Introduction: A 150mm rear dropout is a frame dimension that has to be measured, not assumed, when a 29-inch MTB frame is matched to a high-power hub wheel.
A 29-inch wheel and a 150mm rear dropout describe two different parts of the same build. One is about wheel diameter; the other is about axle width and the space between the rear frame ends. That distinction matters when a workshop is studying Ebike conversion kits for a high-power build, because a frame can accept a 29-inch wheel and still have the wrong rear spacing for a given hub motor. This guide maps the measurement, the common spacing families, and the wheel-width difference that separates a 29er MTB kit from a fat-bike kit.
What rear dropout spacing measures on a 29-inch MTB frame
Rear dropout spacing is the distance between the inside faces of the two rear frame ends where the hub axle sits. On a 29-inch MTB frame, that number has nothing to do with the 29-inch rim diameter. A 29-inch wheel can be paired with different rear axle widths, and a frame with 150mm spacing can be built around different wheel sizes. The dropout measurement tells you whether the hub axle will seat between the frame ends with the right alignment, brake position, and tyre clearance. It is a frame-and-axle fit number, not a tyre-size label. In a workshop, the useful measurement is taken with the rear wheel out and the dropout faces clean. A caliper or dropout-width tool is placed against the inside faces at the axle centre line, with the frame supported so the ends are not squeezed together. The axle should later seat fully into both dropouts, not rest on one shoulder. The referenced 29-inch kit uses a 150mm rear dropout and a 29-inch Sunringle MTX39 rim, so the frame has to show a stable 150mm reading at the same points. Frame material matters here: steel, aluminium, and carbon rear ends behave differently under load and should be judged by the actual frame, not by a generic rule. A frame is a candidate for this hub only after the real dropout faces and axle seating are checked; widening a frame is not a universal safe action.
How 135mm, 142mm, 148mm and 150mm spacing differ in frame design
These numbers belong to different frame and axle families, and the label alone does not tell you whether a conversion will fit. A 135mm rear end is an older quick-release mountain bike standard, while 142mm and 148mm are common thru-axle and Boost-era dimensions. A 150mm rear end sits in a heavier-duty space, often linked to downhill or high-load frames. The important point for a 29er build is that the frame has to be designed or confirmed for the axle width, the brake mount, the chainline, and the tyre clearance as a package.
- Common MTB spacing families: 135mm, 142mm, and 148mm cover many quick-release and thru-axle mountain bikes, but each family has its own axle standard, frame end shape, and brake alignment. Treat them as common starting points that still need frame-specific evaluation, not as interchangeable numbers.
- Downhill or heavy-duty spacing: 150mm spacing appears on heavier-duty rear ends and some high-power hub builds because it gives the hub and frame more room to work with a wide axle assembly. It is a frame design choice, not simply a 135mm frame pulled wider by 15mm.
- The frame-strength question behind 150mm: moving from 135mm, 142mm, or 148mm to 150mm changes dropout alignment, chainline, brake position, and tyre clearance. The 72V 5000W rear hub motor in this example is rated at 150 N. m nominal maximum torque and uses a 16T single-speed freewheel, so the rear end has to be evaluated as a complete load path, not as a gap that happens to measure 150mm.
A 29-inch frame with 150mm spacing can be a valid starting point for a high-power hub build, but it is a specific frame type. The iEEPower ebike kit in this example includes dual torque arms, a 29-inch Sunringle MTX39 rim, and 36-hole 12G spokes as part of its hardware package; those parts support the build, while the dropout measurement remains the first fit question. The 5000W system is an off-road or closed-course power level, so the frame discussion should stay tied to that use rather than to a road-legal bicycle claim.
Why a 29-inch MTB conversion kit is not a fat-bike kit
A fat-bike kit is defined by very wide tyres and the frame clearance to hold them, not by a large wheel diameter. Fat bikes commonly run tyres around 3.8 to 5.0 inches wide, with wide rims, wide hub spacing, and broad frame and fork clearances. A 29-inch MTB wheel is a large-diameter wheel, but it usually carries a narrow or mid-width mountain tyre. The 29-inch Sunringle MTX39 rim in this kit is a 29er MTB rim, and the 150mm rear dropout is an axle-width specification. Neither detail turns the kit into a fat-bike system. That difference shows up in the frame space around the wheel. A 29er frame needs clearance for the tyre height and the tyre width it is designed around, plus room for mud and frame flex. A fat frame needs much more width through the rear triangle and fork. A 150mm rear dropout can be part of a heavy-duty 29er build, but the tyre and rim still have to match the frame’s 29er clearance. The same logic applies at the front: fork clearance is a separate check from rear dropout spacing, and both have to suit the chosen wheel and tyre. The practical reading is simple: treat this as a narrow or mid-tyre 29er system with a high-power rear hub motor, not as a fat E-Bike Conversion kit. The 72V 5000W motor, 150 N. m nominal maximum torque, 16T single-speed freewheel, and dual torque arms describe a high-load off-road build. The 29-inch wheel size describes the rolling diameter. A buyer who needs a fat-bike kit should look for the wide-tyre frame and clearance package, while a 29er build should confirm the 150mm dropout, rear triangle space, and fork clearance before matching the wheel.
Conclusion
A 150mm rear dropout is a measurement to confirm on the actual 29-inch MTB frame, not a promise that every 29er can take a high-power hub wheel. The spacing families of 135mm, 142mm, and 148mm are common reference points, while 150mm belongs to a heavier-duty frame discussion that includes axle seating, brake alignment, chainline, and tyre clearance. A 29-inch kit with a Sunringle MTX39 rim and a 150mm hub is a 29er system, not a fat-bike kit. The right next step is to compare the frame’s measured dropout width and clearance with the kit’s stated dimensions, then keep 5000W use to off-road or closed-course settings.
FAQ
Q: How do I measure a 150mm rear dropout on a 29-inch MTB frame?
A: Remove the rear wheel, clean the dropout faces, and measure the inside-to-inside distance at the axle centre line with a caliper or dropout tool. Support the frame so the ends are not squeezed, take the reading more than once, and confirm that the hub axle seats fully into both dropouts. A stable 150mm reading at the axle line is the starting point; the actual frame still needs a clearance and material check.
Q: What is the difference between 135mm, 142mm, 148mm and 150mm rear spacing?
A: The numbers describe different rear axle and frame-end families. 135mm is an older quick-release MTB spacing, 142mm and 148mm are common thru-axle and Boost dimensions, and 150mm is a heavier-duty spacing used on some downhill or high-load frames. Each family has its own axle standard, brake alignment, chainline, and frame clearance, so the label is a reference point rather than a universal fit guarantee.
Q: Is a 29-inch MTB conversion kit the same as a fat E-Bike Conversion kit?
A: No. A 29-inch MTB kit uses a large-diameter 29er wheel with a narrow or mid-width mountain tyre, while a fat E-Bike Conversion kit is built around very wide tyres and the wider frame and fork clearance they need. A 150mm rear dropout and a 29-inch Sunringle MTX39 rim describe a 29er high-power build, not a fat-bike system.
Sources / References
Hub Overhaul and Adjustment: Cup and Cone Style | Park Tool
Bicycle Requirements Business Guidance | CPSC.gov
Related Examples
iEEPower 29-inch 72V 5000W Ebike Conversion Kit