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Front fork special made for e-bike hub motor mounting

Published: 04 December 2022
Last updated: 30 January 2024

Why is a special front fork for hub motor mounting needed?

For DIY converting bicycles to ebikes, hub motors are available. However, surprisingly, no suitable front forks exsist. Standard front forks are not optimized for hub motor mounting, which can pose a risk. Therefore, I have had a carbon fork produced - the Maxun fork - specifically designed for hub motors:

Carbon front fork for hub motorMaxun front fork special made for e-bike hub motor mounting

The advantages of the Maxun front fork

  • With the Maxun fork, you can make a professional and light weight e-bike.
  • Some hub motors, such as the dual-speed Xiongda, require a 110mm-wide front fork instead of the standard 100mm width.
  • There are no 110mm-wide rigid forks on the market with U-shaped dropouts.
  • Dropouts are often made of aluminum, which is too weak for hub motors. This necessitates an additional torque arm, but this is not an elegant construction.
  • The current trend is moving toward forks with thru-axles, but these are incompatible with hub motors. The Maxun fork presents an excellent alternative.
  • Common U-shaped dropouts have a slot diameter of 9mm, so these need to be filed down to 10mm. It is manually challenging to execute with high precision.

Hub motor torque armCommon hub motor torque arm

The Maxun carbon fork offers a nicer solution for mounting hub motors:

Maxun fork4Maxun carbon fork with integrated torque arm

The need for extra strong U-shape dropouts

Most hub motors have around 50 Nm of torque. This is not a problem at all for front forks itself, because the stress during braking is many times higher. The big problem lies in the strength of the dropouts and their attachment. Common U-shaped dropouts, often made of aluminum, are far too weak for the direct mounting of hub motors; they will easily break or bend apart. Extra strong dropouts are required that can withstand the motor torque without the necessity of using torque arms.

Aluminium fork dropout failureAluminium fork dropout failure

Flat mount disk brakes

The fork must be compatible with flat mount disc brakes. The reason for this is that disk brakes are the standard these days. Flat mount makes it possible to use the lightweight M9100 Shimano brakes without an adapter:

Shimano XTR M9100 caliper 193gramShimano XTR M9100 caliper 193gram

Larger dropout spacing of 110mm

Since I will use the disc brake version of the Xiongda 2-speed hub motor, the fork dropout spacing should be 110mm. However, the normal width of a hub-motor is 100mm. So, if we want to use a 100mm hub motor, two 5mm spacers are needed on each side. However, this will course that the disc brake is 5mm out of line with the caliper.

The solution is to mount 100mm hub motors asymmetrically with a 10mm spacer on the right side. The rim must of course be aligned in the center of the front fork. But that is not a problem since we can spoke the rim asymmetric, that is a common practice.

The dropout spacing of 110 mm has an additional advantage. Some hub motors with disc brake seem to be a little too wide to mate decently with the caliper when used in a 100mm wide fork.

Maxun front fork technical specification summary

  • Comply with the standard ISO 4210-6
  • Wheel diameter: 28” and 29”
  • Dropout spacing: 110mm
  • Flat mount disk brakes
  • For hub motors with 12 and 14mm axle diameters
  • Compatible for all kind of headsets

Bicycle Safety Standards

Standards have been drawn up for bicycle frames. In Europe, the ISO 4210 standard is used. Taiwan derived its standard from this: the TBIS 15194. In these standards, mountain bikes are subjected to the heaviest test forces. In the tests, the front fork is maximally exposed to the forces shown in the figure below. The front fork is also tested for fatigue during approximately 100,000 test cycles.

Front fork Safety StandardsFront fork Safety Standards

Building a 13kg e-bike

I want to build lightweight e-bikes of about 13kg, these will also be used for my Maxun One solar bike. Carbon forks are needed to save weight. I have figured out which motor is most suitable for this. In my opinion, the Xiongda 2-speed front wheel hub motor is the best choice. You can read more about it here: Building the most lightweight ebike with the Xiongda 2-speed hub motor.

Because I do not want to make any concessions to weight, quality and design, I want to have custom carbon forks manufactured specially made for hub motor mounting.

Gravel carbon frames metallic blueGravel carbon frames metallic blue