Best mid-drive motor for 2020 Stumpjumper?

[Fat Rabbit]

Hey, 2020 stumpjumper mid-drive motor recommendation

 

[Greg Watts]

Hey, 2020 stumpjumper mid-drive motor recommendation

@Fat Rabbit, welcome to the forum. I should be upfront: converting a 2020 Stumpjumper (the acoustic version) to a mid-drive eMTB is a project that ranges from "ambitious" to "inadvisable" depending on how attached you are to your frame warranty, bottom bracket shell, and general sanity.

The 2020 Stumpjumper wasn't designed around a motor, so you're looking at aftermarket conversion kits rather than anything from Bosch, Shimano, or the other OEM-only manufacturers. That narrows your options considerably.

The Bafang BBSHD is the most established DIY conversion motor out there. It's been in production for close to a decade, it's powerful enough for trail riding, and spare parts are readily available on AliExpress if anything goes wrong. The main drawback is that it lacks a torque sensor (it's cadence-sensing), so the power delivery isn't as refined as what you'd get from a purpose-built eMTB. It also adds a fair chunk of weight low and centre, which changes the handling character of a bike that was designed to be light and nimble. The Tongsheng TSDZ2 is the other popular option, notably cheaper and does have a torque sensor for more natural-feeling assistance, though it's less powerful and better suited to lighter duty riding.

There are some real concerns worth flagging though. The Stumpjumper's carbon frame (if that's what you've got) was absolutely not engineered for the stresses a mid-drive motor puts through the bottom bracket area. Even the alloy version wasn't designed for it. You're voiding any frame warranty, and there's a genuine risk of structural damage over time. The rear triangle and linkage will also be under more stress than Specialized ever intended. Battery mounting is another headache, as you'll be strapping something to the downtube or running a bag setup, neither of which is elegant.

If you're set on this, the Bafang BBSHD with a 52V battery is the most proven route. But if you're open to the honest answer: selling the Stumpjumper and putting the conversion budget (easily £800-1500 for motor, battery, display, and installation) towards a purpose-built eMTB would give you a vastly better riding experience, better integration, better warranty, and likely better resale value down the line. Even something second-hand with a Bosch Gen 4 motor would run rings around a conversion kit in terms of refinement.

Happy to dig into either path in more detail if you let me know which way you're leaning, and whether your frame is carbon or alloy.