Converting a bike to mid drive power means giving up the bottom bracket to a motor. The reward is assist that works through your own gears, and the best versions of that feel like your legs got stronger rather than like a switch flipped on.
Torque sensing is what makes that feel possible. A motor with it meters how hard you press the pedals and matches its output to it. Most kits at the low end sense only that the pedals are turning, which is why they lurch.
The Varstrom Z8 claims the better sensor at a kit price, with a 140 N.m peak torque figure behind a 750W rating.
The catch is what sits around the claim. This is a Tongsheng motor sold under a store name, it arrives with no battery and no printed manual, and the listing manages to describe its sensor two different ways on one page. Three owners have left ratings so far, all of them four stars or better, and two of them wrote up the details that matter.
You're buying a Tongsheng motor under the Varstrom name here. The Varstrom Z8 puts a torque-sensing 750W mid drive on a standard bottom bracket, and the three owners in its record treat it as a competent, slightly loud peer of the Bafang BBS02B we reviewed, weaker on the hills and audible under load. The motor itself has held up in their hands.
The claims stacked around it, the 1056W peak, the 15 percent after-sales figure, and two different sensor stories, deserve more caution than the motor does.
Three ratings, all purchase-confirmed, none below four stars
The kit holds a 4.4 score from three global ratings, where 40 percent of buyers gave it five stars and the other 60 gave it four, and all 3 written reviews are purchase-confirmed. Two of these owners live in the United States and one is in Australia.
The most detailed owner has ridden over a thousand miles each on both a TSDZ2b and a Bafang BBS02, and this motor feels comparable to both, even though it makes a noticeable whine.
The deepest use report in the record sits at 130 miles, so nobody has lived with this kit for years yet. Nobody reports a failure or a warranty claim either. Three ratings is a thin record, and it says almost nothing about durability beyond that.
What the owners measured on the road
One experienced converter noted that the motor makes an audible whine under load, which is the only noise from the three motors they tested, yet it still doesn't feel as strong as the BBS02 on hills. Another owner has found how it climbs hills is about on par with a TSDZ2, though it feels weaker than the BBS02 when climbing.
Since the kit uses a torque sensor, riders report little help is felt below PAS 3, because what the motor gives scales based on how hard they push the pedals. What they saw shows 8 mph unassisted on flats, while the bike hits 12 to 13 mph in PAS 5 while still pedaling. This isn't about moving for free.
The torque sensor buys push-matched oomph rather than raw speed, so the motor feels like an extension of the legs instead of a simple push-button. This means the bike works harder when you do more, which keeps the ride natural on varied terrain.
The paperwork is the install
Two of the three owners have found no documentation in the box, and the one who complained about being locked out of the settings menu got the display manual as a download link. The menu opens when you double-click the middle button, and it holds about ten settings, while the 5 PAS levels themselves cannot be configured.
The speedometer reads fast against GPS until you set the wheel diameter to 24 on a 26-inch wheel, while the display stays too dim in sunlight even at max backlight.
The double-ended wrench for the locking rings didn't fit one owner's inner ring well, so they used a plastic mallet to get it tight. Their cranks creaked at 130 miles until they pulled the arms, cleaned mill scale off the spindle and re-torqued it with anti-seize.
One listing, two sensor stories
The listing claims a torque sensor measures pedal pressure in real time, yet a later bullet mentions an integrated cadence sensor, which leaves you wondering how the kit actually works. One spot calls the display the SW2-M58, but the text says just the SW-M58, which is more confusing.
The page advertises a 1056W peak with no duration and no voltage condition attached. It's a number you can't check. The upgrade bullet also says the new cable reduces after-sales issues by 15 percent, with no basis stated for the figure.
Since the kit hits 55km/h, it's far past every legal e-bike class in the US, which matters for where it can legally be ridden. Class 1 and 2 cut out at 20 mph, while class 3 stops at 28 mph per electric bicycle laws.
Budget for the whole build, not the motor
The asking price for the motor alone sits among the most expensive of the 191 kits we price, and a battery is on top of that. That's a below-average value position, said plainly, and the live position renders in the specs box of our conversion kit comparison table. Check it before you commit to the whole project.
The hardware fits standard JIS bottom brackets with 68mm or 73mm shells, so it won't work on carbon fiber or wood frames. The listing weighs the kit at 15 pounds, and the owner who totaled the build reports about 20 pounds added once the battery is on the bike.
Because this is a motor-only kit, the actual current limit depends on your battery's BMS and your controller rather than any label written on the motor itself. One owner fit the kit to a recumbent only with extension cables for the speed sensor and the display, and still called it functional. Long frames need that planning.
Frequently asked questions
Will it fit my frame?
It fits standard JIS bottom bracket shells of 68mm or 73mm, and the listing explicitly rules out carbon fiber and wooden frames. Press-fit shells and one-piece cranks sit outside that standard, so measure before you order.
What sets the power limit?
The motor label doesn't. The battery's BMS and your controller decide how much continuous current the system allows, so size the battery to the build rather than to the rating on the motor.
What warranty comes with it?
The spec table lists 30 days to return, 365 days of warranty service, and lifelong technical guidance. Nobody in the record has reported using it.
Is it noisy?
One owner with a thousand miles each on a TSDZ2b and a Bafang BBS02 calls this the only one of their three motors they can hear, a whine when it is engaged. They still rated it four stars.
Does it come with documentation?
No printed manual ships in the box. The display's manual reached one owner as a download link only after they complained about being locked out of the settings menu.