using 4 bolts and a rubber sleeve that makes for a solid no slip fit.
Seat post mounted braze on or even braze on with side loads.
If you need the panniers it s better to have the higher load capacity.
To fit other racks to such bikes.
I d use a frame mounted rack if i could but the frame isn t set up for it.
If your bike has mudguard eyelets but none on the seatstays a simple and elegant solution is to replace the seat clamp with one that s threaded for rack attachments.
In the world of quick release seat post racks there appears to be only one that has 3 point attachment system.
Upanbike 22lbs capacity mountain bike bicycle adjustable length rear rack quick release mounted at seatpost luggage carrier cargo 4 3 out of 5 stars 145 20 99 20.
Top 3 best bike rack reviews 1.
Axiom dlx streamliner disc.
Anyway the biggest draw back to seatpost racks is the weight limit 20 to 30lbs as compared to 40 to 60lbs for a frame rack.
You can also get racks that fit on the seatpost though these should only be used with very light loads as they tend to sway.
Cyclingdeal bicycle bike alloy seatpost mount rear back cargo pannier rack universal mountain or fat bike luggage carrier lightweight 4 3 out of 5 stars 62 1 offer from 59 90.
Most quick release beam seat post racks connect directly to the seat post.
The market offers various types of racks.
Upanbike 22lbs capacity mountain bike bicycle adjustable length rear rack quick release mounted at seatpost luggage carrier cargo 4 2 out of 5 stars 111 20 99 20.
If you are a mountain biker you definitely pick the seat post mounted rack as it will not interrupt you in your cycling session and will keep your tools or any other things safe.
I replaced an older mountain bike with a similar seatpost rack but it used a quick release lever and shims which would not fit the large seatpost size of the new bike.