Vivi F26FUL 500W Long-Range Fat-Tire E-Bike

The 30-second verdict

The F26FUL sits third in Vivi's own best-seller ranking for a simple reason: it is the cheapest way we know of to get a UL 2849-certified 26x4" fat-tire e-bike with a battery upgrade path. The base bike is $599.99 with a 48V 12.8Ah (614 Wh) pack; $100 more buys the "Pro" 48V 18Ah (864 Wh) pack, which is more watt-hours than the Kingbull Hunter 2.0 or Vivi ACE07 carry at hundreds of dollars more. The compromises are real and visible: mechanical disc brakes with 160 mm rotors where the ACE01 gets hydraulics with 180 mm, a cadence sensor rather than a torque sensor, and an 80 mm coil fork that locks out but does not adjust. We have not ridden this bike; every figure here is from Vivi's published spec sheet, and where the numbers don't survive arithmetic we say so below. It is the right buy when range-per-dollar is the whole question — and the wrong one when braking or ride refinement is.

Power: 500W rated, 1000W peak — and why the rated number is the one that matters

Vivi's marketing leads with "1000W peak," but the bike is a 500W rated rear hub — that's the number Vivi's own catalogue tags it with, and it's the number that matters legally: state power caps regulate rated watts, and 500W clears the 750W ceiling used by every state except Oregon's 1,000W limit. 75 Nm of torque is healthy for the class — more than the ACE07's 80 Nm would suggest given the 250W rated gap, and enough to move a fat-tire bike off the line without drama. The pedal assist is a 12-magnet cadence sensor — power arrives when the cranks turn, not in proportion to how hard you push. That's the budget standard, and on a 74 lb fat bike it means the surge-and-coast character every bike in this tier shares.

Range: run the arithmetic before trusting the headline

Vivi claims "up to 50-60 miles." The arithmetic: a 26x4.0" fat-tire bike typically burns 20-30 Wh per mile in mixed pedal-assist riding. The base 614 Wh pack ÷ 25 Wh/mi ≈ 25 miles; the 864 Wh Pro pack ≈ 35 miles. Hitting 60 miles requires ~14 Wh/mi — a light rider, PAS 1, flat ground, no wind, on tires this wide. Treat the claim as a lab ceiling: plan on 20-30 miles from the base battery and 30-40 from the Pro, and the bike will never disappoint you. This is also the strongest argument for the $100 Pro upgrade — on range-per-dollar it is one of the best battery upsells we've seen at any price tier.

Speed and class: 25 mph pedal assist, 20 mph throttle

The spec table lists pedal assist to 25 mph and the half-twist throttle capped at 20 mph — the textbook Class 3 configuration (throttle-equipped Class 3 is legal in most, not all, states). Class 3 brings real restrictions: banned from many shared-use paths by default, mandatory helmets in several states, and outright illegal to sell as street-legal in DC, Kentucky, Massachusetts, Montana, North Carolina, Pennsylvania and South Carolina. Check your state's page before assuming the 25 mph mode is street-legal where you ride.

Build: where the $599 shows

Mechanical disc brakes with 160 mm rotors are the biggest compromise on the sheet. They work, but on a 74 lb bike that reaches 25 mph, 180 mm hydraulics (as on the ACE01) are a meaningful safety upgrade — budget for pad adjustment every few hundred miles. The 80 mm lockable coil fork takes the edge off broken pavement but is not trail suspension. Shimano 7-speed drivetrain, a 54.6V 2A charger (slow — a full Pro-pack charge takes overnight), and 26x4.0" all-terrain tires round it out. Certification is the quiet headline: SGS-tested UL 2849 (system) and UL 2271 (battery) — the certs New York City's Local Law 39 and California SB 1271 key on. Vivi's warranty runs 1 year on motor and battery — shorter than Kingbull's 2 years, standard for the Amazon-refugee price tier.

Who should buy it

Buy the F26FUL if you want maximum watt-hours per dollar in a certified fat-tire bike — the $699.99 Pro variant's 864 Wh is unmatched under $1,000 — and your riding is fire roads, gravel, sand, or snow where 4" tires earn their drag. Skip it if you brake late and hard (get the ACE01's hydraulics), want natural-feeling assist (the ACE01's torque sensor), or ride purely on pavement (the C26UL does that job for $100-200 less and 20 fewer pounds).