HALLOMOTOR BBSHD 42T Chainring

Wider chains are the reason that range matters. A 7- or 8-speed chain is noticeably broader than an 11-speed one. A ring designed only for narrow 10- to 12-speed chains can bind or run rough with a wide chain, while a 6–11 speed ring leaves room for it.

The teeth use a positive-and-negative design, the same idea as narrow-wide: alternating tooth widths that match the chain's inner and outer plates. On a single-ring conversion with a basic derailleur, that grip is the main defence against chain drops.

• Material: 6061-T6 aluminium, with 7075 also named in the product title.
• Size: 42T, four teeth below the stock 46T.
• Motor: the BBSHD on 48V or 52V packs at 1000W.
• Finish: plain black.

It is a direct replacement for the stock ring, so the conversion keeps its original motor mounting and simply gains a lower gear and a chain-friendly tooth profile. For many riders on budget drivetrains, that is the single most useful change to make after the kit goes in.

Chain length changes only slightly with this swap. Going from 46T to 42T removes about one inner-plus-outer link pair, so most riders can reuse the existing chain after a quick shortening with a chain tool and a fresh quick link.

Every ring type, from offset rings to spider adapters, is lined up in the BBSHD chainring guide.

With the stock 46T ring and a 28T largest cog, the lowest gear is still tall. The motor has to turn slowly under heavy current on a grade, and that is exactly the situation that heats windings and controller. A 42T ring lowers every gear by about a tenth, which is often the difference between a bike that climbs steadily and one that bogs.

Here is the effect at 90 rpm on a 2.1 m wheel.

The top end gives up a few mph, which rarely matters on a 48V build. The bottom end gains a gear the motor can pull cleanly.

Lower gears ease the load on the drive's internal nylon primary gear, too. It takes its hardest hits at low speed and high torque, which is what a tall gear on a steep hill creates. Learn the warning signs from the nylon gear guide.

Motor speed rises with voltage. A 52V pack spins the BBSHD faster at full throttle than a 48V pack, so the same ring reaches a higher road speed before the motor runs out of revs. That also means a 52V build can pull a taller ring up the same hill with a little more margin.

• 48V, hilly routes: 42T is often the right step down from stock; 40T for steeper ground.
• 48V, flat routes: 42T still helps with launches and short climbs.
• 52V, mixed routes: 42T keeps the motor fast on climbs with plenty of road pace.
• 52V, flat and fast: a bigger ring, 46T or above, uses the extra speed.

Heat is the thread running through all of it. The BBSHD makes most of its heat when it turns slowly under high current, and ring size is the simplest lever a rider has over that. Voltage, current and torque interact in ways the BBSHD 1000W guide spells out.

1. Note how fast you cruise on your usual flat road.
2. Note how long your longest climb takes at full assist.
3. If the climb is long and the cruise is modest, go smaller than stock.
4. If the climb is short and the cruise is fast, stay near stock.

A 42T answers most combinations of those four notes on either pack, which is why it is such a common BBSHD replacement size.

6061-T6 is tough and machines cleanly; 7075 is harder and wears more slowly. On a 1000W mid-drive, either grade is a wear part, and how long the ring lasts depends mainly on chain care. Replace a stretched chain before it hooks the teeth, and swap ring and chain together if a new chain skips under power.

Fitting takes standard tools: a crank puller that matches your crank arms, hex and Torx keys, medium-strength threadlocker and a torque wrench. Pull the crank with the steps in the BBSHD install guide.

1. Remove the drive-side crank arm with the puller.
2. Unbolt the stock ring and slide it off the output.
3. Clean the faces, fit the 42T ring and tighten its bolts in a star pattern with threadlocker.
4. Refit the crank and shorten the chain by about one inner-plus-outer link pair.
5. Measure chainline from the frame centre to the teeth and compare with the middle cog.

The offset is not published, so that chainline check tells you whether the ring suits your frame. A chain that grinds in the largest or smallest cog is running at too sharp an angle.

• Running 12-speed: the Partora 42T for 10–12 speed.
• Want 40T in 7075: the EfforPrac ring.
• Fat bike chainline: see the fat bike guide.