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Carbon front fork for hub motors

Carbon forks are not suitable for direct mounting of hub motors. That's why I want to have strong carbon forks made especially for hub motors. Actually, I need these for my Maxun One solar bike, you can read more about this later on. Because expensive molds have to be made for production, I will sell this fork also to other people.

Carbon front fork for hub motor
Carbon front fork for hub motor

Normal carbon front forks are not suited well for hub motors

  • Common dropouts are too weak for direct mounting hub motors, this is why we always need a separate torque arm, which is of course a very ugly solution.
  • Hub motors have so-called slotted axles. The axle and slot-diameter are resp 12 and 10mm. Common U-shape dropouts are made for quick release wheels, have a slot-diameter of 9mm, so these need to be filed out.
  • The trend is going towards true-axles, but hub motors require U-shape dropouts for the mounting.

Hub motor slotted axle
Hub motor slotted axle

No more ugly torque arms needed
No more ugly torque arms needed

Building a 13kg e-bike

I want to build some 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.

Workswellbikes has made for me some carbon frames in metallic blue. 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.

Workswellbikes gravel carbon frames metallic blue
Workswellbikes gravel carbon frames metallic blue

Is a carbon fork strong enough for a hub motor?

Presumably, all carbon front forks are strong enough for small 250W hub motors. I have driven more than 15000km with a 1.8kg Cute Q-85SX hub motor in a carbon front fork, with the use of a torque arm. Read more about this in the article HERE.

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 the above 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 Standards
Front fork Safety Standards

The need for extra strong U-shape dropouts

Most ebikes have around 50 Nm of torque. This is not a problem at all for front forks itself, because the stress with braking is many times higher. The big problem is the strength of the dropouts and its attachment. Common U-shape dropouts, which are often made of aluminum, are far too weak for direct mounting of hub motors, they will easily break or be bent apart.
Extra strong dropouts have to be made that can withstand the motor torque, without using torque arms:

Dropout for hub motor
Dropout for hub motor

I want to make the dropouts of 7mm thick forged chromium-vanadium alloy steel, because this is also the favourite material of wrenches:

Forged chromium vanadium alloy steel for wrenches 1
Forged chromium vanadium alloy steel for wrenches

Forged chromium vanadium alloy steel for wrenches 2
Forged chromium vanadium alloy steel for wrenches

Special attention should be given to the attachment strength of the dropout to the front fork. With a hub motor, the weight to which the dropout is subjected becomes more than 2x higher. Also consider the vibrations when driving.

A potential problem can be that a hub motor can get very hot and so can be its axle. That heat is passed on to the dropout and fork and that has to be taken into account too.

The hub motor that I'm going to use is the 3.2kg Xiongda 2-speed. A bicycle wheel of 2.5 kg becomes more than twice as heavy with this hub motor. The fork must be suitable for hub motors up to 4kg. With heavier motors, it is nonsense to use a carbon fork to save weight.

It is important to know that in the test standards, the dropout is very heavily loaded with 5000N, see for example the following description in the Taiwanese TBIS 15194 EPAC standard:

Test method: Mount the fork steerer securely in a suitable rigid mount, keeping any clamping forces away from the fork-crown, and apply a tensile force of 5000 N distributed equally to both drop-outs for 1 min in a direction parallel to the axis of the fork steerer.

So, we shouldn't have to worry about the strength of the attachment of the dropout to the front fork anyway.

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 193gram
Shimano XTR M9100 caliper 193gram

Dropout distance 110mm

Since I will use the disc brake version of the Xiongda 2-speed hub motor, the fork dropout distance 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 distance 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.

Front fork technical specification summary

  • Comply with the standard ISO 4210-6
  • Wheel diameter: 28” and 29”
  • Dropout distance: 110mm
  • Flat mount disk brakes
  • U-shape dropout wide: 10mm
  • Compatible for all kind of headsets

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