Partora BBSHD 42T Narrow-Wide Chainring

Narrow-wide teeth alternate between thick and thin. The thick teeth fill the wider gaps between the chain's outer plates, the thin ones fit between the inner plates, and together they leave the chain little room to wander. On a single-ring drivetrain with no front derailleur, that profile is the main thing keeping the chain on over roots, rocks and potholes.

The red finish makes it the obvious pick for a build with red details on the frame, hubs or brake calipers. It also makes chain wear easier to spot, since a worn tooth shows bare metal against the colour.

With a 12-speed cassette, the range looks like this at 90 rpm.

The 10T top cog on many 12-speed cassettes gives back almost all the stock ring's top-end speed, while the big climbing cog keeps the motor spinning on steep ground. That combination is why 42T is such a common choice on 12-speed BBSHD builds.

Partora rates the ring for 10-, 11- and 12-speed chains. That covers most current 1x trail and commuter drivetrains. Wider 6- to 9-speed chains are outside that range and may bind on the narrow-wide teeth; for those, a ring rated for 6–11 speed suits better.

Narrow chains wear faster under motor loads, which makes chain care the biggest factor in how long this ring lasts.

1. Measure chain wear often: a gauge takes seconds and catches stretch before it damages teeth.
2. Replace the chain early: a stretched chain rides up the teeth and hooks them.
3. Shift off the power: ease pressure, or fit a gear sensor that cuts the motor during shifts.
4. Keep chainline straight: a steep angle wears one side of the teeth and pulls the chain off.

Chain drops on a narrow-wide ring usually have a cause beyond the ring itself: a worn chain, a chain that is too long, a derailleur without a clutch or a chainline far from the cassette centre. Fix those and the narrow-wide teeth do the rest.

A dropped chain on a BBSHD can wedge between the ring and the frame under power, which damages paint and sometimes the chainstay. Prevention is worth the effort.

On the road, the 42T keeps up with traffic in the mid-20s on an 11T cog, or close to 30 mph on a 10T cog. On the trail, a wide-range cassette brings the climbing gear down to walking pace. The motor stays in a comfortable speed band across the whole ride.

Compared with the stock 46T, the 42T ring raises motor speed on every climb by about a tenth. That lowers current and heat on long grades, and it eases the load on the drive's internal nylon primary gear, a known wear point on heavy, tall-geared builds. Grinding, slipping and the other symptoms are listed in the nylon gear guide.

• Good fit: trail bikes with road links, hilly commuters on 1x drivetrains, 52V builds that still climb.
• Consider 36T–38T instead: steep singletrack every ride, cargo loads, towing.
• Consider 44T–48T instead: mostly flat road riding with the occasional trail.

Rider weight and pack voltage shift the choice. A heavy rider on 48V who climbs a lot is often happier a size smaller. A light rider on 52V can hold a 42T up surprisingly steep grades.

Pack voltage changes the motor's speed ceiling (the BBSHD 1000W guide has the numbers), and that is what sets the useful top gear on any ring.

The kit of tools is short: a crank puller that matches your crank arms, hex and Torx keys, medium-strength threadlocker, a torque wrench and a chain tool. First time pulling the arms? The BBSHD install guide has the puller steps.

1. Remove the drive-side crank arm with the puller.
2. Unbolt the stock ring and slide it off the output.
3. Clean the mounting faces, fit the Partora ring and tighten its bolts in a star pattern with threadlocker.
4. Refit the crank, then shorten the chain by about one inner-plus-outer link pair (46T to 42T).
5. Measure chainline and run through every gear on the stand.

Recheck the ring bolts with the torque wrench after the first few rides. New parts settle against each other under the motor's torque, and a bolt that has backed off slightly is far easier to catch in the garage than on a trail.

The offset is not published, so the chainline check matters. Measure from the frame centre to the middle of the ring's teeth and compare with the middle cog; aim to match within a few millimetres. The ring fits the BBSHD only, since BBS01 and BBS02 motors use a different spline.

• Need a stated offset: the mGYDY 12 mm offset ring.
• Running an older 6–11 speed chain: the HALLOMOTOR 42T.
• Want a chain guard: compare guarded rings in the chainring guide.