Mountainsmith Zerk 40 vs Mountainsmith Zerk 55 UL
Quick take: The Mountainsmith Zerk 40 costs $49.95 less; the Mountainsmith Zerk 55 UL carries more (53 vs 40 liters); the Mountainsmith Zerk 40 is lighter (2.13 vs 3.5 lbs).
Which one to buy
Mountainsmith lists the Zerk 40 at 40 L (44 L with the roll top extended), but one review measured about 51 L, and the Mountainsmith Zerk 55 UL holds 53 L, or 58 L with the roll top extended. So the bigger Zerk doesn't buy you much more volume. It buys carrying capacity and fit. Mountainsmith suggests up to 40 lb for the 55 and says it tolerates up to 50. For the 40 it suggests 30 lb (up to 45), and reviewers find it comfortable only to about 20-25 lb before it degrades.
The 55 also comes in two torso sizes, Medium for 16-20 in and Large for 18-22 in, with a 28-48 in belt. The 40 is one size for 16-21 in torsos, so tall hikers get a better fit on the 55. Mountainsmith reshaped both interiors to take 700 cubic inch bear canisters, hedging the 55's fit to some makes, so check your canister's dimensions first. In the 40, reviewers advise carrying a canister vertically inside and skipping the top straps, which let it slide off the side. The 55 has a 210D bottom and back panel at 3 lb 8 oz, so it's a heavy pack for the ultralight label.
The 40's advantage is going lighter. Both frame sheets come out, but the 40 is the one suited to running frameless once your base weight comes down. Since volume is nearly the same, decide by your heaviest day on trail, not the label. The $280 Zerk 55 is worth it only if your load passes the point where the 40 starts to sag.
Mountainsmith Zerk 40
For hikers whose loads stay around 20-25 lb and who want to drop the frame sheet as their kit gets lighter.
Mountainsmith Zerk 55 UL
Worth it for long food and water carries or canister routes where loads near 40 lb, and for tall hikers who need the Large torso size.
| Spec | Mountainsmith Zerk 40 | Mountainsmith Zerk 55 UL |
|---|---|---|
| Price | $230 | $279.95 |
| Rating | — | — |
| Type | Ultralight Pack | Ultralight Pack |
| Frame Type | Internal Frame | Internal Frame |
| Capacity | 40 L | 53 L |
| Weight | 2.13 lbs | 3.5 lbs |
| Max Load | 30 lbs suggested (up to 45 lbs) | 40 lbs suggested (maker says up to 50 lbs) |
| Torso Fit | One size, 16-21 in torso | Sized MD 16-20 in / LG 18-22 in; waistbelt 28-48 in |
| Material | Robic nylon X-Grid Extreema ripstop with UTS coating | 100D Robic nylon X-Grid Extreema (UHMWPE) ripstop with UTS coating, 210D bottom and back panel |
| Back Panel | Removable frame sheet | Airflow EVA foam over a removable EVA/Atilon frame sheet |
| Adjustable torso | No | No |
| Ventilated back | No | No |
| Raincover included | No | No |
| Hipbelt pockets | Yes | Yes |
| Removable lid | No | No |
| Hydration sleeve | Yes | Yes |
| Fits BV500 bear can | Yes | Yes |
| Warranty | Mountainsmith lifetime warranty against material and workmanship defects; repair or replacement at Mountainsmith's discretion | Mountainsmith lifetime warranty against material and workmanship defects; repair or replacement at Mountainsmith's discretion |
Pros & cons
Mountainsmith Zerk 40
- ✓The removable frame sheet means one pack covers both framed and frameless use.
- ✓One review measured about 51 L against the 40 L badge, where most packs here measure under.
- ✓Robic X-Grid Extreema ripstop (100D body, 210D base and back) is a tough woven fabric that repairs easily.
- ✓$230 is mid-range for the segment and below most cottage packs of similar volume.
- ✓Mountainsmith is a mainstream brand, so returns and support are simpler than cottage brands'.
- ✓1 lb 14 oz stripped (2 lb 2 oz full) is competitive with dedicated ultralight packs.
- ✓Hipbelt pockets included.
- ✓Widely available rather than made to order.
- ✗A removable frame sheet is less effective at load transfer than stays or a hoop.
- ✗No adjustable torso.
- ✗Not waterproof, unlike the laminate packs at this price.
- ✗Mountainsmith has less ultralight credibility than the cottage brands it competes with.
- ✗The 40 L badge understates the pack, which makes cross-shopping confusing.
- ✗Fewer configuration options than a made-to-order pack.
- ✗Limited presence on long trails, so fewer real-world reports.
Mountainsmith Zerk 55 UL
- ✓Robic nylon with an Extreema UHMWPE grid and a 210D base is tougher than the thin fabrics most 50-plus litre UL packs use.
- ✓Suggested 40 lb load, with Mountainsmith stating it tolerates 25% more, gives real headroom for food and water carries.
- ✓The interior was re-patterned to take some 700 cubic inch bear canisters.
- ✓Two zippered hipbelt pockets and a hydration sleeve with hose port come standard.
- ✓Two torso sizes and a 28-48 in waistbelt cover a wide range of hikers.
- ✓The frame sheet is removable, so the pack can be stripped down for lighter trips.
- ✗3 lb 8 oz is heavy for the ultralight label; carbon-stay competitors of similar volume weigh closer to 2 lbs.
- ✗Bear-canister fit is hedged to 'some makes', so check your canister's dimensions first.
- ✗No lid; the optional 7 L Toplid is a $34.95 accessory.
- ✗No raincover and no ventilated back panel.
- ✗Torso length is fixed within each size rather than adjustable.
Mountainsmith Zerk 40 vs Mountainsmith Zerk 55 UL: common questions
Is the Zerk 55 UL much bigger than the Zerk 40?▾
Less than the names suggest. One review measured the Zerk 40 at about 51 L, and the Zerk 55 holds 53 L, or 58 L with the roll top extended. The real difference is the 55's suggested 40 lb load and its two torso sizes.
Which Zerk carries a bear canister better?▾
Mountainsmith reshaped both for 700 cubic inch canisters, and the 55 leaves more room around one, though Mountainsmith hedges its fit to some makes. In the Zerk 40, reviewers advise carrying a canister vertically inside and avoiding the top-mounted straps, which let it slide sideways.
Does a tall hiker need the Zerk 55 instead of the Zerk 40?▾
If your torso is over 21 in, yes. The Zerk 40 is one size for 16-21 in torsos, while the Zerk 55 comes in Medium for 16-20 in and Large for 18-22 in, with a 28-48 in waistbelt.