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Will it work with a dropper post?
Yes. This one mounts to a rack rather than the seatpost, so the post is free to drop. You are carrying the rack's weight to get that.
From how Tailfin describe the attachment. See the source.
How much room does it need?
This pack measures 55cm long and 24cm deep. Length is how far it reaches behind your saddle, and it's what drives sway — a long pack is a longer lever on the seatpost. Depth is what competes with your rear tire once the pack sags under load.
Across the 18 seat packs we hold dimensions for — which run from 24cm to 67cm — this one reaches further back than 16 of them, and hangs deeper than 15.
The other number that decides this — how much seatpost you have showing above the collar — depends on your leg length rather than your frame, and makers almost never publish a minimum. So measure from the top of your seat collar to the bottom of your saddle rails, then check it against the mounting hardware on the product page. If you run a dropper post, look for a pack that mounts to the saddle rails alone.
Check against your measurementsTailfin CargoPack Alloy — Specs & Analysis
Manufacturer specifications, compared against every other seat bags in our database. We do not test gear in the field — we measure, verify, and compare.
Tailfin's integrated seat pack carries its load on an alloy arch bolted to the rear axle, with a quick-release seatpost connector rather than a seatpost clamp. That is what gives it rack-like stability and almost no sway. Tailfin state 18 litres in the main bag plus 3 litres across the expandable side pockets, and 1,129g for the alloy version with Fast Release Dropouts.
Who Is the Tailfin CargoPack Alloy Best For?
Riders who want maximum seat pack capacity with zero compromise on stability. If that describes your riding style, the CargoPack Alloy should be at the top of your shortlist. It sits in the Seat Bags category alongside 17 other products we track.
How It Compares
At 1129g and $420, the CargoPack Alloy is 987g heavier than the lightest option in this category (the Revelate Designs ECOPAK Shrew Seat Bag at 142g). The most budget-friendly option is the Topeak BackLoader 10L at $70.
The price-to-weight ratio comes in at $37 per 100g — a premium price point, but you're paying for quality, materials, and design that justify the investment for serious bikepackers.
What We Like
- +Load is carried by the rear axle, not the seatpost
- +Tailfin state the connector suits dropper, carbon and aero posts
- +18L in the bag plus 3L of side pockets
- +Modular system — add panniers later
Watch Out For
- −1,129g is heavy for a seat pack
- −From $420, among the most expensive available
- −Overkill for lightweight setups
The Bottom Line
We haven't scored the Tailfin CargoPack Alloy, and neither has anyone else we can cite. It's here for its verified specifications and dimensions, which is what the fit tools run on — a score is an editorial judgement, and we don't hand one out until someone has a basis for it. What this page can tell you is measured: 1129g, $420, and how both sit against every other seat bags product we track.
Price and Weight checked against the manufacturer on 22 August 2026 — see the source.
Specifications and dimensions come from the sources listed above. Where a page draws on first-hand riding, it says so.
How the CargoPack Alloy compares
Against the bags people cross-shop it with, measured rather than reviewed — including how many real frames each one fits.
How it compares
Against the 18 seat bags we hold published specs for.
At 1129g the CargoPack Alloy is the heaviest in the category we track, against a category median of 466g.
At $420 it undercuts 0 of 17 of them, against a median of $170. That works out at $37 per 100g — above the $36 median for seat bags.
Weight and price come from published specifications. Cost per 100g is a blunt measure — it rewards light gear and says nothing about durability or whether the thing works — so treat it as one input, not a verdict.
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Seat bags extend behind the saddle, carrying gear without the need for a rear rack.