A loose left crank can turn a short commute into a wobbly, noisy ride. That small arm connects the pedal to the bottom bracket spindle, the axle inside the frame that transfers leg power to the wheel. Matching length and hole shape matters more than brand for a clean swap.
You can measure centre to centre and look at the spindle end before ordering. The picks below cover 170mm and 152mm lengths, steel and aluminium builds, single left arms and matched pairs for electric bikes.
The Picks In Brief
Epessa Crank Arm Left 170mm
BLLREMIPSUR 2 PCS Left Crank Arm 170mm
Rhinenet Left 170mm Bicycle Crank Arm Black
BAFANG E-Bike Crank Arm Aluminium Alloy 2 Pieces
BlueStars Crank Left Arm Diamond Hole 170mm Black
S SYDIEN 152mm Bike Crank Arm Diamond Hole
Left Crank Arm Diamond Hole Square Hole
What Each Pick Brings To The Table
Epessa Crank Arm Left 170mm suits riders replacing a worn left arm on a standard 170mm setup. Two spindle interfaces are offered, diamond and square, which covers many older bottom brackets without forcing a full drivetrain change.
Aluminium alloy construction keeps weight low at 1.6 ounces for the arm itself. You need to match the existing hole shape exactly, since both sides should share the same interface for even pedal height. Gym bikes are called out as a possible mismatch, so length and port checks matter there.
Practical Pluses
- 170mm centre to centre length matches common road and mountain setups
- Aluminium alloy keeps the arm light for daily riding
- Square or diamond interface choice eases matching an existing spindle
- 9/16 inch pedal thread fits standard pedals
Things To Consider
- Left side only, so the right arm stays unchanged
- Interface must match the original crank port before ordering
BLLREMIPSUR 2 PCS Left Crank Arm 170mm takes a different route from the aluminium Epessa arm, using solid steel throughout. That material choice suits riders who prioritise strength over saving a few grams on a commuter or trail bike.
The pack contains two left cranks at 170mm centre to centre, black in colour. On longer rides the extra heft is noticeable, yet steel construction gives a sturdy feel for heavier riders and rougher paths. Matching the diamond hole spindle remains essential, since a square hole version will not seat correctly.
Reasons It Performs
- Solid steel build resists bending under load
- Two arms in one pack covers a spare or second bike
- 170mm length aligns with the existing right crank
- Listed for mountain, road, folding and fixed gear bikes
- Compact size keeps storage simple in a garage
Worth Noting
- Heavier than aluminium options at roughly 330 to 400 grams each
- Diamond hole fit only, so spindle shape needs checking
- Left side only, length must match the right crank
Who It Suits Best
Riders wanting a steel left crank in a two pack for mountain or road bikes.
Where Another Pick Suits Better
Epessa Crank Arm Left 170mm suits riders preferring lighter aluminium over steel weight.
Rhinenet Left 170mm Bicycle Crank Arm Black targets hybrid, folding, electric and mountain bikes with a single left arm. The black finish and polished surface give a clean look that blends with many frame colours.
Steel and aluminium alloy are listed for this crank set, with a 170mm length measured without overlap from the chainring bolt. You get one arm per pack, which keeps the purchase focused when only the left side needs replacing. In practice, that single piece approach works well for a quick repair, though a spare is not included.
A Strong Showing
- 170mm length fits a broad range of bike types
- Black polished finish suits many builds
- Lightweight, compact arm simplifies fitting
- Single arm pack avoids paying for unneeded parts
Trade Offs
- Diamond hole version listed, spindle shape needs confirmation
- Left side only, right crank remains original
BAFANG E-Bike Crank Arm Aluminium Alloy 2 Pieces stands apart as a matched pair for Bafang BBS middle drive motors. Both left and right arms arrive together, which removes the guesswork of pairing a new left arm with an older right arm.
Aluminium alloy construction aims to handle tough terrain and harsh weather on electric bikes. For daily e-bike commuting, a matched pair helps keep pedal feel consistent on both sides. Compatibility with the specific BBS motor setup remains the key check before ordering.
Bright Spots
- Left and right arms supplied as a matched set
- Aluminium alloy build for e-bike use
- Designed for Bafang BBS middle drive systems
Minor Gripes
- Pair format suits Bafang setups, not standard bikes
- Motor compatibility needs confirming before fitting
Made For
Owners of Bafang BBS middle drive e-bikes needing both arms replaced.
Consider Something Else When
Rhinenet Left 170mm Bicycle Crank Arm Black suits standard bikes without a mid-drive motor.
BlueStars Crank Left Arm Diamond Hole 170mm Black focuses on finish and fit for everyday riding. Solid aluminium alloy with a polished surface gives a tidy black look for road, mountain and folding bikes.
Measurements list 170mm centre to centre and 198mm total length, with a diamond hole at the rear. For regular commuters, that precise sizing helps keep cadence and pedal height aligned with the right side. The arm cannot move to the right pedal, so side marking matters during fitting.
The Case For It
- Aluminium alloy balances strength and low weight
- Polished finish resists fading in outdoor use
- 170mm centre to centre length matches common setups
- Precision forging supports steady pedal force transfer
- Easy to clean surface for regular maintenance
Room For Improvement
- Diamond hole only, square spindle bikes need another option
- Left side only, right crank must already match in length
S SYDIEN 152mm Bike Crank Arm Diamond Hole brings a shorter option to the list, at 152mm rather than the common 170mm. That shorter length can suit folding bikes and smaller frames where a full length crank feels awkward.
Iron material construction gives a sturdy, hard-wearing feel in a compact shape. Rounded end ergonomics and a diamond hole connection aim to simplify fitting on city, folding and exercise bikes. Shorter cranks change pedal circle size, so matching the original length keeps leg extension consistent.
Where It Delivers
- 152mm length fits folding and city bike geometry
- Iron build offers high strength and hardness
- Rounded end shape aids comfortable fitting
- Lightweight, small profile eases storage
Points To Note
- 152mm length differs from 170mm models, original length must match
- Diamond hole only, spindle shape needs checking
- Left side only, incorrect side fitting will not work
Right Fit For
Riders with folding, city or smaller bikes that use a 152mm left crank.
Not Your Match When
BlueStars Crank Left Arm Diamond Hole 170mm Black suits standard 170mm geometry better.
Ebike Crank Arm Diamond Hole offers unusual sizing flexibility, with lengths from 89mm up to 175mm listed. Shorter cranks in that range can suit extreme sports riding, while longer sizes cover standard e-bike and mountain bike setups.
Forged Alloy 6061 construction provides high strength and hardness in a sturdy arm. You can choose a single arm for mountain, road, folding, cargo, city or e-bike use, or a pair for electric bicycles with mid mounted motors. Torque guidance of 35 to 45 N/m helps prevent damage during fitting.
Points In Its Favor
- Ten length options from 89mm to 175mm
- Forged Alloy 6061 for strength and hardness
- Single arm or pair formats for different bike types
- Left and right markings simplify correct side fitting
- Crank bolts included in the package list
Softer Spots
- Diamond hole interface only, spindle shape must match
- Pair format limited to electric bicycles
- Torque limit of 45 N/m needs care during installation
Left Crank Arm Diamond Hole Square Hole covers five lengths from 152mm to 175mm in forged Alloy 6061. That range helps match junior bikes, folding bikes and standard mountain or road setups without hunting through separate listings.
The arm fits 9/16 inch pedals and includes crank bolts in the package. For riders replacing a single left side, the focused pack keeps things simple across city, belt drive and exercise bikes. Torque guidance of 35 to 45 N/m applies here as well, with a clear warning against exceeding 45 N/m.
Positive Takeaways
- Five lengths from 152mm to 175mm
- Forged Alloy 6061 for strength and hardness
- 9/16 inch pedal fit for standard pedals
- Crank bolts included for fitting
Small Caveats
- Left side only, marked L for correct fitting
- Spindle interface must match the existing setup
- Torque should stay within 35 to 45 N/m
Length, Hole Shape, and Material: What Matters Most
Crank Length and Pedal Height
Crank length is measured centre to centre from the spindle to the pedal hole. A 170mm arm creates a larger pedal circle than a 152mm arm, which changes leg extension at the bottom of the stroke. Keeping both sides equal prevents an uneven pedal height that feels awkward on longer rides.
Spindle Interface Compatibility
The spindle interface is the shape where the crank meets the axle, commonly square or diamond. Both sides should share the same shape, since mixing interfaces can cause poor seating and uneven rotation. Checking the original arm before ordering saves a return trip.
Material and Bike Type
Steel arms offer strength with added weight, while aluminium alloy arms save weight for commuters and road riders. E-bike mid-drive systems often need matched pairs rather than a single left arm. Matching material and format to the bike type keeps the replacement practical.
Two Checks That Save Headaches Later
Common Shopping Slip Ups
Ordering by brand alone causes many mismatches, since length and hole shape vary across models. Skipping a spindle check leads to a crank that will not seat properly. Measuring the original arm first avoids both problems.
What The Higher Tier Buys
Higher tier options often add forged construction, broader length ranges and matched pairs for electric systems. Those features help riders with unusual sizing or mid-drive motors. Standard 170mm bikes rarely need that extra flexibility.
Frequently Asked Questions
How do I know whether a crank arm will fit?
Measure the centre to centre length and check the spindle hole shape on the original arm. Matching both details gives the strongest chance of a clean fit.
What is the difference between square and diamond holes?
Square and diamond describe the shape of the hole that slides onto the spindle. The two shapes are not interchangeable, so the replacement must match the existing spindle.
Can a left crank arm go on the right side?
No, left and right arms are shaped for opposite sides and marked accordingly. Fitting a left arm on the right will not work correctly.
Why do some packs include two arms and others only one?
Single arms suit repairs where only one side is worn or damaged. Pairs suit e-bike systems or riders replacing both sides at once for matched wear.
What torque should crank bolts use?
Several forged alloy models list 35 to 45 N/m for crank bolts. Staying within that range helps protect the crank during installation.
Closing Remarks
For a standard 170mm replacement, Epessa Crank Arm Left 170mm and BlueStars Crank Left Arm Diamond Hole 170mm Black cover the most common needs. Shorter folding bikes point toward S SYDIEN 152mm Bike Crank Arm Diamond Hole, while Bafang owners should look at the matched pair. Matching length and hole shape first keeps the choice simple.