Outdoor Vitals Carbon Evo 50 vs Mountainsmith Zerk 55 UL
Quick take: The Mountainsmith Zerk 55 UL costs $0.02 less; the Outdoor Vitals Carbon Evo 50 carries more (58 vs 53 liters); the Outdoor Vitals Carbon Evo 50 is lighter (1.88 vs 3.5 lbs).
Which one to buy
Price and suggested load match at about $280 and 40 lbs, yet the Carbon Evo 50 weighs 1.88 lbs and the Zerk 55 UL weighs 3.5 lbs. That 1.62 lb gap is the story. Outdoor Vitals gets there with two curved carbon-fiber stays, true load lifters and a Robic and Spectra ripstop body; Mountainsmith uses a removable EVA and Atilon frame sheet and a burlier Robic fabric with an Extreema UHMWPE grid.
Fit is where the Zerk claws back ground. It comes in MD 16-20 in and LG 18-22 in, overlapping ranges that help a between-size hiker, with a waistbelt spanning 28-48 in. The Carbon Evo is sold in just two fixed lengths, Short at 17.7 in and Regular at 19.2 in, though its hipbelt comes in XS to XL independently. If your torso sits outside those two numbers, the Zerk is the safer order.
Both have two zippered hipbelt pockets, a hydration sleeve and room for a bear canister, though Mountainsmith hedges its fit to some makes. Mountainsmith says the Zerk tolerates up to 50 lbs and its frame sheet comes out for lighter trips. For most thru-hikers, the Carbon Evo is the better pack for the money; the Zerk suits hikers who are hard on gear or whose torso falls between the Evo's sizes.
Outdoor Vitals Carbon Evo 50
The better fit for thru-hikers whose torso matches its Short or Regular length and who want a carbon frame rated to 40 lbs at under 2 lbs.
Mountainsmith Zerk 55 UL
For hikers who need overlapping torso sizes or a 28-48 in waistbelt, or who value a tougher fabric over weight.
| Spec | Outdoor Vitals Carbon Evo 50 | Mountainsmith Zerk 55 UL |
|---|---|---|
| Price | $279.97 | $279.95 |
| Rating | — | — |
| Type | Ultralight Pack | Ultralight Pack |
| Frame Type | Internal Frame | Internal Frame |
| Capacity | 58 L | 53 L |
| Weight | 1.88 lbs | 3.5 lbs |
| Max Load | 40 lbs | 40 lbs suggested (maker says up to 50 lbs) |
| Torso Fit | Fixed torso, sold in Short (17.7 in) and Regular (19.2 in); hipbelt XS-XL | Sized MD 16-20 in / LG 18-22 in; waistbelt 28-48 in |
| Material | 100D Robic nylon with 200D Spectra ripstop; 210D Robic / 400D Spectra bottom | 100D Robic nylon X-Grid Extreema (UHMWPE) ripstop with UTS coating, 210D bottom and back panel |
| Back Panel | Foam back panel over two curved carbon-fibre stays | 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 | Outdoor Vitals Performance Promise | Mountainsmith lifetime warranty against material and workmanship defects; repair or replacement at Mountainsmith's discretion |
Pros & cons
Outdoor Vitals Carbon Evo 50
- ✓A 40 lb rating at 1.88 lb is a strong load-to-weight ratio for a framed roll-top pack.
- ✓Twin curved carbon stays plus true load lifters, a frame layout usually found on heavier packs.
- ✓Mixed Robic and Spectra ripstop body with a 400D Spectra bottom where abrasion is worst.
- ✓The maker names a BearVault 475 as fitting inside, so canister trips are planned for.
- ✓Two zippered hipbelt pockets and a removable hydration sleeve come standard.
- ✓Hipbelt sizes run XS to XL independently of the two torso lengths.
- ✗Only Short and Regular torsos are sold, although the page lists a Tall 21 in spec.
- ✗No adjustable torso, so between-size hikers have to pick one of two fixed lengths.
- ✗Raincover is sold separately.
- ✗The $251.97 member figure is a rebate price; the regular price is $279.97.
- ✗Foam back panel against the body, not a suspended mesh back.
- ✗New in February 2026, so there is little long-term durability record yet.
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.
Outdoor Vitals Carbon Evo 50 vs Mountainsmith Zerk 55 UL: common questions
Why is the Zerk 55 UL so much heavier than the Carbon Evo 50 at the same price?▾
It uses a removable EVA and Atilon frame sheet and a heavier UHMWPE-grid Robic fabric with a 210D bottom and back panel. The Carbon Evo uses lighter carbon stays and a Robic and Spectra ripstop, which is where the 1.62 lb difference comes from.
Which one fits if my torso is between sizes?▾
The Zerk 55 UL is the easier fit, since its MD (16-20 in) and LG (18-22 in) ranges overlap. The Carbon Evo 50 is sold only at fixed lengths of 17.7 in and 19.2 in, with no adjustment.
Will a bear canister fit in either pack?▾
Outdoor Vitals names a BearVault 475 as fitting in the Carbon Evo 50. Mountainsmith says the Zerk 55 UL takes some 700 cubic inch canisters, so check your canister's dimensions before relying on it.