GX DH Rear Derailleur vs. ZEE FC-M640 Crankset: A Downhill Match

These two parts solve different problems

The SRAM GX DH Rear Derailleur and the Shimano ZEE FC-M640 Crankset are not competing upgrades. One controls chain tension and shifting at the back of a downhill drivetrain; the other transfers your pedal input through the bottom bracket and crank arms. Comparing them directly only makes sense if you are deciding where to spend money on a downhill build, not choosing between two versions of the same component.

This comparison focuses on what each part is designed to do, what the supplied catalog confirms, and where the evidence stops. That matters for a downhill drivetrain setup because a rear derailleur and a crankset have different compatibility checks.

What the catalog confirms for each part

• GX DH Rear Derailleur: SRAM GX DH Rear Derailleur: X-ACTUATION offers precise and dependable 7-speed performance. The 7-speed figure is the key drivetrain constraint: this derailleur is intended for a 7-speed downhill system, not a general-purpose trail cassette.

  

• GX DH Rear Derailleur: SRAM GX DH Rear Derailleur: X-HORIZON design reduces shift force, ghost shifting and cage slap. These are the behaviors downhill riders notice most under rough, high-speed descending, so they are relevant to the rear-derailleur side of the decision.
• GX DH Rear Derailleur: SRAM GX DH Rear Derailleur: fully compatible with the SRAM X01 DH drivetrain. This is the only explicit compatibility statement in the supplied record; it points to a SRAM DH ecosystem rather than a Shimano one.
• ZEE FC-M640 Crankset: Shimano ZEE FC-M640 Crankset: thick wall aluminum construction with a reinforced steel axle. The catalog describes the crank as built for strength, which is the relevant quality for downhill use where crank arms take impacts.

  

• ZEE FC-M640 Crankset: Shimano ZEE FC-M640 Crankset: includes SM-BB52 68/73 bottom bracket. The included bottom bracket is the practical starting point for frame fit, but the supplied record does not state which shell width your frame needs.
• ZEE FC-M640 Crankset: Shimano ZEE FC-M640 Crankset: reinforced steel axle with 30% greater stiffness compared to Trail designated crank options. This is a manufacturer comparison claim in the catalog, not an independent test result; treat it as a design description rather than a measured guarantee.

How to choose between spending on the rear derailleur or the crankset

Use the state of your current build to decide, not the price alone. The catalog prices are USD 65.00 for the GX DH rear derailleur and USD 72.50 for the ZEE FC-M640 crankset, with comparison prices of USD 130.00 and USD 145.00 respectively; those are catalog figures, not a statement about performance.

• Your rear shifting is the weak point: If you already run a 7-speed SRAM DH drivetrain and the rear derailleur is worn or damaged, the GX DH is the more direct replacement. Its catalog description centers on shift precision and chain-slap control, which are rear-end concerns.
• Your crank arms or bottom bracket are the weak point: If the crankset is the part you need to replace, the ZEE FC-M640 is the relevant choice. The catalog lists two crank-length variants, but the supplied record does not give a rule for choosing between them; that decision needs your own fit information.
• You are building a mixed-brand drivetrain: The supplied evidence does not confirm that the GX DH rear derailleur and ZEE crankset are designed to work together. Each record describes its own ecosystem, so do not assume a SRAM derailleur and Shimano crank form a verified pairing just because both are downhill-oriented.
• You need a complete drivetrain answer: Neither record supplies cassette, shifter, chainring, or chain specifications. A downhill drivetrain setup needs those parts matched too, and the catalog evidence here is not enough to specify them.

Fit and compatibility checks before you buy

For the GX DH rear derailleur, the clearest check is the 7-speed requirement and the stated compatibility with the SRAM X01 DH drivetrain. The record does not list cage length, mounting standard, or maximum cassette size, so confirm those against your frame and existing drivetrain before ordering.

For the ZEE FC-M640 crankset, the catalog confirms an included SM-BB52 68/73 bottom bracket and two crank-length options. It does not state chainline, spindle type beyond the bottom bracket inclusion, or which frame shell widths are supported in practice. The 68/73 figure belongs to the bottom bracket model name in the catalog, not a recommendation for your bike.

Neither product record includes installation torque values, safety-critical setup steps, or maintenance intervals. If you need those, they are not confirmed by the supplied information and should come from the manufacturer's documentation for your specific frame and components.

Where each part makes the most sense

The GX DH rear derailleur makes sense when you are maintaining or completing a 7-speed SRAM downhill drivetrain and want the rear shifting behavior described in the catalog. The ZEE FC-M640 crankset makes sense when the crankset itself is the part being replaced and you can confirm bottom bracket fit for your frame.

What the catalog does not support is a claim that one part is better than the other overall. They are different components with different jobs. The honest takeaway is to identify which part your build actually needs, verify the specific compatibility points the catalog does list, and treat everything else as information to confirm before purchase.